Extraction Optimization in Food Engineering (Food Science and Technology)

Extraction Optimization in Food Engineering

edited by

Coustantina Tzia National Technical Universig of Athens Athens, Greece

George Liadakis 700SE Pireas, Greece

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FOOD SCIENCE AND TECHNOLOGY A Series of Monographs, Textbooks, and Reference Books EDITORIAL BOARD Senior Editors Owen R. Fennema University of Wisconsin-Madison Y. H. Hui Science Technology System Marcus Karel Rutgers University (emeritus) Pieter Walstra Wageningen University John R. Whitaker University of California-Davis Additives P. Michael Davidson University of Tennessee-Knoxville Dairy science James L. Steele University of Wisconsin-Madison Flavor chemistry and sensory analysis John H. Thorngate 111 University of California-Davis Food engineering Daryl B. Lund University of Wisconsin-Madison Food lipids and flavors David B. Min Ohio State University Food proteindfood chemistry Rickey Y. Yada University of Guelph Health and disease Seppo Salminen University of Turku, Finland Nutrition and nutraceuticals Mark Dreher Mead Johnson Nutritionals Phase transifion/food microstructure Richard W. Hartel University of Wisconsin-Madison Processing and preservation Gustavo V. Barbosa-Canovas Washington State University-Pullman Safety and toxicology Sanford Miller University of Texas-Austin

1. Flavor Research: Principles and Techniques, R. Teranishi, 1. Hornstein, P. Issenberg, and €. L. Wick 2. Principles of Enzymology for the Food Sciences, John R. Whitaker 3. Low-Temperature Preservation of Foods and Living Matter, Owen R. Fennema, William D. Powrie, and Elmer H. Marth 4. Principles of Food Science Part I: Food Chemistry, edited by Owen R. Fennema Part II: Physical Principles of Food Preservation, Marcus Karel, Owen R. Fennema, and Daryl 5.Lund 5. Food Emulsions, edited by Stig E. Friberg 6. Nutritional and Safety Aspects of Food Processing, edited by Steven R. Tannenbaum 7. Flavor Research: Recent Advances, edited by R. Teranishi, Robert A. Flath, and Hiroshi Sugisawa 8. Computer-Aided Techniques in Food Technology, edited by Israel SWUY

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9. Handbook of Tropical Foods, edited by Harvey T. Chan 10. Antimicrobials in Foods, edited by Alfred Larry Branen and P. Michael Davidson 11. Food Constituents and Food Residues: Their Chromatographic Determination, edited by James F. Lawrence 12. Aspartame: Physiology and Biochemistry, edited by Lewis D. Stegink and L. J. Filer, Jr. 13. Handbook of Vitamins: Nutritional, Biochemical, and Clinical Aspects, edited by Lawrence J. Machlin 14. Starch Conversion Technology, edited by G. M. A. van Beynum and J. A. Roels 15. Food Chemistry: Second Edition, Revised and Expanded, edited by Owen R. Fennema 16. Sensory Evaluation of Food: Statistical Methods and Procedures, Michael O'Mahony 17. Alternative Sweeteners, edited by Lyn O'Brien Nabors and Robert C. Gelardi 18. Citrus Fruits and Their Products: Analysis and Technology, S. V. Ting and Russell L. Rouseff 19. Engineering Properties of Foods, edited by M. A. Rao and S. S. H. RiZVi 20. Umami: A Basic Taste, edited by Yojiro Kawamura and Morley R. Kare 21. Food Biotechnology, edited by Dietrich Knon 22. Food Texture: Instrumental and Sensory Measurement, edited by Howard R. Moskowitz 23. Seafoods and Fish Oils in Human Health and Disease, John E. Kinsella 24. Postharvest Physiology of Vegetables, edited by J. Weichmann 25. Handbook of Dietary Fiber: An Applied Approach, Mark L. Dreher 26. Food Toxicology, Parts A and B, Jose M. Concon 27. Modern Carbohydrate Chemistry, Roger W. Sinkley 28. Trace Minerals in Foods, edited by Kenneth T, Smith 29. Protein Quality and the Effects of Processing, edited by R. Dixon Phillips and John W. Finley 30. Adulteration of Fruit Juice Beverages, edited by Steven Nagy, John A. Attaway, and Martha E. Rhodes 31. Foodborne Bacterial Pathogens, edited by Michael P. Doyle 32. Legumes: Chemistry, Technology, and Human Nutrition, edited by Ruth H. Matthews 33. Industrialization of Indigenous Fermented Foods, edited by Keith H. Steinkraus 34. International Food Regulation Handbook: Policy 0 Science 0 Law, edited by Roger D. Middlekauff and Philippe Shubik 35. Food Additives, edited by A. Larry Branen, P. Michael Davidson, and Seppo Salminen 36. Safety of Irradiated Foods, J. F. Diehl

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37. Omega3 Fatty Acids in Health and Disease, edited by Robert S. Lees and Marcus Karel 38. Food Emulsions: Second Edition, Revised and Expanded, edited by KAre Larsson and Stig E. Friberg 39. Seafood: Effects of Technology on Nutrition, George M. figott and Barbee W. Tucker 40. Handbook of Vitamins: Second Edition, Revised and Expanded, edited by Lawrence J. Machlin 41. Handbook of Cereal Science and Technology, Klaus J. Lorenz and Karel Kulp 42. Food Processing Operations and Scale-Up, Kenneth J. Valentas, Leon Levine, and J. Peter Clark 43. Fish Quality Control by Computer Vision, edited by L. F. Pau and R. Olafsson 44. Volatile Compounds in Foods and Beverages, edited by Henk Maarse 45. Instrumental Methods for Quality Assurance in Foods, edited by Daniel Y. C. Fung and Richard F. Matthews 46. Listeria, Listeriosis, and Food Safety, Elliot T. Ryser and Elmer H. Marth 47. Acesulfame-K, edited by D. G. Mayer and F. H. Kernper 48. Alternative Sweeteners: Second Edition, Revised and Expanded, edited by Lyn O'Brien Nabors and Robert C. Gelardi 49. Food Extrusion Science and Technology, edited by Jozef L. Kokini, Chi-Tang Ho, and Mukund V. Karwe 50. Surimi Technology, edited by Tyre C. Lanier and Chong M. Lee 51. Handbook of Food Engineering, edited by Dennis R. Heldman and DarylB. Lund 52. Food Analysis by HPLC, edited by Leo M. L. Nollet 53. Fatty Acids in Foods and Their Health Implications, edited by Ching Kuang Chow 54. Clostridium botulinum: Ecology and Control in Foods, edited by Andreas H. W. Hauschild and Karen L. Dodds 55. Cereals in Breadmaking: A Molecular Colloidal Approach, Ann-Charlotte Eliasson and KAre Larsson 56. Low-Calorie Foods Handbook, edited by Aaron M. Altschul 57. Antimicrobials in Foods: Second Edition, Revised and Expanded, edited by P. Michael Davidson and Alfred Larry Branen 58. Lactic Acid Bacteria, edited by Seppo Salminen and Atte von Wright 59. Rice Science and Technology, edited by Wayne E. Marshall and James 1. Wadsworth 60. Food Biosensor Analysis, edited by Gabriele Wagner and George G. Guilbault 61. Principles of Enzymology for the Food Sciences: Second Edition, John R. Whitaker 62. Carbohydrate Polyesters as Fat Substitutes, edited by Casimir C. Akoh and Barry G. Swanson 63. Engineering Properties of Foods: Second Edition, Revised and Expanded, edited by M. A. Rao and S. S. H. Rizvi

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64. Handbook of Brewing, edited by William A. Hardwick 65. Analyzing Food for Nutrition Labeling and Hazardous Contaminants, edited by Ike J. Jeon and William G. lkins 66. Ingredient Interactions: Effects on Food Quality, edited by Anilkurnar G. Gaonkar 67. Food Polysaccharides and Their Applications, edited by Alistair M. Stephen 68. Safety of Irradiated Foods: Second Edition, Revised and Expanded, J. F. Diehl 69. Nutrition Labeling Handbook, edited by Ralph Shapiro 70. Handbook of Fruit Science and Technology: Production, Composition, Storage, and Processing, edited by D. K. Salunkhe and S. S. Kadarn 71. Food Antioxidants: Technological, Toxicological, and Health Perspectives, edited by D. L. Madhavi, S. S. Deshpande, and D. K. Salunkhe 72. Freezing Effects on Food Quality, edited by Lester E. Jeremiah 73. Handbook of Indigenous Fermented Foods: Second Edition, Revised and Expanded, edited by Keith H. Steinkraus 74. Carbohydrates in Food, edited by Ann-Charlotte Eliasson 75. Baked Goods Freshness: Technology, Evaluation, and Inhibition of Staling, edited by Ronald E. Hebeda and Henry F. Zobel 76. Food Chemistry: Third Edition, edited by Owen R. Fennema 77. Handbook of Food Analysis: Volumes 1 and 2, edited by Leo M. L. Nollet 78. Computerized Control Systems in the Food Industry, edited by Gauri S. Miffal 79. Techniques for Analyzing Food Aroma, edited by Ray Marsili 80. Food Proteins and Their Applications, edited by Srinivasan Damodaran and Alain Paraf 81. Food Emulsions: Third Edition, Revised and Expanded, edited by Stig E. Friberg and K5re Larsson 82. Nonthermal Preservation of Foods, Gustavo V. Barbosa-Canovas, Usha R. Pofhakamury, Enrique Palou, and Bany G. Swanson 83. Milk and Dairy Product Technology, Edgar Spreer 84. Applied Dairy Microbiology, edited by Elmer H. Marth and James L. Sfeele 85. Lactic Acid Bacteria: Microbiology and Functional Aspects: Second Edition, Revised and Expanded, edited by Seppo Salrninen and Affe von Wright 86. Handbook of Vegetable Science and Technology: Production, Composition, Storage, and Processing, edited by D. K. Salunkhe and S. S. Kadam 87. Polysaccharide Association Structures in Food, edited by Reginald H. Walter 88. Food Lipids: Chemistry, Nutrition, and Biotechnology, edited by Casimir C. Akoh and David 6. Min 89. Spice Science and Technology, Kenji Hirasa and Mitsuo Takemasa

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90. Dairy Technology: Principles of Milk Properties and Processes, P. Walstra, T, J. Geurts, A. Noomen, A. Jellema, and M. A. J. S. van Boekel 91. Coloring of Food, Drugs, and Cosmetics, Gisbert Offerstijffer 92. Listeria, Listeriosis, and Food Safety: Second Edition, Revised and Expanded, edited by Elliot T. Ryser and Elmer H. Marth 93. Complex Carbohydrates in Foods, edited by Susan Sungsoo Cho, Leon Prosky, and Mark Dreher 94. Handbook of Food Preservation, edited by M. S h a h Rahman 95. International Food Safety Handbook: Science, International Regulation, and Control, edited by Kees van der Heijden, Maged Younes, Lawrence Fishbein, and Sanford Miller 96. Fatty Acids in Foods and Their Health Implications: Second Edition, Revised and Expanded, edited by Ching Kuang Chow 97. Seafood Enzymes: Utilization and Influence on Postharvest Seafood Quality, edited by Norman F. Haard and Benjamin K. Simpson 98. Safe Handling of Foods, edited by Jeffrey M. Farber and Ewen C. D. Todd 99. Handbook of Cereal Science and Technology: Second Edition, Revised and Expanded, edited by Karel Kulp and Joseph G. Ponte, Jr. 100. Food Analysis by HPLC: Second Edition, Revised and Expanded, edifed by Leo M. L. Nollet 101. Surimi and Surimi Seafood, edited by Jae W. Park 102. Drug Residues in Foods: Pharmacology, Food Safety, and Analysis, Nickos A. Botsoglou and Dimitrios J. Fletouris 103. Seafood and Freshwater Toxins: Pharmacology, Physiology, and Detection, edited by Luis M. Botana 104. Handbook of Nutrition and Diet, Babasaheb 6. Desai 105. Nondestructive Food Evaluation: Techniques to Analyze Properties and Quality, edited by Sundaram Gunasekaran 106. Green Tea: Health Benefits and Applications, Yukihiko Hara 107. Food Processing Operations Modeling: Design and Analysis, edited by Joseph lrudayaraj 108. Wine Microbiology: Science and Technology, Claudio Delfini and Joseph V. Formica 109. Handbook of Microwave Technology for Food Applications, edited by Ashim K. Daffa and Ramaswamy C. Anantheswaran 110. Applied Dairy Microbiology: Second Edition, Revised and Expanded, edited by Elmer ti.Marth and James L. Steele 111. Transport Properties of Foods, George D. Saravacos and Zacharias B. Maroulis 112. Alternative Sweeteners: Third Edition, Revised and Expanded, edited by Lyn O’Brien Nabors 113. Handbook of Dietary Fiber, edited by Susan Sungsoo Cho and Mark L. Dreher 114. Control of Foodborne Microorganisms, edited by Voay K. Juneja and John N. Sofos 115. Flavor, Fragrance, and Odor Analysis, edited by Ray Marsili

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116. Food Additives: Second Edition, Revised and Expanded, edited by A. Larry Branen, P. Michael Davidson, Seppo Salminen, and John H. Thorngate, 111 117. Food Lipids: Chemistry, Nutrition, and Biotechnology: Second Edition, Revised and Expanded, edited by Casimir C. Akoh and David B. Min 118. Food Protein Analysis: Quantitative Effects on Processing, R. K. Owusu-Apenten 119. Handbook of Food Toxicology, S. S. Deshpande 120. Food Plant Sanitation, edited by Y. H. Hui, Bernard L. Bruinsma, J. Richard Gorham, Wai-Kit Nip, Phillip S. Tong, and Phil Ventresca 121. Physical Chemistry of Foods, Pieter Walstra 122. Handbook of Food Enzymology, edited by John R. Whitaker, Alphons G. J. Voragen, and Dominic W. S. Wong 123. Postharvest Physiology and Pathology of Vegetables: Second Edition, Revised and Expanded, edited by Jerry A. Bartz and Jeffrey K. Brecht 124. Characterization of Cereals and Flours: Properties, Analysis, and Applications, edited by Goniil Kalefung and Kenneth J. Breslauer 125. International Handbook of Foodborne Pathogens, edited by Marianne D. Miliotis and Jeffrey W. Bier 126. Food Process Design, Zacharias B. Maroulis and George D. Saravacos 127. Handbook of Dough Fermentations, edited by Karel Kulp and Klaus Lorenz 128. Extraction Optimization in Food Engineering, edited by Constantina Tzia and George Liadakis 129. Physical Principles of Food Preservation: Second Edition, Revised and Expanded, Marcus Karel and Daryl B. Lund

Additional Volumes in Preparation Handbook of Vegetable Preservation and Processing, edited by Y. H. Hui, Sue Ghazala, Dee M. Graham, K. D. Murrell, and Wai-Kit Nip Food Emulsions: Fourth Edition, Revised and Expanded, edited by Stig E. Friberg, K i r e Larsson, and Johan Sjoblom Handbook of Frozen Foods, edited by Y. H. Hui, Paul Cornillon, Isabel Guerrero Legarreta, Miang Lim, K. D. Murrell, and Wai-Kit Nip Handbook of Food and Beverage Fermentation Technology, edited by Y. H. Hui, Lisbeth M. Goddik, Aase Solvejg Hansen, Jytte Josephsen, Wai-Kit Nip, Peggy S. Stanfield, and Fidel Toldra

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Industrialization of Indigenous Fermented Foods: Second Edition, Revised and Expanded, edited by Keith H. Sfeinkraus Genetic Variation in Taste Sensitivity, edited by John Prescott and Beverly J. Tepper Handbook of Food Analysis: Second Edition, Revised and Expanded: Volumes 1, 2, and 3, edited by Leo M. L. Nollet

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Preface

Extraction is an important separation process that is extensively used in various applications in food engineering. Extraction is used either to recover important food components, being a main processing stage for the production of certain food products (sugar, oils, proteins), or to isolate desired components (antioxidants, flavors). It also removes contaminants and other undesirable components (alkaloids, cholesterol) from food sources. Extraction, as a major operation of the manufacturing process in many food industries, requires a food engineer to have knowledge of the thermodynamics, theory, methods, and systems of extraction, while the feasible design and efficient operation of an extraction process requires knowledge of the technological parameters of the process and of optimization methods respectively. Solvent or water extraction, liquid–liquid extraction, and leaching have already been incorporated in basic food processing, while supercritical fluid extraction (SFE) is a promising alternative method for new or improved applications in food engineering; SFE is recommended for the production of high-value food components as natural food products. Despite the great variety of extraction applications already in use, it is difficult to interpret and transfer theory to application at the industrial scale. Even in recent applications of extraction, optimal extraction or separation conditions are obtained empirically. One primary goal of food process engineering is finding out how an extraction process works best—achieving the optimum in respect of yield, quality, or cost—and consequently finding the adjustable processing factor values that produce the optimum.

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Preface

Optimization of extraction processes is needed either for extraction processes that are already in use or for future process designs for certain applications in food engineering. Consequently, it is important for the food industry to optimize food extraction processes; that is, to attain the optimum operating conditions (yield, quality, operation time, and cost) using appropriate methods and techniques. In this book, we review the extraction methods used in food processing from fundamental theory to optimum practical application using the relevant equipment and the appropriate methods of optimization. All of the contributors are experts in the field of extraction, in research as well as in applications in industrial food processes. This book will be useful for food process engineers who employ extraction processes. In order to introduce and apply an extraction process in the food industry, the knowledge of thermodynamic principles and the existing extractive systems is necessary. Irrespective of the extraction method used, the feasibility of the process or the system is affected by the operation parameters of the process or system. Consequently, the best result in respect of yield or quality characteristics of the final product should be achieved. That is the objective of the optimization of a process or a system. An elementary review of thermodynamics is presented in Chapter 1, containing the basic laws, glossary, concepts, and relations of thermodynamics, particularly of chemical thermodynamics covering mixtures, solutions, phase equalibria, chemical reactions, etc. Chapter 1 presents fundamental knowledge useful for the understanding of extraction theory, operation, and applications in food processing. Solid–liquid extraction or the leaching proces is described in Chapter 2. The characteristics and steps, as well as the basic variables affecting the extraction operation in relation to the solvent and the microstructure of the solid being extracted, are described. The relationship between the solid microstructure and the extraction rate based on the mass transfer mechanism due to liquid diffusion inside the solid is also explained. Supercritical fluid extraction (SFE) using supercritical fluids (SCFs), a novel method providing the possibility for “green” processing of foods, is reviewed in Chapter 3. The basic (physical and chemical) properties of SCFs, particularly of the commonly used SC-CO2, and the key factors that play significant roles in the success of the SFE operation (as solubility of solutes in SCFs, phase equilibria, mass transfer) are discussed; the effective use of SCFs in extraction and fractionation of food-related materials is also discussed. The methods and equipment used in conventional solvent extraction and the relative extraction systems that have been applied in food industrial processes are presented in Chapter 4 with the equipment required for solid pretreat-

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Preface

v

ment and solvent recovery. Similarly, for the alternative SFE method the selection and design of the required equipment are discussed. Optimization theory and appropriate methods involved in design and process operation having economical and technological interest for food engineers are discussed in Chapter 5. An extraction process should operate efficiently by determining the operation time in order to achieve the best yield. The applications of optimization in food engineering and especially in food extraction (conventional or SFE) processing are reviewed. In the following chapters, the most important applications of extraction processes in food production industry are presented under the aspect of optimization of the already operating extraction systems; novel and alternative operations and case studies are also presented. Vegetable oil extraction of oil-bearing materials is covered in Chapter 6. The most efficient method of solvent extraction and the factors affecting the operation such as the solvent characteristics, extraction temperature, solid pretreatment, modes of operation, and equipment are discussed. Commercial units used for oil extraction and extraction technology for oilseeds with the pre- and post-treatment are presented; extraction of essential oils is also discussed. Extraction processes for protein isolation from various edible protein sources (oilseeds, cereals, etc.) and production of protein products with many applications in food systems are covered in Chapter 7. Commercial processing and advanced methods of vegetable protein isolates are discussed from the viewpoint of yield and quality (nutritional, organoleptic, functional properties) of protein products. Sugar extraction from sugar beet and sugar cane is presented in Chapter 8, as extraction is one of the major stages in the sugar manufacturing industry. Characteristics of sugar beet and cane, parameters affecting the extraction processes, and the optimization of the industrial process of beet sugar are discussed. Techniques of extraction, extractors, and other equipment required for industrial sugar production are presented. Extraction of starch and carbohydrates from corn is also discussed. The extraction processes for isolation of desired components such as flavor and aroma substances or compounds with antioxidant activity (natural antioxidants) from plant sources are reported in Chapters 9 and 10, respectively. The main sources of natural antioxidants and synergists, the relative extraction processes, and the effect of processing parameters, as well as the conventional or SFE extraction procedures and purification of extracts, are discussed. Solvent extraction is one of the most important methods for producing natural extracts of aroma and flavor compounds; extraction solvents used, SFE processes, and applications of solvent extraction of flavor and aroma compounds are discussed. The extraction processes for removal of undesirable or toxic constituents,

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such as alkaloids from natural products, or compounds with adverse effect on human health (e.g., cholesterol) from various food products with emphasis on SFE processes are covered in Chapters 11 and 12, respectively. The physiological effects and applications of alkaloids, the commercial-scale operations for extraction and isolation of the most valuable alkaloids, especially the decaffeination of coffee and black tea using conventional extraction or SFE, are described. Similarly, the physiological effects of cholesterol on health and the extraction and fractionation processes of cholesterol from milk, fat, eggs, and meats are described. Both for alkaloids and cholesterol removal, future prospects using SFE are offered. The safety, health, and environmental issues of solvent extraction, concerning mainly the edible oil and fat extraction industry, are described in Chapter 13, along with regulatory concerns and toxicity of solvents (hexane or alternative solvents) in commercial processes. Constantina Tzia George Liadakis

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Contents

Preface Contributors 1. Introduction: Fundamental Notes on Chemical Thermodynamics Petros Tzias 2. Solid–Liquid Extraction Jose´ Miguel Aguilera 3. Supercritical Fluid Extraction in Food Engineering Nurhan Turgut Dunford, Jerry W. King, and Gary R. List 4. Extraction Systems Rudolf Eggers and Philip T. Jaeger 5. Optimization Constantina Tzia 6. Fats and Oils from Plant Materials Lei Xu and Levente L. Diosady 7. Proteins from Plant Materials Lei Xu and Levente L. Diosady

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8. Sugars and Carbohydrates Pascal Christodoulou 9. Flavor and Aroma Substances Youn-Woo Lee and Youn Yong Lee 10. Extraction of Natural Antioxidants Vassiliki Oreopoulou 11. Extraction of Alkaloids from Natural Plants Using Supercritical Fluids Marleny D. A. Saldan˜a and Rahoma S. Mohamed 12. Removal of Cholesterol from Food Products Using Supercritical Fluids Rahoma S. Mohamed, Marleny D. A. Saldan˜a, Alvaro B. A. de Azevedo, and Uiram Kopcak 13. Solvent Extraction: Safety, Health, and Environmental Issues Phillip J. Wakelyn and Peter J. Wan

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Contents

Contributors

Jose´ Miguel Aguilera Universidad Cato´lica de Chile, Santiago, Chile Pascal Christodoulou

Hellenic Sugar Industry S.A., Thessaloniki, Greece

Alvaro B. A. de Azevedo pinas, Sa˜o Paulo, Brazil

The State University of Campinas–Unicamp, Cam-

Levente L. Diosady Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada Nurhan Turgut Dunford Food and Agriculture Products Research and Technology Center, and Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, Oklahoma, U.S.A. Rudolf Eggers Technische Germany

Universita¨t

Hamburg-Harburg,

Hamburg,

Philip T. Jaeger Technische Universita¨t Hamburg-Harburg, Hamburg, Germany Jerry W. King National Center for Agricultural Utilization Research, Agricultural Research Service, U.S. Department of Agriculture, Peoria, Illinois, U.S.A.

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Contributors

Uiram Kopcak The State University of Campinas–Unicamp, Campinas, Sa˜o Paulo, Brazil Youn-Woo Lee Supercritical Fluid Research Lab, Korea Institute of Science and Technology, Seoul, Korea Youn Yong Lee Korea Institute of Science and Technology, Seoul, Korea Gary R. List U.S. Department of Agriculture, Peoria, Illinois, U.S.A. Rahoma S. Mohamed School of Chemical Engineering, The State University of Campinas–Unicamp, Campinas, Sa˜o Paulo, Brazil Vassiliki Oreopoulou Greece

National Technical University of Athens, Athens,

Marleny D. A. Saldan˜a The State University of Campinas–Unicamp, Campinas, Sa˜o Paulo, Brazil Constantina Tzia National Technical University of Athens, Athens, Greece Petros Tzias Athena S.A., Athens, Greece Phillip J. Wakelyn

National Cotton Council, Washington, D.C., U.S.A.

Peter J. Wan U.S. Department of Agriculture, New Orleans, Louisiana, U.S.A. Lei Xu University of Toronto, Toronto, Ontario, Canada

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1 Introduction Fundamental Notes on Chemical Thermodynamics Petros Tzias Athena S.A. Athens, Greece

I. INTRODUCTION Thermodynamics is a very useful tool for many scientists and engineers, including geologists, geophysicists, and mechanical engineers, and is considered the scientific cornerstone for chemical engineers. When we study a handbook containing papers from a certain field of chemical engineering, it is very useful to have some notes on thermodynamics on hand to help us understand what we’re reading. This is the purpose of this chapter; to give a brief, elementary review of thermodynamics, reminding to the reader the basic laws, glossary, concepts, and relations of this important branch of science so as to make the study of the present volume much easier and pleasant. This chapter is intended for chemical engineers and chemists, and for this reason we devote most of this chapter to dealing with chemical thermodynamics, covering mixtures, solutions, phase equilibria, chemical reactions, etc. We are going to cite simple definitions of the different thermodynamic functions and quantities, and we will not enter to any analytical description or philosophical discussion about them; similarly we will not prove the mentioned thermodynamic relations, as these are beyond the scope of this chapter. We will deal mainly with reversible processes, so far as an interest is in chemical thermodynamics, and we will not refer to statistical or nuclear and relativistic phenomena.

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Tzias

II. SCOPE Thermodynamics is a branch of physical sciences concerned with the study of the transformation of heat, work, and other kinds of energy (electrical, light energy, etc.) from one form to another, in the different physicochemical phenomena and determines the laws and relations governing and describing these energy transformations. Historically (1), many scientists studied the interconversion of heat and work, from N.L.S. Carnot with his famous ideal gas cycle, to Clausius, who laid the foundations of the classical thermodynamics with the expression of the first and second laws of thermodynamics. Later, J. Willard Gibbs extended the application of the thermodynamic postulates and relations to chemical reaction and phase equilibria putting the foundations for the development of the field, which we call today chemical thermodynamics. Thermodynamics consists of a collection of equalities and inequalities which interrelate physical and chemical properties of substances as well as some physical or chemical phenomena. These relations are deduced in a mathematical way from some laws, the thermodynamic laws, which are derived directly from experience. The physical quantities used are taken either from physics, or they are introduced in thermodynamics. Using these relations, we can predict the possible direction of chemical reactions or the final result of a physical process, as well as the quantities of the different kinds of energy involved in these transformations. Thermodynamics is an experimental science (2). All the physical or chemical quantities used in its relations are independently measurable, but some of them are easier to be measured than others. Another feature of thermodynamics that makes it very important and useful is the capability of calculation, through its relations, quantities measured with difficulty or low accuracy by others measured easier and more accurately. Another advantage is that very often from existed data of some physical quantities we can calculate through thermodynamic relations the values of the physical quantities we are interested in, in this way avoiding long and difficult experiments and also saving time and money. Therefore, it becomes obvious that thermodynamics can be a very useful tool to a chemist or a chemical engineer.

III. GLOSSARY OF BASIC THERMODYNAMIC TERMS (2–6) There are a variety of thermodynamic terms, the most common of which are defined below, together with their SI units (3).

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Introduction

3

The amount of substance (2) nb of an entity b is a physical quantity proportional to the number nb of entities b in the system. The SI unit of the amount of substance is the mol. A. Pressure, Volume, Temperature Pressure and volume are concepts taken from physics. Pressure is defined as the ratio of a perpendicular force to a surface by the area of this surface. The SI unit of pressure is the Pa. Volume (m3) is the three-dimensional space occupied by a substance. Specific volume (m3/kg) is the volume per unit mass. Molar volume is the volume divided by the amount of substance. Density is the reciprocal of the specific volume. Temperature is a fundamental concept used in thermodynamics, and its definition is a very difficult matter. However we could simply describe it as the degree of hotness of a substance or as the property of matter which has equal magnitude in systems, connected by diathermic walls and where thermal flow does not exist (3). B. Temperature Scales (4) The Celcius scale (°C) is established by assigning the value 0.01°C to the triple point of water and the value 100°C to the boiling point of water at an atmospheric pressure of 1 standard atm (760 torr). The absolute temperature scale (T) is a scale which is related to the Celcius scale by the relation: T = t (°C) + 273.15 T is given in degrees Kelvin. In this scale it is enough to define just one point and this is the triple of water equal to 273.16 K. The freezing point of water at 1 atm is 273.15 K. The ideal gas temperature scale defined as: T = 273.16K lim

P273.16→0

冉 冊 PT

P273.16

where PT and P273,16 are the pressures of a gas trapped in a gas thermometer at the temperatures T and 273.16 K. The ideal gas temperature scale is the same with the absolute scale. Constant volume gas thermometers are used to determine the thermodynamic absolute temperatures. The International Practical Temperature Scale (IPTS-68). This scale gives the temperatures at some reproducible fixed points together with some interpo-

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4

Tzias

lating instruments and functions, by which we find the temperatures between these points. The fixed points as well as the functions have been determined by constant volume gas thermometers. The IPTS-68 gives the possibility by the above mentioned instruments and functions to measure in an easy and accurate way the absolute temperature. In the literature (4), we can find temperatures at different fixed points as well as the instruments used to measure the absolute temperature between these points. In the SI system for the absolute temperature we use the degree K. C. System In thermodynamics, we call any part of the real world we are choosing to study a system. All the rest of the parts of the world are the surroundings of the system. Practically by surroundings we consider the part of the world around the system which can interact with it. A system is closed if no matter enters or leaves it during any process we study. Otherwise it is open. A system is called isolated if neither matter nor energy enters or leaves it. The state of a system is defined by the values of its properties. Properties are physical quantities like temperature, volume, pressure, etc., which are related with a system and they have fixed values at any given state of the system. In order to define the state of a system, it is not necessary to know the values of all its properties, but only a certain number of them, which are called independent and all the rest (dependent) can be calculated by the values of the independent ones. Since a property is fixed by the state of a system it is also called a state function. A mathematical relationship between thermodynamic state functions is called an “equation of state.” D. Extensive and Intensive Properties Considering that we divide a system in different parts, if for one property its value for the whole system is equal to the sum of its values for the different parts, then this property is called extensive. Extensive properties are the volume, the mass, the internal energy, etc. If the value of a property for the whole system is equal to the values for its different parts, then the property is called intensive. Intensive properties are the temperature, the pressure, the molar volume, the density, etc. E. Phase If for a system, throughout all its parts, all its intensive properties have the same value, then the system is a homogeneous one and is called a phase. One system

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Introduction

5

can consist of more than one phases. In that case it is heterogeneous and some of its intensive properties have not the same value at all its parts. One phase can be open when it exchanges matter either with its surroundings or with another phase within the system. In the opposite case it is called closed. F. Process Process is the pathway through which one system passes from one state to another. If during a process the temperature of the system remains constant, the process is called isothermic, if the volume of the system remains constant, the process is called isometric or isochoric, and if the pressure remains constant, the process is called isobaric. If no heat enters or leaves a system undergoing one process the system and the process are called adiabatic. A process is called reversible if it takes place slowly and in such a way that at any stage of the process the properties of the system differ from equilibrium by infinitesimal amounts. Otherwise the process is called irreversible. All natural processes are irreversible. G. Molar Quantities From any extensive quantity X of a phase, it is defined an intensive quantity Xm by the relation: Xm =

X ∑ ni

(1)

i

where

∑i ni is the sum of the amounts of the different substances contained in

this phase. H. Mole Fraction The mole fraction Xa of a substance a in a phase is defined by the ratio: Xa =

na ∑ ni

(2)

i

where na the amount of substance of a and

∑i ni the sum of the amounts of all

substances in this phase. It follows immediately from the above definition that

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6

Tzias

∑i Xi = 1

(3)

I. Molality Molality m is the number of moles of a substance in 1 kg of solvent. J. Molarity Molarity c is the number of moles of a substance in 1 L of solvent. K. Partial Molar Quantities From any extensive quantity X of a phase we define an intensive quantity called the partial molar quantity Xa of the substance a in the phase by the relation: Xa =

冉 冊 ∂X ∂na

(4)

T,P,ni≠na

where ni ≠ na means all n’s except na in this phase.

IV.

THE CONCEPTS OF W, PE, KE, U, Q AND S (6–8)

W (work), PE (potential energy), and KE (kinetic energy) are concepts borrowed from physics. Work (W) is produced by a force (F) acting on a system and replacing it by a distance (ds) in the direction of the force and equals to: dW = F ⴢ ds In the case of a uniform pressure P acting on a system’s wall of surface S and replacing it by a distance x dW = −P ⴢ s ⴢ dx

or

dW = −P ⴢ dV

(5)

By energy we mean the ability of a system to produce work. Potential energy (PE) is the energy possessed by a system because of its position. PE = m ⴢ g ⴢ h

(6)

where m is the mass of the system, g the gravity acceleration and h the height of the system from zero level. Kinetic energy (KE) is the energy possessed by a system of mass m, because of its velocity (ν) and is equal to:

Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved.

Introduction

KE =

mE2 2

7

(7)

Internal energy (U) is the total energy, except potential and kinetic, contained within a system at a certain state. It is a magnitude determined by the state of a system. We can’t measure the absolute value of U, but only differences ∆U = U2 − U1 between two states (1) and (2). ∆U depends only on the states (1) and (2) and not on the path followed to pass from one to the other. Heat (Q) is the amount of energy, transferred from one system to another because of the difference in the temperature of the two systems. The amount of heat depends on the followed path and not on the original and final states of a procedure. For a system receiving heat Q, Q = C ⴢ (T2 − T1)

(8)

where C is the heat capacity of the system and T2, T1 its final and original temperatures. The entropy (S) (6) is an extensive property, depending on the state of a system. It can be defined as: dS =

dQ T

(9)

where dS is the heat received by it in a reversible way at a temperature T. In natural processes S always increases and dQ ≤ T ⴢ dS

(10)

The SI unit for W, PE, KE, and Q is the Joule. Joule . The SI unit for S is K

V. THERMODYNAMIC LAWS (2, 5, 6, 7) Thermodynamic laws are laws formed from experience, and there is no exception to them. There are several expressions for them all equivalent with each other. Below are given the most common ones. A. First Law of Thermodynamics In any process the total energy is conserved. In other words, there is no device which can create or eliminate energy. Considering the transformation of heat to work or vice-versa in a system, the first law can be expressed as:

Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved.

8

Tzias

dU = dW + dQ

(11)

where dU is the variation of the internal energy of the system, dW is the work produced, and dQ is the heat transferred. dU is an exact differential depending only upon the original and final state of the system, but dQ and dW are not exact differentials and depend upon the pathway of the process. By substituting Eqs. (5) and (9) to (11), the first law can be expressed as: dU = −P ⴢ dV + T ⴢ dS

(12)

B. Second Law of Thermodynamics It is not possible to transfer heat from a lower temperature to a higher temperature without the expenditure of work. In other words, in any process the total entropy of an isolated system increases. C. Third Law of Thermodynamics The expression of the third law is not possible without reference to statistical mechanics (2). As expression for the third law we could give Plank’s postulate (7): “At 0K the entropy of a pure crystalline is zero.” D. Law of Thermal Equilibrium If two systems are in thermal equilibrium with a third one, then they are also in thermal equilibrium between them. This law is not an independent one but it is derived from the first and second thermodynamic laws (2).

VI. THERMODYNAMIC FUNCTIONS AND SOME RELATIONS FOR ONE-PHASE CLOSED SYSTEM (6, 7) We have already met the properties P, V, T and the thermodynamic functions U, Q, W, and S and for the last two functions: W = −P ⴢ dV and dS =

dQ T

Furthermore, there are the following very important thermodynamic functions: the enthalpy H = U + P ⴢ V the Helmholtz free energy A = U − T ⴢ S

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(13) (14)

Introduction

9

and the Gibbs energy or Gibbs function G=H−TⴢS=A+PⴢV=U−TⴢS+PⴢV

(15)

All the three above functions are state functions and they have the dimensions of energy. Since they are state functions their differentials are exact differentials and for reversible processes we have: dH = T ⴢ dS + V ⴢ dP dA = −S ⴢ dT − P ⴢ dV dG = −S ⴢ dT + V ⴢ dP

(16) (17) (18)

By applying the properties of exact differentials we can obtain the following very useful relations:

冉 冉 冉 冉

冊 冊 冊 冊

冉 冊 冉 冊 冉 冊 冉 冊

∂U = −P, ∂V S

∂U =T ∂S

(19)

∂A = −P ∂V

(20)

V

∂A = −S, ∂T V

T

∂G = V, ∂P T

∂G = −S ∂T

(21)

∂H =V ∂P

(22)

P

∂H = T, ∂S P

S

and also the following:

冉 冉 冉 冉 冉

冊 冉冊 冊 冉 冊 冊 冉冊 冊 冉 冊 冊冉 冊冉 冊

∂T ∂P =− ∂V ∂S S

∂T ∂V = ∂P ∂S S

∂S ∂P = ∂V ∂T T

∂P ∂T

V

∂T ∂V

P

(24)

P

(25)

V

∂S ∂V =− ∂P ∂T T

(23)

V

(26)

P

∂V = −1 ∂P T

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(27)

10

Tzias

a=

冉 冊 冉 冊 冉 冊 冉 冊 冉 冊

1 ∂V is the coefficient of thermal expansion V ∂T

(28)

P

1 ∂V is the isothermal compressibility V ∂P

(29)

1 ∂V KS = − is the adiabatic compressibility V ∂P

(30)

K=−

T

S

CV =

∂U is the heat capacity at constant volume ∂T

(31)

∂H is the heat capacity at constant pressure ∂T

(32)

V

CP =

P

VII. FUNDAMENTAL INEQUALITIES We have already seen (Sec. IV.) that although for a reversible process dQ = T ⴢ dS and for natural processes dQ ≤ T ⴢ dS. Similarly for natural processes from Eqs. (12), (16), (17), and (18) we derive: dU ≤ T ⴢ dS − P ⴢ dV dH ≤ T ⴢ dS + V ⴢ dP dA ≥ −S ⴢ dT − P ⴢ dV dG ≥ −S ⴢ dT + V ⴢ dP

(33) (34) (35) (36)

This means that for a natural process at equilibrium the above functions get the lowest value, except the entropy which gets the maximum. In other words for any closed isolated system (2):

冉冊 ∂S ∂t

>0

(37)

U,V,ni

which means, that if anything happens in that system then S is increasing, and if

冉冊 ∂S ∂t

=0

U,V,ni

then the system is in equilibrium. Similarly, from:

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(38)

Introduction

冉 冊 ∂U ∂t

11

<0

(39)

S,V,ni

that if anything happens in a system at constant V, S and content then U is decreasing, from:

冉 冊 ∂H ∂t

<0

(40)

S,P,ni

that if anything happens in a system at constant S, P and content then H is decreasing, from:

冉 冊 ∂A ∂t

<0

(41)

T,V,ni

that is anything happens in a system at constant T, V, and content then A is decreasing, and from:

冉 冊 ∂G ∂t

<0

(42)

T,P,ni

that if anything happens in a system at constant T, P, and content then G is decreasing. The above (42) inequality is very important for chemists since most chemical reactions take place at constant T and P. If in the place of the above inequalities we consider the respective equalities, this will mean that the system to which they are referred is in equilibrium.

VIII. RELATIONS OF THERMODYNAMIC FUNCTIONS IN ONE-PHASE OPEN SYSTEM (2, 6, 7) We have already seen the Eq. (12) dU = −P ⴢ dV + T ⴢ dS, which is the expression of the first law of thermodynamics for a change involving only the transformation of energy. If we suppose that in the system under consideration there is also addition or removal of matter, then the above equation should be written under the following form: dU = T ⴢ dS − P ⴢ dV + ∑ µidni

(43)

i

where dni is the amount of substance of species i transferred and µi its molar energy.

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12

Tzias

In a similar way, from Eqs. (16), (17), and (18) we derive: dH = T ⴢ dS + V ⴢ dP + ∑ µidni

(44)

dA = −S ⴢ dT − P ⴢ dV + ∑ µidni

(45)

dG = −S ⴢ dT + V ⴢ dP + ∑ µidni

(46)

i

i i

Eqs. (43) through (46) are called Gibbs equations. Considering U, H, A and G as the functions U(S, V, ni), H(S, P, ni), A(T, V, ni) and G(T, P, ni) we can prove in a very easy way that the µi’s in the previous four relations are equal and that: µi =

冉 冊 ∂U ∂ni

S,V,nk≠i

=

冉 冊 冉 冊 ∂H ∂ni

=

S,P,nk≠i

∂A ∂ni

T,V,nk≠i

=

冉 冊 ∂G ∂ni

(47)

T,P,nk≠i

where n k≠i indicates all the other species nk except ni. The above defined quantity µi is called the chemical potential of the substance i; it is an intensive thermodynamic function, it has the dimensions of energy per amount of substance and its unit in the SI system is the Joule per mole. By definition: µi = R T lnλi

(48)

where λi is called the absolute activity of the species i in the multicomponent system. Integration of the Eq. (43) by keeping P, T, and ni constant (2) leads to: U = S ⴢ T − P ⴢ V + ∑ niµi

(49)

G = ∑ niµi

(50)

i

or i

Differentiation of (50) gives: dG = ∑ nidµi + ∑ µidni i

(51)

i

Equating the expressions for dG in Eqs. (46) and (51) yields: S ⴢ dT − V ⴢ dP + ∑ nidµi = 0

(52)

i

which is known as the Gibbs-Duhem equation. Replacing in (52) µi by its form in function of activity (48) the GibbsDuhem equation takes the form:

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Introduction

13

S ⴢ dT − V ⴢ dP + ∑ niRTdlnλi = 0

(53)

i

From Eqs. (46) by simple mathematical manipulations we can derive the following useful relations:

冉 冉 冉 冉 冉 冉

冊 冊 冊 冊 冊 冊

∂µi ∂T

∂µi ∂P

∂µi ∂nk

∂µi ∂T

∂µi ∂V

∂µi ∂nk

冉冊 冉 冊 冉 冊 冉冊 冉冊 冉 冊

=−

P,ni

=

T,ni

∂V ∂ni

=

T,P,nk≠i

=−

V,ni

=−

T,ni

T,V,nj≠i

∂S ∂ni

(55)

T,P,nk≠i

∂µk ∂ni

∂S ∂ni

∂P ∂ni

=−

(54)

T,P,nk≠i

(56)

T,P,nk≠i

(57)

T,V,nk≠i

(58)

T,V,nk≠i

∂µk ∂ni

(59)

T,V,nj≠i

IX. MIXTURES (2, 6, 7) A system consisting of more than one substance is called mixture. A mixture may exist in gaseous, liquid, or solid phase. We shall confine ourselves to binary mixtures, from which the extension to multicomponent ones is straightforward. By definition a mixture is said to be ideal if for any component i (6): µi = µ0i + R T lnXi

(60)

where µi, µ0i are the chemical potentials of component i with a mole fraction Xi in the mixture and of pure i respectively, both at the same P and T. In other words if for any component i in the mixture (2): λi = Xi λ0i

(61)

where λi, λ0i are the absolute activities of the component i with a mole fraction Xi in the mixture and of pure i respectively, again at the same P and T. A mixture for which (60) or (61) are not valid is called a real mixture.

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14

Tzias

In an ideal mixture the interactions between like and unlike species are the same and the components in the mixture they behave as in the pure components. A. Mixing Functions For a binary mixture of components A and B with mole fractions (1-x) and x respectively and for any extensive thermodynamic quantity X, such as G, A, H, S, or V the mixing function is defined as: ∆mix Xm = Xm (T, P, x) − (1-x) Xm (T, P, 0) − x Xm (T, P, 1)

(62)

∆mix Xm = (1-x) [XA (T, P, 1-x) − Xm (T, P, 0)] + x [XB (T, P, x) − Xm (T, P, 1)]

(63)

or

where Xm (T, P, x) is the molar function of the mixture at (T, P, x), XA (T, P, 1-x), XB (T, P, x) the molar functions of A and B in this mixture at mole fractions 1-x and x and Xm (T, P, 0), Xm (T, P, 1) the molar functions of the pure components A and B respectively. From (62) and (63) using Eqs. (50) and (61) and for Xm = Gm we obtain for an ideal mixture: ∆mix Gmid = R T[(1-x) ln (1 − x) + x lnx] > 0

(64)

From (21) and (64) at constant P, n we obtain: ∆mix Smid = −R [(1-x) ln (1 − x) + x lnx] < 0

(65)

From (64), (65), and (15): ∆mix Hmid = 0

(66)

and from (21) and (64): ∆mix Vmid = 0

(67)

B. Excess Functions By definition Excess function XmE is the difference between the real ∆mixXm and the ideal one. That is: XmE = ∆mixXm − ∆mixXmid

(68)

C. Thermodynamic Functions of Dilution If in one binary or multicomponent one-phase homogeneous mixture of nonreacting species, one component is in excess related to the others, and more of this is added to the mixture, this process is called dilution.

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Introduction

15

The thermodynamic functions of the dilution process are given as the difference of thermodynamic functions of mixing between the final diluted state and the original one, that is ∆Xm,dil = (∆Xm,mix)2 − (∆Xm,mix)1

(69)

D. Standard and Reference States of Thermodynamic Functions (8, 9) We define as standard thermodynamic function of a component i in a system at any temperature and at a fixed pressure the thermodynamic function of i at a given composition. The state thus defined is called standard state. Historically the standard states are defined at the fixed pressure of 1 atm (= 101.325 Pa) and in that case the standard Thermodynamic functions depend only on the temperature. Usually, for gases as standard state is accepted the pure ideal gas at 1 atm and for liquids and solids the pure liquids or solids at certain P, T where P can be defined as 1 bar. As reference state is called one state that is used as reference for the calculation of the different thermodynamic functions. Any thermodynamic function can be expressed in function of its standard or reference state.

X. GASES AND GASEOUS MIXTURES (5, 6, 8) The PVT behavior of a pure fluid, e.g., a gas, can be expressed by the equation: P ⴢ Vm = R ⴢ T (1 + B ⴢ P + C ⴢ P2 + . . . )

(70)

or P ⴢ Vm = R ⴢ T (1 +

b c + +...) Vm V2m

(71)

V where Vm is the molar volume of the gas, Vm = , V the volume and n the mol n of the gas, and R is the gas constant, which in SI units is equal to 8.3144 J ⴢ mol−1 ⴢ K−1. Eqs. (70) and (71) are called virial equations of state of a gas and the coefficients b, c, . . . of (71) virial coefficients and they depend on the temperature and on the type of chemical species of the gas. The coefficients B, C, . . . can be calculated from b, c, . . . . when P → 0 then P ⴢ Vm = R ⴢ T

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(72)

16

Tzias

This is the equation of state of an ideal gas. The virial coefficients show the deviation of a real gas from ideality. The PVT behavior of the gases is also expressed by the aid of the compressibility factor: z=

PVm RT

(73)

For an ideal gas z = 1 and P ⴢ Vm = R ⴢ T One other attempt to express the PVT behavior of the real gases is the van der Waals equation:

冉 冊

a (Vm − b) = R ⴢ T V2m

P+

(74)

which tries to take in account the volume (coefficient b) of the molecules and the interactions between them (coefficient a). For an ideal gas we can derive the following relations:

冉 冊

∂H =V−T ∂P T

CP = CV + n R

冉 冊

冉 冊 ∂

nRT P ∂T

=V−

nRT =0 P

(75)

P

(76)

The Gibbs-Duhem equation at constant T and for a pure gas becomes: n dµ = V ⴢ dP

(77)

If the gas is ideal, substituting in (77) the volume from (72) we obtain: n dµ =

nRTdP P

(78)

or dµ = R T d lnP

(79)

and for a change at constant T from P1 to P2 µ2 − µ1 = R T ln

冉冊 P2 P1

(80)

A. Fugacity (7, 10, 11) To express the properties of a real gas in the same way with an ideal gas, Lewis and Randall (10) originated the term fugacity (f), which has the dimensions of pressure.

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Introduction

17

So, for a real gas or vapor instead of (79) or (80) we write (10): dµ = R T dlnf

(81)

or µ2 − µ1 = R T ln

f2 f1

(82)

where f is the fugacity of the gas or vapor. The ratio Φ =

f P

(83)

is called fugacity coefficient. Since ideal gas behavior is approached as P → 0 then f lim Φ = lim = 1 P→0 P→0 P

(84)

As we will see in the phase equilibrium the fugacity of a liquid or a solid which is in equilibrium with its vapor is equal to the fugacity of its vapor. The fugacity of a real pure gas at a given P, T can be evaluated (7) through the relation:

冉 冊 冋∫冉 冊 f ln = P

P

0

Vm 1 − dP RT P

册

(85)

T

if we know the equation of state of that gas. For multicomponent real gases, the partial fugacity fi of a component i is defined in terms of the chemical potential µi as follows: dµi = R T d(lnfi)

(86)

In this case: lim P→0

fi =1 xiP

(87)

where xi is the mole fraction of component i and P is the total pressure (not the partial pressure Pi). For multicomponent gases, the fugacity fi can be also evaluated (7) through the following relation, if we know the equation of state of that gas mixture:

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18

Tzias

冋冉 P

lnfi = ln(xi P) +

∫

0

冊册

(Vm)i 1 − dP RT P

册

(88)

T

where (Vm)i is the partial molar volume of species i. B. The Standard State of a Gas Component (2) The standard state of a gas component in a mixture of gases is given by:

冉 冊 冋 P

µB (g, T, P, xc) = µ0B (g, T) + R T ln xB

册

P RT + ∫ VB(g, T, P, xC) − dP P0 0 P

(89)

where T, P, xC are the temperature, pressure and composition of the gas mixture, xB the mole fraction of B in the mixture and µ0B (g, T) is the chemical potential of the pure ideal gas, at temperature T and pressure P0. Historically P0 = 1 atm.

XI. LIQUID MIXTURES (2, 8) All that was mentioned in Sec. IX and what it will follow are applicable to liquid and to solid mixtures as well. For a liquid mixture using Eq. (48) we obtain: µi = µ0i + R T ln

λi λ0i

(90)

or µi = µ0i + R T lnai

(91)

where µi, µ are the chemical potentials of component i with mole fraction xi and of the pure liquid i at certain P, T, λi, λ0i the absolute activities of µi and µ0i respectively and 0 i

ai =

λi λ0i

(92)

is what we call relative activity of component i in the mixture. Some authors call ai simply activity. For the absolute activity, the activity coefficient fi is defined by the relation: λi = fi xi

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(93)

Introduction

19

and for the relative activity the corresponding activity coefficient γi is defined by the relation: ai = γi xi

(94)

When ai = 1 from (91) we obtain µi = µ , that is the term R T lnai gives the difference between µi and its reference state µ0i at certain P, T and if P = 1 bar then the difference of µi from its standard state. 0 i

From (91) and (94) we get µi = µ0i + R T lnxi + R T lnγi

(95)

Since for the case of an ideal mixture µi = µ + R T lnxi (Eq. [60]), the term R T ln γi expresses the deviation of µi from ideality. After defining the standard state for µi the standard states for the other thermodynamic functions can be derived in an easy way. In particular 0 i

S0i = −

dµ0i dT

H0i = µ0i − T

(96) dµ0i dT

G0i = µ0i

(97) (98)

where Si0, Hi0, and Gi0 are the standard molar functions of S, H, and G respectively.

XII. EQUILIBRIUM OF PHASES (2, 6, 8, 11, 12) In many industrial processes there is coexistence of two or more phases. When there is mass transfer from one phase to the other the phases are not in equilibrium. The study of the mass transfer in these processes requires exact knowledge of the phases at equilibrium. In this section, we will treat liquid-vapor equilibrium states. Similar results can be derived from the treatment of liquid–solid and solid–vapor equilibrium. We say that two or more phases are in equilibrium regarding several intensive properties if these properties have the same value in both phases. For example we have: Thermal equilibrium, when the temperatures T of the different phases are equal Hydrostatic equilibrium, when the pressures P of the different phases are equal Chemical equilibrium, when there is no reaction between the constituents of all the phases Osmotic equilibrium, when P, T, and several µi are the same Diffusive equilibrium, when P, T, and all µi are the same

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20

Tzias

We will deal here with equilibrium states between phases where P, T, µi are equal in all phases. A. The Phase Rule (4, 6, 8, 11) For a closed, isolated system consisting of several phases a, b, c, . . . in thermal, hydrostatic and chemical equilibrium (fixed composition) and where there is no transfer of mass, at macroscopic observation, from one phase to the other, writing the Gibbs-Duhem equations for each phase, since dP, dT and dni are zero it is derived that, for every component i, µai = µbi = µci = . . . The minimum number of intensive properties (pressure, temperature, mole fractions, etc.) needed so that the state of a closed, isolated nonreacting system is completely defined is called the degrees of freedom F. Gibbs derived a very important rule involving phase equilibria which connects the degrees of freedom F with the number of phases P and the number of different substances C in the system. This rule is given by the following relation: F=C+2−P

(99)

For example in the P, T diagram of Fig. 1 for a pure substance we observe that for the regions of only one phase (P = 1), the relation (99) yields F = 2, which means that both P, T are needed for the definition of the state. For points on the curves we have two phases P = 2 and F = 1 which means that only one of P, T is needed for the definition of the state and finally at the triple point T,

Figure 1 P-T diagram for a pure substance.

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Introduction

21

P = 3 and F = 0, which means that there is no degree of freedom and only one pair of P, T corresponds to the state of coexistence of three phases. If a reaction is taking place in the system or we want to take in account a peculiarity of the equilibrium phases, for instance an azeotrope, then the phase rule must be modified. For a single-component two phase system, following the vaporization curve (Fig. 1) up to the end point C, we observe that by increasing gradually the temperature of the system we pass to vapor and liquid phases which they become more and more similar in density and molar volume and the meniscus separating the two phases becomes more indistinct. Finally at the point C the two phases become identical and the meniscus between the two phases disappears. Beyond C there is no liquid or vapor phase, but only one single-fluid phase. At the critical point

冉 冊 冉 冊 ∂P ∂V

= 0,

TC

∂2P ∂V2

=0

(100)

TC

Point C is called the critical point and the corresponding P, T the critical Pressure PC and the critical Temperature TC of the studying substance. B. The Chemical Potential in Phase Equilibria (7, 11) In the previous section we saw that between two phases a, b in equilibrium for every substance i µai = µbi

(101)

µi0,a + R T ln aai = µi0,b + R T ln abi

(102)

or

Since the µi0,a and µi0,b are the chemical potentials of the pure i at the same P, T then µi0,a = µi0,b

(103)

and consequently from (103) aai = abi

(104)

but this does not mean that necessarily the activity coefficients γai, γbi will be equal since usually xai ≠ xbi For the case of liquid–vapor equilibrium we have seen for the vapor phase that

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22

Tzias

µvi = µi0,v + R T ln

f vi f i0,v

(105)

while for the liquid phase µRi = µi0,R + R T ln

λRi = µi0,R + R T ln aRi λi0,R

(106)

From the previous relations it is obtained the following relation, connecting the absolute activity, the relative activity and the fugacity of a component i in a system at equilibrium fi λi = = ai f 0i λ0i

(107)

The equations relating the thermodynamic functions of phases at equilibrium are very important since from data of one phase we can calculate the properties of the other phase. C. Binary Vapor-Liquid Systems (7, 9, 11) There are important differences between the behavior of a single component vapor–liquid system and a multicomponent one. For instance, in a single component the vapor and liquid phases have the same composition, but not in a multicomponent system. During the evaporation at constant pressure for a single-component system the temperature remains constant, but in a multicomponent system at constant P the temperature changes during the evaporation. It is obvious that the behavior of a multicomponent system is more complicated than that of a single component. As an example of multicomponent system we study here a two-phase binary system. Fig. 2 shows a vapor-pressure, composition (X = mole fraction) diagram for an ideal mixture of two liquids. At any mole fraction X between 0 and 1 the vapor pressure Pmix of the mixture is Pmix = PA (1 − X) + PBX

(108)

where PA, PB the vapor pressures of the pure components A, B. Fig. 3 shows the P, X diagram of a real mixture of two component liquids A and B, completely miscible through the whole range of X. At any pressure a composition of the liquid phase Xliq corresponds to a different composition Xvap of the vapor phase, which is in equilibrium with the liquid. The compositions of the liquid phase form a curve called bubble point line and the corresponding compositions of the vapor phase form another curve called dew point line. A similar to Fig. 3 diagram can be drawn relating T with X.

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Introduction

23

Figure 2 Vapor-pressure composition diagram for an ideal mixture of two liquids, A and B.

Figure 3 Vapor-pressure composition diagram for a real mixture of two liquids.

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24

Tzias

Each liquid component, when its mole fraction tends to 1, behaves like in an ideal solution. Some binary liquid mixtures, at a fixed composition, have identical composition in both the liquid and vapor phase. This composition is called azeotropic and the mixture azeotrope. In these cases, although the compositions of the two phases are equal, it does not mean that the mixture is an ideal one. There are positive azeotropes with azeotropic pressure Paz higher than the vapor pressure of the two pure components and negative azeopropes with Paz lower than the vapor pressure of the two pure components of the mixture. Fig. 4 shows the P, T diagram of a binary liquid mixture at a constant composition. Since in this case there is one more degree of freedom from the single component system, in order to define the state of the system for the vapor + liquid region we need both P, T (for the single component system it is needed either only P or only T). In Fig. 4, the region included inside the ABCDE curve is in the place of the vaporization line of Fig. 1. To pass from the pure liquid to the vapor under constant pressure the temperature is changing. This passage becomes shorter as we approach the critical point C. The end temperatures, at the start and the end of this process, are called bubble point and dew point temperatures respectively at the pressure of vaporization. At each composition there is one curve ABCDE and one critical point. Similar behavior is observed for the processes of fusion and sublimation and it is not necessary to study them separately.

Figure 4 P-T diagram at constant composition for a mixture of two liquids.

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Introduction

25

D. Principle of Corresponding States (7, 12) For different real gases at the same pressure and temperature the molar volumes PVm Vm are different. The compressibility factor z = defined in Sect. X expresses RT the deviations of the real gases from ideality and it is different for the different gases at the same P and T. However, it has been observed, by defining the reduced pressure Pr, the reduced temperature Tr and the reduced molar volume Vm,r by the relations: Pr =

P , PC

Tr =

T , TC

Vm,r =

Vm Vm,C

(109)

where PC , TC, Vm,C are the critical P, T and Vm that for equal Pr, Tr the Vm,r of all gases are approximately equal. This is known as the van der Waals principle of corresponding states. The critical compressibility factor zC =

PC Vm,C R TC

(110)

is also found experimentally to be in the narrow range 0.2–0.3 and it can be considered as a universal constant. So we finally have z = F (Pr, Tr)

(111)

where F is the same function for all the gases. E. Enthalpy and Entropy Change in a Two-Phase Transition (2, 7, 12) When there is transition from one phase to another we define a property called change of transition of state by the relation ∆ Miab = Mbi − Mai

(112)

Thus for the case of vaporization we have: ∆ ViRv = Vvi − VRi ∆ HiRv = Hvi − HRi ∆ SiRv = Svi − SRi

(113) (114) (115)

where by R and v we mean liquid and vapor respectively. Knowing that at equilibrium µRi = µvi

or

GRi = Gvi

for a two-phase one-component system at each phase:

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(116)

26

Tzias

Gi = Hi − T Si

(117)

and dGi = Si dT − Vi dP

(118)

From (117) and (118), it is finally derived dPsat ∆SRv i = iRv dT ∆Vi dPsat ∆HiRv i = dT T∆ViRv

(119) (120)

where Psat i is the vapor pressure at the equilibrium of the two phases. From Eqs. (119) and (120) we can calculate the entropy and enthalpy change of vaporization.

XIII. SOLUTIONS (2, 5, 11, 13) For several mixtures it is convenient to distinguish some components from the others, for instance when one solid, liquid, or gas has a limited solubility in a liquid and its mole fraction in the mixture does not cover the whole range from 0 to 1. In this case, by convention we call the liquid which has the higher mole fraction the solvent, the component with the limited solubility solute, and the mixture the solution. When the solvent is in excess and the solute in low concentration, the solution is called a dilute solution. For a binary solution the chemical potential of the solvent is given as in mixtures of liquids by the relation: µi = µ0i + R T lnai = µ0i + R T lnγi xi

(121)

where µ0i is the reference chemical potential of pure solvent at certain P, T. ai,γi are, respectively, its relative activity and activity coefficient. When xi → 1 then also γi → 1, and the solvent behaves as in an ideal mixture. At these compositions near the pure solvent the partial pressure and the fugacity of the solvent are proportional to its mole fraction. Pi = P0ixi or fi = f 0ixi

(122)

where Pi, fi, xi are the partial pressure, fugacity, and mole fraction of the solvent and P0i, f 0i are the vapor pressure and the fugacity of the pure solvent respectively at the temperature of the solution. In this region, the solvent behaves in an ideal way and the relations, Eq. (122), express what we call as Raoult’s law (Fig. 5).

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Introduction

27

Figure 5 Fugacity of a solvent and a solute as function of their mole fraction.

The standard chemical potential of the solvent is that of the pure liquid at 1 atm and the temperature of the solution. For the solute, however, it is not possible to define a similar standard state since there cannot be solutions after a certain composition, with higher solute concentrations. In this case, it is adopted as standard state for the solute the hypothetical, ideal unit concentration of solute solution, at certain pressure and temperature (reference state) or at the fixed pressure of 1 atm (standard state). This solute standard state is derived by the extrapolation of the fugacity of the solute at conditions of infinite dilution (the mole fraction of all the solutes in the solution tend to zero), where the fugacity of the solute fi is proportional to its mole fraction xi, fi = K xi

(123)

to the hypothetical state of solution with solute mole fraction 1. The relation (123) is called Henry’s law and K is a constant called Henry’s constant. In the region of very dilute solutions where the relation (123) is valid the solute behaves in an, by convention, ideal way which is different from the ideal conditions near the pure solvent. From the above mentioned the chemical potential of the solute in a solution is given by: µi = µ0i + R T lnγixi = µ0i + R T ln

fi f 0i

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(124)

28

Tzias

where γi, fi are the activity coefficient and the fugacity of i at a mole fraction xi and f0i, µ0i are the fugacity and the chemical potential of pure i at the reference state of infinite dilution conditions and at the temperature and pressure of the solution. The standard chemical potential of a solvent is given by the relation (2): P0

µ (T, P ) = µ*2 (T, P) + 0 2

0

∫ V*2 (T, P)dP

(125)

P

where P0, P are the standard pressure and the pressure of the solution respectively, * means is pure solvent, and V*2 the volume of pure solvent. The standard chemical potential of a solute is given by the relation (2):

冋

册

∞

m µ (T) = µ1(T,P,m1) − RTln 10 + m1 0 i

P0

∫ V∞1 (T,P)dP

(126)

P

where ∞ means conditions at infinite dilution, µ1(T,P,m1) the chemical potential at T, P, m1, m01 the standard molality, P0 the standard pressure and V∞1 the volume of solute at conditions of infinite dilution. All the other thermodynamic functions and relations for the solution can be derived in a straight mathematical way from the above relations.

XIV. ELECTROLYTE SOLUTION (2, 8, 14) Electrolytes are a special class of solute substances, which involve several complications in their thermodynamic study, not found in solutions of nonelectrolytes. The difficulties arise from the fact that the electrolytes in solution are found under complete or partial dissociation in the ions of which they are consisted. Because of the restriction of electrical neutrality in an electrolyte solution it is not possible to define thermodynamic functions of one ion, for example its chemical potential, since this would imply the change of the amount of substance of this ion by keeping constant the amount of substance of all the rest ions, which has no physical meaning. To overcome this difficulty, we consider all the thermodynamic functions with both the anions and the cations of one electrolyte. For example, for the case of a strong, completely dissociated electrolyte of the type Mν+Aν−, where ν+ and ν− are the number of positive and negative ions, respectively, in the molecule of the electrolyte, we can write: µMA = ν+ µ+ + ν− µ−

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(127)

Introduction

29

and using molalities µ+ (T, P, m) = R T lnm+ + R T lnγ+ (T, P, m) + µ0+ (T, P) µ− (T, P, m) = R T lnm− + R T lnγ− (T, P, m) + µ0− (T, P)

(128) (129)

Substituting (128) and (129) to (127) we obtain: µMA = R T lnmν++ mν−− + R T lnγν++ γν−− (T, P, m) + ν+µ0+ (T, P) + ν−µ0− (T, P)

(130)

The mean activity coefficient is defined as: γ±ν = γν++ γν−−

(131)

and the mean molality: m±ν = mν++ mν−−

(132)

where ν = ν+ + ν− From (131), (132) and (130) and by defining: µ0MA (T, P) = ν+ µ0+ (T, P) + ν− µ0− (T, P) µMA (P, T) = ν R T lnm± + ν R T lnγ± (T, P, m) + µ0MA (T, P)

(133) (134)

The m±, γ± and µ0MA (T, P) have a similar meaning as for the nonelectrolyte solutions when m± → 0 then γ± → 1 and µMA (T, P) is equal to µ0MA (T, P) at the reference state. To obtain the standard state for an electrolyte solute it is followed a similar procedure to that of a nonelectrolyte solute providing that the appropriate quantities are plotted (13, 14). For the case of weak electrolytes the method to obtain expressions for the chemical potentials are the same as for strong electrolytes. A. The Debye–Hu¨ckel Limiting Law (2, 6) In 1923 Debye and Hu¨ckel developed a theory about the behavior of strong electrolytes in dilute solutions. This theory was a mathematical treatment of some ideas previously assumed by Arrhenius regarding the dissociation of electrolytes in solution. By this theory after a series of mathematical calculations we arrive to a formula giving the mean ionic activity coefficient, which for very dilute solutions by approximation takes the form: ln γ± = C I1/2 z+ z− where C = (2 π N0ρs)1/2 (e2 / 4 π ε K T)3/2

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(135)

30

Tzias

N0 = Avogadro’s number ρs = density of the pure solvent e = the charge on a proton ε = ε0 εr, where εr is the dielectric constant of the solvent and ε0 is the dielectric constant of a vacuum T = temperature of the solution R K = , R = gas constant N0 I is called ionic strength and is given for a 1-1 type electrolyte by: 1 I = (m+ z2+ + m− z2−) 2

(136)

m+, m−, the molalities of the ions z2+, z2−, the electrical charges of the ions. The Debye-Hu¨ckel law is very accurate for very dilute solutions of strong electrolytes. For mixed electrolytes the theory is still valid with I=

1 ∑ miz2i 2 i

(137)

XV. THERMOCHEMISTRY—CHEMICAL REACTION EQUILIBRIUM (11, 15) The application of thermodynamics to chemically reacting systems is very important. Together with mass balance the first thermodynamic law under certain conditions gives exactly the energy absorbed (endothermic) or released (exothermic) by a chemical reaction. For instance we can calculate the energy needed to produce from some substances other useful substances or the energy released from the combustion of a fuel. The second thermodynamic law can predict if one chemical reaction will proceed to one direction or to the opposite and at which extend it will stop (equilibrium state). However, in several cases, although from the second thermodynamic law it results that one reaction should proceed to one direction, this reaction does not start and to overcome this hindrance, catalysts or other means are used. But still, in these cases thermodynamics is useful, since through the prediction by the second thermodynamic law of the possibility of realization of a reaction, it can allow us or release us from the trouble, to seek finding the appropriate catalyst for this reaction.

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Introduction

31

A. Enthalpy of Formation and Enthalpy of Reaction (4, 5) The heat of formation of any compound is the heat required to form that compound from its elements at a certain temperature and pressure. When this formation is considered under constant pressure then the heat of formation is equal to the enthalpy of formation. The standard enthalpy of formation, ∆HF, of a compound is defined as the heat required to form the compound in its standard state of 1 atm pressure and 25°C from its elements at the same standard conditions. ∆HF = hcompound − ∑ (νihi)elements

(138)

i

νi is the stoichiometric coefficient hcompound, hi the standard molar enthalpies of formation of the compound and of its elements respectively. By convention the standard enthalpies of formation hi of all elements at their more stable state are considered as zero, therefore ∆HF = hcompound

(139)

Tables with standard enthalpies of formation of many compounds are given in books on thermodynamics. The heat of reaction is the heat absorbed or rejected by the reaction. If the reaction takes place at constant P, the heat of reaction is equal to the enthalpy of reaction. The standard enthalpy of reaction is defined as the change in enthalpy from a reaction taking place at a constant pressure of 1 atm and constant temperature of 25°C. The heat of combustion of any compound is defined as the heat of reaction resulting from the oxidation of this compound with oxygen. Quantities of heat transferred at constant T, like heat of vaporization, or fusion for a single compound at constant pressure are called heat effects. These heat effects together with the heat of mixing, the heat of solution (heat of mixing for the case of a solution), as well as the heat of reaction are studied by the branch of thermodynamics, called thermochemistry, and the instruments used for the experimental determination of these functions are called calorimeters. B. Determination of the Enthalpy of Reaction (4, 5, 7) For a reaction under constant P, where variations in potential and kinetic energy are negligible and no work is produced the change in enthalpy is given by HP − Hr = (HP − HP0) + ∆HR − (Hr − Hr0)

Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved.

(140)

32

Tzias

where HP, Hr enthalpies of products and reactants at a pressure P respectively and HP0, Hr0 enthalpies of products and reactants at a pressure of 1 atm and temperature of 25°C and ∆HR = HP0 − Hr0 =

∑

(νh) −

products

∑

(νh)

(141)

reactants

where the ν’s are the stoichiometric coefficients and the h’s are the standard molar enthalpies of formation of the compounds in the reaction. ∆HR is the standard enthalpy of reaction and can be calculated from the standard enthalpies of formation of the products and reactants using existed relative tables. (HP − HP0) and (Hr − Hr0) can be calculated either from known data, experimentally or by simplification considering, for instance, that the reactants and products behave as ideal gases. In several cases we can determine the enthalpy of a certain reaction by simply adding or subtracting other reactions of which we know the enthalpies of reaction. C. Equilibrium Constant-Affinity of a Reaction (6, 7, 11) Let us consider the reaction ν1c1 + ν2c2 W ν3c3 + ν4c4

(142)

at constant T, P, where ci are the constituents and νi the stoichiometric coefficients. Eq. (46) at P, T constant gives dGT,P = µ1dn1 + µ2dn2 − µ3dn3 − µ4dn4

(143)

and for a reaction −

dn1 dn dn dn = − 2 = 3 = 4 = dξ ν1 ν2 ν3 ν4

(144)

where ξ is the extent of the reaction (16). Based on (143), (144), and (42) it follows dGT,P = (µ1dn1 + µ2dn2 − µ3dn3 − µ4dn4)dξ ≤ 0

(145)

the sign of the parenthesis determines the sign of the ξ and consequently the direction of the reaction. At equilibrium where dGT,P = 0

(146)

µ1dn1 + µ2dn2 = µ3dn3 + µ4dn4

(147)

The quantity µ1dn1 + µ2dn2 − µ3dn3 − µ4dn4 was introduced by De Donder (16) and called by him the affinity Af.

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Introduction

33

From (147) using µi = µ0i + RTlnai, [Eq. (90)] it is obtained: Af = A0f − RTln

冉 冊 a3ν3 ⴢ a4ν4 a1ν1 ⴢ a2ν2

(148)

where A0f is the affinity for the µ0i’s. The quantity

冉 冊

a3ν3 ⴢ a4ν4 = Qa a1ν1 ⴢ a2ν2

(149)

is called reaction quotient. At equilibrium where Af = 0, the Qa depends only on the temperature and on A0f and not on the activities and is called the equilibrium constant Ka. Thus we have A0f = R T lnKa K Af = RTln a Qa

(150) (151)

Since one reaction proceeds only when Af > 0 this means that: when Ka > Qa the reaction proceeds to the right when Ka < Qa the reaction proceeds to the left and when Ka = Qa there is equilibrium.

REFERENCES 1. SW Angrist, LG Hepler. Order and Chaos. New York: Basic Books Inc., 1967. 2. ML Mac Glashan. Chemical Thermodynamics. London: Academic Press Inc., 1979. 3. ML MacGlashan. Physicochemical Quantities and Units, 2nd ed. London: The Royal Institute of Chemistry, 1971. 4. TE Daubert. Chemical Engineering Thermodynamics. Singapore: McGraw Hill, 1985. 5. P Rock. Chemical Thermodynamics. California: University Science Books, 1983. 6. CE Reid. Chemical Thermodynamics. Singapore: McGraw Hill, 1990. 7. JS Hsieh. Principles of Thermodynamics. New York: McGraw Hill, 1975. 8. SE Wood, R Battino. Thermodynamics of Chemical System. Cambridge, UK: Cambridge University Press, 1990. 9. JM Smith, HC Van Ness. Introduction to Chemical Engineering Thermodynamics, 4th ed. Singapore: McGraw Hill, 1987.

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10. GN Lewis, M Randall. Thermodynamics. Revised by KS Pitzer, L Brewer. 2nd ed. New York: McGraw Hill, 1961, p. 191. 11. DP Tassios. Applied Chemical Engineering Thermodynamics. Berlin, Germany: Springer-Verlag, 1993. 12. HC Van Ness, MM Abbott. Classical Thermodynamics of Non Electrolyte Solutions, with Applications to Phase Equilibria. New York: McGraw Hill, 1982. 13. JM Prausnitz, RN Lichtenthaler, EG de Azevedo. Molecular Thermodynamics of Fluid-Phase Equilibria, 3rd ed. New Jersey: Prentice Hall, Inc., 1999. 14. JW Tester, M Modell. Thermodynamics and its Applications, 3rd ed. New Jersey: Prentice Hall, Inc., 1997. 15. MK Karapetyants. Chemical Thermodynamics. Trans. by G Leib. Moscow, Russia: MIR Publishers, 1978. 16. De Donder, P Van Rysselberghe. The Thermodynamic Theory of Affinity. Stanford, California: Stanford University Press, 1936.

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2 Solid–Liquid Extraction Jose´ Miguel Aguilera Universidad Cato´lica de Chile Santiago, Chile

I. INTRODUCTION Solid–liquid extraction or leaching is a separation process affected by a fluid involving the transfer of solutes from a solid matrix to a solvent. It is an extensively used unit operation to recover many important food components: sucrose in cane or beets, lipids from oilseeds, proteins in oilseed meals, phytochemicals from plants, and functional hydrocolloids from algae, among others. Solid– liquid extraction (or simply extraction) may also be used to remove undesirable contaminants and toxins present in foods and feeds. “Solid–liquid” extraction may be a misnomer for it gives the impression that mass transfer occurs at a sharp interface between a “dry” solid and the liquid phase. In most food extractions, either the “solid” naturally contains a liquid phase or it becomes impregnated by the extraction liquid, so that liquid phase diffusion inside the solid is a major mass transfer mechanism during leaching. A. Characteristics of Food Extraction From an engineering viewpoint, solid–liquid extraction of foods is a multicomponent, multiphase, un-steady state mass transfer operation. It involves transfer of more than one chemical species—the solute—from a solid to a solvent. The solute is sometimes referred to as the extract, when the chemical species being recovered are ill defined, as occurs in the extraction of phytochemicals from plants. Commonly used solvents in extraction of food components are water,

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36

Aguilera

ethanol (or ethanol-water mixtures), hexane, and carbon dioxide, but the trend is toward the use of natural chemicals. During extraction, the concentration of solute inside the solid varies leading to the nonstationary or unsteady condition. A series of phenomenological steps have to occur during the period of interaction between the solute-containing particle and the solvent effecting the separation (1) as represented schematically in Fig. 1. These include: 1. 2. 3. 4.

Entrance of the solvent into the solid matrix Solubilization and/or breakdown of components Transport of the solute to the exterior of the solid matrix Migration of the extracted solute from the external surface of the solid into the bulk solution 5. Movement of the extract with respect to the solid (i.e., extract displacement), and 6. Separation and discharge of the extract and solid

Figure 1 Scheme of the main steps in solvent extraction of solid food particles. Mass transfer of solute in liquid occupying pores is an important mechanism. External resistance is caused by the static liquid layer surrounding the particle.

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Solid–Liquid Extraction

37

As a result of these phenomena, extraction takes place at a rate expressed in terms of (mass of solute leached)/unit time or, more commonly, as change in solute concentration in the solid/unit time (dc/dt or dx/dt). In multicomponent extraction the relative rate at which different chemical species migrate through the solid may also be of concern. Since the aforementioned elementary steps occur at their own rate and in some cases sequentially, the overall rate of the extraction process is determined by the step having the slowest rate or the ratecontrolling step. As discussed later, transport through the solid matrix is usually the rate-controlling step in food extractions. Foods are unique in that the microstructure of the solid plays a major role in the rate and quality of the extraction process. To attain a fundamental understanding of the relationship between rate and food microstructure, it is necessary first to introduce some theoretical notions of mass transfer. A rigorous treatment on the subject may be found in the classical texts for chemical engineering. Gekkas (2) presents concepts of transport phenomena with applications to foods and biological materials. A recommended book on mass transfer for beginners and experts as well is that by Cussler (3). B. Solvent Selection Solvent selection is based on several properties: 1. Solubility of the specific compound (or compounds) in the solvent. 2. Recovery, since the solvent will be reused in subsequent extractions. If distillation or evaporation is used, the solvent should not form azeotropes and the latent heat of vaporization should be small. Removal of the solvent from the miscella (and from the spent solids) can pose serious problems if the residual level of the solvent must be minimized. 3. Interfacial tension and viscosity. The solvent should be capable of wetting the solid matrix and its viscosity should be sufficiently low so it can flow easily. Wettability is also important if the solvent must penetrate through pores and capillaries in the matrix. 4. Ideally, the solvent should be nontoxic, stable, nonreactive, nonflammable, harmless to the environment, and cheap. Laws relating to extraction solvents for use in foods are concerned primarily with human health requirements. Accepted solvents for use in compliance with good manufacturing practice (GMP) provided the presence of residues are unavoidable are as follows: propane, butane, propyl acetate, ethyl acetate, ethanol, carbon dioxide, acetone and nitrous oxide. Substances that have been found acceptable by the Enzyme Commission (EC) when used under specific conditions are hexane, methyl acetate, ethylmethylketone, and dichloromethane.

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Aguilera

II. EXAMPLE OF STRUCTURAL FEATURES OF A SOLID MATRIX: PLANT TISSUE Food materials of plant origin have an intricate microstructure formed by cells, intercellular spaces, capillaries, and pores. There are four major types of mature plant tissue: (a) storage or parenchyma; (b) conducting or vascular, composed of phloem (transport of organic materials) and xylem (transport of water); (c) supporting: and (d) protecting tissue. Only two of these contribute to the microstructure of edible parts of plant foods: parenchyma cells and the conducting tissue forming an intricate network throughout. An excellent introduction to plant cells and microscopy techniques can be found at the web site http://www. rrz.uni-hamburg.de/biologie. The desired solute in the tissue may be present inter- or intracellularly. In the first case, intact cell walls and adhering membranes constitute a major resistance to diffusion. They affect the permeability of solutes so that small molecules pass at a faster rate than larger ones, resulting in a selective transfer. The permeability of cell walls and membranes is increased and selectivity reduced by heat-induced denaturation. Moreover, cell walls forming most of the supporting tissues and certain types of conducting cells (tracheids, vessel elements) are lignified to a different extent, further reducing the passage of molecules. During preparation for extraction the solid is reduced to small particles because, theoretically, extraction time varies inversely with the square of the characteristic dimension of the solid. As particle size decreases the ruptured outer cells constitute a larger proportion of the particle volume and extraction characteristics per unit mass of material change. Size reduction is limited because of pressure drop in the extractor, slow drainage rates, presence of fines in the extract, flow instability, and entrainment. It has to be kept in mind that extraction is just one of the unit operations in a separation process and downstream purification depends strongly on how “cleanly” the extract was obtained.

III. FUNDAMENTAL ASPECTS OF EXTRACTION A. Extraction as a Diffusion Process Molecular diffusion is the process by which molecules are transported from one part of the system to another by random movement as a result of a concentration gradient. In leaching of foods the interior of the solid cannot be agitated and turbulence is unlikely to occur in small capillaries and pores, leaving molecular diffusion as the main transport mechanism within the solid phase. Solvent extraction may be considered as a diffusion process in the liquid (fluid) state since

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solute transfer, even inside a solid, exists as a dilute solution. In some cases, solvent influx may occur due to pressure gradients due to capillary forces or by mechanical relaxation of the cellular matrix. Fick’s laws provide the semiempirical bases for analysis of molecular diffusion. Fick’s first law is useful for defining a diffusion coefficient or diffusivity (D). It simply establishes that under steady-state conditions (concentration does not change with time) the unidirectional flux of solute 1 (J1, mol/s) in the r direction is directly proportional to the diffusivity of the solute, to the area traversed by the flux and to the gradient of solute concentration between two points, expressed in terms of absolute concentration (dc/dr) or molar fraction (dx/dr). Fick’s first law describes diffusion referred to a fixed coordinate system and for the unidirectional case it takes the form: j1 =

J1 dx1 Ddc1 −cD =− A dr dr

(1)

where j1 is the flux in moles per unit time and unit area, r is the direction of flow, A is the area across which diffusion occurs, and c is the total molar concentration (moles/volume). The minus sign gives a positive flux term since the gradient is negative (flow occurs down a concentration gradient, from high to low concentration). This equation can be regarded as a limit for long times, when transient conditions have disappeared and a linear gradient is established in the system. It can be applied without problems whenever the solute is in high dilution in the solvent; diffusion in concentrated systems involves convection and a more complex mathematical treatment (3). In practical situations and for short times, unsteady or nonstationary conditions exist and the concentration of solute varies with time (t) and position (r) inside the solid. In such cases, Fick’s second law (or the diffusion equation) applies and takes the general form:

∂c1 1 = ν−1 ∂t r

冉

∂ rν−1D

冊

∂c1 ∂r

∂r

(2)

where the index ν equals 1 for an infinite slab, 2 for an infinite cylinder, and 3 for a sphere. It is easier to understand Fick’s second law when Eq. (2) is written as:

冉 冊

∂c1 ∂2c1 ∂ ∂c1 =D 2 =D ∂t ∂r ∂r ∂r

(3)

for it says that the flow of solute is directly proportional to the change of the concentration gradient with position. So, when the gradient is constant (i.e.,

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Aguilera

linear concentration profile), then ∂c1/∂t = 0, meaning that steady-state conditions exist and we are back to Eq. (1). Analytical solutions for Eq. (3) under several simple initial and boundary conditions are found in many textbooks (4, 5); solutions to more complex situations and for different geometries are analyzed in the classical book by Crank (6). Schwartzberg and Chao (7) present a detailed analysis of the assumptions and conditions under which solutions to Eq. (3) can be applied to extraction in foods. In general, solutions relate the dimensionless extent of extraction of solute 1 as X = (c − cf)/(c0 − cf) with time in a series expression represented by Eq. (4), where c is the average solute concentration inside the solid at any time and c0 and cf are the initial and final equilibrium concentrations, respectively.

冉 冊

X = Σ Bn exp −

q2nDt L2

(4)

where X depends on α = mE/R (where m is the equilibrium distribution ratio between the solute concentration in the bulk solution and inside the solid, and E/R is the extract-to-solid volume ratio), and on Fick’s dimensionless number Dt/L2 (where L is a characteristic length, e.g., the particle size). Parameters Bn and qn are functions of α. When only the first term of the series is considered (e.g., for Dt/L2 > 0.06), plots of log X vs. t are straight lines with a slope equal to −Dq12/2.303L2, from which D can be obtained as an overall diffusion coefficient (effective or apparent diffusion coefficient). The dimensionless number Dt/L2 can be used as a criterion of closeness to the steady state. If it is much larger than unity (e.g., Dt >> L2) an equilibrium or steady-state condition may be assumed. Also, two parameters may be established for un-steady-state conditions: a “velocity of diffusion,” √D/πt and a “penetration distance,” √4Dt (3). Note that both parameters involve the square root of time. Solid–liquid extraction of various food materials is controlled by internal diffusion except in the case of very small particles, poor agitation, or presence of a skin (8–11). The extent of control between external and internal diffusion is indicated by the Sherwood number NSh = kcL/D, where kc is the liquid-phase mass transfer coefficient, L a characteristic dimension of the solid (e.g., particle size), and D the internal diffusion coefficient. If NSh > 200, internal control can be safely assumed (7). B. Determination of Diffusion Coefficients Data for diffusion coefficients are necessary to make calculations using the Fickean approach. Diffusivities may be determined experimentally or predicted. Experimental methods used for liquids are, among others, the diaphragm cell, the

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rotating disk (used for drug dissolution), nuclear magnetic resonance (NMR) spin-echo techniques, and interferometer methods (Gouy interferometer). For details of these methods, see Cussler (3). Orders of magnitude of these coefficients are important to remember (all units in cm2/s): for gases, 10−1; for liquids, around 10−5; and for solids, between 10−8 and 10−30 (if m2/s is used as unit, values must be divided by a factor of 104). Consequently, perfume in air diffuses 10,000 times faster than the tea extract in a cup of hot water! Diffusion coefficients of polymers in solvents under dilute conditions may be as low as 10−6 to 10−8 (cm2/s) and of gases through synthetic membranes vary widely between 10−8 and 10−11 (cm2/s). Numerous attempts have been made to predict diffusivities in liquids. According to the Stokes-Einstein equation, the diffusion coefficient of large spherical solute species in a pure solvent of viscosity η is given by: D=

kT 6πηrs

(5)

where k is the Boltzmann constant [gas constant divided by Avogadro’s number (R/N0)], T the absolute temperature, and rs the effective radius of the diffusing molecule. This expression is misleading in the sense that in practice viscosity and solute radius effects are more important than those of temperature. Also, adaptations of Eq. (5) for dilute solutions of macromolecules must encompass consideration of their size and shape. Nevertheless, the Stokes-Einstein equation has provided the foundation for several useful semiempirical equations. Most data for diffusion coefficients of liquids or gases (vapors) in solid foods come from solutions to the diffusion equation for standard geometries combined with experimental results, so they are “effective diffusion coefficients.” The main problem is that all steps involved in the diffusion of a solute through a solid matrix are ignored and the process is characterized by a single coefficient that is highly dependent on experimental conditions, solid geometry, and microstructural arrangement. It is not surprising, then, that “diffusion coefficients” for solute extraction vary by several orders of magnitude, even for the same solute. As explained before, effective or apparent diffusivities appear as a calculated value of D obtained from a transient diffusion experiment. Defining M as the total amount of solute that has diffused in (impregnation) or out (extraction) of a solid of regular geometry (slab, cylinder, or sphere) at any time and M∞ as the amount transferred after equilibrium is reached, the ratio M/M∞ may be calculated by integrating Fick’s second law under appropriate boundary conditions. For a slab of thickness L when 1 − M/M∞ is plotted against t, the slope is −π2Dapp/4L2 (the expression changes for other geometries) from which a value of Dapp can be determined. The term “apparent” confirms that we do not know the exact mechanism of transport, which in most cases may be complex.

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C.

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Diffusion Through the Solid Matrix

It should be evident from the previous analysis that microstructure influences molecular diffusion through its effect on the diffusion coefficient. Solute diffusivities can be defined in the liquid phase (DL) or in the wet solid (DS), and expressions for Fick’s laws can be written accordingly. Differences between DL and DS can be attributed to factors such as membrane resistance, complexity of the diffusion path, sorption of the solute by the inert solid, etc. If all of these factors were accounted for, DS could be related to DL by an expression of the form: DS = Fm × DL

(6)

where Fm is a correction factor including all phenomena related to microstructure. Schwartzberg and Chao (7) have reviewed extensively the subject of solute diffusivities in foods. DL values for various food solutes at infinite dilution in water at 25°C vary from 5.4 × 10−6 for sucrose to 1–7 × 10−7 cm2/s for some proteins. Selected DS values listed in the literature where microstructural effects appear important are presented in Table 1. Extraction experiments from which these data were obtained do not lead to easy determination of a single, constant diffusion coefficient. Typical curves for hexane extraction of oil from soybean grits and flakes of different particle size and a theoretical curve are shown in Fig. 2 (12). The slope of the curves becomes less steep as extraction proceeds, meaning a decrease in the value of DS so that usually an initial as well as a final diffusion coefficient has to be determined. The initial DS may be influenced by the previously mentioned release of surface solute (washing); thus, it is not truly Table 1 Diffusion Coefficients in Foods and Reference Values

Food material

Solute

Solvent

Temp. (°C)

Dilute solution Sugar cane (across grain) Sugar cane (with grain) Sugar beets Gelatin gel Dilute solution Pickled cucumbers Dilute solution Small curds Peanut slices Tungseed slices Dry solid matrix

Sucrose Sucrose Sucrose Sucrose Sucrose NaCl NaCl Lactose Lactose Oil Oil Glyceride

Water Water Water Water Water Water Water Water Water Hexane Hexane —

25 75 75 24 5 25 25 25 25 25 30 50

Source: Ref. 1.

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DAB × 106 cm2/s 5.4 5.1 3.0 1.6–2.5 0.1–0.2 16.1 5.3–11.0 4.9 3.0 0.007 0.006 5 × 10−4

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Figure 2 Kinetics of oil extraction from soybean flakes (------) and ground particles ( ) and theoretical behavior. Numbers on curves are the thickness of flakes or particle size (in microns).

representative of diffusion of solute inside the solid. Consequently, data on solid diffusivities listed in Table 1 should be analyzed with caution, always referring to the original work if further conclusions are to be inferred. DL values from the literature and diffusivities calculated for extraction of the same component may be used to determine the order of magnitude of the correction factor Fm in Eq. (6). The DL of caffeine in water is 6.9 × 10−6 cm2/s whereas the diffusivity of coffee solubles during extraction from grinds is of the order of 1.1 × 10−6 cm2/s (13). In sucrose extraction from beets the calculated diffusivities are one-half to one-third the DL at the same temperature. Diffusivities of pure linoleic and oleic acid in hexane are 3.6 × 10−7 and 2.6 × 10−7 cm2/s, respectively, while inside the seed they are reduced to 2.6–6.2 × 10−8 cm2/s (14). Hence, the correction factor in Eq. (6) appears to be of the order of 0.1–0.9 for small solutes, with higher values occurring when membranes have been denatured, as in sugar beet processing. For macromolecules Fm is obviously larger. In fact, Schwartzberg and Chao (7) report that solutes exceeding a molecular weight of 2000 cannot diffuse into intact cells of coffee grounds. The reduction of the diffusion coefficient

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for zein (corn protein) in dilute solution and inside the endosperm is about 1000fold (15). Sorption of solvent and/or solutes by the solid also retards diffusion. Microstructural entrapment seems to play a role at least as important as physicochemical sorption. Lignocellulosic materials are known to sorb water that is no longer available as solvent, particularly if ground into a fine powder. Sorption is critical when organic solvents must be removed from spent solids after extraction, in an operation known as desolventizing. Residual hexane left in rapeseed meals concentrates preferentially in the hulls, dissolved in the residual oil, entrapped inside thick-walled cells (16), or simply adsorbed. In this latter case, internal diffusion coefficients for hexane during adsorption into the solid matrix are very small (about 10−10 cm2/s) and increase with hexane content, probably due to swelling of the structure.

IV. THE MICROSTRUCTURAL APPROACH A. Simple Correction Factors On examination of Eq. (3) and data previously presented it can be concluded that the influence of food microstructure on extraction rate is predominantly by its effect on the diffusion coefficient. Chemical engineers use the effective (or apparent) diffusion coefficient Deff when dealing with impermeable porous solids with fluid-filled pores: Deff = D

ε τ

(7)

where D is the diffusion coefficient of the solute in the fluid filling the pores, ε is the void fraction or porosity of the solid, and τ is the tortuosity of the pores, which attempts to account for the longer distance traversed by the solute along a sinuous path. Porosity may be very low in potato tissue (⬃2%) or high as in apples (⬃20%). For solid materials used in chemical engineering (adsorbents, porous catalysts) tortuosity varies between 2–6, and porosity between 0.3–0.8, thus Deff may be 6 to 15 times lower than D. When the size of the pore and the solute are of comparable magnitude (e.g., in some membranes), Eq. (7) is corrected by a factor λ that depends on the ratio of solute radius to pore radius. The restricted diffusion of spherical molecules within cylindrical pores (Dp) is usually modeled by the so-called Renkin equation (17): Dp = D(1 − λ)2f(λ)

(8)

The squared term of the equation is a partition coefficient and accounts for steric hindrance at the pore entrance. The factor f(λ) is a polynomial function

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of λ and corrects for the friction between the diffusing molecule and the walls of the pore. The previous results may be expressed in a more general form. For a twophase composite where spherical particles of a material 1 are dispersed in a continuous phase 2, an effective diffusion coefficient may be obtained from the expression:

冉

冊

Deff − D1 D − D1 =φ 2 Deff + 2D1 D2 + 2D1

(9)

where D1 is the diffusion coefficient through the interstitial pores and D2 is the diffusion coefficient through the particles. So far we have assumed that pores are quite large. When the size of pores is of the order of magnitude of the distance between molecular collisions, Fick’s laws no longer applies and the so-called Knudsen diffusion takes place in which molecules collide with the pore walls. We leave this section with the impression that a much better work in using Fick’s laws could be made to predict extraction rates if we had an idea of the microstructural architecture of a solid food particle. Microscopy can assist in developing such structural models and in finding parameters such as tortuosity, porosity, and pore size. B. Introducing Architectural Effects Other correction factors where the diffusivity is corrected taking into account the architecture of the structure have been discussed by Cussler (3). A flake with uniformly distributed platelets impermeable to the solute, parallel or perpendicular to the diffusing stream where the volume fraction of the platelets is φF and their aspect ratio is α, results in an effective diffusivity given by the expression:

冉

Deff 1 = 2 2 D 1 + α φF /(1 − φF)

冊

(10)

A plot of Deff/D vs. φF having α (long dimension divided by short dimension of platelets) as parameter is shown in Fig. 3. When platelets are arranged parallel to the diffusion (segmented line), tortuosity is 1 and the effective diffusivity varies linearly with φF, underscoring the unidirectional nature of the diffusion. When platelets are arranged perpendicular to the diffusion Deff/D depends strongly on α and a nonlinear dependence on φF is evident. For a constant value of α, Deff/D becomes smaller as φF increases. It is also interesting to note that when α is very large, say α = 30, even at low values of φF the ratio Deff/D can become very small. Thus, this model shows an important effect of microstructure and architecture on the rate of trans-

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Figure 3 Curves showing the correlation between Deff/D and volume fraction φF of impermeable platelets in a slab (φF) for different aspect ratios α of platelets. The effective diffusion coefficient varies sharply with φF when platelets are placed perpendicular to the diffusional flow.

fer of solute. However, it does not consider the possibility of nonuniform spatial distribution of the impermeable elements. Another model that considers the situation of impermeable spheres occupying a volume fraction φS and uniformly distributed within the matrix giving rise to the expression of Deff/D given by Cussler (3) is:

冋

Deff 2(1 − φs) + D 2 + φs

册

(11)

Evidently the size and distribution of the spheres is not considered in the model. In summary, although some expressions like (10) and (11) are available for

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biphasic systems with uniformly distributed elements, they do not account for nonuniform architectural arrangements.

V. MATHEMATICAL MODEL FOR EXTRACTION FROM A TWO-DIMENSIONAL MATRIX Fick’s first law for the steady state and second law for the transient or unsteady state in two dimensions of a homogeneous and isotropic media are described by the following equations:

冉

冊

∂C ∂C + ∂X ∂Y

(12)

∂C ∂ ∂C ∂ ∂C = D + D ∂t ∂X ∂X ∂Y ∂Y

(13)

J = −D

冉 冊 冉 冊

Applying the precedent equations to a structure with two different phases A and B, continuity of mass flow must be observed at every boundary between the phases. Crank (6) proposed a discrete form (finite difference) for the unidirectional diffusional case of a two-phase compound: 1 C(i,n+1) − C(i,n) DB DA {∆XA + ∆XB} = (C(i+1,n) − C(i,n)) − (C(i,n) − C(i−1,n)) (14) 2 ∆t ∆XB ∆XA where i is the position of the element or node and n the iteration index. Based on this equation and applying explicit finite difference discretization to the Fick’s second law in two dimensions using ∆X = ∆Y = ∆L while taking into account the continuity between phases, equal distance increment in the phases (i.e., ∆XA = ∆XB = ∆X) and constant diffusion coefficients at each point throughout the extraction process, the following expression is obtained: C(i,j,n+1) = C(i,j,n) =

∆t (∆L)2

× [D(i+1,j,n)(C(i+1,j,n) − C(i,j,n)) − D(i−1,j,n)(C(i,j,n) − C(i−1,j,n)) + D(i,j+1,n)(C(i,j+1,n) − C(i,j,n)) − Di,j−1,n)(C(i,j,n) − C(i,j−1,n))]

(15)

which is based on the application of the Schmidt method of the finite difference approach to heterogeneous compounds (e.g., different diffusion coefficients at each specific spatial locations) as presented in Crank (6). The subindex (i,j,n) refers to the position (i,j) of an element in the matrix at the iteration step n, equivalent to a time t = n × ∆t.

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Modeling extraction with Eq. (15) starts by defining a matrix of M × N elements representing the object being extracted bidirectionally and introducing the specific architectural elements in the form of pores, walls, or platelets. Geometrical arrangements of the structure are modeled assuming different diffusion coefficients for each phase or element. The boundary condition for the model assumed that the object was surrounded by pure solvent at all times (bulk concentration of solute in solvent equal to zero or infinite volume of solvent). This is equivalent to saying that the internal resistance is controlling or the Biot number for mass transfer is infinite. The concentration at each point of the matrix and the corresponding diffusion coefficients at time 0 were the initial conditions for this model. Extraction was simulated for different architectures of the matrix. Extraction curves are semilog plots depicting variations in time of the ratio of the instant mass of solute remaining in the solid to the total initial mass (q/q0). A. Modeling Extraction from Cellular Material An example of the previous approach has been applied to extraction from a cellular structure modified by various pretreatments (18). Figure 4 shows the architectural arrangements associated with four pretreatments. The basic structure considers that all cells are intact and completely surrounded by a thick cell wall (A). Enzymatic treatment with a cocktail of cell wall–degrading enzymes may pierce the wall of outer cells at different places, leaving openings that connect the cytoplasm directly to the solvent (B). A third situation may be that induced by blanching, resulting in degradation of the cell wall membrane (and possibly the cell wall itself), diminishing the overall internal resistance for transport of solute as most cells become interconnected (C). Another conceivable architecture is that of a totally connected cytoplasm and broken outer cells exposed to the solvent due to extensive physical destruction of the tissue (D). In all cases, the proportion of cell wall material to cytoplasm is the same. The simulation used data obtained from Schwartzberg and Chao (7) for infusion of salt (NaCl) in pickled cucumbers and water. Data for the cell wall complex were Dcw = 53 µm2/s and Ccw = 1 g/L, and for the cytoplasm D = 219 µm2/s and C = 3 g/L. ∆L was 20 µm and ∆t was 0.2 s. The overall size of the matrix was 380 × 380 µm in a grid of 19 × 19 elements. The resulting extraction curves for the simulations are presented in Fig. 5. The fastest extraction rate corresponded to pieces having complete breakage of the cell wall and a layer of cytoplasm exposed directly to the solvent. Washing of solute from the outermost layers removes 20% of the solute almost instantly. Since it was assumed that the cell wall complex also contained solute and had a finite diffusivity, even the arrangement containing intact external cells showed some extraction. However, if the cell wall complex had been assumed

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Figure 4 Architectural arrangement of cells of pickled cucumbers subject to different treatments (theoretical).

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Figure 5 Resulting curves of the extent of extraction vs. time simulating salt diffusion in pickles with and without pretreatments to modify the microstructure.

impermeable, extraction would have been zero in this case for all times. Actual extraction of triglycerides from oilseeds having intact cells by organic solvents is extremely slow (1). The fastest extraction is achieved in the situation of full rupture of the cell complex, and the slowest for intact cells. In the latter case, the curves start from zero concentration (no washing) and the time to achieve 90% removal of solute is 40% longer than in the previous case. Maintenance of intact cell membranes may be beneficial to selectively extract small solutes from the cytoplasm keeping large molecules and debris within the piece to facilitate downstream separation and purification (as in sucrose extraction from sugar beets). The use of microscopy techniques to correlate the state of the microstructure and its effect on extraction is presented in Aguilera and Garcı´a (19) for aqueous extraction of protein from lupins; Aguilera and Lusas (20) to assess the

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extractability of oil from flakes, grits, and extrudates of high-oil corn using hexane; Fan et al. (21) for the extraction of peanut oil; and Rastogi and Niranjan (22) for osmotic dehydration of pineapple tissue, among others. With the advent of nonintrusive microscopy techniques and real-time video microscopy, structural features prior and during extraction may be observed and different architectural arrangements (presence of elements, phase ratios, geometrical parameters, etc.) characterized by image processing and analysis. Combination of microstructural information, better data on diffusivities of individual phases, and advanced computer techniques should lead to a more fundamental approach to the study of mass transfer phenomena in foods and other biological materials (23).

VI. USE OF MASS TRANSFER COEFFICIENTS The problem with Eq. (1) is that the distance over which concentration changes occur (δ) must be known to determine the gradient. This is not easy task for processes that occur inside process equipment. Moreover, we are in the presence of interfacial mass transfer or transfer between two different phases (solid and liquid). Chemical engineers prefer to study interfacial mass transfer using mass transfer coefficients (individual or overall), which multiplied by a measurable driving force give the rate of mass transfer. Thus, the distance problem becomes hidden in the coefficient, which also contains implicitly the diffusivity. A practical expression for the rate of solute extraction takes the form: Rate = mass transfer coefficient × driving force

(16)

The driving force in extraction should be the difference between the chemical activity of the solute inside the solid and that in the bulk of the solution. For practical reasons, the rate is expressed as the product of the difference between an external solute concentration (cout) and that in the interior of the solid (cin), and a mass transfer coefficient based on concentration. For this difference to be meaningful it must be expressed in the same base; thus, cin is usually taken as that concentration of solute in the liquid phase that would be in equilibrium with the concentration inside the solid c*. Then the driving force for extraction becomes (c* − cs), where cs could be measured in the bulk of the solution and Eq. (16) takes the form: N1 = Kc(c* − cs)

(17)

This simple equation states that the rate of extraction N1 depends on the difference in a thermodynamic variable (expressed as concentration) and a global mass transfer coefficient Kc that includes all physical and microstructural parameters of the process. If the interfacial area for transfer (a) is unknown, the coeffi-

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cient becomes Kca. If each phase is taken separately, individual mass transfer coefficients ki can be defined for transport between the interface and the bulk of the respective phase [see Cussler (3) for details]. If a solid is modeled as series of structures (i.e., a plant cell with protoplasm, plasmalemma, cell walls, etc.), the observed global mass transfer coefficient may be related to individual mass transfer coefficients inside each of the structures as a sum of resistances in series: 1 1 =Σ Kc kci

(18)

and a rate-limiting step having the highest resistance (or the slowest flow) may be identified. This structure then controls the mass transfer process and efforts should be made to increase the rate within this phase. This approach can also be used when studying extraction as a series of sequential steps as shown in Fig. 1. Theoretically, there is a relationship between D and Kc (e.g., Kc is proportional to D/δ, where δ is a thin film over which diffusion occurs) and, as previously concluded, study of the influence of microstructure on extraction rate is reduced to the analysis of its effects on the mass transfer coefficient. Mass transfer coefficients are often correlated to dimensionless numbers, allowing predictions to be made for other experimental conditions. Dimensionless numbers are ratios of transport parameters and have physical meaning. For example, the most relevant of these numbers for solid–liquid extraction is the Sherwood number (NSh), which is the ratio of mass transfer velocity (kL) to diffusion velocity (D). A large NSh in solvent extraction means that the controlling step is diffusion inside the solid, and reduction of particle size or restructuring into a porous material may prove effective in speeding extraction. NSh is in turn related to several other dimensionless numbers, e.g., to the Schmidt number representing flow effects (momentum transfer) by the expression: NSh =

冉冊

kcL ν =f D D

1/3

(19)

where ν is the kinematic viscosity.

VII. CONCLUSIONS Food engineers can make a significant contribution to the study of extraction or leaching by introducing the microstructural variable into the problem. A good architectural description of the solid being extracted together with fundamental data of diffusion coefficients in different structures or phases can be combined with modern computational methods to predict extraction rates from food solids.

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Solid–Liquid Extraction

ACKNOWLEDGMENTS This work was partly funded by project FONDEF D9712026.

NOMENCLATURE a A Bn c C D Deff , Dapp E/R Fm J j k kc Kc L M N0 NSh q qn R r t T X

interfacial area of transfer (m2/m3) area for diffusion (m2) parameter in eq. 4 molar concentration (moles/m3) concentration (g/L) diffusion coefficient or diffusivity (m2/s) effective or apparent diffusion coefficient (m2/s) extract-to-solid volume ratio correction factor in eq. 6 mass flux (moles/s) flux (moles/s m2) Boltzmann constant liquid phase mass transfer coefficient global mass transfer coefficient characteristic length or slab thickness (m) total amount of solute extracted (g) Avogadro’s number Sherwood number mass of solute in the solid (g) parameter in eq. 4 gas constant direction of flow or position inside the solid time (s) absolute temperature (K) extent of extraction of solute (dimensionless)

Symbols α α δ ε φ η τ

aspect ratio (length/width) m.E/R film thickness porosity or void fraction volume fraction of platelets or spheres viscosity tortuosity

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54

λ ν υ

Aguilera

correction factor index (1 for infinite slabs, 2 for infinite cylinders, 3 for spheres) kinematic viscosity

REFERENCES 1. JM Aguilera, DW Stanley. Microstructural Principles of Food Processing and Engineering. Gaithersburg, MD: Aspen Publishers, 1999. 2. V Gekkas. Transport Phenomena of Foods and Biological Materials. Boca Raton, FL: CRC Press, 1992. 3. EL Cussler. Diffusion: Mass Transfer in Fluid Systems, 2nd ed. Cambridge, UK: Cambridge University Press, 1997. 4. RB Bird, WE Stewart, EN Lightfoot. Transport Phenomena. New York: Wiley, 1960. 5. CJ King. Separation Processes, 2nd ed. New York: McGraw-Hill, 1981. 6. J Crank. The Mathematics of Diffusion. London: Oxford University Press, 1975. 7. HG Schwartzberg, RY Chao. Solute diffusivities in leaching processes. Food Technol 2(2):73–86, 1982. 8. U Cogan, A Yaron, Z Berk, S Mizrahi. Isolation of soybean protein: Processing conditions on yields and purity. JAOCS 44:321–324, 1967. 9. HA Leninger, WA Beverloo. Food Process Engineering. Dordrecht, Netherlands: D. Reidel, 1975, p. 502. 10. M Moo-Young, G Andre. An appraisal of mass transfer phenomena in food processing operations. In: P Linko, Y Malkki, J Olkku J Larinkari eds. Food Engineering Progress. London: Applied Science Publishers, 1980, Vol. 1, pp. 342–353. 11. M Spiro, CM Page. The kinetics and mechanism of caffeine infusion from coffee: Hydrodynamic aspects. J Sci Food Agr 35:925–930, 1984. 12. G Karnofsky. The theory of solvent extraction. JAOCS 26:564–570, 1949. 13. A Voilley, D Simatos. Modeling the solubilization process during coffee brewing. J Food Proces Eng 3:185–198, 1980. 14. R Anderson, R Osredkar, C Zagar. Diffusion properties of lipids: pure and in seeds. JAOCS 54:487–489, 1977. 15. MK Russell, GT Tsao. Protein removal from corn endosperm by solvent extraction. AIChE Symp Series No. 218, 78:83–89, 1982. 16. FH Schneider, U Rutte. Zur bindung der resthexans in raps-extraktionsschroten. Fette Seifen Anstrichmittel 86:331–339, 1984. 17. L Lebrun, GA Junter. Diffusion of sucrose and dextran through agar membranes. Enz Microb Techn 15:1057–1062, 1993. 18. J Crossley, JM Aguilera. Modelling the effect of microstructure on food extraction. J Food Process Eng, 24:161–177, 2001. 19. JM Aguilera, HD Garcı´a. Protein extraction from lupin seeds: a mathematical model. Int J Food Sci Technol 24:17–27, 1989. 20. JM Aguilera, EW Lusas. Laboratory and pilot solvent extraction of extruded high oil corn. JAOCS 63: 239–243, 1986.

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21. HP Fan, JC Morris, H Wakeham. Diffusion phenomena in solvent extraction of peanut oil: Effect of cellular structure. Ind Eng Chem 40(2):195–199, 1948. 22. NK Rastogi, K Niranjan. Enhanced mass transfer during osmotic dehydration of high pressure treated pineapple. J Food Sci 63:508–511, 1998. 23. JM Aguilera, PJ Lillford. Microstructure as applied to food engineering. In: P Fito, E Ortega-Rodrı´guez, GV Barbosa-Ca´novas eds. Food Engineering 2000. London: Chapman and Hall, 1996, pp. 20–36.

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3 Supercritical Fluid Extraction in Food Engineering Nurhan Turgut Dunford Oklahoma State University Stillwater, Oklahoma, U.S.A.

Jerry W. King and Gary R. List U.S. Department of Agriculture Peoria, Illinois, U.S.A.

I. INTRODUCTION The origins of supercritical fluid extraction (SFE) are firmly rooted in the phenomenon of solute solubility in gases as typified by the classic studies of Hannay and Hogarth in the 1880s (1). However, this curious and somewhat unexpected phenomenon, i.e., the dissolution of solids in such fluids as supercritical ethanol and water, was not exploited for any useful purpose until the midpart of the next century. From a modern perspective, SFE was first exploited in Russia in the 1950s (2) and later in Germany (3) for the extraction and fractionation of components in natural products. However, it was the appearance of the Zosel patent (4), issued in Germany in the late 1960s, that firmly documented the myriad of possibilities for using supercritical fluids (SCFs) as processing agents. The U.S. version of this patent, entitled, “Process for the Separation of Mixtures of Substances,” provided more than 60 examples of applications of super- and subcritical ethylene or carbon dioxide to food processing and other applications. HowNames are necessary to report factually on available data. However, the USDA neither guarantees nor warrants the standard of the product, and use of the name by USDA implies no approval of the products and the exclusion of other that may also be suitable.

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ever, this broad coverage did not deter other parties from similar patent filings over the next 30 years or the industrial exploitation of critical fluids as versatile processing agents. The classic, often cited applications of SCFs, especially supercritical carbon dioxide (SC-CO2) or liquefied carbon dioxide, are for the decaffeination of coffee (5) and the processing of hops (6). Thus, there is a historical precedent for using critical fluid media in food engineering, which continues up to this present day. Since the above-cited applications, there has been a plethora of additional applications of SFE to foodstuffs, agricultural raw materials, and associated natural products (7). This has resulted in the development of 32 processing plants worldwide employing SCFs. Currently, most of these facilities are centered in Germany, the United States, France, and Japan. Nations such as Great Britain, Australia, Canada, India, and Italy continue to develop more plant capacity for critical fluid processing; and other nations with a rich litany of natural products will undoubtedly enter the marketplace as users of SCF technology in the future. It should be noted that not all of the new processing schemes employing SCFs will be wholly based on extraction, but may include fractionation schemes or reactions conducted in critical fluid media. Table 1 shows a list, which is not inclusive, of many users of critical fluid extraction for food and natural product processing throughout the world. Key players in the various market segments are as follows: decaffeination—General Foods, SKW Trostberg, Kaffee HAG, Hermsen; hops processing—HVG Barth (NATECO2), John Haas, Yakima Chief, Carlton United Breweries, Steiner Hops, English Hops, SKW Trostberg; flavors/spices—Cultor, Quest, Flavex,

Table 1 Organizations Processing or Offering Critical Fluid–Derived Products Flavex (Germany) Hermsen (Germany) HVG Barth (Germany) Kaffe HAG (Germany) SKW Trostberg (Germany) KD-Pharma-IQA (Germany–Spain) General Foods (United States) John Haas (United States) Praxair (United States) Yakima Chief (United States) Carlton United Breweries (United Kingdom) English Hops (United Kingdom) Steiner Hops (Germany–United Kingdom– United States)

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Fuji Flavor (Japan) Kobe (Japan) Mori Oil Mills (Japan) Ogawa (Japan) Takasago (Japan) Takeda (Japan) Cultor (France) HITEX (France) Norac (Canada) Aroma Tech OY (Finland) Quest (Holland) Wells Investment Ltd. (New Zealand)

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Norac, Ogawa, Fuji Flavor, Kobe, Mori Oil Mills, Takeda. Many of the processors listed under the generic heading of flavors/spices also produce a variety of other natural products such as specialty oils, natural pigments, and antioxidants. For example, Flavex is also involved in the processing of ginseng among other moieties, and Norac in Canada processes rosemary antioxidants, saw palmetto, kava kava, and other botanicals (8). Since the early 1990s, an awareness of the potential of critical fluid processing as a viable component for “green” processing has arisen (7). This coupled with an increasing consumer awareness of the identity and use of chemical solvents in food and natural products processing has provided further impetus for the use of benign solvents as SC-CO2, ethanol, and water. Recently, the use of SC-CO2 for the processing of certain nutraceutical products (9) has provided additional possibilities for exploiting SCFs. As governments worldwide are also currently making regulations on the use of organic solvents even more strict (10), including the banning of some traditional solvents, environmentally benign SCFs such as SC-CO2 or non-ozone-depleting fluorocarbons will become even more attractive. In this chapter, we have reviewed some of the basic fundamentals of SCFs and their use for the extraction and fractionation of food-related materials. The relevant physical and chemical properties of SCFs, particularly SCCO2, are initially discussed, followed by the key factors that contribute to successful SFE, namely, the solubility of solutes in SCFs, phase equilibria, and mass transfer considerations. A discussion of the fundamentals required in effectively using SCFs, such as the type of extraction or fractionation, along with choice of solvents, entrainers, and condition of the matrix to be extracted, follows. The various application areas of SCF extraction are discussed at length in following chapters.

II. PROPERTIES OF SUPERCRITICAL FLUIDS The mathematical modeling, simulation, and design of an SFE process require knowledge of physicochemical properties of compounds of interest. Further interpretation of physicochemical property values can be obtained by an understanding of molecular behavior. For example, the ideal gas law [Eq. (1)] is a prime example of the correlation of properties. PV = NRT

(1)

Critical temperature and pressure, compressibility, density, heat capacity, dielectric constant, viscosity, diffusivity, and thermal conductivity are important properties, which are needed for the characterization of both solvent and solutes in an extraction process. Pure-component properties as well as mixture proper-

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ties are used to describe individual solute and solvent characteristics and how the solute and solvent behave as constituents of mixtures. A. Critical Constants The critical point of a pure substance is usually defined as the temperature and pressure at which the gas and liquid phases become indistinguishable. However, in the case of a solid and a liquid, these two phases do not become identical when their densities are equal. Therefore, solid–liquid, solid–gas, and solid–solid equilibrium lines do not end by critical points, as do gas–liquid lines (Fig. 1). When a substance is compressed and heated to its critical point it enters a phase referred to as “supercritical phase.” The matter that is in the supercritical region is called a supercritical fluid (SCF). Temperature, pressure, and molar volume of a substance at the critical point are defined as the critical temperature (Tc), critical pressure (Pc), and critical molar volume (Vc), respectively. These parameters are collectively referred to as critical constants. Each substance has a unique set of critical constants (Table 2). Applications of SFE in food engineering involve processing of many materials, which contain a number of components. Identification and characterization of natural material components are usually very difficult. Therefore, in the literature SFE often characterizes processes above the critical point of the solvent. In this case, mixture of solvent and solute usually exists as two distinct, partially miscible phases below the critical point of the mixture. Similarly, extraction processes just below the critical point of the solvent are usually referred to as near-critical extraction based on the pure solvent properties rather than that of the mixture. Critical properties of small molecular weight compounds can be found in standard handbooks (12, 13). Critical properties of many biological materials cannot be determined experimentally due to the fact that these materials will be decomposed before their critical point is reached. Usually, semiempirical methods are used to estimate critical properties of natural products. The group contribution method is also widely used to estimate critical properties of biological materials. Reid et al. (13) reviewed Ambrose, Fedors, and Joback modification of Lydersen’s method in detail. The Ambrose group contribution method is widely used to estimate critical properties of organic materials. In this method, the critical properties are estimated using the following relations: Tc = Tb[1 + (1.242 + Σ∆T)−1]; Pc = M(0.339 + Σ∆P)−2; Vc = 40 + Σ∆ ν

(2)

The ∆ values for a number of compounds are given in Reid et al. (13). When the compound of interest contains functional groups for which ∆ parameters are not available, use of group contribution methods becomes difficult. In such cases, more empirical models are used (14, 15).

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Figure 1 Phase diagram for a pure substance.

B. Physicochemical Properties A unique feature of the SCF is the adjustability of their density in the supercritical region by regulating the temperature and pressure of the system. An increase in temperature leads to a decrease in density in all cases. Density of a fluid is extremely sensitive to temperature and pressure near the critical point (Pr = 1, Tr = 1). Reduced density (ρr = ρ/ρc) of a pure compound at reduced pressure 1.0

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Table 2 Critical Pressure and Temperature of Common Fluids for SCFE

Fluid Carbon dioxide Chlorotrifluoromethane Ethylene Ethane Propane Propylene Cyclohexane Isopropanol Benzene Toluene p-Xylene Trichlorofluoromethane Ammonia Water

Crit. temp., Tc (°C)

Crit. pressure, Pc (× 105 Pa)

31.1 28.9 9.3 32.3 96.7 91.9 280.3 235.2 289.0 318.6 343.1 198.1 132.5 374.2

73.7 39.2 50.3 48.8 42.4 46.2 40.7 47.6 48.9 41.1 35.2 44.1 112.7 220.4

can be changed from a value of about 0.1, a gas-like density, to about 2.0, a liquid-like density, by regulating reduced temperature in the range of 0.9–1.2 (Fig. 2, Table 3). As the reduced densities become liquid-like, SCF begins to act like a liquid solvent. However, as the reduced temperature is raised to a value of about 1.6, the fluid becomes gas-like due to the decreased density (expansion of fluid) with increasing temperature. Data on the physical properties of mixtures is scarcer than for the pure compounds. An interesting publication of Magee (16) includes a list of references, which report density calculations for CO2-rich mixtures from various predictive models or theories. At high pressures, gases deviate from ideal behavior due to enhanced physical forces between ions, dipoles, induced dipoles, and higher poles, which contribute to the molecular interactions in the system. The potential energy of interaction, Ep, of point charges q1 and q2 can be defined as a function of the permittivity of the medium, ε, and the distance between the charges, r [Eq. (3)]. Ep =

q1q2 4πεr

(3)

The relative permittivity, εr, also known as dielectric constant of the medium, is defined as follows:

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Figure 2 Variation of reduced density of a pure substance in the near-critical region (20).

Table 3 Comparison of Physical Properties of Gas, Liquid, and SCF (98)

Physical property

Gas (1 atm, 15–30°C)

Supercritical fluid Tc, Pc

Tc, 4Pc

Liquid 15–30°C

Diffusion coefficienta (cm2/s) 0.1–0.4 0.7 × 10−3 0.2 × 10−3 (0.2–2) × 10−5 −4 −4 −4 Viscosity (g/cm.s) (1–3) × 10 (1–3) × 10 (3–9) × 10 (0.2–3) × 10−2 −3 Density (g/mL) (0.6–2) × 10 0.2–0.5 0.4–0.9 0.6–1.6 a

Self-diffusion for gas and dense gas; binary mixture for liquid.

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εr =

ε ε0

(4)

where, ε0 is the permittivity in a vacuum. The static dielectric constant (DC) is a useful property to estimate the solvent properties of the relatively polar fluids such as ethanol, methanol, and water. DC is also a density-dependent property and can be changed significantly by modest changes in temperature and pressure of the system. The DCs for SCFs become important because they are a measure of intermolecular force enhancement through dipole–dipole interactions. For example, DC values for CO2 rise rapidly between 70 and 200 × 105 Pa at 40°C and reach liquid-like values around 200 × 105 Pa (Fig. 3) (17). This partially explains the greater solvent power of SC-CO2 for low-volatility compounds at elevated pressures. C. Transport Properties 1. Viscosity Viscosity is an important property, which is needed for the characterization of momentum, mass, and energy transport within a system. The viscosity of a gas increases with increasing temperature at a moderate pressure whereas that of an SCF decreases (18). This phenomenon is attributed to the higher molecular velocity and increasingly hindered “collision transfer” of momentum at high temperature and pressures. Viscosity of a substance changes by a factor of 3–5 in the critical region rather than orders of magnitude (Table 3). Thus, when pressure is increased, gas viscosity approaches liquid-like value more slowly than does density (19). The viscosity of CO2 as a function of pressure is shown in Fig. 4. The viscosity of CO2 is only 0.09 cP even at pressures as high as 300– 400 atm, which is an order of magnitude lower than the typical viscosities of liquid solvents (20). As another example, viscosity of supercritical water (Fig. 5) is less than one-tenth of liquid water, and as a consequence its diffusion coefficient and ion mobilities are one order of magnitude greater than those of the liquid water. Viscosity measurement and relevant estimation techniques for pure and SCF mixtures have been discussed by Vesovic and Wakeham (21) in detail. Magee (16) also reviews the literature related to the viscosity for gas mixtures; however, this kind of information is virtually nonexistent for the SCF-natural product mixtures. 2. Diffusivity Knowledge of diffusion coefficients in SCF is important to the design and efficient operation of SFE processes. The diffusion coefficient is the proportionality

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Figure 3 Density and dielectric constant of carbon dioxide as a function of pressure at 50°C (17).

constant between the molecular flux of a compound and its composition gradient. Fick’s law defines it as follows: J1 = −D12

∂C1 ∂Z

J2 = −D21

∂C2 ∂Z

(5)

where C1, C2, and J1, J2 are the molar concentrations and fluxes of two components, respectively. Fick’s law holds for binary, isobaric mixtures and assumes presence of no external force field. The diffusivity of a pure SCF is orders of magnitude greater than that of a liquid, i.e., liquid CO2, which results in im-

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Figure 4 Viscosity behavior of carbon dioxide (20).

Figure 5 Viscosity of water as a function of density and temperature (18).

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proved mass transfer rates during SFE. In general, the binary diffusion coefficient in SCF increases with temperatures and decreases with pressure at constant pressure and temperature, respectively (Fig. 6). Although the effect of composition on diffusion coefficient is quite small at low pressures, it becomes significant at higher densities. Solute diffusion coefficients decrease with increasing solute molecular mass. Furthermore, molecular mobility is hindered if the solute and solvent molecules are significantly different in size. The experimental methods used to determine diffusion coefficients in SCF

Figure 6 Diffusivity behavior of carbon dioxide (20).

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systems, such as solid dissolution, capillary peak broadening, photon correlation spectroscopy, nuclear magnetic resonance, and radioactive tracer response techniques, have been reviewed by Giddings and Seager (22) and Liong et al. (23). Examples of measured diffusion coefficients reported in the literature are for C16 fatty acid esters (24), caffeine (25), and C18 unsaturated fatty acid methyl esters (26). 3. Heat Capacity and Thermal Conductivity Heat capacity and thermal conductivity data are needed for the characterization of heat transfer behavior in a system. In the critical region, heat capacity at constant pressure is large and goes through a maximum (Fig. 7) (27). However, heat capacity at constant volume shows only small changes in the critical region (Fig. 7). Angus et al. (28) have reviewed the heat capacity data published up to 1976. Magee and Ely (29) have measured the heat capacity of CO2 at constant volume, at a larger density range, 0.2–2.5 times the critical density. They also compared their experimental results with those calculated using an extended Benedict-Webb-Rubin equation of state. In general, the measured and calculated heat capacity values agreed well, except that relative errors were higher at the near-critical region. Magee (16) also reviewed the heat capacity data for pure CO2 and CO2-rich mixtures.

Figure 7 Heat capacities for carbon dioxide at 320 K (27).

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Thermal conductivity of fluids has been recognized as an important transport property, which is needed for the design of the SFE process equipment. Thermal conductivity “λ” is a proportionality constant between the heat flux, Q, and the temperature gradient that exists in the fluid. The Fourier’s law defines it as follows: Q = λ 䉮T

(6)

For most SCFs thermal conductivity increases with increasing temperature and density of the system. Table 4 shows the thermal conductivity data for water and CO2 as a function of temperature at several pressures. The Transient Hot-Wire Technique and The Coaxial Cylinder Method are two techniques that are used to measure thermal conductivity of SCFs over a wide range of conditions (21). Although the Euken expression and correlations based on the work of Dymond had been used to estimate thermal conductivity of dense fluids, the calculated values did not agree well with the experimental data (21). Nieto de Castro (30) has reviewed the literature on the thermal conductivity in SCF.

III. EQUILIBRIUM PROPERTIES A. Phase Equilibrium Process modeling and operating conditions, as well as economic feasibility for an extraction process, require information on phase equilibria between the solute(s) and solvent(s) involved. This information can be obtained experimentally or via phase equilibrium calculations. Phase equilibrium calculations require an equation of state, which describes the mathematical relation between volume, pressure, temperature, and composition in the system. Deviations from ideal gas law are expressed in the equations of state such as van der Waals [Eq. (7)] and Redlich-Kwong (31) or Peng-Robinson equations (32).

Table 4 Thermal Conductivity [λ(mW/mK)] of Some Fluids Used in SCF Technology (21) T = Tc + 20 K

CO2 H2O

T = Tc + 100 K

P = 0.1 MPa

P = 10 MPa

P = 0.1 MPa

P = 10 MPa

Tc (K)

18.8 54.0

51.1 67.8

25.5 63.7

31.9 73.0

304.1 647.1

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冉 冊 P+

a (V − b) = RT V2

(7)

where a and b are the constants specific for each gas. The van der Waals equation of state assumes that molecules restricted to a volume (V − b) and repulsive interactions cause molecules to behave as small but impenetrable spheres. Attractive forces between molecules reduce the pressure exerted by a real gas. The critical constants can be calculated from the van der Waals equation by setting the first and second derivatives of Eq. (7) with respect to zero to obtain the following relations: Vc = 3b; Pc =

a 8a ; Tc = 2 27Rb 27b

(8)

Through algebraic manipulation, then, the critical compression factor, Zc, can be given as: Zc =

PcVc 3 = RTc 8

(9)

In a similar fashion, dimensionless reduced properties of a gas can be expressed as ratios of the respective critical properties, as: Pr =

P V T ; Vr = ; Tr = Pc Vc Tc

(10)

The “law of corresponding states” (33) states that if reduced variables are used to define the state of gases, all gases show the same PVT behavior; in another words, two different gases, each at the same Pr and Tr, will have the same Vr. Using this analogy, van der Waals’ equation can also be written in terms of reduced variables as: Pr =

8Tr 3 − 3Vr − 1 V 2r

(11)

This equation eliminates the constants a and b and provides a more direct method of estimating physical properties. Other equations of state, their mixing rules (13, 34–36) along with PVT surface measurements, and their correlations for a number of fluids (37), are available in the literature. Models describing equilibria between SCF and condensed phases are classified in two groups: fluid–liquid and fluid–solid equilibria (31, 38–41). Van Konyenburg (42, 43) developed a system for classification of phase behavior for binary fluid mixtures. This system groups the phase behavior of fluid mixtures into six general types, which are distinguished by the behavior of their critical lines (20, 32, 44). Rizvi et al. (32) has reviewed the phase diagrams of

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class I and class III mixtures for representative of typical biomaterials of interest to the food industry. Modeling of such phase equilibria shows highly nonideal behavior due to the complexity of these natural products, which vary enormously in molecular size and chemical nature. Despite these complexities, equations of state have been developed to predict phase behavior of natural substances such as fatty acid esters (45), vegetable oils (46), rapeseed oil (47), and limonene (48) in SCFs. The equations of state calculations require the knowledge of acentric factor, ω, which is defined as follows: ω = −logPvpr − 1

(12)

where Pvpr is the reduced vapor pressure at Tr = 0.7. The acentric factor is a macroscopic property that measures the extent to which the force field around a molecule deviates from spherical symmetry. The acentric factor is practically zero for small, spherical, or highly symmetrical and nonpolar molecules. Several experimental and predictive methods for the determination of the acentric factor of compounds have been reported in the literature (13). Araujo and Meireles (45) used Tu’s (49) indirect method and correlation of Vetere (50) to estimate the acentric factor for high and low molecular weight fatty acids, respectively. The study showed that acentric factor of the compounds increased with increasing carbon number and linearly decreased with increasing double bond in the chemical structure. B. Solute Solubility The development of mathematical models describing the solubility behavior of solutes in SCFs requires an understanding of intermolecular interaction among solute and solvent, as well as solute and solute. Thus, the vapor pressure and the solute–solvent intermolecular interactions are key properties that determine the solubility of a solute in a supercritical solvent. The effect of temperature on solubility manifests itself in the solute’s vapor pressure, the SCF’s density, and molecular interactions in the supercritical phase. At low pressures, solubility decreases with increasing temperature due to the lower SCF density. However, at higher pressures, the SCF’s density is only slightly affected by temperature, so that solute solubility increases with temperature through its effect on solute vapor pressure. In the highly compressible near-critical region, it is also known that the solvent “clusters” around the solute due to the attractive intermolecular interactions (23). This effect is especially important in entrainer systems because abnormally high local solvent density around the solute may preferentially attract entrainer to the solute. Unfortunately, many solubility models assume infinite

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dilution of a solute in a solvent (SCF) and no solute–solute interactions. However, there is experimental and computational evidence that strength of solute– solute interaction may be strong in very dilute solutions (14). One measure of solvent power (capability of a solvent to dissolve various solutes) of an SCF can be described qualitatively by the solubility parameter approach (51). The concept of solubility parameter was first defined for liquids in the condensed phase by Hildebrand and Scott (51) as follows: δ=

冉 冊 −Ev Vl

1/2

(13)

where Ev and Vl are heat of vaporization and molar liquid volume, respectively. A liquid’s solubility parameter is theoretically calculated knowing the heat of vaporization and molar volume of the liquid. Relating the solubility parameter to the energy of vaporization for dense gases is conceptually difficult because the heat of vaporization becomes zero at a solvent’s critical temperature and the vaporization process cannot occur under supercritical conditions. However, Giddings (52) has combined the van der Waals equation with the theory of corresponding states, to yield and empirical relationship, where the solubility parameter is defined by the following formula: δ = 1.25P1/2 c

冋册 ρ ρliq

(14)

where Pc, ρ, and ρliq are critical pressure and density of gas and liquid, respectively. When the solubility parameter defined by Eq. (14) is plotted against pressure, the relationship depicted in Fig. 8 resembles that of density vs. pressure (Fig. 2). At infinite compression, the SCF’s solubility parameter in Eq. (14) reduces to: δliq = 1.25P1/2 c

(15)

where ρ = ρliq, approximating the solvent power of a gas in its liquid state. Returning to Eq. (14), the solubility parameter is a function of density, which is a state variable. The contribution of δliq to δ is known as the “chemical effect” and is dependent on the choice of SCF, while the contribution of ρ/ρliq is attributed to the “state effect” (53). Under supercritical conditions, a large δ is obtained when pressure is high and temperature is slightly over the Tc. The solubility parameter of the fluid may vary from zero at low pressures up to liquid-like values ⬃10 (cal/cm3)1/2 at ultrahigh pressures. SCFs have solubility parameter values in the range of 0–10 (cal/cm3)1/2 (53). King and Friedrich (54) utilized the concept of reduced solubility parameter as a measure of the solute–

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Figure 8 Solubility parameter vs. pressure plot for carbon dioxide (97).

solvent interactions to correlate solubility and distribution coefficients in supercritical and near-critical fluids. The molecular structure of a solute influences its solubility in an SCF (54, 55). Stahl and Quirin (56) were the first to report that the presence of polar functional groups such as OH, C=O, and COOH in a molecule significantly reduced the solubility of a compound in SC-CO2. Solubility studies utilizing sterols indicated that increasing the number of carbon–carbon double bonds decreased the solute’s solubility in SCF (55). Substantial work has been carried on the measurement and prediction of solubility of lipid components in SCF, specifically SC-CO2. The literature on solubility of seed oils in SC-CO2 has been reviewed by King (56). For example, Eisler and Friedrich (57) used the Fujishiro and Hildebrand equation (58) to

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estimate vegetable oil solubility parameters in SC-CO2 and reported that calculated values compared reasonably well with the experimental literature data. Similarly, Chastril (14) determined solubilities of a number of lipid components such as stearic and oleic acid, tripalmitin, butyrin, and olein. Solubilities of αtocopherol, cafestol, cholesterol, and water were also measured in pure CO2 in the range of 40–80°C and 8–25 × 106 Pa (14). Chastril’s study (14) has related solute solubilities directly to density of SCF rather than using the equations of state for the prediction. The solubility of relatively polar compounds such as free amino acids, e.g., glycine, phenylalanine, tryptophan, and leucine, and sugars such as D-glucose and D-xylose in pure CO2 at 40°C and 5–20 × 107 Pa were also reported, and solubilities of these compounds were very low, of the order of 10−7 –10−8 mole fraction (59). C. Solvent Selectivity The selectivity can be defined as the ability of a solvent to dissolve the desired compound to a greater extent than the other constituents of the mixture. Selectivity of an SCF for component 2 vs. component 1 is defined as: S=

y2 y1

(16)

where y1 and y2 are the binary solute solubility of components 1 and 2, respectively. The selectivity of an SCF can be tuned by regulating temperature and pressure of the fluid. This phenomenon allows the use of SCFs for selective extraction and fractionation processes. Selectivity and solubility of a compound are primarily related to the solute vapor pressure and only secondarily to the intermolecular forces in the supercritical phase (55). For example, although the vapor pressure of both cholesterol and ergosterol is negligibly small, selectivity of SC-CO2 for cholesterol is almost two orders of magnitude higher than that of the ergosterol due to the fact that vapor pressure of ergosterol is lower (10−5 –10−6 Pa) than that of cholesterol (10−3 –10−4 Pa), showing that selectivity follows the ratio of vapor pressures (55). D. Enhancement Factor Frequently in SCF technology, a term called the enhancement factor, E, is utilized to describe the propensity of a solute to partition into a SCF. The enhancement factor normalizes the effect of vapor pressure, allowing an estimate to be made of solute–SCF interactions in the supercritical phase, and is defined as follows (44):

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E=

75

yactual yideal

(17)

Ps P

(18)

and yideal =

where yactual = actual solute solubility in SCF, yideal = solubility in ideal gas, Ps = vapor pressure of the solid, and P = pressure of the system. The solubility of a solute in a solvent changes rapidly near the critical region, and this is reflected in the enhancement factor. Enhancement factors of the order of 105 –107 are common for solubilization of solutes in SCF. For example, enhancement factors are the same (⬃107) for cholesterol, stigmasterol, and ergosterol in CO2 at 35°C and 200 atm, which indicates that the sterol-CO2 attractive forces are similar for these sterols (55). Therefore, as noted above, fractionation of the sterols via SFE is basically dependent on the vapor pressure of the components.

IV. MASS TRANSFER A. Mass Transfer Principles The economical scale-up and design of commercial SFE processes requires mass transfer data for the system. Even though SFEs have excellent mass transfer properties, such as gas-like diffusivity and viscosity, practically no surface tension contribution at interface, liquid-like density, or pressure-dependent solvent power; SFE processes still have mass transfer limitations. For example, if the rate-limiting step or the largest mass transfer resistance of a separation process is the transfer of the solute from the surface of a solid to the SCF phase, the gas-like diffusivity of a SCF will enhance diffusion and consequently speed up the extraction. However, if the extraction is from a liquid phase into a SCF phase, mass transfer in the liquid phase will be the rate-determining step. Similarly, if the extraction is occurring from within a solid, then the internal solid phase diffusion will control the rate of mass transfer. It has been established by the experimental measurements that initially at high solute concentrations in the solid matrix, the mass transfer rate from a solid matrix into a SCF is constant. This region of the extraction curve is referred to as steady-state or solubilitycontrolled mass transfer region. However, after a certain amount of material is extracted, this rate starts to decline and the region is called diffusion-controlled mass transfer region (56). A typical SFE curve for oil production from a solid matrix is shown in Fig. 9 (60). Interfacial tension (IFT) is an important parameter that influences mass

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Figure 9 A typical curve for an SCFE process (60).

transfer and solubility of compounds in SCFs. Packed columns have been used for fractionation of liquid mixtures utilizing SCF technology. The design of such systems requires mass transfer data, including the wetting characteristics of the packing surface by the fluid. Literature on the study of IFT in SCF is scarce and usually deals with model systems. However, Simoes et al. (61) reported the data on the IFT of crude and refined corn, coffee, and citrus oils in SC-CO2. It was shown that IFT of the oils decreased with increasing pressure (61). This phenomenon was explained by the breakup and eventually disappearance of the oil droplets on the packing material surface, with increasing pressure. Experiments carried out with corn oil demonstrated that the decrease in IFT was larger above 20 MPa, and at lower pressures only slight changes in IFT were recorded (61). B. Mass Transfer Models Fundamentals of SFE processes for natural materials and food systems are not well defined due to the complex nature of the matrix and difficulty in obtaining

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mass transfer data under high pressure. The majority of the SFE mass transfer models published in the literature are based on extraction of oil (60, 62, 63). A number of researchers have attempted to simulate mass transfer rates in SCFs by using different approaches, such as empirical kinetic models, linear driving force approximations, the shrinking core leaching model, and local adsorptiondesorption technique. For example, Brunner (64) used dimensionless numbers such as Sherwood, Schmidt, and Reynolds numbers to derive correlations for mass transfer coefficients in various solid fixed-bed SFE processes, i.e., coffee beans and oil seeds. It was shown that extraction rate of whole coffee beans was limited by the diffusion rate of caffeine within the solid matrix. Peker et al. (65) also studied a similar system and developed a mathematical model based on a linear driving force approximation of mass transfer and partitioning of caffeine between the water and the SC-CO2. A similar approach was applied to spice extraction by Goto et al. (66). In this study, the SFE of peppermint oil was described by a mathematical model that accounted for both local adsorption of essential oil on lipids in the peppermint leaves and mass transfer from a solid matrix to SCF. Bulley et al. (67) and Lee et al. (68) used a one-dimensional, un-steadystate mathematical model to describe the SFE of a fixed bed of crushed canola flakes. This method assumed plug flow within the seed bed and that axial dispersion was negligible. They calculated canola oil concentration profiles in both SC-CO2 and solid phases and determined overall volumetric mass transfer coefficients. The model adequately described the constant-rate period of the extraction process. Cygnarowicz-Provost (60) used a similar model but included mass transfer coefficients that represented both constant-rate and diffusion-controlled regimes for the determination of caffeine extraction rates in SC-CO2. Likewise, the SFE of grape oil has been simulated using a plug flow model, allowing mass transfer coefficients in both solid and supercritical phases to be calculated (69). Another mass transfer model, which was applied to SFE of oils from herbaceous matrices (70), assumed that diffusivity of the solute in the solid matrix was the only variable in the system. The shrinking core leaching model may be used when the solute to be extracted constitutes a large portion of the solid matrix and is held in the macropores by mechanical or capillary forces as a condensed phase. This model assumes that a sharp boundary exists between a core of material to be extracted, and an outer region, in which only partially saturated SCF exists in the pores. The shrinking core leaching model is valid where the diffusivity of the solute in SCF is much larger than that of the solvent in the solid. Simulation of such a system requires mathematical description of a moving core boundary, which shrinks nonlinearly due to the depleting solute in the pores during the extraction process. Therefore, the following assumptions may be made to simplify the calculations; a pseudo-steady-state diffusion mechanism for extraction from the outer porous layer, negligible fluid velocity at the interface, and the radius of

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the core can be determined by a simple mass balance (62). The shrinking core leaching model has applied to ginger oil (71) and freeze-dried mackerel powder (72). Another example of the mass transfer simulation efforts in food-related systems was published by Dunford et al. (73). They developed a mathematical model to describe the dynamic extraction behavior of oil and water from Atlantic mackerel at different moisture levels. Their model incorporated the interactions between oil and water in the SC-CO2. This model accurately simulated the experimental data in the solubility-controlled region for the above extraction.

V. ENTRAINER EFFECT Addition of an entrainer, which is also referred to as cosolvent, or modifier to a mixture containing a supercritical component can modify the extraction selectivity markedly. Entrainers, which are mostly liquids usually having solubility parameters larger than the SCF component, can be used to increase solvent power of an SCF in addition to density adjustment of the extraction fluid. Hexane, benzene, chloroform, isopropanol, methanol, ethanol, acetone, and water have all been used as entrainers. Ethanol is the preferred entrainer for food applications because it is not toxic and is approved as GRAS (generally recognized as safe) status component. Entrainers are usually chosen to interact specifically with targeted solute, through hydrogen bonding, acid–base interactions, or strong dipole–dipole interactions. Entrainers also increase yields, then, by decreasing the pressure and solvent requirements in SFE. Entrainers can affect the extraction process in many ways: by increasing the volatility of the solutes, by increasing the density of an SCF, by potentially dilating a liquid condensed phase, or by enhancing the miscibility of the components. An entrainer normally increases the solubility of nonvolatile compounds in an SCF by shifting the equilibrium of the binary mixture and/or by shifting the critical point of the solvent phase. Entrainers, which as noted previously are more polar compounds than SC-CO2, can form electron donor–acceptor complexes, i.e., hydrogen bonds, with polar solutes to increase solubilities and selectivities beyond what would be expected based on solubility in the neat SCF. Supporting evidence for the above trend, particularly increasing solute solubility in SCF, was recorded for the addition of a modest amount of cosolvent (less than 10%). For example, Dobbs et al. (74) showed that the solubility of 2-aminobenzoic acid was increased by 600% by the addition of 3.5% methanol into SC-CO2. Several thermodynamic models have been developed to correlate and in some cases to predict the effects of entrainer on solute solubilities in SCF (3). In a related phenomenon, when dry almonds are extracted with SC-CO2 at 40°C and 6 × 107 Pa, an oily extract containing the flavor components is

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achieved (75). However, when the extraction is carried out in the presence of ethanol, two phases—an oil phase free of flavors and an ethanol phase containing flavor components with a small amount of oil—were obtained (75). Influence of an entrainer has also been shown for the extraction of caffeine from coffee beans, where neat SC-CO2 extraction of dry coffee beans cannot efficiently extract the caffeine as a moist coffee bean (5). Another study showed that the pleasant flavor of tea could be extracted with SC-CO2 in the presence of water; however, isolation of flavors by evaporating the water was not possible since most of the flavor components were removed azeotropically during evaporation (75). An excellent example of where entrainer-based SCF separations are necessary is in the fractionation of polar solutes from nonpolar moieties using SCCO2. Thus, substantial differences in the polarity of molecules, such as neutral oil components (triglycerides) and polar lipids (phospholipids), can be exploited for fractionation purposes with the aid of an entrainer such as a GRAS-approved ethanol. For example, Dunford and Temelli (76) and Montanari et al. (77) studied the extraction of phospholipids from canola and soybean lipids, respectively, using SC-CO2 and ethanol mixtures. Initially, neutral lipids were extracted with SC-CO2 and then ethanol was added to SC-CO2 as an entrainer to enhance the extraction of the relatively polar phospholipids.

VI. CRITERIA FOR SOLVENT SELECTION The critical temperature and pressure of a compound are two very important physical properties, when selecting an appropriate solvent for an SFE application. If fluids with high Pc are chosen as solvents, the extraction process will be potentially more expensive due to the higher cost of equipment, which will need to be designed and manufactured for use at high pressures. If Tc of the solvent is exceedingly high, then heat-sensitive materials could be affected adversely during the SFE process. The chemical stability of the solvent under the processing conditions is another important criterion that should be considered in solvent selection. The chemical decomposition of a number of SCF–entrainer mixtures has been reported in literature (37). As an example, methanol can be very unstable in the presence of stainless steel (37). SFCs should also be inert with respect to the processed raw material to avoid reactions during the extraction process. Lowboiling fluids should be preferred as supercritical solvents for ease of removal after processing. The ideal solvent of choice should also be inexpensive, nonflammable, nonexplosive, noncorrosive, nontoxic, and readily available in high purity. Carbon dioxide fulfills those requirements as a fluid for applications of SFE technology for food processing, and it is preferentially used.

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VII. EFFECT OF MATRIX COMPOSITION AND STRUCTURE ON SFE YIELDS AND RATES Extraction yield is an important parameter, which greatly affects feasibility of a production process. Stahl et al. (78) and Friedrich et al. (79) concurrently reported that the yield of an SFE process is influenced by the size and physical structure of the oilseeds. They showed that oilseeds had to be ground to assure complete extraction of the oil. It was also noted that the shape of the ground seeds as well as their size affected the SFE of oil. Similarly, Snyder et al. (80) illustrated that oil yields were quite low with cracked soybeans; however, theoretical yields were obtained from ground or thinly flaked seeds. Similar results were reported with rapeseed (80) (Fig. 10) and canola (81). Water is present in all biological materials at varying concentrations and plays an important role in the SFE and mass transfer kinetics. In some cases, a matrix’s moisture content determines the surface structure and/or activity of the components, such as enzymes. High moisture content may provide an aqueous barrier inhibiting diffusion of SC-CO2 into the matrix as well as diffusion of oil out of the matrix. An example of this phenomenon has been demonstrated by King et al. (82). Here it was reported that the yield of fat extraction with SCCO2 decreased with increasing moisture content of the beef samples. The presence of water in the matrix may act as an entrainer and improve the selectivity of SCF for a solute, i.e., caffeine extraction from coffee beans in the presence of water, which was mentioned earlier in this chapter (5). During

Figure 10 SFE yields as affected by the size of the oilseeds (78).

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essential oil extraction, polar component solubilities in SC-CO2 were improved at higher moisture content in the sample matrix (83, 84). Swelling of plant materials in the presence of water may also be quite significant for extraction because it widens plant cell capillaries, thereby increasing the matrix porosity with subsequent improvement in solute diffusion (85). This phenomenon reduces the extraction time or may even make an extraction possible. It is important to note that water present in the feed material may be coextracted along with the compounds of interest, affecting the quality and purity of the final product. This effect was demonstrated for oil extraction from canola flakes and fish muscle using SC-CO2 by Dunford and Temelli (86, 87).

VIII. SCF PROCESSING SCHEMES A. Extraction A schematic diagram of a basic SFE process is shown in Fig. 11. As noted in the figure, the major equipment components are the extractor, separators, heat exchangers, pumps, and compressors. If desired, more than one extractor can be connected to the same system. The selection of equipment for a food processing system must ensure a sanitary and safe process. All surfaces, including seals and gaskets, in contact with process fluids and solids must be suitable for food processing and sterilization. Construction materials and fittings must be suitable for the pressure and temperature conditions required by the process. The solvent, i.e., CO2, is pumped into the system as a liquid or gas. If the solvent is pumped as a liquid, it will be cooled in a reservoir or it will be kept in liquid phase by cooling the pump head. Depending on the process requirements a system for entrainer addition can be incorporated into the system. Methods for entrainer addition to an SCF system were reviewed by Dunford (88). The fluid is then pressurized and heated to the desired processing conditions. A solvent preheater can be installed into the system to avoid temperature fluctuations due to the solvent being pumped into the system at a lower temperature. In the case of solid feed material processing, the matrix to be extracted will be packed into the extraction cell in a mesh basket or between frits to prevent its being carried from the cell during extraction. Following extraction, the pressure is reduced to precipitate the extract through a control valve. A high-pressure metering valve can control the flow rate of the fluid. The pressure of the system is controlled by the rate of pumping and a back-pressure valve setting. A computer system can be used to control the extraction conditions. Choosing a continuous process rather than a batch or semicontinuous process may significantly improve the economics of an industrial scale process. Continuous transport of a large volume of solid feed, such as oilseeds, into and out of a high-pressure extractor is costly and difficult. However, advances in

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Figure 11 Typical flow diagram for a SFE process.

high-pressure technology may allow continuous feed of solid materials. Today an extractor design that allows intermittent loading and unloading of solid material through the lock-hopper vessels fitted below and above the extractor while it is pressurized is used in the coffee-decaffeinating plant in Houston, Texas (20). A portion of the solid is discharged to the bottom hopper, while fresh feed is simultaneously charged to the extractor from the top hopper. In the meantime, SCF continuously passes through the extractor countercurrently relative to feed. Such an extractor design has several advantages. Sequencing these operations minimizes raw material feeding and vessel unloading times. The compression costs are lowered since feed loading and unloading are carried out simultaneously while maintaining pressure. Simulated moving-bed technique has been developed (89) in an effort to approach a countercurrent solid–fluid extraction process. In this method, a single fixed-bed column is subdivided into several zones. The apparent movement of the solid is achieved by switching between valves connected at the junctions

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between zones. The solid and extract withdrawal ports are periodically shifted in the direction of the solid movement and opposite to the fluid flow, thus simulating a countercurrent process. This technique has been applied to extraction and fractionation of tocopherol from oleic acid (89) and fragrance and essential oil extraction (90). B. Fractional Extraction Fractionation of components of a mixture can be achieved by various modes of fractional extraction. One fractional extraction method is to collect fractions as a function of time throughout a semicontinuous process. In such a case, the extract composition changes over time due to the changes in the composition of residual material in the extractor. Components that have higher solubility under the initial processing conditions will be selectively extracted in the beginning. Then, as these components are depleted from the matrix, the extraction selectivity will shift to other components. A good example of this behavior would be during SFE of oilseeds. Friedrich and Pryde (91) showed that the amount of phospholipids extracted from full-fat soybean flakes with SC-CO2 increased in the fractions collected toward the end of the extraction process. Fractional extraction can also be achieved by increasing solvent density over time. This mode of extraction using incremental pressure programming has been applied to fish oil ethyl esters (92), rice bran oil (93, 94), and phytosterol recovery from vegetable oil deodorizer distillates (95). Fractional extraction using an entrainer in the supercritical phase is another approach, which was discussed earlier in this chapter (76, 77). C. Column Fractionation SFE from a liquid phase can be performed utilizing a vertical column. This process is also frequently referred to as supercritical fluid fractionation (SFF). The feed may be a liquid mixture or a solution containing solutes that are solids under extraction conditions or at slurry of solid particles. The column is usually filled with a packing material or has an internal structure such as trays or baffles to increase the contact area between fluid solvent and feed material and to enhance mass transfer. The process can be carried out in semicontinuous or continuous mode. The feed and solvent could be in the batch and continuous modes during the semicontinuous and continuous processes, respectively. Continuous column operations tend to be more efficient and the feasibility of the operation can be improved. Countercurrent operations are inherently more efficient than concurrent operations due to the larger concentration difference between solvent and feed stream, which results in better mass transfer rates.

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A schematic diagram of a typical countercurrent SFF process is shown in Fig. 12. The feed is pumped into the column near the top, or at a point between the column top and base, or from the bottom of the column. Because of their relative densities, the liquid phase descends while SCF phase rises through the column during a countercurrent operation. The stream leaving the top of the column contains the extract and the liquid leaving the bottom of the column is the raffinate. The column can be operated at constant temperature, or a temperature gradient can be imposed above the feed entry point. Thermal gradient imposed on the column causes condensation of larger molecules in the extract

Figure 12 Flow diagram for a SFF process.

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stream upon their encountering the lower solvent density at higher temperatures upward in the column under isobaric conditions, thus creating an internal reflux in the column. This process is similar to liquid–liquid extraction with reflux and fractional distillation. Refluxing can improve the extract purity if the process is designed properly. Analogous to terms used in distillation and liquid–liquid extraction process with reflux, the lower and upper parts of the SFF column are referred to as stripping and enrichment sections, respectively. The book by Clifford (27) is an excellent reference for a discussion of the fundamentals and mathematical modeling of SFE and SFF processes.

IX. EXTRACT AND SOLVENT RECOVERY Recovery of extracted compounds is usually done by lowering the pressure. Hence, high molecular weight solutes in an SCF can usually be precipitated or phase separated at higher collection pressures than more volatile substances. Fractional separation of extracted solute mixtures can also be achieved in a series of collection vessels by successively lowering pressure on each vessel. This method has been successfully applied to essential oils and spice extracts by Reverchon (96) and Nguyen et al. (8), respectively. Increasing the extract temperature at constant pressure may be another alternative for extract recovery. This method can be used if the solubility of a compound decreases with increasing temperature. This is what occurs mostly with nonvolatile compounds. This method also improves the feasibility of an SFE process by eliminating recompression of the solvent for recycling. Solute recovery by partial depressurization might not be very successful if the solute is a very volatile compound. In such a case, adsorptive recovery methods might be more effective. For example, the solute may be adsorbed on silica, alumina, or active carbon and then separated from the adsorbent at a higher pressure or by thermal desorption, among other options. Use of molecular filters such as zeolites is another method for extract recovery from SCF. In such a case, pore size of the filter media is chosen such that only fluid passes through the filter (permeate) and extracted solute is recovered as retentate (27). Solvent separated from the solute by means of one of the methods mentioned above can be either vented out to atmosphere or recycled to the system depending on the size of the operation. For small-scale equipment such as laboratory scale SFE units, solute-free gas is vented to the atmosphere due to the higher cost of installing a recycling system as compared with the cheaper gas cost. If the solvent is to be recycled, which is the case for pilot and industrial scale operations, it is usually passed through an adsorbent bed before it is pumped back into the extractor.

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X. CONCLUDING REMARKS As was noted in this chapter, SCF extraction offers considerable versatility for the extraction of food-related products and raw materials. In general, the overall SFE process is simple, embodying many of the principles inherent in other extraction processes. New approaches for the continuous handling of solid and liquid materials, in and out of extraction vessels, continues to evolve, and improved equipment design and operational dynamics are making SFE processes much more efficient and competitive. However, in some product applications, SCF technology faces competition from conventional techniques such as molecular (vacuum) distillation, a time-honored technique, although greater selectivity is potentially available from SFE using the described methods. In closing, we would like to offer the following short list of “critical” references that will supplement the material discussed in this chapter. Although this list is not inclusive of all available literature resources, it should assist the reader wishing to learn more about this novel technology and abets the formal references cited in this chapter.

NOMENCLATURE P V N R T Tc Pc Vc ∆T ∆P ∆V Pr Tr Tb ρ ρc ρr a b δ Ev

pressure volume number of moles gas constant temperature critical temperature critical pressure critical volume group contribution constant at constant temperature group contribution constant at constant pressure group contribution constant at constant volume reduced pressure reduced temperature boiling temperature density critical density reduced density van der Waals constant van de Waals constant solubility parameter heat of vaporization

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Supercritical Fluid Extraction

Ep ε εr ε0 qi r Ji Ci Dij Vl ρliq δliq S yi E Ps M S Q ω Pvpr Zc λ 䉮T

87

potential energy of interaction permittivity of the medium relative permittivity, or dielectric constant permittivity in a vacuum point charge for compound i distance between the charges molar flux for compound i molar concentration for compound i binary diffusion coefficient for component i molar liquid volume liquid density liquid solubility parameter selectivity binary solubility of component i in the supercritical phase enhancement factor saturation pressure of solvent molecular weight solubility heat flux acentric factor reduced vapor pressure critical compression factor thermal conductivity temperature gradient

BIBLIOGRAPHY G Brunner. Gas Extraction. Berlin: Springer-Verlag, 1997. BA Charpentier, MR Sevenants (eds.). Supercritical Fluid Extraction and Chromatography: Techniques and Applications. Washington, DC: American Chemical Society, 1988. T Clifford. Fundamentals of Supercritical Fluids. New York: Oxford University Press, 1999. I.S.A.S.F. Proceedings of the 5th Meeting on Supercritical Fluids-Materials and Natural Products Processing. Nancy, France: Institut National Polytechnique de Lorraine, 1998. I.S.A.S.F. Proceedings of the 6th Meeting on Supercritical Fluids-Chemistry and Materials. Nancy, France: Institut National Polytechnique de Lorraine, 1999. KP Johnston, JML Penninger (eds.). Supercritical Fluid Science and Technology. Washington, DC: American Chemical Society, 1989. JW King, GR List (eds.). Supercritical Fluid Technology in Oil and Lipid Chemistry. Champaign, IL: AOCS Press, 1996. MB King, TR Bott (eds.). Extraction of Natural Products Using Near-Critical Solvents. Glasgow: Blackie Academic, 1993.

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E Kiran, JF Brennecke (eds.). Supercritical Fluid Engineering: Fundamentals and Applications. Washington, DC: American Chemical Society, 1993. E Kiran, JMH Levelt Sengers (eds.). Supercritical Fluids. Amsterdam: Kluwer, 1994. MA McHugh, VJ Krukonis. Supercritical Fluid Extraction, 2nd ed. Boston, MA: Butterworth-Heinemann, 1994. M Mukhopadhyay. Natural Extracts Using Supercritical Carbon Dioxide. Boca Raton, FL: CRC Press, 2000. SH Rizvi (ed.). Supercritical Fluid Processing of Food and Biomaterials. London: Blackie Academic, 1994. E Stahl, K-W Quirin, D Gerard. Dense Gases for Extraction and Refining. Berlin: Springer-Verlag, 1987.

REFERENCES 1. JB Hannay, J Hogarth. On the solubility of solids in gases. Proc R Soc London 29: 324, 1879. 2. PMF Paul, WS Wise. The Principles of Gas Extraction. London: Mills & Boon Ltd., 1971. 3. GM Schneider, E Stahl, G Wilke. Extraction with Supercritical Gases. Deerfield Beach, FL: Verlag-Chemie, 1980. 4. K Zosel. Process for the separation of mixtures of substances. U.S. Patent 3,969,196 (1976). 5. K Zosel. Process for the decaffeination of coffee. U.S. Patent 4,260,639 (1981). 6. DRJ Laws. Hop extracts-a review. J Inst Brew 87:24–29, 1981. 7. MB King, TR. Bott. Extraction of Natural Products Using Near Critical Solvents. Glasgow: Blackie Academic, 1993. 8. U Nguyen. Supercritical fluid extraction applications in the nutraceutical industry. Presented at the Annual Meeting of the Canadian Institute of Food Science and Technology, June 6–9, Kelowna, BC, 1999. 9. PT Anastas, TC Williamson (eds.). Green Chemistry. Washington, DC: American Chemical Society, 1996. 10. F Temelli, JW King. Application of critical fluid technology to nutraceutical extraction and refining. Presented at the Short Course on Nutraceuticals and Functional Foods. February 20–24, College Station, TX, 2000. 11. M McCoy. Industry intrigued by CO2 as solvent. Chem Eng News 77(24):11–13, 1999. 12. RH Perry, CH Chilton (eds). Chemical Engineers’ Handbook. 5th ed. Japan: McGraw-Hill Kogakushu, Ltd., 1973. 13. RC Reid, JM, Prausnitz, BE Poling. The Properties of Gasses and Liquids, 4th ed. New York: McGraw-Hill, 1987. 14. J Chrastil. Solubility of solids and liquids in supercritical gases. J Phys Chem 86: 3016–3021, 1982. 15. JM del Valle, JM Aguilera. An improved equation for predicting the solubility of vegetable oils in supercritical CO2. Ind Eng Chem Res 27:1551–1553, 1988. 16. JW Magee. Properties of carbon dioxide rich mixture. In: TJ Bruno, JF Ely (eds.).

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17. 18. 19. 20. 21.

22. 23. 24. 25.

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38. M Gitterman, I Procaccia. Quantitative theory of solubility in supercritical fluids. J Chem Phys 78(5):2648–2654, 1983. 39. I Procaccia, M Gitterman. Quantitative analysis of supercritical extraction. AIChE J 29(4):686–687, 1983. 40. RT Kurnik, RC Reid. Solubility extrema in solid-fluid equlibria. AIChE J 27(5): 861–863, 1981. 41. RT Kurnik, RC Reid. Solubility of solid mixtures in supercritical fluids. Fluid Phase Equil 8(1):93–105, 1982. 42. PH van Konyenburg, R Scott. Critical lines and phase equilibria in van der Waals mixtures. Phil Trans R Soc (London) A. 298:495–540, 1980. 43. R Scott, PH van Konyenburg. Static properties of solutions. Part 2. Van der Waals and related models for hydrocarbon mixtures. Discuss Faraday Soc 49:76–87, 1970. 44. JM Prausnitz, RN Lichtenthaler, EG de Azevedo. Molecular Thermodynamics of Fluid-Phase Equilibrium, 3rd ed. Englewood Cliffs, New Jersey: Prentice Hall, 1999. 45. ME Araujo, MAA Meireles. Improving phase equilibrium calculation with PengRobinson EOS for fats and oils related compounds/supercritical CO2 systems. Fluid Phase Equil 169:49–64, 2000. 46. E Batista, S Monnerat, L Stragevitch, CG Pina, CB Goncalves, AJA Meirelles. Prediction of liquid–liquid equilibrium for systems of vegetable oils, fatty acids and ethanol. J Chem Eng Data. 44:1365–1369, 1999. 47. L Coniglio, Kobayashi, T, R Gani. Model prediction of supercritical fluid-liquid equilibria for carbon dioxide and fish oil related compound. Ind Eng Chem Res 34: 2473–2484, 1995. 48. Y Iwai, T Morotomi, K Sakamoto, Y Koga, Y Arai. High pressure vapor-liquid equilibria for carbon dioxide+limonene. J Chem Eng Data 41:951–952, 1996. 49. A Vetere. Predicting the vapor pressures of pure compounds by using Wagner equation. Fluid Phase Equil 62:1–10, 1991. 50. CH Tu. Group-contribution method for the estimation of vapor pressures. Fluid Phase Equil 99:105–120, 1994. 51. JH Hildebrand, RL Scott. The Solubility of Nonelectrolytes, 3rd ed. New York: Reinhold Publishing Corp., 1950. 52. JC Giddings, MN Myers, JW King. Dense gas chromatography at pressures to 2000 atmospheres. J Chromatogr Sci 7:276–283, 1969. 53. JC Giddings, MN Myers, L McLaren, RA Keller. High pressure gas chromatography of nonvolatile species. Science 162(10):67–73, 1968. 54. JW King, JP Friedrich. Quantitative correlations between solute molecular structure and solubility in supercritical fluids. J Chromatogr 517:449–458, 1990. 55. JM Wong, KP Johnston. Solubilization of biomolecules in carbon dioxide based supercritical fluids. Biotechnol Prog 2:29–39, 1986. 56. E Stahl, KW Quirin, D Gerard. Dense Gases for Extraction and Refining. Berlin: Springer-Verlag, 1987. 57. RL Eissler, JP Friedrich. Estimation of supercritical fluid-liquid solubility parameter differences for vegetable oils and other liquids from data taken with a stirred autoclave. J Am Oil Chem Soc 65:764–767, 1988. 58. R Fujishiro, JH Hildebrand. The liquid–liquid solubility of cyclohexane and perfluorotributylamine at 25°C. J Phys Chem 66:573–574, 1962.

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59. E Stahl, W Schilz. Solubility behavior of polar natural substances in supercritical gases in the pressure range up to 2500 Bar. Chem-Ing-Tech 50:535–537, 1978. 60. M Cygnarowicz-Provost. Design and economic analysis of supercritical fluid extraction process. In: JW King, GR List (eds.). Supercritical Fluid Technology in Oil and Lipid Chemistry. Champaign, IL: AOCS Press, 1996, pp. 155–179. 61. PC Simoes, R Eggers, PT Jaeger. Interfacial tension of edible oils in supercritical carbon dioxide. Eur J Lipid Sci Technol 263–265, 2000. 62. MC Jones. Mass transfer in supercritical extraction from solid matrices. In: TJ Bruno, JF Ely (eds.). Supercritical Fluid Technology: Reviews in Modern Theory and Applications. Boca Raton, FL: CRC Press, 1991, pp. 365–381. 63. M Poletto, E Reverchon. Comparison of models for supercritical fluid extraction of seed and essential oils in relation to the mass-transfer rate. Ind Eng Chem Res 35:3680–3686, 1996. 64. G Brunner. Mass transfer from solid material in gas extraction. Ber Bunsenges Phys Chem 88:887–891, 1984. 65. H Peker, MP Srinivasan, JM Smith, BJ McCoy. Caffeine extraction rates from coffee beans with supercritical carbon dioxide. AIChE J 38(5):761–770, 1992. 66. M Goto, M Sato, T Hirose. Extraction of peppermint oil by supercritical carbon dioxide. J Chem Eng Jpn 26:401–407, 1993. 67. NR Bulley, M Fattori. Supercritical fluid extraction of vegetable oil seeds. J Am Oil Chem Soc 61:1362–1365, 1984. 68. AKK Lee, NR Bulley, M Fattori, A Meisen. Modeling of supercritical carbon dioxide extraction of canola oilseed in fixed beds. J Am Oil Chem Soc 63:921–925, 1986. 69. H Sovova, J Kucera, J Jez. Rate of the vegetable oil extraction with supercritical CO2. II. Extraction of grape oil. Chem Eng Sci 49:415–420, 1994. 70. E Reverchon, G Donsi, OL Sesti. Modeling of supercritical extraction from herbaceous matrices. Ind Eng Chem Res 32:2721–2726, 1993. 71. M Goto, BC Roy, T Hirose. Shrinking core leaching model for supercritical fluid extraction. J Supercrit Fluids 9:128–131, 1996. 72. Y Ikushima, N Saito, K Hatakeda, S Ito, T Asano, T Goto. A supercritical carbon dioxide extraction from mackerel (Scomber japonicus) powder: Experiment and modeling. Bull Chem Soc Jpn. 59:3709–3713, 1986. 73. NT Dunford, M Goto, F Temelli. Modeling of extraction with supercritical carbon dioxide from Atlantic mackerel (Scomber scombrus) at different moisture contents. J Supercrit Fluids 13:303–309, 1998. 74. JM Dobbs, JM Wong, KP Johnston. Nonpolar co-solvents for solubility enhancement in supercritical fluid carbon dioxide. J Chem Eng Data 31(3):303–308, 1986. 75. JP Calame, R Steiner. Extraction in flavor and perfumery industries. Chem Ind 10: 399–403, 1982. 76. NT Dunford, F Temelli. Extraction of phospholipids from canola with supercritical carbon dioxide and ethanol. J Am Oil Chem Soc 72(9):1009–1015, 1995. 77. L Montanari, P Fantozzi, JM Snyder, JW King. Selective extraction of phospholipids from soybeans with supercritical carbon dioxide and ethanol. J Supercrit Fluids 14:87–93, 1999. 78. E Stahl, E Shutz, HK Mangold. Extraction of seed oils with liquid and supercritical carbon dioxide. J Agric Food Chem 28:1153–1157, 1980.

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79. JP Friedrich, GR List. Characterization of soybean oil extracted by supercritical carbon dioxide and hexane. J Am Oil Chem Soc 30:192–193, 1982. 80. JM Snyder, JP Friedrich, DD Christianson. Effect of moisture and particle size on the extractability of oils from seeds with supercritical CO2. J Am Oil Chem Soc 61:1851–1856, 1984. 81. M Fattori, NR Bulley, A Meisen. Carbon dioxide extraction of canola seed: Oil solubility and effect of seed treatment. J Am Oil Chem Soc 65:968–974,1988. 82. JW King, HJ Johnson, JP Friedrich. Extraction of fat tissue from meat products with supercritical carbon dioxide. J Agric Food Chem 37:951–954, 1989. 83. E Stahl, D Gerard. Solubility behavior and fractionation of essential oils in dense carbon dioxide. Perfum Flav 12:29–37, 1985. 84. N Gopalakrishnan, CS Narayanan. Supercritical carbon dioxide extraction of cardamom. J Agric Food Chem 39:1976–1978, 1991. 85. R von Eggers, W Stein. Hochdruck-extraktion von olsaates. Fette Seifen Anstrich 86:10–16, 1984. 86. NT Dunford, F Temelli. Extraction conditions and moisture content of canola flakes as related to lipid composition of supercritical CO2 extracts. J Food Sci 62:155–159, 1996. 87. NT Dunford, F Temelli, E LeBlanc. Supercritical CO2 extraction of oil and residual proteins from Atlantic mackerel (Scomber scombrus) as affected by moisture content. J Food Sci 62:289–294, 1997. 88. NT Dunford. Use of SC-CO2 for edible oil processing. PhD dissertation. University of Alberta, Edmonton, AB, Canada, 1995. 89. A Bertucco, F Sanmartin, G Storti. Simulated moving bed technology for continuous, countercurrent solid-fluid supercritical extraction. J Supercrit Fluids 8:138– 148, 1995. 90. M Bork, JP Korner. Description of an industrial-size supercritical gas extraction plant for semicontinious operation. Proceedings of 2nd International Symposium on Supercritical Fluids, Boston, MA, May 20–22, 1991, pp. 368–369. 91. JP Friedrich, EH Pryde. Supercritical CO2 extraction of lipid-bearing materials and characterization of the products. J Am Oil Chem Soc 61:223–228, 1984. 92. WB Nilsson, EJ Gauglitz, JK Hudson. Supercritical fluid fractionation of fish oil esters using incremental pressure programming and a temperature gradient. J Am Oil Chem Soc 66:1596–1600, 1989. 93. NT Dunford, JW King. Using a supercritical carbon dioxide fractionation technique for phytosterol enrichment in rice bran oil. J Food Sci, 65(8):1395–1399. 94. NT Dunford, JW King. Supercritical carbon dioxide fractionation of crude rice bran oil using a packed column with characterization of the resultant fractions. Proceedings of the 5th International Symposium on Supercritical Fluids (ISSF) (proceedings on CD). Atlanta, Georgia, April 8–12, 2000. 95. NT Dunford, JW King. Phytosterol-enriched trygliceride fractions from vegetable oil deodorizer distillate using supercritical fluid fractionation technology. Separation Science and Technology 37(2):1–2, 2002. 96. E Reverchon. Supercritical fluid extraction and fractionation of essential oils and related products. J Supercrit Fluids 10:1–38, 1997.

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97. KP Johnston. New directions in supercritical fluid science and technology. In: KP Johnston, JML Penninger, eds. Supercritical Fluid Science and Technology. Washington, DC: American Chemical Society, 1989. 98. LG Randall. Present status of dense (supercritical) gas extraction and dense gas chromatography: Impetus for DGS/MS development. Sep Sci Technol 17(1):1–7. 99. G Brunner, S Peter. Zum Stand der extraction mit komprimierten gasen. ChemIng-Tech 53(7):529–542, 1981.

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4 Extraction Systems Rudolf Eggers and Philip T. Jaeger Technische Universita¨t Hamburg-Harburg Hamburg, Germany

This chapter deals with different methods and equipment used for solvent extraction of food components from natural sources. A main distinction is made between systems using common liquid solvents that are industrially applied at greater scale since the first decades of the 20th century and an alternative extraction method using compressed gases, which is called supercritical fluid extraction (SFE). SFE started gaining importance in the mid-1970s, especially as a result of rising consciousness with respect to environmental aspects and the recent development of appropriate high-pressure equipment. In the first section, conventional extraction systems using liquid solvents are discussed, including equipment necessary for pretreatment and solvent recovery. The second section is concerned with selection and design of equipment required for SFE.

I. CONVENTIONAL EXTRACTION SYSTEMS Design of extraction systems and detailed selection of suitable equipment depends on the objective of the process and physical properties of the material to be extracted as well as of the obtained product. The solubilizing ability and selectivity of liquid solvents that are mainly based on water, hydrocarbons like hexane, or alcohol are used to leach or extract certain desired components from the applied source material, which is a naturally obtained solid in this case. This section first focuses on the extraction process itself describing different processing principles and giving an overview of the required equipment. Product quality and extraction efficiency depend very much on conditioning of the solid feed material ahead of solvent extraction, which will also be discussed. Posterior to

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extraction, the solvent has to be recovered from the product and the exhausted meal, taking care of the product quality and increasing environmental demands. Finally, an example of a complete plant used for hops extraction by alcohol is described. A. Classification In the past, extraction or leaching was often divided into percolation and immersion methods, referring to whether the solid is completely submerged or the solvent is just trickled through a solid bed (1). In general, classification has become clearer since technical development has concentrated on a few different types of extractors, most of them working continuously as percolators (2). On the other hand, immersion leaching of solid-solvent slurries using agitators or screws has largely lost industrial relevance. Even though many of the former extractor types (3) have nearly disappeared, they are still worth mentioning because older equipment may still be perfect for certain applications. Therefore, it is useful to start out by listing all possible operation modes and the relative working principles in order to document technical improvements in the past decades but finally focusing on the actual state of the art. 1. Operation Mode Batch Extraction. Within batch processing, extraction is carried out in vessels that are filled with the solid matter to be extracted. Afterward solvent is either percolated through the solid bed or added to the vessel until the solid is completely submerged. In the latter case, the solvent-solid mixture may be stirred so as to enhance mass transfer. After a designated holding time, the solvent-extract mixture, called miscella, is drawn from the vessel and the solid matter is discharged. This way of processing has almost completely disappeared because of the required interruptions of the operation for charging and discharging and high amounts of required solvent. Only for some special applications with small product rates, such as extraction of flavors, is batch extraction still used (e.g., extraction of lavender). Operating several batch extractors in parallel with cross-current solvent flow allows achievement of higher product rates but does not make use of the entire solvent capacity. Quasi-continuous Extraction. In order to increase extractor efficiency, several batch extractors can be operated in series using the solvent that is loaded in one extractor for passing it through another solid bed that still contains higher amounts of extractable substances. In this way, solute concentration is enhanced, gradually approaching the total solvent capacity. One or two extractors are always off-stream for discharging and charging. After a certain interval, input and

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output ports are changed so that the solid bed extracted to the highest extent is shut off for discharging and a freshly charged extractor is switched in-line. Usually, extraction is performed countercurrently for maintaining continuously a sufficient concentration gradient of the solute in the solid matter and the solvent. Therefore, the freshly entering solvent is contacted with the solid extracted to the highest extent. Afterward, the solvent is passed through the extractors in the order of decreasing bed age until the solvent is almost completely loaded and is brought into contact with the latest charged solid. Continuous Extraction. For complete continuous operation, the solid matter has to be charged and discharged continuously to and from the extractor. Different types of conveying systems exist depending on whether percolation or immersion extraction is applied. These are presented in the following. In the case of extraction performed under conditions other than atmospheric pressure, either adequate sluice systems for the solid matter are required or quasi-continuous operation must be applied. If, for instance, proper extraction temperatures are above boiling temperature at atmospheric pressure, extraction is to be performed at elevated pressures. With the use of flammable organic solvents their escape through leakages can be prevented by operating under slight vacuum. Table 1 gives an overview of different types of extractor systems, including their field of application with some specific examples. 2. Working Principle Single-Stage Extraction. The simplest apparatus for liquid solvent extraction is a single-stage vessel used for immersion extraction. In order to confine the extraction period, the vessel may be stirred. Therefore, the solvent-solid mixture should form a flowable slurry. The extraction time has to be optimized because after a while extraction kinetics becomes quite slow and additional leaching becomes time consuming. The loaded solvent is retrieved and replaced by fresh solvent. This procedure is repeated several times until the solid matter is extracted to an acceptable extent. Multistage Extraction. In multistage static-bed extraction using a countercurrent operation mode, the fresh solvent enters the vessel containing the most exhausted solid. The extract being retrieved from this vessel is successively passed through a battery of extractors until arriving at the vessel most recently loaded. The extract retrieved from this one is discharged for further processing, i.e., desolventizing. The purpose of this operation is to increase the solute load of the solvent to a maximal value for posterior product separation and for solvent recovery to be as economical as possible. This method of quasicontinuous processing can be carried out with immersed solid as well as a solid bed exposed to percolating solvent. A typical application is the extraction of

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Table 1

Extraction Systems: Characteristics and Applications

Operation Batch

Working principle

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Stirred vessel Single-stage percolator Multi-stage percolator

Field of application Pharmacy Spices

Multistage percolator battery Instant material, sugar

Examples Alkaloids Pepper

Instant coffee, sugar from beets

Rotocel

Vegetable oil

Carrousel

Vegetable oil, spices, instant material

Soybean oil, paprika, pepper, hops

Stationary basket

Vegetable oil, spices

Wheat germ, paprika

Sieve tray belt; sliding cell

Sugar

Sugar from beets/cane

Crown loop extractor

Sugar, vegetable oil

Sugar cane, soybean oil

Basket elevator

Vegetable oil

Flaked oil seeds

Screw conveyer

Sugar

Sugar beets

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Immersion extraction Static bed percolation Static bed cross-current percolation Quasi-continuous Stationary bed, countercurrent percolation Continuous Rotating cells, countercurrent percolation Rotating bed, countercurrent percolation, stationary sieve tray bottom Stationary bed, countercurrent percolation, rotating feed/ discharging locations Horiz. moving bed, countercurrent percolation Horiz. moving bed, co-/ countercurrent percolation Vert. moving bed, co-/ countercurrent percolation Moving bed, countercurrent, immersion

Extraction system

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coffee for instant coffee production by a battery of percolators. At temperatures of up to 180°C and pressures of up to 20 atm, around 40–60% of water-soluble material is extracted from coffee grounds by water in an upstream or downstream fashion. At temperatures above 140°C hydrolysis of polysaccharides takes place enhancing their solubility, which increases the total extraction yield. These sugars are also capable of retaining flavors. A series of four to eight percolators allows the establishment of temperature gradients along the extraction line for taking influence on product yield and quality due to differing solubilities of the various substances present in roasted coffee (4). Figure 1 shows a battery of percolators for instant-coffee extraction. Vessel diameters range from 0.25 to 0.75 m. Typical capacities are around 1 t resulting in bed heights of 4.5–6 m. Applying vacuum to the vessel facilitates loading of the percolators. The ball valve at the top is opened and the grinded coffee enters the vessel. Grind sizes range from 3 to 5 mm, the result of a compromise between favoring mass transfer and limiting pressure drop. After being sufficiently leached, the spent coffee ground is discharged by “shooting” through the ball valve at the bottom of the vessel. Holdup is maintained at 30–35 min showing a tendency to shorter times. An extractor system developed by NIRO (Denmark) for coffee extraction, called FIC (fast instant coffee), comprises seven percolators arranged in a circle as a compact unit (Fig. 2). The percolators themselves are about half the size of conventional percolators reducing residence time to half. By their relatively compact design, high liquid velocities are reached that enhance mass transfer from the ground coffee particles. Operation is carried out in two steps based on water temperatures of 100°C for aroma extraction and 180°C for hydrolysis of polysaccharides. The process works with high efficiency although concentration of the produced liquid extract is somewhat lower than in the case of normal percolators. The complete extraction cycle takes about 1.5 h from the fresh water entering the unit to withdrawing the concentrated coffee solution. For extraction of sugar from sugar beets 10–16 columns are used, each having a diameter of around 1.6 m and a height of 2.6–3.2 m, resulting in volumes of 3–4 m3. The scalded beets (cossettes), resembling shoestring potatoes of 3–7 mm width and 5–8 cm length, are loaded to the vessel by means of hinged covers. Extraction is carried out using water at 40–75°C. Holdup is kept between 50 and 120 min. Different apparatus alternatives also exist such as versatile, chain belt, and ring extractors. Moving-Bed Extraction. Extractors with continuously moving beds are distinguished according to the mechanism of transporting the solid matter and contacting the solvent. Moving-bed extractors can be divided into percolators and immersion extractors. In percolation extraction, the solvent phase is passed through the solid bed, which is mainly stationary with respect to the solid-

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Figure 1 Schematic of a percolator battery. Each percolator can be bypassed for discharging/charging as indicated only in the case of the second percolator.

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Figure 2 FIC extractor for instant-coffee production. (Courtesy of Niro, Denmark.)

containing cell. Most extractors that make use of the percolation principle have conveyer elements such as conveyer belts, frames, cells, or baskets that carry the solid matter through the extractor. Depending on the actual position, either fresh solvent or miscella is sprayed on the solid passing by. Commonly, fresh solvent is sprayed on the most exhausted solid. The miscella leaving here is recirculated and used for extraction of freshly entering solid. Recirculation is also used for enhancing contact time by submerging the bed to a greater extent. As much as possible countercurrent flow of solid with respect to solvent must

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be achieved. Accounting for the flow direction of the draining miscella, the general flow pattern is a cross-countercurrent flow. Nevertheless, when making use of gravity for miscella flow, as in basket elevators or loop extractors, some places of cocurrent flow are unavoidable. If the static solid bed is mixed up at least once during extraction, e.g., by dumping the content of one basket into another, a slightly higher yield may be achieved. To a certain extent, solid mixing already occurs in loop extractors. Cross-current operation of a moving bed means that fresh solvent is used throughout the entire extractor, resulting in rapid leaching of the solid but also causing a high consumption of solvent. An intensive solid–liquid contact, which includes continuous mixing of the solid, is carried out in immersion extraction using screws for conveying the solid through the extractor that is filled with solvent. The extractor may even consist of several different compartments dividing zones of different miscella concentrations. In this case, solid transport is achieved by screws from one trough to the other throughout the whole extractor. The solid is submerged partly within the solvent realizing countercurrent flow between the solid and the solvent. Immersion extraction is keener in facilitating entrainment of small particles than percolation where there is always some sort of self-filtration. Thus, solid content in the miscella after immersion extraction may easily rise to 1– 10%, whereas there are only several parts per million after percolation. B. Industrial Extractors Some general trends concerning efforts for improving extraction efficiency and performance can be observed. Development of an extractor implies a compromise between simplicity with respect to the number of moving parts and transporting systems, such as solvent pumps and solid-conveying systems, and efficiency of recirculation pattern for the miscella. Extractors containing compartments, as opposed to simple conveyer belts, facilitate submersion of the solid, which usually leads to higher extraction efficiencies. Shallow beds that are not divided into separated stages are in danger of forming lakes of solvent freely flowing on the bed surface and mixing miscella of different concentrations. Instead, in deep beds the absolute residence time is often enhanced so that the liquid (miscella or solvent) is sprayed on such a moving bed and dissolving solute on its way, then is drained from the solid in a subsequent section that does not correspond to its solute concentration. For example, full miscella drained from the descending leg of a basket elevator might be collected in the half-miscella tank below the rising leg. In the following, different types of extractors working with a moving bed are presented. Most of these extractor types have existed for several decades. Technical improvements are mostly confined to some details concerning the position of inlet/outlet streams and recirculation of miscella. The extractors are

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subdivided into those containing rotating parts and those in which longitudinal movements are carried out. The latter machines make use of conveying elements such as conveyor belts or chains. In general, capital costs are lower than in case of the rotating principle. Moreover, they can be delivered preassembled to a great extent. Since they mostly work with a shallow bed, extraction results may differ from the previously mentioned rotating deep-bed extractors depending on the material being extracted. In general, the range of application of shallow-bed extractors is somewhat confined due to the extractability and permeability of the meal. The Rotocel extractor is a well-established extraction system developed by Blonox in the early 1960s (5). Then the successor Dravo closed down and fusion of licensees Krupp and Extraktionstechnik in 1990 resulted in production of the Carrousel extractor. The Rotocel consists of up to 18 cells located in a circle, each of them containing perforated bottoms that are periodically opened when the cells pass the discharging section during their rotation. One rotation lasts more than 1 h depending on the extractability of the solid. The rotational energy requirement is fairly low because in spite of large diameters there is little frictional force to be overcome. The Rotocel is a so-called “deep-bed” extractor, with a bed height of 1.8–3 m. The flakes, which may also be introduced as a slurry using miscella, are not moved relative to their neighbors. In this way fragile solid structures can be handled, but there is no additional mixing effect that otherwise could enhance extraction yields. Miscella drawn from the bottom of the cells is collected and used for leaching solid of less bed age. This recirculation system consists of up to 10 miscella pumps. The Carrousel extractor is similar to the Rotocel, except that only the frame of the extraction compartments rotates atop a static sieve tray, the structure of which is shown in Fig. 3. In this way, friction of particles scrapping along the sieve tray bottom needs to be overcome; but on the other hand, particles are kept in movement relative to each other, which enhances mass transfer.

Figure 3 Segment of the sieve tray bottom of a carrousel extractor. Original (concentric) (a) and actual (b) orientation of sieve gaps.

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One rotation takes about 1 h, which means that at a diameter of 8 m the highest relative velocities of particles with respect to the sieve tray bottom do not exceed 7 mm/s. After being exposed to different extraction stages during one cycle, each compartment arrives at the solid outlet position where the complete solid bed of the respective compartment is discharged through the open bottom. The separation walls of the compartments have a conical cross-section (Fig. 4) that helps in discharging the exhausted meal downward. Carrousel extractors have diameters of up to 15.3 m. At a bed height of 2 m an extractor of 8 m in diameter has a capacity of 75 m3. Double-deck carrousels containing diameters of up to 8 m are also in use (Fig. 4). After passing one cycle in the upper deck, the solid drops onto the lower level for extraction during another rotational period. Such an extractor is capable of extracting oil from 2000 t/d soybeans containing 18% of oil using hexane; the residual oil content amounts to 0.8%. Triple-deck carrousels have also been constructed, but poor accessibility to the middle deck for cleaning and repairing is a disadvantage. Figure 5 gives an overview of solid and liquid flow, also indicating the respective extract concentrations in countercurrent extractors like the Carrousel extractor. The weight of rotating parts can be reduced by using a stationary basket extractor originally fabricated by French Oil Mill Machinery Co. In such an extractor, the compartments filled with solid are stationary and only feed spouts and the positions of the solid discharging facility and draw-off of miscella rotate. Usually, 12–20 cells are applied, each of them 1.8–3 m deep. In the meantime, the French Company switched from the stationary basket principle to carrousel-type extractors. Next to rotating extractors also the conveyer belt principle is applied to oil extraction. DeSmet uses sieve tray belts for transporting the meal in their belt extractor. Either fresh solvent or miscella provided by a system of miscella recirculation is sprayed onto the shallow solid bed usually containing heights lower than 0.6 m. The solvent containing the freshly extracted solute drops from the belt into collecting trays. Before leaving the extractor, the solid is treated once more with pure solvent (benzene). A slight vacuum within the extractor is obtained by Venturi nozzles. Belt extractors have the advantage of low apparatus costs and fairly uncomplicated installation. Extraction results depend on a precise adjustment of transporting velocity, amount of liquid sprayed on the solid, and solid bed permeability. Following efforts to submerge the solid bed to the highest extent possible, lakes may be formed on top of the solid moving bed freely flowing to all sides and mixing half and full miscella with one another. Similar to rotational extraction systems, horizontal/vertical working extractors offer different ways of moving the solid for passing it through different stages of extraction. In general, recent development is leaning toward stationary screen plates or sieve trays on which the solid is pushed by cells or frames fixed

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Figure 4 Interior of a double floor carrousel extractor. (Courtesy of Krupp, Hamburg.)

to chain conveyors, as in the case of the Crown loop extractor (Fig. 6). The loop extractor is quite compact with one cocurrent and two countercurrent extraction zones. In the sliding cell extractor from Lurgi (Fig. 7), U-shaped cells run on roller tracks pushing the solid material over stationary screen plates. The screen plates consist of rods aligned in a flow direction. The cross-section of the rods

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Figure 5 Concentration profiles of countercurrent extraction, e.g., Carrousel extractor.

Figure 6 Crown loop extractor.

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Figure 7 Lurgi sliding cell extractor.

is V shaped to prevent clogging. The feed material is introduced into the cells of the upper belt by a filling device. After approximately half of the extraction time, the screen plate ends and the feed is dumped through the open cell bottom into the cell, just arriving at the lower screen plate in the feed transfer stage, as indicated in Fig. 7. Before being discharged, the feed is passed through a final drainage zone. The solvent passes through the feed material countercurrently by spraying the collected miscella onto feed at a previous position; it becomes enriched with extract until finally leaving the extractor. Recently, a sliding bed extractor was developed by Krupp containing a chain-frame conveying system on top of a static sieve tray passing two levels (Fig. 8). Miscella collection is also divided into two levels with a sophisticated recycling system. Full miscella is liberated from fines by integrated hydrocyclones. Bed height is adjusted at 0.5–1.3 m. Baskets fixed to a conveyer belt allow either for construction of tall extractors that take little floor space or for horizontal extractors in case one-floor operation is required. A variety of methods concerning collection and recirculation of the miscella and solvent-solid feed positions exist, always intending to maximize the number of regions of countercurrent flow while confining the number of pumps required. The basket elevator (Fig. 9), also known as the Bollman extractor (6), contains a rising and a descending leg, each of which contains around 15 baskets that continuously descend and rise undergoing different stages of charging/discharging of solid, fresh solvent, and circulated ex-

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Sliding bed extractor. (Courtesy of Krupp, Hamburg.)

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Figure 8

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Figure 9 Basket elevator.

tract. The solid bed formed in the baskets is rather shallow taking heights between 0.5 and 0.7 m. The solid is fed to the top basket of the descending leg. At this place, half miscella originating from drainage through the more exhausted solid beds of the rising leg is introduced draining downward through the descending baskets in a cocurrent fashion. The miscella draining from the bottom basket of the descending leg is collected in a sump and drawn off for further processing. Fresh solvent is sprayed on the top basket of the rising leg draining downward through the rising baskets in a countercurrent fashion, while extract concentration is enhanced resulting in the formerly mentioned half miscella, which is collected in a sump at the bottom. After arriving at the top and being contacted to fresh solvent, the exhausted solid is discharged by inverting the basket. A couple of extractor types make use of the screw transporting principle, such as the vertical screw tower from Buckau Wolf, Germany, the Hildebrandt extractor formerly applied to soybeans, and the horizontal helix from Raffinerie

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Tirlemontoise used for sugar beets, all of which have not been in operation to a great extent (5). Two helicoidal screws transport the solid through the doublescrew conveyor called Contex, offered at present by Niro, Denmark. The extraction liquid flows through the solids as a submerged stream by means of gravity due to a slight inclination of the vessel. Complications during operation are caused by disintegration of solid particles and flow conditions that depend on solid compacting. Multiplate tower extractors like the Bonotto extractor make use of rotating plates or paddles for transporting oilseed flakes until they fall through openings in the plates to the floor below. Solvent and miscella are transported upward in a countercurrent fashion. Such problems as bypassing, fines entrainment, and back mixing of miscella inhibited commercial application. C. Safety Aspects Working with volatile and flammable solvents implies risks. During normal operation, a number of measures, such as the use of explosion-protected equipment, working at slight vacuum and continuous control of escaping gases by ignition detectors can for the most part guarantee safe handling. Thus, most accidents involving ignition of solvents or even explosions occur as a result of equipment failure (7). When the plant is shut down and vessels are opened for repair, strict safety guidelines might be missing and residual solvent vapors might come into contact with air, producing flammable mixtures. The U.S. National Fire Protection Association has formed a committee dedicated to safety of solvent extraction plants, which issued the following statement given in part: NFPA 36 Committee on Solvent Extraction Plants: Par. 5–8.3: Extractors, Desolventizers, Toasters, Dryers, Spent Flake Conveyers shall be of a design that minimizes the possibility of ignition of product deposits. Such equipment shall be protected by extinguishing systems using inert gas, steam, or a combination of the two, controlled from a safe remote location. Par. 5–8.1.7: The extractor shall be provided with means to remove solvent vapors so that the concentration of vapors inside the unit in the area where work is required can be maintained at or below 25% of the lower flammable limit, e.g., by a purge fan sized so that it changes the empty air volume of the vessel once every 3 min. D. Conditioning 1. Mechanical Pretreatment Extraction efficiency is highly contingent on preparation of the solid matter that undergoes extraction. Small particles are advantageous to small diffusion

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resistance within the particles. On the other hand, powders of very small particle size require great efforts of milling or grinding. While preparing the solid bed, reagglomeration may even occur and during operation as a trickle bed extractor channels might be formed, resulting in insufficient extraction yields. If the bed’s permeability decreases pressure drop might increase and lakes may be formed on top of the solid bed. Drainage is retarded, which may result in undesired back mixing in case the solid bed is moved through successive extraction zones within the extractor. Furthermore, the so-called fines of very small particle sizes are at risk of being entrained. In general, the content of particles of diameter below 0.5 mm should not exceed 5–10%. Oilseed processing preparation methods may be distinguished depending on whether they are combined with mechanical deoiling or are applied for obtaining solid material of defined size and structure. In the case of oil content above 25 wt%, mechanical deoiling by expression is suitable for economical reasons. Rapeseeds and sunflower seeds both containing more than 40% and corn germs with about 50% oil always undergo previous pressing. Cottonseeds containing about 25% are the limiting case for previous mechanical deoiling. Corn and soy, containing less oil, are usually directly extracted after conditioning. Mechanical deoiling implies changed characteristics of the feed material for extraction, e.g., moisture and cell structures differ from their natural state due to high pressing temperatures. Even agglomerates may be formed that have to be crushed prior to further processing. Furthermore, different equipment for mechanical pretreatment exist due to the type of force applied. The fluted rolling mill (Fig. 10) mainly cuts larger particles into pieces. A hammer mill (Fig. 11) inserts kinetic energy in particles, which is converted to surface energy by increasing surface area during impact with the agitator or the wall. In this case, the fines content is higher than that of the formerly described cutting principle. A roller mill works by pressing two cylinders against each other. In case there is no differential circumferential speed, the seeds are flaked only by com-

Figure 10 Working principle of a fluted breaker rolling mill.

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Figure 11 Working principle of a hammer mill.

pressing. A slight differential speed gives rise to shear forces, and seeds may also be ruptured. Extruders being high-shear devices are also capable of rupturing oil cells prior to solvent extraction or mechanical pressing. For processing of high oil content material the extruder may be provided with a drainage cage similar to those used for screw presses described below. In order to obtain particles of defined size, pelletizing or granulating may be applied by pressing, cutting, or adding moisture. The adequate pretreatment depends on the individual feed material and must be determined by experience. As already mentioned, if the content of extractable substances is fairly high, prepressing is carried out prior to extraction. Commonly, the screw presses used easily reduce the oil content in solids to 10% or less. Following solvent extraction requires less solvent because of the reduced amount of extractable substances. In addition, cell structures are destroyed, mechanically liberating enclosed substances. Using a pressure measurement technique described by Eggers et al. (8), the screw geometry may be adjusted to obtain an adequate pressure profile for achieving an optimal pressing result. Presses of the EP series manufactured by Krupp contain a shaft with a rising outer diameter in flow direction at a constant inner diameter of the strainer cage (Fig. 12) in order to achieve adequate compacting. Recent developments tend to obtain the complete final product only by mechanical treatment (full or final pressing; see section

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Figure 12 Screw press. (Courtesy of Krupp, Hamburg.)

below). Therefore, different steps of pressing according to different operating temperatures may be applied for obtaining products of specified quality and quantity. Usually, elevated operating temperatures result from frictional forces. In order to avoid temperature rising, cooling of the press is needed. On the other hand, rising solid temperature before feeding may help performance and enhance product yield. A press recently developed by Krupp contains different temperature zones within one machine. Here prepressing is carried out at lower temperatures resulting in higher product quality but low yield. Final pressing at elevated temperatures has the objective of enhancing the product yield. Monforts has developed a compact double-acting screw press called Komet having a broad range of applicable source material (9) but without any special facility for cooling. Screws of various slopes are used for solids of different hardness, but the shafts always have the same constant outer diameter. Compacting is additionally regulated by use of different nozzles at the cake discharge spout. Very hard solids should be broken or crushed before being fed to the press. If the residual press cake still contains a considerable amount of extractable substances, solvent extraction follows pressing but the compacted solid cake should pass through a cake breaker first. Presses manufactured by Anderson International under the name Expeller are in use for prepressing and full pressing of a variety of oil containing material such as cottonseed, peanuts, corn germ, and sesame seed. After conditioning for careful adjustment of its temperature, the feed material enters the downspout where it receives a first pressing by a vertical screw (Fig. 13). Leaving this section, the solid arrives at the horizontal barrel containing a second screw for final pressing. The compacting pressure of the solid cake is regulated by a hydraulic cone choke at the position of solid discharge. A couple of different

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Pressing by Anderson Expeller. (Courtesy of Anderson International Corp.)

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Figure 13

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Expeller types are offered to meet specific demands. The Expeller 33 prepress leaves 15% residual oil content in the cake for further solvent extraction working at rates of up to 90 t/d feed material. The 33 duplex press was developed to handle especially hard fibrous and high oil content materials such as copra and palm kernels. The residual solid contains about 7% oil. The high-capacity Model 55 allows processing of more than 25 t/d with residual oil contents of 4–5%. Commonly, Anderson expellers are combined with thermal conditioning like cooking and drying of the feed prior to pressing. 2. Thermal Pretreatment Materials that are destined for oil production and contain a large amount of proteins, such as cottonseeds, soybeans, flaxseeds, sesame seeds, and peanuts, must be cooked prior to pressing in order to coagulate proteins and allow efficient recovery of the liberated oil. Therefore, the raw material is maintained for about 20 min at a moisture content of around 10% and temperatures of 90– 95°C. Afterward the material must be dried in a separate step to approximately 3% moisture before entering mechanical pressing so as to stiffen the particles. Materials containing small amounts of proteins only undergo the drying procedure. Drying time strongly depends on the physical properties of the solid particles, e.g., size, moisture, and porosity. Cooking of proteins and starches is combined to adjustment of moisture and porosity by the expander technology. Vapor is introduced into the solid structure during extrusion in a screw press–like device containing steam inlet nozzles at the circumference (Fig. 14). The solid-vapor mixture is expanded at the outlet gap resulting in increased porosity due to explosive-like evaporation from the solid cells. Amandus Kahl offers a ring gap expander disposing of an adjustable ring-formed gap that keeps a defined pressure of the solid moving bed. The resulting material is homogeneous having a high porosity at an increased bed density (compacting). Penetration of liquid in posterior solvent extraction is facilitated while the resulting homogeneous structure and high density facilitate enhanced solid throughput and safe performance of percolators. The expander-extruder-cooker fabricated by Anderson also allows blending of water into the feed material for cooking at a prescribed moisture level. An alternative method of preparing source material for extraction is so-called electropermeabilization. High-intensity electric field pulses cause cell damage and release of liquid contained in the cells (10). Because of its high viscosity, oil cannot be obtained by this technique but it may be applied to fruit juice extraction. E. Solvent Recovery The extracted cake and the product need to be liberated from any solvent residuals by stripping or distillation procedures. Therefore, the miscella leaving the

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Schematic of an expander.

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Figure 14

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extractor is passed through a distillation step for stripping the solvent and obtaining a solvent-free product. This product is ready for further treatment, such as liquid extraction of undesired components, enrichment of desired ones, or chemical modification. The exhausted meal is discharged from the extractor and transported to the desolventizing step by conveyer belts or screws. Here the solvent is evaporated in order to obtain a solvent-free solid residual that can be used, e.g., as animal food. Usually, the desolventized meal is toasted to increase its nutritional value as food material. Desolventizing and toasting is usually carried out in two separate steps. Lurgi offers a toaster including so-called pre-desolventizer stages at the top end of the same tower. Steam is passed from below through vapor duct trays holding the meal at various heights of the tower. Double-arm agitators move the meal atop these trays until it drops through outlet holes to the tray below. Within the trays there are also steam-heated sections for indirect heating. Using solvents that are partly miscible with water (e.g., alcohol, acetone), steam cannot be applied in view of posterior separation and solvent recovery. In this case, the so-called flash desolventizing process can be applied contacting the meal with superheated solvent. If volatile flammable organic solvents are used, operation of extraction, distillation, and desolventizing is carried out at slight vacuum to prevent the risk of explosion in case of leakage. If less volatile solvents are used, e.g., water, high temperatures must be applied, e.g., in spray drying of the extract-solvent mixture. Often, high temperatures are disadvantageous with respect to valuable compounds such as flavors and fragrances. Since the 1930s freeze drying is increasingly applied in order to evaporate water, e.g., from the extracted solubles of coffee. Here temperatures of 60–85°C are applied for sublimation of water at 50 Pa from coffee that is frozen at temperatures below −30°C. The highly concentrated frozen coffee is either filled into trays that are moved onto heated plates through a vacuum tunnel or directly scrapped over heated plates by agitators. In the former case, heat transfer is worse but the latter method leads to abrasion of the solid particles. In general, environmental protection and consumer demands afford development of solvent recovery systems that provide adequate cleaning of exhaust air and careful treatment of conserving valuable product components. F. Final Pressing Systems For production of small amounts of edible oil, mere mechanical pressing may be suitable. Physical limits of the minimal residual oil content exist due to strong adhesive forces. In case of rapeseed, a content of 7% of oil remaining in the solid matrix can be achieved only by pressing. Further treatment using solvents is not economic. Accounting for the loss in residual oil compared with a content

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of 1.5% after combined prepressing and solvent extraction and high costs of maintenance in the case of full pressing, production costs of oil coming from either process are usually equal at production rates above 500 t/a. Fruit juices are commonly produced only by mechanical treatment. Belt filter presses, e.g., manufactured by Flottweg or Zentrifuges from Westphalia, both in Germany, are used to mechanically extract juice, such as from apples and cherries. In processing of citrus juice, crushing of the fruit must be avoided since oil and bitter compounds originating from the peel have to be kept low in the juice. The FoodTech Extractor System from FMC, Florida, extracts juice and oil simultaneously by scratching off the peel using two approaching cups while the peeled fruit moves into a strainer tube. Here the juice is separated from the seeds and the rest of the fruit. In general, around half of the weight remains as solid residue subsequently processed to cattle feed supplement. Furthermore, about 0.4% is formed by citrus oil situated in the peel. Next to pressing of the peel, water is used for rinsing the oil, which on its turn requires separation of the obtained emulsion by centrifugation. Quality of peel oil depends on the amount of water used. While the oil obtained directly from pressing may be sold as cold-pressed citrus oil, the oil obtained from a subsequent aroma recovery step using multistage evaporation and distillation, and thus lower in quality is applied for production of soaps and detergents. Within the juice extractor offered by Brown International Corp., the peel oil is obtained before juice extraction. The peel is cut and rinsed with water. Centrifuges carry out separation of the resulting emulsion. G. Complete Extraction Plants Figure 15 shows a flow diagram of a hop extraction plant using ethanol as solvent. Cone hops is fed to a double-deck Rotocel extractor with 16 compartments after being dried. The miscella drawn from the extractor is concentrated by passing a four-stage vacuum evaporator at gentle temperatures. Complete elimination of the alcohol is carried out in a posterior separation step. The spent hops discharged from the extractor is desolventized in a dryer and subsequently pelletized for animal feed application. Recovered ethanol-water mixture is adjusted to the desired composition by rectification and recycled. Compared to alternative CO2 extraction (discussed in the next section), ethanol possesses little selectivity resulting in a product that contains almost the natural composition of extractables given by the feed material.

II. SUPERCRITICAL FLUID EXTRACTION (SFE) Extraction from solid material using supercritical fluid extraction (SFE), especially carbon dioxide (CO2) extraction, is established on an industrial scale for

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Figure 15 Flow diagram of a hops extraction plant using ethanol as solvent. (Courtesy of Hallertaler Hopfenveredelungsgesellschaft.)

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a wide range of applications. Besides some large plants used for industrial production of decaffeinated coffee and hops extract, there are a number of smaller multipurpose plants that obtain extracts from a variety of natural materials, such as spices, herbs, and valuable vegetable/essential oils (11). A. Extraction System In Fig. 16 a general flow sheet of a supercritical fluid extraction is shown. Such a processing line mainly consists of a pressurizing device, a pressure vessel for extraction, one for separation (i.e., solvent recovery), and a couple of heat exchangers. If separation is performed in the most common way by pressure release, the fluid must be vented through a butterfly valve before entering the separator. Separation is carried out at lower pressure than extraction. The type of pressurizing device used to recirculate the fluid by rising pressure back to extraction conditions depends on the state of the solvent fluid coming from the separator. If the pressure in the separator is high enough for the fluid to be liquified by chilling in a reasonable temperature range, piston pumps may be applied. On the other hand, gas compressors working with much lower volumetric efficiencies are needed if the fluid at the pump inlet is in a gaseous state. Nevertheless, the corresponding low separation pressures might be of interest for highly volatile components that are well solubilized by the compressed fluid. Pressure must be reduced considerably for precipitating these solutes. Alternative methods of solvent recovery also exist for a complete isobaric solvent cycle. The solute can

Figure 16 General flow sheet of supercritical fluid extraction.

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either be absorbed by an additional liquid, e.g., water, or adsorbed on a fixed bed, e.g., of activated carbon. Therefore, the pump is just needed for maintaining fluid flow and overcoming relatively low-pressure drops along the processing line. For arriving at the operating pressure and compensating solvent losses, a relatively small additional pump has to be installed. The solvent cycle is commonly represented by a T-S diagram from which energy balances may be drawn that are needed for heat exchanger design. For a usual SC-carbon dioxide (CO2) extraction with liquid CO2 at the pump inlet, Fig. 17 shows this clockwise turning cycle in such a T-S diagram. After being liquified in the chiller C1 the fluid is compressed by the pump P1. Pressure increase is supposed to take place nearly reversibly (isentropically). HE1 heats up the fluid to extraction temperature. Adiabatic expansion (no significant change in enthalpy) occurs in the butterfly valve RV1. Due to the JouleThompson effect, the carbon dioxide is cooled down arriving at saturation conditions. For better separation, the loaded fluid is completely evaporated just before entering the separator in HE2. Besides some large-scale plants for extraction of α-acids from hops (12), there are quite a number of smaller, multipurpose plants for obtaining extracts of a variety of natural materials such as spices, herbs, and solids containing flavors or fragrances, all of them using similar setups as depicted in Fig. 16 (13). For

Figure 17 CO2 cycle, depicted in a T-S diagram.

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decaffeination of green coffee beans, either the beans are moistened or moist carbon dioxide is used (14). Separation and regeneration of the supercritical solvent after decaffeination is usually performed by isobaric adsorption on activated carbon or absorption by water (15). In a similar way, caffeine-free tea can be produced (16). An alternative procedure has been proposed by Buse GmbH for decaffeination of green coffee beans using water saturated with carbon dioxide at pressures of up to 30 MPa (17). B. Batch and Continuous Processing Up to now, supercritical extraction of solids at industrial scale is mostly performed in a discontinuous manner due to the lack of reliable sluice systems for continuous inflow and outflow of solids to and from high-pressure vessels. In order to save expensive operation time, two to four extractors are often operated in turns. Figure 18 schematically shows a plant for solid extraction containing two extractors and an optional second separator in case stepwise pressure reduction is necessary. Figure 19 shows a time schedule of discontinuous supercritical extraction including charging and discharging of four vessels in turns. One vessel is always off-line for depressurizing, discharging, charging, purging, and pressurizing. Different methods have recently been proposed to achieve continuous charging and discharging of solids to and from high-pressure vessels. By using screw conveyers or extruders, the solid is compacted entering into the extractor (18, 19). The resulting pressure drop along the moving solid bed prevents pressure loss while charging the extractor. An alternative multistage sluice system containing various compartments on an axially moving frame has problems of scaling up due to lack of appropriate sealing systems. In case a slurry is formed, continuous processing can be carried out using common piston and membrane pumps (20). The solid–liquid dispersion is introduced into rather tall and thin extractors or columns where the supercritical fluid is directed either co- or countercurrently with respect to the liquid flow. Anyway, using a liquid “carrier” enhances mass transfer resistance from the solid particles into the supercritical fluid phase, which can partly be overcome by finely dispersing the liquid within the supercritical solvent. Finally, discharging of solid powders formed inside the extractor by dissolving the original carrier phase completely within the supercritical fluid may be realized by simply opening a discharge valve at the bottom and blowing out the solid particles together with the supercritical solvent. For pressure release of vessels containing, for example, pressurized carbon dioxide, one must also take into account temperature drops toward the vapor–liquid line or even surpassing the triple point that is situated at a slightly elevated pressure. As a consequence, outflow from the vessel partly takes place as a two-phase flow and blocking by

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Figure 18

Flow diagram of an extraction plant with two extractors and fractionated separation.

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Figure 19 Time schedule of supercritical–solid batch extraction using four extraction vessels in turns, 120 kg per batch, 1200 kg CO2/h at 250 atm.

dry ice (solidified carbon dioxide) may occur (21). The interior of the vessel is easily cooled down to −30°C (Fig. 20). C. Recovery Systems As mentioned above, different principles of solvent (supercritical fluid) recovery have been proposed that are also applied at industrial scale. Next to the energyconsuming method of depressurizing the supercritical solvent, adsorption and absorption methods are used especially in case high amounts of supercritical

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Figure 20 Pressure and temperature decrease during depressurizing of moist carbon dioxide from a 50-L vessel.

fluid must be circulated and the substances dissolved by this solvent are destined for elimination. If the objective is to obtain a valuable product as the extract phase, separation by pressure reduction is usually most suitable. Nevertheless, caffeine that is dissolved by supercritical carbon dioxide for decaffeination of green coffee beans may be absorbed by water in some type of high-pressure liquid–fluid extraction plant such as a cocurrent spray tower, countercurrent packed column, or mixer-settler (22), concentrating the caffeine in a posterior membrane module and obtaining a product of elevated purity (23). The highpressure countercurrent principle is described by Jaeger focusing on the wetting behavior of packing materials at elevated pressures (24). At industrial scale, adsorption of caffeine on activated carbon dominates, but up to now the caffeine is burned for thermal recovery of the adsorbent. Each recovery implies a 10% loss of activated carbon. Recent developments are aimed at obtaining the adsorbate without being destroyed during regeneration. The use of alternative adsorbents, such as ion exchangers (25), is also proposed to enhance processing rates or others with hydrophilic properties in order to facilitate their regeneration by water (26). For dimensioning an adsorber, one has to consider a sufficient length in order to establish plug flow within the adsorbent bed. Therefore, the minimal length-to-diameter ratio is about 10. The required length further depends on the amount of substance to be adsorbed.

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Adsorption capacities range between 25 and 80 g/kg adsorbent. Back mixing should be prevented by keeping fluid velocities at values below a few centimeters per second by choosing an adequate inner diameter. D. Pretreatment Similar to conventional liquid extraction, the solid feed material has to be conditioned properly. Mass transfer should be favored by maintaining small particles, eventually removing skins by pealing or dehulling. On the other hand, the structure of the solid bed should be porous and as homogeneous as possible. Large particles result in high void fraction, but at the same time the mass transfer from inside the particles into the bulk fluid is slowed down. Solid particles that tend to stick to each other (e.g., at high moisture content) often form channels by the fluid or even complete blocking. As a consequence, the solid material is extracted heterogeneously and to a low extent. Figure 21 shows the influence of pretreatment on the kinetics of supercritical extraction of sunflower oil. Extraction of dehulled and flaked sunflower seeds proceeds faster. In general, flaking

Figure 21 Kinetics of oil extraction from sunflower seeds at 50 MPa, 60°C. DHF = dehulled, flaked; UDHM = undehulled, milled; UDHP = undehulled, pelletized.

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appears to be an appropriate approach to pretreatment since the thin layers minimize transport resistance while leaving a firm and porous structure to the bed. E. Vessel Design In supercritical extraction, pressure vessels are needed for supply and recovery of the solvent, the extraction (loading of the solvent) itself, and a number of heat exchangers. According to their dimensions and way of operation, different types of construction and factoring of these vessels are applied (27). Figure 22 summarizes the most important methods of pressure vessel construction. In general, vessels with solid walls and those containing compound (layered) walls are distinguished. Solid-walled vessels are normally produced as single forgings. Using this method, the diameter is limited to about 1 m and the wall thickness to about 150 mm. Greater heights may be obtained by joining several cylinders by circumferential welding. Wall thickness in excess of 200 mm is achieved by joining two half shells by longitudinal welding. The technological limitations of weight and size imposed on the mentioned methods for solid-walled vessels are overcome to a large extent by the use of vessels with laminated walls. The presence of such multiple layers while normally beneficial, can produce complications; the insertion of an adapter or a nozzle in a layered wall requires very careful design. Working with corrosive substances, the multilayer principle has the advantage of separating the corrosion problem from the required pressure resistance. The inner layer is made of a corrosion-resistant material with less tensile strength, whereas the outer layers resist the high pressure. A disadvantage of this type of vessel is reduced thermal conductivity. Usually, contact between the layers is not ideal leaving small gaps that give rise to enhanced thermal resistance. However, at high pressure, this effect is partially reduced due to additional pressing of the layers toward each other. Calculation of wall thickness is carried out according to national codes, such as the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers (ASME). Especially for design of extractor vessels, rules concerning some construction principles are based on experience gained in the past two decades of industrial application of SFE. The ratio of inner length to inner diameter influences the performance of extraction and should therefore be carefully chosen. At too low an inner diameter, wall effects become noticeable. Although axial dispersion may be relatively low, back mixing becomes relevant within a tall extractor. On the other hand, high inner diameters may result in heterogeneous extraction with respect to the radial position. In most cases, solid extraction is performed upflow. The fluid enters the bottom of the extractor at its center. The extractor contains a so-called product basket that has a sieve tray bottom or, even better,

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Methods of pressure vessel construction.

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Figure 22

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a sinter metal bottom. At its circumference, the basket must be sealed towards the inner wall of the extractor to prevent bypassing of the fluid. Since the basket enters the extractor by opening the extractor top, this top must be removable. Therefore, any fixed tubing at the top would have to be replaced for opening the extractor. Therefore, the outlet is positioned just below the top to one side of the extractor. This has to be considered for a homogeneous flow within the extractor because having the outlet on one side results in asymmetrical flow. In the case of discontinuous charging and discharging of solids, the vessel must contain quick-acting closures. Furthermore, cleaning must be facilitated if there is a risk of accumulating precipitates. F. Heat Exchangers The specific heat transport properties of the respective supercritical fluid and their variation depending on operating conditions must be taken into account for heat exchanger design. The rate of heat to be transferred within the fluid cycle, e.g., CO2, may be deduced from the T-S diagram taking the respective points of specific enthalpy. Coming from the pump (Fig. 16) the supercritical fluid, e.g., CO2, enters the heater HE1 at, say, 15°C and 30 MPa. The corresponding enthalpy of CO2 amounts to about −284 kJ/kg. Raising the temperature to 100°C, which gives an enthalpy of −116 kJ/kg, 168 kJ/kg of heat needs to be transferred. At a CO2 mass flow of 500 kg/h the transferred heat flux comes out to be 23 kW. For cleaning purposes, heat exchangers should be constructed as tube bundles or double tubes. In particular, the evaporator following the throttling valve is at risk of being blocked by precipitated extract. Vertical orientation of this heat exchanger helps downflow of precipitating liquid. For the condenser position, different possibilities are discussed leaving some doubts for the best solution. In the course of cooling, a condensate film is formed on the walls increasing heat transfer resistance to the gaseous phase. A diagonal orientation down toward the fluid outlet allows the freshly formed liquid to drop off the tube walls right away, leaving only a thin film of condensate. For laboratory-scale plants, heat exchangers may also be constructed as a coil of high-pressure tube placed in a thermostat bath to keep dimensions small despite relatively high heat transfer area. G. Pumps and Compressors In general, pumps are used to transport liquids through pipes. Every pump has certain operating characteristics due to which mass flow and pressure increase are related for the installed piping. Figure 23 qualitatively shows operating characteristics for centrifugal and piston pumps. Centrifugal pumps that are usually

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Figure 23 Characteristic curves of different types of pumps.

not applied to high-pressure technology show a reciprocal behavior of pressure with respect to mass flow: at higher pressure mass flow diminishes. The pump is actually operated at the point of intersection of the operating line and the characteristic line of the piping due to pressure losses caused by friction. On the other hand, piston pumps that are commonly used for high-pressure purposes ideally maintain a constant mass flow no matter the pressure at the outlet, assuming a noncompressible liquid. Cavitations, i.e., evaporation due to pressure loss at the pump inlet, must be avoided in the case of both centrifugal and piston pumps. So-called net pressure suction height (NPSH) is a measure of the minimal inlet pressure that ensures complete liquid pumping, usually based on properties of water. Since piston pumps have a certain constant volume per stroke, it is very dangerous to shut valves right behind the pump. Pressure will increase until either tubes burst or major damages occurs, e.g., at the pump transmission. Therefore, security devices such as security valves and rupture discs are obligatory. If the fluid to be circulated is highly compressible and the pressure is to be increased, compressors with multistage working principle need to be used. Pressure is increased stepwise with an intermediate cooling step. In each step pressure may be raised by a factor of 3–4. Compressors have a much lower volumetric efficiency than pumps but can work with almost any suction pressure. Supercritical fluid extraction works with fluids under conditions ranging from the need of compressors to the possibility of applying pumps. Usually, the

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circulating fluid enters the pump at elevated pressure. Whether or not a liquid state can be assured at this point depends on conditions of the previous separation step and the chilling capacity. In the following we will assume that a pump is used. The two main different types of reciprocating pumps used for supercritical fluids are plunger (piston) and membrane pumps. Membrane pumps have certain advantages, especially concerning sealing. While piston pumps need special dynamic sealing packing to guarantee hermetic operation, the membrane itself separates the pressure chamber from the pressure-transmitting liquid and the transmission of the pump (Fig. 24). For food and pharmaceutical applications sealing toward, say, lubricants may be decisive for pump selection. On the other hand, the fact that the efficiency of piston pumps is higher than that of membrane pumps is important for realizing high mass flows. For circulating fluids on a high-pressure level but relatively small pressure drops in the cycle, canned centrifugal pumps (hermetic) may be applied. 1. Pump Efficiency (Volumetric Efficiency) Multiplying the stroke volume and the pumping frequency gives a theoretical value of the volume flow of the pump. This volume flow is never reached because of various factors that are quantified by using the so-called pump efficiency, ηp. ηp is composed of a systematic efficiency accounting for back flow through valves, fluid losses, and so forth, as well as an elastic efficiency taking into account elasticity of the pumping head, stagnant volumes, and compressibility of the fluid. The elastic efficiency that usually comes close to the total pump efficiency is defined by (28): ηE = 1 − (εTκ + λE)∆p

H h

with H/h denominating the stroke ratio. In case of a nonelastic piston (λE = 0) and a normal stagnant volume (εT = 1), the pump efficiency for a single acting piston pump comes out as: ηE = 1 − ∆V/V0 where ∆V/V0 is the relative compressibility, which can also be defined as ∆V/ V0 = κ ∆p, κ being the compressibility coefficient. In Fig. 25, the relative compressibility is depicted as a function of pressure for CO2. The compressibility is not only important to the pump efficiency but also to the pulsation of the pump. The volume flow characteristics of a double-acting plunger pump can be seen in Fig. 26. In the case of incompressible fluids one peak is directly followed by the other. For compressible fluids the first part of the peak is cut away and the flow becomes similar to that of a single-acting pump.

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Schematic of a piston (a) and a membrane (b) pump.

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Figure 24

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Figure 25 Relative compressibility of CO2.

Figure 26 Volume flow characteristics of a double-acting pump.

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2. Dimensioning of Pumps For solid extraction using supercritical CO2, velocity with respect to the empty cross-section of the extractor should amount to a few millimeters per second. In many applications the resulting mass flow (in kilograms per hour) comes out as about 20 times the content of the extractor (in kilograms), of course still depending on the solubilities in detail. Whether or not a sufficient mass flow can be achieved depends strongly on conditions at the pump inlet and on the desired extraction pressure. Taking into account conditions on the suction and the highpressure side of the pump, an estimation may be carried out using the volumetric efficiency described above. 3. Formation of Gas Hydrates In general, low temperatures are required for assuring liquid state of the fluid at the pump inlet. For many gases, including CO2, there are limits given in case water is present in the cycle. In spite of moist CO2 containing only around 0.02–0.2 wt% water, gas-hydrate crystals may be formed in or near the condenser, growing on the inner walls and possibly blocking tubes and pumps. These hydrates were found to be formed below 10°C and be quite stable, eventually lasting for days after stopping operation (31). H. Industrial Plants In the meantime, several applications for SFE have been established. Two of the most important industrial scale applications are extraction of α acids from hops and caffeine from green coffee beans. The purpose of using supercritical fluids is mainly to obtain a solvent-free product of high quality, carefully treated with respect to temperature and to save costs for environmental protection. A selection of industrial operated plants is shown in Table 2. The product value depends on whether the objective of the process is to obtain the fluid-soluble extract or the remaining nonsoluble substances. Because of rather low solubilities within supercritical CO2 in most cases, product costs are much higher when aiming for the extract. In detail, the costs depend on the solubility of the desired component in the solvent phase; on the possibilities of continuous, quasi-continuous, or semicontinuous operation; and on the type of regeneration of the solvent. Since the used supercritical solvent may have an increased selectivity in comparison with conventional liquid solvents, the final composition of the extract mostly differs from its natural composition within the solid matrix. This fact has to be taken into account considering product stability since many natural components disintegrate rapidly after being isolated or simply when nonsoluble natural stabilizing agents or antioxidants are absent in the obtained extract.

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Table 2 Industrial Applications of SCF-Extraction Company/Country Yasuma (Japan) Fuji Flavor (Japan) Sago Koryo (Japan) Mori Seiya (Japan) Idemitsu Sekiyu (Japan) Takeda Seiyaku (Japan) Essences (Italy) CUB (Australia) Universal Flavors Ltd. (UK) HAG (Germany) Barth (Germany) SKW (Germany) Sumitomo Seika (Japan)

Capacity (L) 100 300 300 500 1000 1200 1200 1000 1000 45.000 12.000 18.000 100

Products Spices, pigments, food additives (19) Tobacco, flavors Flavors, pharmaceutical compounds (19) Flavors, pigments (19) Flavors (19) Pharmaceutical compounds (19) Essential oils (20) Flavors (21) Flavors (21) Decaffeinated coffee (21) Hops (22) Hops (22) Coffee extract (19)

REFERENCES 1. FA Norris. Extraction of fats and oils. In: Bailey’s Industrial Oil and Fat Products, Vol. 2, 4th ed. New York: John Wiley & Sons, 1982, pp. 230–237. 2. MA Williams. Extraction of lipids from natural sources. In: FD Gunstone, FB Padley eds. Lipid Technologies and Applications. New York: Marcel Dekker, 1997, pp. 127–135. ¨ le. Stuttgart: Ulmer, 1993, pp. 319–333. 3. M Bockisch. Nahrungsfette und -O 4. General Foods Corp.: Verfahren zur Herstellung von Kaffee-extrakt. German Patent 2125545, May 22, 1971. 5. HG Schwartzberg. Continuous counter-current extraction in the food industry. CEP: 67–85, 1980. 6. RE Treybal. Mass-Transfer Operations, 3rd ed. New York: McGraw-Hill, 1980, pp. 740–744. 7. CL Kingsbaker: Extractor failure: Safety procedures. Inform 10:1142–1147, 1999. 8. R Eggers, H Boeck, W Stein. Druck- und Temperaturprofile beim Abpressen von o¨lsaaten in seiherschneckenpressen. Fette Seifen Anstrichmittel 87:494–499, 1985. 9. L Falla, J Ospina, E Posada. Determinacio´n de las condiciones de extraccio´n de aceite de cafe´ por extrusio´n. Proceeding of the 13th ASIC Colloquium, Paipa, 1989, pp. 232–235. 10. A Angersbach, MN Eshtiaghi, D Knorr. Impact of high intensity electric field pulses on plant cells and tissues. In: G Barbosa-Canovas (ed.), COFE 97. New York: AICE, 1997, pp. 11, 18–21. 11. E Stahl, K-W Quirin, D Gerard. Dense Gases for Extraction and Refining. Berlin: Springer Verlag, 1987. 12. Brewing Research Foundation. Hop extraction with carbon dioxide. US Patent 4,218,491, Aug. 19, 1980.

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13. Norac Technologies Inc. Process for the supercritical extraction and fractionation of spices. European Patent 0511673 A1, Apr. 30, 1992. 14. HAG AG. Method for the decaffeination of raw coffee. US Patent 3,879,569, Apr. 22, 1975. 15. Studiengesellschaft Kohle mbH. Process for recovering caffeine. US Patent 3,806,619, Apr. 23, 1974. 16. O Vitzthum, P Hubert. Method for the manufacture of caffeine free black tea. US Patent 4,167,589, Sept. 11, 1979. 17. Buse GmbH. Verfahren zur Entcoffeinierung von Rohkaffee. European Patent 90122400.6, Nov. 23, 1990. ¨ berkritischem CO2. Fat Sci Technol 18. R Eggers. Extraktion von Fettrohstoffen mit U 96:513–518, 1994. 19. G Kaschel. Die Schneckenmaschine mit Gassperre als Ein- und Austragsystem fu¨r die Hochdruckextraktion von Pfanzlichen Naturstoffen. Dissertation, Technische Universita¨t Hamburg-Harburg, 1992. 20. Nestle´. Decaffeination process, European Patent 0010637, Oct. 3, 1979. 21. A Fredenhagen, R Eggers. Vessel release of pressurized carbon dioxide containing nitrogen or water. Two-Phase Flow Modeling Exp 1:623–627, 1999. ¨ berkritischem Kohlendioxid an den 22. A Pietsch. Die Gleichstromverspru¨hung mit U Beispielen Hochdruckentcoffeinierung und Carotinoidaufkonzentrierung. Dissertation, Technische Universita¨t Hamburg-Harburg, 2000. 23. A Pietsch, R Eggers. The mixer-settler principle as a separation unit in supercritical fluid processes. J Supercrit Fluids 14:163–171, 1999. 24. PT Jaeger: Grenzfla¨chen und Stofftransport in Verfahrenstechnischen Prozessen am Beispiel der Hochdruck—Gegenstromfraktionierung mit u¨berkritischem Kohlendioxid. Dissertation, Technische Universita¨t Hamburg-Harburg, 1998. 25. HAG AG. Verfahren zur Entcoffeinierung von Ro¨stkaffee. German Patent DE 3303679 A1, Aug. 16, 1984. 26. D.E.J. International Research. Process for extracting caffeine from solutions thereof in carbon dioxide under high pressure. US Patent 4,364,965, Dec. 21, 1982. 27. R Eggers. Design and operation of the pressure vessels used in near-critical extraction processes. In: MB King, TR Bott (eds.). Extraction of Natural Products Using Near-Critical Solvents. Glasgow: Blackie Academic, 1993, chapter 8. 28. R Eggers, G Fischer. Zum Einfluss der Kompressibilita¨t verdichteter Fluide auf den Fo¨rdervorgang in Verfahren zur Extraktion mit Gasen. Pumpentagung Karlsruhe Oct 6th –8th, 1992, B4-02. 29. V Hung, MA Unger: Applications of supercritical fluid extraction in the Japanese food industry. Food Aust 46:235–239, 1994. 30. E Reverchon. Supercritical fluid extraction and fractionation of essential oils and related materials. J Supercrit Fluids 10:1–38, 1997. 31. MV Palmer, ST Ting. Application for supercritical fluid technology in food processing. Food Chem 52:345–352, 1995.

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5 Optimization Constantina Tzia National Technical University of Athens Athens, Greece

I. INTRODUCTION Optimization is the act of obtaining the best result possible or the effort for achieving the optimal solution under a given set of circumstances (1). In design, development, processing operation, and maintenance of engineering systems, common goals are either to minimize the cost or maximize the desired profits as product quality and operation yield. A systematic and efficient way to meet these goals is to place the emphasis on design and process optimization for manufacturability/performance (or yield), quality, and cost (2, 3). Such operations should be made efficient by applying the relevant optimization methods and taking the appropriate technological and managerial decisions among all possible alternatives. Optimization can be defined as the process of finding the conditions that give the optimum (maximum or minimum) value of a function of certain decision variables subject to restrictions or constraints that are imposed (2). Optimization may be the process of maximizing a desired quantity or minimizing an undesired one. The conditions (values of the processing variables) that produce the desired optimum value are called optimum conditions while the best of all feasible designs is called optimal design. In its most general meaning, optimization is the effort and process of making a decision, a design, or a system as perfect, effective, or functional as possible. Optimization for a system may mean the design of system parameters or the modification of its structure to minimize the total cost of the system’s products under boundary conditions associated with available materials, finan-

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cial resources, protection of the environment, and governmental regulation, taking into account the safety, operability, reliability, availability, and maintainability of the system. Optimizers or decision makers use optimization in the design of systems and processes, in the production and in systems operation. Some examples of the optimization use are: selection of processes or size of equipment, equipment items and their arrangement, operation conditions (temperature, pressure, flow rate, chemical composition of each stream in the system), equipment combination in specific processes to increase the overall system availability, etc. Formal optimization theory encompasses the special methodology, techniques, and procedures used to decide on the one specific solution in a defined set of possible alternatives that will best satisfy a selected criterion or function. The application of scientific methods and techniques to decision-making problems based on mathematical programming techniques may achieve the optimum of the operation result—the maximization or minimization of the criterion or function. A number of optimization methods have been developed for solving different types of optimization problems and hence various methods of efficient experimentation and simulation are available for performing the optimization. In many applications, the process to be optimized can be formulated as a mathematical model and the optimization experiments may be contacted or simulated in software. Today, even very large and complex systems can be modeled by means of computers, and optimization can yield substantially improved benefits. Optimization methods have been found effective in many areas of engineering design and operation or even in business systems. In chemical processes, optimization methods were developed in an effort to produce high-quality products under normal manufacturing and working conditions, reducing their functional variations under real manufacturing circumstances (low quality of raw materials, poor manufacturing equipment, etc.) and further lowering the cost of making the product (including development and manufacturing). A wide variety of problems in the design, construction, operation, and analysis of chemical plants or industrial processes can be solved by optimization. These methods have particular economical and technological interest for food engineers, as they are widely used in food processes to achieve high efficiency in design, development and manufacturing. Now optimization methods are used routinely in industrial processes of foods. In this chapter, an overview of the principles involved in design and process optimization is presented, the appropriate methods of optimization are described, and the applications of optimization in food engineering, especially in food extraction processing, are reviewed.

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II. OPTIMIZATION THEORY A. Quality Engineering—The Design Optimization Problem The quality of a product or a manufacturing process can be quantified in terms of the deviation from the target performance due to undesirable effects by factors external to the product, manufacturing imperfection or deterioration/changing of product performance characteristics (3). In a representative design optimization problem of a product or a process the common elements must be considered (Fig. 1). The response (y) is influenced by various factors; the dependence of the response y on the factors can be denoted by a function (f) or a mathematical model. Input or signal factors (or pre-assigned parameters) (s) are selected by the engineering knowledge and are set by the operator to attain the intended output (target performance). Control factors (or design or decision variables) (x) are the product or process parameters specification, and their best values (levels) are determined by the designer using a number of criteria. Noise factors (z) are the uncontrollable factors and their levels change from the environmental conditions of manufacture. One approach to reduce the variation of a product or process is to limit or eliminate the noise factors. The role of design optimization is first to center the design parameters specifying the levels of control factors in order to minimize sensitivity to all noise factors. Product or process design can make the product or process robust against all factors, whereas manufacturing process design and actual manufacturing can reduce only the variation due to manufacturing imperfection. A typical engineering problem can be posed as a process represented by some equations or by experimental data, while a performance criterion as minimum cost, maximum yield, etc. The process or model and the performance criterion comprise the optimization problem. The goal of optimization is to find

Figure 1 Block diagram of a product or a process in an optimization problem.

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the values of the variables in the process that yield the best (maximum or minimum) value of the performance criterion while satisfying the constraints—this is the optimization of a given problem. B. Classification of Design Problems Design problems can be broadly classified into static and dynamic problems depending upon the absence or presence of signal factors, respectively (3). Static refer to problems in which the values of the dependent variables remain constant with respect to time, while dynamic problems represent the ones where the process-dependent variables change with time (2). Static problems are also classified depending whether the response is continuous or discrete. The desired value may be the maximum or the minimum value or be categorized into ordered categories. Examples of static problems are the maximum yield of a desired constituent during an isolation process, the minimum product loss during a manufacturing process, or the best sensory quality and acceptability of a food. Dynamic problems are classified depending on the nature of the signal factor and the response variable (continuous or discrete). Examples of dynamic problems are fermentations in food processing in which the response (i.e., yield as product weight or concentration of a constituent, etc.), results by continuous monitoring of a process (viscosity increasing during curd formation in yogurt production, ethanol content increasing during wine fermentation). C. Elements of Optimization Problems—Definitions Optimization determines the values of independent variables that result in an optimal value of a dependent variable. This involves a process for finding the unique set of process conditions that produces the best results, usually after establishing a mathematical model of the product or process. The theory, methods, and techniques used to attain an optimum and to locate the optimum operating conditions are the subject of optimization. Following are definitions of the basic elements and terms that are common to all optimization problems (2, 4, 5). The representation of the relationship of the elements in an optimization problem and solution is in Fig. 2. Performance function or objective function or response or criterion or criterion function or performance measure: This is a function of the design variables that quantifies the desired result. It is the measured variable that determines and reflects the performance of the system or process; it is the quantity to be optimized (maximized or minimized). It is often the profit, cost, product yield, energy consumption, and particularly for food systems the shelf life, consumer preference, quality characteristics, physical properties, etc.

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Figure 2 Elements of an optimization problem.

Decision (independent) variables or parameters or (control) factors: These are the parameters in the process that affect the value of the objective function and can be adjusted to improve performance. They are free and independent variables that must be specified in the process model. In food systems, these commonly include temperature, pressure, flow rates, pH, concentration, moisture content, concentration of ingredients, etc. Constraints: These are the constraints on the allowed solutions that may have a problem to be optimized. The constraints, equality or inequality, limit the values of the dependent variables, the decision variables or even the performance functions of the process. Equality constraints are the fixed values that variables will have in the boundaries. Inequality constraints refer to upper or lower values of the variables that the system may approach at boundaries. Mathematical model: The model is the mathematical representation of the process that determines the performance functions in terms of the decision variables or other independent variables subject to constraint. Modeling of a process is the application of engineering sciences and knowledge to describe mathematically (using mathematics, statistics, numerical analysis, computer software) a process and its performance. Simulation is the use of the model to assess different scenarios. The simulation of the process with a mathematical model facilitates the process optimization against to costly experiments predicting process results for any one set of decision variables. Furthermore, simulation helps to gain understanding, to evaluate alternatives, and to answer specific questions. Optimization is described as the simulation performed aiming to maximize (or minimize) a certain process objective; the search for the desired optimum is usually done using mathematical algorithms.

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Optimization method or technique or procedure: This is the method of searching to find the optimum combination of decision variables possible within the boundary of the constraints. It can be as simple as selecting the combination among all possible combinations that produces the best results from the objective functions in the mathematical model. However, in complex problems where there are many decision variables with a wide range of values, a structured technique based on mathematical algorithms must be followed for solving of the problem. Computers and associated software make the computations involved in the selection (solution) feasible and cost-effective. Feasible solutions: These are values of the design variables that satisfy all the constraints. D. Problem Optimization Procedure In a general case of optimization, the problem must first be formulated and then the system to be optimized must be defined. Following, a function (mathematical model) that describes the system must be constructed and then the system’s optimum solution must be found. The simulation of the process by a mathematical model as well as the selection of the appropriate optimization method is fundamental to a successful process optimization. In all optimization problems, the above elements must be considered (6). A typical optimization problem procedure including the individual optimization steps and the relationships between the important optimization elements are presented in Fig. 3. 1. Optimization Problem Formulation Formulation of problem optimization requires identifying the essential elements of a given system and organizing them into a mathematical form, namely: (a) the objective function (criterion) and (b) the process model and constraints (2). It means that the variables that affect the performance of the system and the variables that determine and reflect the system’s behavior must be specified. The objective function represents profit in terms of the (key) variables of the process being analyzed and the process model and constraints describe the interrelationships of the (key) variables. A systematic approach for assembling the physical and empirical relations and data involved in an optimization problem and procedures are recommended. The need for formulation of the problem has been illustrated by examples of relative optimization problems. Problem formulation and establishment of a satisfactory function that describes the behavior of the criterion as a function of the independent parameters is the most critical aspect in a problem optimization and usually the most difficult step of a successful optimization study; artistry, judgment, and experience is required during the problem formulation step of optimization.

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Figure 3 Problem optimization procedure.

For the formulation of problem optimization the following steps must be carried out: 1. Description of the system to be optimized. The system boundaries must be clearly defined so system parameters become independent of external parameters. All the subsystems that significantly affect the performance of the system should be included in the optimization problem. When dealing with complex systems, they are usually divided to subsystems that are optimized individually (suboptimization); similarly, during suboptimization the subsystem boundaries must be carefully selected.

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2. Definition of a criterion (parameter). A single dependent parameter is defined, that serves as the overall evaluation criterion for the specific optimization problem. It will be maximized or minimized to satisfy the objectives and measures the degree the solution satisfies the desired objectives of the problem. Often, only one primary criterion is used as an optimization performance measure and all other criteria are treated as problem constraints or parameters. However, in complex problems, where the specification of the optimum solution is complicated by conflicting criteria, a single parameter must be established for optimization needs, taking into account the relative importance of all relevant criteria. In practice, if it is desirable to develop a design that is “best” with respect to many, usually competitive, criteria, advanced techniques for solving the optimization problem must be applied, i.e., simultaneously minimizing cost and environmental impact, while maximizing efficiency and reliability. The selection of criteria on which the system design will be evaluated and optimized is a key element in formulating an optimization problem. Optimization criteria may be economical (total capital investment, total annual costs, annual net profit, return on investment), or technological (profitability evaluation criteria) (thermodynamic efficiency, production time, production rate, reliability, total weight, etc.), and environmental (e.g. emitted pollutants). An optimized design is characterized by a minimum or maximum value, as appropriate, for each selected criterion. 3. Definition of controlled or independent parameters. These are parameters that affect the performance of the process or system and can be adjusted, changed, and controlled. Their values determine the criterion parameter value and must lie within the boundaries of the defined problem. The most critical element in optimization problems is the selection of all the appropriate variables that contribute to the decision-making process of attaining the optimum. The variables must include all the important variables that affect the performance and cost effectiveness of the system, while not including details or variables of minor importance. Functional and regional constraints: Functional constraints represent physical or functional interrelationships that exist among the independent parameters, while regional constraints limit the range over which the independent parameters can vary; they must be considered in addition to the parameters in optimization problems. Other parameters: Formal optimization depends on a clear definition of the system to be optimized. It only works for the specific system described where all criteria and parameters are defined within the system and are isolated and independent from other parameters outside the boundaries of the system. Therefore, formal optimization theory should be used only to find optimum parameters in well-defined systems with unique, quantitative-dependent and -independent parameters, criterion function, and functional and regional con-

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straints. So, the clearer the description of a given system, the more sure will be in obtaining the best solution. Furthermore, after the optimum set of parameters has been determined, other parameters, not included in the original problem, may be used and then compared with the others. Suboptimization: It is usually applied to complex systems, where the optimization of the entire system may not be feasible, either due to the problem formulation or the inadequate optimization techniques. It can be also applied for economic and practical reasons. Then, the optimization of one subsystem as part of the whole problem is done, ignoring some variables that affect the objective function or other subsystems; it does not however ensure the optimization of the overall system. 2. Development of Mathematical Model—Design and Evaluation The further procedures for formal optimization and solution techniques include the development of a model, model design, and application. That means the forming of the model, establishing and treating constraints, determining feasible solutions, and assigning of performance measures (6). In most cases of optimization, models are used instead of trial-and-error experiments and statistic techniques are applied. Design includes general description and specification of the programming technique and algorithms, formulation of the mathematical description, and simulation of the model. In order to obtain a mathematical description of the process or system to be optimized, a mathematical model of the process or system is formed (7, 8). A mathematical model is a description in terms of mathematical relations of the functions of a physical system; the way variables are interrelated and the independent variables affect the performance criterion are described by mathematical expressions. Models should closely represent the system and allow the determination of the performance functions in terms of the decision variables; consequently, development of a model requires a good understanding of the system. The optimizer has to select the specific mathematical representation to be used in the model, as well as the assumptions and limitations of the model. All the important aspects of the problem should be included, so that they will be taken into account in the solution (specific values of the variables, assigned variables that are functions of time, other independent variables, satisfaction of constraints or certain goals, etc.). Care must be taken not to omit any significant factor to save time. The degree of accuracy needed in the model must be determined. A successfully developed model helps to solve of the optimization problem. Process design simulations and flow-sheeting software are also very useful during modeling. Model building is completed by evaluation and application; it

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is an iterative process as improved models can be produced by feedback of information. The mathematical model for an optimization problem consists of: 1. Objective function to be maximized or minimized f(x) objective function 2. Equality and inequality constraints h(x) = 0 equality constraints g(x) ≥ 0 inequality constraints

(a) (b) (c)

where x is a vector of n variables (x1, x2, . . . , xn), h(x) is a vector of m1 equations, and g(x) is a vector of m2 inequalities. The total number of constraints is m = (m1 + m2). 1. The objective function expresses the optimization criterion (C) as a function of the dependent and independent variables (x1, x2, . . . , xn) and may be represented by: C = f(x1, . . . , xn)

optimization criterion function

(d)

The equality and inequality constraints are provided by appropriate models as well as by appropriate boundary conditions. These models usually may be a cost function, a benefit or profit function, or functions associated with material and energy balance with engineering design. The models also contain equations and inequalities that specify the allowable operating ranges, the maximum or minimum performance requirements, and the bounds on the availability of resources. System models may be linear or nonlinear (set of algebraic, differential, or partial differential equations), static (focused on steady-state operation), or dynamic (focused on transient state), deterministic (not allowing variations), or stochastic (or probabilistic with time-varying parameters). Mathematical models exist for some food processes; for the rest they must be developed based on relevant engineering and scientific knowledge. Their complexity varies depending on the application, so in some food systems model development may be difficult or impossible. Then, an approximating function is developed as closely as possible to the system by performing an appropriate experimental design with limited number of experiments and by model-fitting techniques. 2. The constraints, which in most optimization problems may limit the values of dependent or independent variables and consequently the region of allowable solutions, must be identified. Constraints may be imposed by the particular characteristic of the system or represent external restrictions (equipment, legal, etc.). The system model must account for all the imposed constraints. Constraints restrict the values of the system variables. Constraints often are classified as being either equality or inequality constraints. The types of constraints involved in any given problem are determined by the physical nature

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of the problem and by the level of complexity used in forming the mathematical model. Constraints may be rigid, referred to as physical variables (restricted to non-negative), government regulations, or customer requirements, etc., or may be soft-negotiable to some degree (soft constraints). The former are viewed as absolute goals, while the latter as goals associated with target values. Functional constraints: These are physical principles of operation, which govern the relationship among the various independent parameters (x1, x2, . . . , xn) of the problem and are represented by equations: fci = fi(x1, x2, . . . , xn)

(e)

Regional Constraints: These are practical limits on the range in which each parameter (xi) or function of the parameters can be varied and are expressed as inequalities: l1 ≤ ri(x1, x2, . . . , xn) ≤ l2

(f)

In an optimization problem, functional constraints can be used to eliminate the number of independent parameters. Normally, the number of functional constraints must be less than the number of independent parameters, while there can be any number of regional constraints. Feasible solutions: After the constraints are established, the existence of feasible solutions for the given problem (points or region) that simultaneously satisfy all of the constraints must be examined, whereas the soft constraints may be relaxed in order to minimize the deviations from goals. Evaluation of the model: This is carried out according to the evaluation criteria established in the problem definition and sensitivity testing of the model inputs and parameters. Use actual data, which may entail statistical analysis of the fitted parameters, in the model when possible. Fitting functions to empirical data: A model relates the output, i.e., the dependent variable(s) to the independent variable(s). Each equation in the model usually includes one or more coefficients that are presumed constant. The term parameter as used here will mean coefficient and possibly input or initial condition. With the help of experimental data, the form of the model can be determined, and subsequently (or simultaneously) the value of some or all of the parameters in the model estimated. Model validation: This consists of validation of model assumptions and model behavior by comparison with historical input–output, literature, and performance data and simulation. In general, data used in formulating a model should not be used to validate it. No single validation procedure is appropriate for all models. The model should predict the desired features of the process performance with suitable accuracy.

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3. Performing of Optimization—Attaining the Optimum During the final step of the problem optimization process—attaining the optimum—the optimization must be performed by the appropriate selected method (5). Optimization methods have as scope to adjust, re-adjust, and locate the values of the decision variables that optimize (maximize or minimize) the performance function, while insuring that all variables (dependent or independent) satisfy the constraints of the system. The already developed model of the system usually dictates the approach of optimal solution. Such approaches are: unconstrained optimization, constrained optimization with a single performance measure or multiple performance measures, and optimization of systems that evolve over time (dynamic optimization). Assigning of performance measures: The performance measures that are to be optimized must be assigned (6). The selection of meaningful performance measures is critical for the optimization results. Usually, one of the performance measures is assigned as a target and the remaining are converted into soft constraints. Performance measures are incorporated into the optimization process either by contacting actual experiments or using numerical searching techniques. Solution of optimization problems: The reliability of the solutions of optimization problems depends on well-defining the problems. In complex problem optimization, initial considerations may be primarily based on the optimizer’s judgments of the relevant parameters until a clearer configuration of the problem parts can be defined, together with the relevant criteria, parameters, and constraints. Procedures for defining competing systems or alternate strategies and formulating the problem in order to apply formal optimization techniques are also common to operations research, systems analysis, and systems design. The general formal optimization problem is formulated in terms of the three previously referred equations: the criterion function, and the functional and regional constraints. The techniques used for solving the optimization problem depend on the complexity of the optimization equations. Detailed treatment of the various techniques for solving of equations represent the literature about optimization. Consequently, the formulation and solution of an optimization problem involves the establishment of an evaluation criterion based on the problem objectives, followed by the determination of the optimum values of the controllable or independent parameters that will best satisfy the evaluation criterion. It is accomplished either by performance of measures or analytically. In most optimization problems where there are conflicting criteria a compromise must be done weighting their relative value.

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III. OPTIMIZATION METHODS A mathematical theory of optimization has been developed and has found application in a variety of engineering situations (1, 2, 5, 9, 10). The development of the digital computer allowing rapid numerical calculations has made the utilization of optimization procedures practical in many design situations. Mathematical programming techniques are useful in finding the minimum of a function of several variables under a prescribed set of constraints. Stochastic process techniques can be used to analyze problems described by a set of random variables having known probability distributions. Statistical methods enable one to analyze the experimental data and build empirical models to obtain the most accurate representation of the physical situation. An optimization or a mathematical programming problem can be stated as follows:

再冎 再冎

x1 x2 find X = ⯗ which minimizes or maximizes f(X) xn in unconstrained optimization problems or x1 x2 find X = ⯗ which minimizes or maximizes f(X) under the constraints: xn hj(X) ≤ 0, j = 1, 2, . . . . . , m gj(X) = 0, j = 1, 2, . . . . . , p in constrained optimization problems, where X is an n-dimensional vector called the design vector and f(X) is termed the objective function; in the case of constrained problems hj(X) and gj(X) are known as inequality and equality constraints, respectively. The minimizing/maximizing point or minimizer/maximizer is denoted by x*. A. Basic Theoretical Background (1, 2, 9, 10) In any engineering system the design vector, defined during the design process by a set of variables, represents the selected design variables as X:

再冎

x1 x2 X = ⯗ where xi are the design variables i = 1, 2, . . . , n. xn

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A vector is also represented by x and usually refers to a column vector. A matrix is referred as X and its elements as Xij or xij. A column vector of n variables is also represented in n-dimensional space Rn by: x = [x1, x2, . . . , xn]T, where the superscript T signifies the transpose of a row vector to form the column vector x. In the n-dimensional space (Rn), each coordinate axis represents a design space (or design variable space). A point x in n-dimensional space is the vector (x1, x2, . . . , xn)T, where xi is the component in the i-coordinate direction. For a given function f(x) of n variables, points at which f(x) assumes local maximum and local minimum values are of interest. Each point in the n-dimensional design space, called design point, represents either a possible or an impossible solution to the design problem. Optimization methods by iterative processes generate a sequence of points x(k) or {x(k)}(where k is the iteration number) intending to find x*, the solution of the problem. In general, it is assumed that the problem functions are smooth, continuous and continuously differentiable (C1). Therefore for a function f(x) at any point x, there is a vector of first partial derivatives, or gradient vector.

再 冎

∂f/∂x1 ∂f/∂x2 䉮f(x) = ⯗ ∂f/∂xn

where 䉮 denotes the gradient operator (∂f/∂x1, ∂f/∂x2, . . . . . . , ∂f/∂xn)T. If f(x) is twice continuously differentiable (C2), then there is a matrix of second partial derivatives 䉮2f(x) with elements ∂2f/(∂xi ∂xj) that is called Hessian matrix and denoted by H(x). This matrix is square and symmetric. Since any column is 䉮(∂f/∂xj), the matrix can be written as 䉮(䉮fT). Special cases of many variable functions include the general linear function, which can be written as: n

l(x) = ∑ aixi + bi = aTx + b i=1

where a and b are constants. The general quadratic function, can be written as: q(x) = 1⁄2xTGx + bTx + c where G, b, and c are constant and G is symmetric. The basis for all optimization methods is the classic theory of maxima and minima. Mathematically, the theory concerned with finding the minimum or maximum (extreme points) of an unconstrained function of n variables f(x1, x2, . . . , xn) that can be interpreted geometrically as finding the point in an ndimension space at which the function has an extremum. An optimal point x*

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is completely specified by satisfying the necessary and sufficient conditions for optimality. Following the desirable features of functions, as well as the necessary sufficient conditions to guarantee the defined extremum (minimum or maximum), are presented. The knowledge of the basic properties of objective functions and constraints is considered necessary. It is preferable and more convenient to work with continuous functions of one or more variables as well as with functions having continuous derivatives. Then the extreme points may lie at the stationary points and consequently a necessary condition for a minimum or maximum is: f′(x) = 0 or 䉮f(x) = 0 (except in case of saddle points). Hence, in order to locate the points where the partial derivatives are zero, the solving of n-algebraic equations should be done: ∂f/∂xj (x1, x2, . . . , xn) = 0 (j = 1, . . . , n). The equations can be solved directly or in some cases an approximate solution can be obtained by minimizing the sum of squares of the residuals (least squares method). It is also better to work with a unimodal function that has a single extremum (minimum or maximum); that is, a stationary point. Multimodal function has two or more extrema (maximum or minimum); these are multiple stationary points. The global extremum is the biggest or smallest among a set of extrema; local extrema may be significant in practical optimization problems involving nonlinear functions. Sufficient conditions for a unique (isolated) or extremum to exist are: f″(x) = 䉮2f (x) = H(x) > 0 for unique (global) minimum f″(x) = 䉮2f (x) = H(x) < 0 for unique (global) maximum f″(x) = 䉮2f (x) = H(x) = 0 neither maximum nor minimum (inflection point). The determination of convexity or concavity is helpful to establish if a local optimal solution is also a global optimal solution. A convex region is useful in optimization involving constraints. Equality constraints limit the feasible set of points on hyper-surfaces, curves, or even a single point, while inequality constraints specify a feasible region comprised of the set of points that are feasible. To simplify the above, the case of a function of one variable x is assumed. A point x* is called local minimum if f(x*) ≤ f(x + h) for all sufficiently small positive and negative values of h. Similarly, a point x* is called a local maximum if f(x*) ≥ f(x + h) for all values of h close to zero. A point x* will be a global minimum at x* if f(x*) ≤ f(x) or a global maximum of f(x) if f(x*) ≥ f(x) respectively for all x, and not just for all x close to x*, in the domain over which f(x) is defined. Examples of local and global optimum points are presented in Fig. 4. Local minimum or maximum can be located at the boundaries or at points at

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Figure 4 Types of stationary points of a function. A, inflection point (scalar equivalent to a saddle point); B, global maximum; C, local minimum; D, local maximum.

which the first derivative (f′) is zero or discontinuous (stationary points). One of the local minimum/maximum can be a global minimum/maximum. Points whose first derivative is zero, may be neither a maximum nor a minimum, but a mini-max, or better known as a saddle point and movement away form it will result in an increase or decrease on f(x) depending on the direction of the movement. Difficulties are caused when f(x) is a non-smooth function as its minima do not satisfy the same conditions as smooth minima. B. Basic Optimization Methods Many methods have been developed for the efficient solution of optimization problems; there is no general mathematical method for conducting the search for the optimal value. The optimization methods can be categorized according to: (a) the nature of the objective function; (b) the constraints; and (c) the decision variables involved. The objective function may: 1. Contain only a single decision variable (single or one-dimensional optimization) or many decision variables (multidimensional optimization) 2. Be continuous or contain discontinuities, and 3. Be linear or nonlinear (linear programming—LP or nonlinear programming—NLP) 4. Be one or more than one (single or multi-objective programming) Constraints may or may not exist in a problem and constrained or unconstrained optimization are the two main categories. The constraints may be expressed as linear or nonlinear equations or inequalities.

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Optimization

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The decision variables may be continuous, integers, or a combination. Depending on the nature of the design variables static or dynamic optimization methods may exist. Also, based on the deterministic nature of the variables, deterministic, or stochastic programming is classified. Graphical methods involve a procedure of finding a maximum or minimum point of the objective function by the graphical plotting of the objective function values. These methods are elementary, have reduced accuracy and are usually applied in the case of one or two-variable functions. Methods may be indirect or direct. Indirect methods determine extremum (a) by using derivatives and values of the objective function. A necessary condition for an extremum point of a differentiable objective function is that the point is a stationary point. Direct methods search for an extremum by directly comparing function values at a sequence of trial points without involving derivatives. These methods generally can more easily treat problems involving objective functions with discontinuities, points of inflection, and boundary points, while computers have enabled their application. Numerical methods are search techniques that have been developed to determine maxima and minima of functions. The objective function is computed at a starting set of the independent variables. A second set is then selected and the comparison of the new value of the objective function with the initial one indicates if the objective function is improving toward an optimum. Searches are simultaneous, when all sets of evaluation values are preselected, or are sequential when new sets of data are selected based on information from the previous sets of data. In applying numerical optimization methods, their efficiency should be taken into account before selecting the appropriate optimization method. When the objective function is continuous and continuously differentiable and not near the region limits, the optimization may be done analytically. This implies solving a set of differential equations of the first derivatives of the objective function with respect to each independent variable and following the usual mathematical procedures for maxima and minima. Additional mathematical procedures include the use of Lagrange multipliers and variational calculus. Given that a function and its derivatives are continuous, a method that exhibit quadratic convergence may be best; such methods can locate the exact maximum/minimum of a quadratic function (assuming it has a well-defined optimum) in a finite number of calculations. Gradient-based methods are search techniques that use derivatives; the gradient vector 䉮f(x) is central to these methods. At each point the gradient vector is perpendicular to the contour of the function and points in the direction of the greatest incremental increase in f(x). The gradient search works as follows: (a) evaluate a search direction r = 䉮f(x) at the current best x = x0; (b) let x = x0 + νr, where ν is a real scalar; (c) search for a maximum of f with respect to values of ν ≥ 0; and (d) reassign the current best x and return to step (a). This

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method is simple, however is inefficient when the contours are elongated or irregular in any way. Following, the usual cases of a single-variable function or a multivariable function with no constraints, and a multivariable function with equality and inequality constraints are presented. Also, some commonly used techniques applicable to these types of optimization problems are briefly discussed (1, 2, 5, 9–12). 1. Unconstrained Optimization Unconstrained optimization methods are applicable when searching for a minimum or maximum of a function that is not subject to any constraints. Unconstrained optimization may be single or one-dimensional or multi-dimensional optimization. Single or One-Dimensional Unconstrained Optimization. This is the most elementary type of optimization problem; the function f(x) has only one independent variable. The techniques applicable to this type of problem are important because some techniques applicable to multivariable functions involve repeated use of a single-variable search. To develop the necessary and sufficient conditions for a minimum or maximum of a function f(x), a Taylor series expansion (with n terms) about the presumed extremum x* can be performed. f(x* + h) = f(x*) + hf′(x*) +

h2 f″(x*) + . . . . + 2!

hn-1 (n-1) hn f (x*) + f (n)(x* + θh) (n-1)! n! If f′(x*) = f″(x*) = . . . . = f then f (x* + h) − f(x*) =

(n-1)

for 0 < θ < 1

(x*) = 0, but f

(n)

(1)

(x*) ≠ 0

hn (n) f (x* + θh) n!

(2)

f(x*) is a minimum if f (x*) > 0 and n is even, f(x*) is a maximum if f (x*) < 0 and n is even, and neither a minimum nor a maximum (inflection point) if n is odd. For example when n = 2, given that f′(x*) = 0 at the stationary point, the higher order terms are negligible compared to the second-order terms and hence equation becomes: h2 f(x* + h) = f(x*) + f″(x*) 2! (3) (n)

(n)

Then, the nature of f(x*) depends on the value of f″(x*) as already referred. That is at x* exists a minimum if f″(x*) > 0, a maximum if f″(x*) < 0 or an inflection point if f″(x*) is indefinite.

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Optimization

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Multidimensional Unconstrained Optimization. The theory for the minimum or maximum applied in one-dimensional optimization is generalized and extended in the case of a function of n independent variables. Therefore, for the necessary and sufficient conditions for minimum or maximum of an unconstrained function of several variables, the Taylor’s series expansion of a multivariable function about a point X* is: f(X) = f(X*) + d f (X*) +

1 2 d f (X*) + . . . . + 2!

1 N d f(X*) + RN (X*, h) N!

(4)

where the last term, called the remainder, is given by: RN (X*, h) =

1 dN+1f(X* + θ h) (N + 1)!

(5)

where 0 < θ < 1 and h = X − X*. It must he noted that the rth differential of f(x) at X* (partial derivative of r order) is the polynomial: n

n

n

d f(X*) = ∑ ∑ ⴢ ⴢ ⴢ ∑ hihj . . . hk r

i=1 j=1

k=1

∂rf(X*) ∂xi∂xj ⴢ ⴢ ⴢ ∂xk

(6)

r summations

The necessary condition for f(X) having an extreme point (maximum or minimum) at X = X* is the first partial derivatives of f(X) to exist at X* and to be 0. ∂f ∂f ∂f (X*) = (X*) = . . . . = (X*) = 0 ∂x1 ∂x2 ∂xn

(7)

A sufficient condition for a stationary point X* to be an extreme point depends on the nature of the matrix of second partial derivatives (Hessian ma∂f trix) of f(X*). This results to the quantity Q (given that (X*) = 0 for ∂xi ∂2f i = 1, . . . , n and second partial derivatives of (X*) are continuous in the ∂xixj vicinity of X*): n

n

Q = ∑ ∑ hihj i=1 j=1

where J



x = x*

=

冋

∂2f(X*) = hT J h* x=x* ∂xi∂xj

∂2f (∂xi ∂xj)



x = x*

册

(8)

is the Hessian matrix of f(X). Consequently, if

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J*x = x* > 0 then X* is a relative minimum point, and if J*x = x* < 0 then X* is a relative maximum point. In case that the Hessian matrix is semidefinite (but not definite), the stationary points should be investigated for sufficiency in actual practice. However, in the general case where the partial derivatives of f of all orders up to the order k ≥ 2 are continuous in the vicinity of a stationary point X*, and dk-2*x = x* = 0 dk*x = x* ≠ 0 (the first nonvanishing higher-order differential of f at X*) then, if k is even, when dk*x = x* > 0, X* is a relative minimum point, when dk*x =x* < 0, X* is a relative maximum point, and when dk*x = x* is semidefinite (but not definite), no general conclusion can be drawn (X* is not an extreme point, if k is odd). In the case of a function of two variables f(x,y), the Hessian matrix may ∂f ∂f be neither positive nor negative definite at a point (x*,y*) at which = = 0. ∂x ∂y That point is a saddle point. Unconstrained nonlinear multivariable optimization. The unconstrained nonlinear programming methods used for multivariable optimization are iterative procedures in which the following two steps are repeated: (a) starting from a given point choose a search direction and (b) minimize or maximize in that direction to find a new point. These methods mainly differ in how they generate the search directions. Direct/indirect methods. Direct methods for single variable functions include the region elimination methods, the two-point equal interval search, the bisecting method (or dichotomous search), the Fibonacci method, and the golden section method; the last two methods are considered the most efficient. The point estimation methods, or polynomial approximation methods usually involve a quadratic or cubic approximation of the objective function and for this Powell’s method is considered the most efficient. For multivariable functions, direct methods include the random search, the grid search, the univariate search, the sequential Simplex method, the HookeJeeves pattern search method, and Powell’s conjugate direction method. These methods are relatively simple to understand and execute. However, they are not as efficient and robust as many of the indirect methods. The indirect methods include the steepest descent/ascent gradient method (or Cauchy’s method), the conjugate gradient methods, Newton’s method, Marquardt’s method, the Secant methods, and the Broyden-Fletcher-GoldfarbShanno (BFGS) method. 2. Constrained Optimization Constrained optimization methods are applicable to locate stationary points of a function, but the solutions are subject to equality or inequality constraints.

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Optimization

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Optimization of the function must be carried out over a restricted domain of the independent variables. Optimization of Systems with Equality Constraints. Constrained optimization of a continuous function f(x1, x2, . . . , xn) of n independent variables subjected to m equality constraints gi(x1, x2, . . . . , xn) (i = 1, 2, . . . , m) will be considered. The problem is defined when m ≤ n. Three methods are mainly used for the solution of such problems: direct substitution, constrained variation, and Lagrange multipliers. Direct substitution methods involve the substitution of the m constraint equations directly into the objective function. The resulted objective function is not subject to any constraint and hence its optimum can be found by applying the unconstrained optimization techniques. This method is suitable for solving simpler problems, but cannot be applied to many practical problems due to nonlinearity of the constraints equations. Constrained variation methods in the general case of n variables with m constraints scope in finding an expression for the first-order differential of f (df) at all points where the constraints are satisfied. Each constraint equation is expressed as a linear equation of the variations dxi, i = 1, 2, . . . , n (m equations of n variations). Each one of m variations as well as df are expressed by the remaining n-m variations. The optimum points are obtained when df = 0. ∂f ∂f dx1 + . . . . . . . + dxn = 0 ∂x1 ∂xn ∂g ∂g dg1 = 1 dx1 + . . . . . . + 1 dxn = 0 ∂x1 ∂xn ............ ∂g ∂g dgm = m dx1 + . . . . . . + m dxn = 0 ∂x1 ∂xn df =

(9)

Hence, the necessary conditions for the extremum of f(X) are given by the n-m equations (Jacobian determinants):

J

冉

冊

f, g1, g2, . . . , gm = xk, x1, x2, . . . , xm

∂f ∂xk ∂g1 ∂xk ∂g2 ∂xk ⯗ ∂gm ∂xk

∂f ∂x1 ∂g1 ∂x1 ∂g2 ∂x1 ⯗ ∂gm ∂x1

∂f ∂x2 ∂g1 ∂x2 ∂g2 ∂x2 ⯗ ∂gm ∂x2

where k = m + 1, m + 2, . . . . . , n.

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∂f ∂xm ∂g1 ... ∂xm ∂g2 ... ∂xm O ⯗ ∂gm ... ∂xm ...

=0

(10)

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Similarly, in the case of two independent variables x1, x2 where the function f(x1,x2) is subject to the constraint g(x1,x2), a necessary condition for f to have an extreme point at (x1*,x2*) is: df =

∂f ∂f dx1 + dx2 = 0 ∂x1 ∂x2

(11)

dg =

∂g ∂g dx1 + dx2 = 0 ∂x1 ∂x2

(12)

or by combining the equations:

冉

冊

∂f ∂g ∂f ∂g − ∂x1 ∂x2 ∂x2 ∂x1

=0

(13)

(x1*, x2*)

It is noted that the variations dx1, dx2 about the point (x1*,x2*) are called admissible variations. A sufficient condition for X* to be a constrained relative extremum is resulted by the Taylor series expansion of f, in terms of n-m variables (xm + 1, xm + 2, . . . , xn) about the extremum point X*: n

f(X* + dX) ⬇ f(X*) +

∑

i=m+1

where

冉冊

n

n

冉 冊

∂f 1 ∂2f dxi + dxidxj ∑ ∑ ∂xi 2! i=m+1 j=m+1 ∂xi∂xj g

(14)

g

冉冊 冉 冊

∂f ∂2f and are the first and second partial derivatives of f with ∂xi ∂xi∂xj g

g

respect to xi and xi xj (holding all the n-m variables constant) respectively, when x1, x2, . . . , xm are allowed to change so that the constraints gj (X* + dX) = 0, j = 1, 2, . . . , m, are satisfied. The quadratic form Q defined by: n

Q=

n

∑ ∑

i=m+1 j=m+1

冉 冊

∂2f dxidxj ∂xi∂xj

(15)

g

or the matrix:

冤

冉 冊冉 冉 冊冉

冊 冉

冊

∂2f ∂2f ∂2f ⴢ ⴢ ⴢ ∂xm+1∂xn ∂x2m+1 g ∂xm+1∂xm+2 g ⯗ ⯗ O ⯗ ∂2f ∂2f ∂2f ⴢⴢⴢ ∂xn∂xm+1 ∂xn∂xm+2 ∂x2n g g g

冊

冉 冊

g

冥

(16)

depending on their positive or negative value for all nonvanishing variations

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Optimization

159

dxi are determinant for the optimum point X*. This method involves difficult computations as the constraints are more than two. Lagrange Multipliers method is the most commonly used method in constrained optimization problems with equality constraints. The Lagrange function L, for a problem of n variables and m equality constraints is defined for each constraint gj (X) as: L (x1, x2, . . . , xn, λ1, λ2, . . . , λm) = f (X) + λ1 g1 (X) + λ2 g2 (X) + . . . . . . + λm gm (X)

(17)

where the quantities λj are called Lagrange multipliers. The necessary conditions for the extremum of L are produced by the solution of a system of n+m equations in terms of the unknowns, xi and λj, and are given by: m

∂L ∂f ∂g = + ∑λj j = 0, i = 1, 2, . . . , n ∂xi ∂xi j=1 ∂xi

(18)

∂L = gj (X) = 0, j = 1, 2, . . . , m ∂λj

(19)

From the solution, the relative constrained extremum X* and the λ* result. It must be noted that the vector λ* of Lagrange multipliers is a sensitivity factor that indicates how tightly the constraint is binding at the optimum point. A sufficient condition for f(X) to have a relative extremum at X* is determined by the sign of the quadratic form Q: n

n

Q=∑∑ i=1 j=1

∂2L (X*,λ*) dxidxj ∂xi∂xj

(20)

or by the sign of the roots of the polynomial zi:



L11−z L21−z ⯗ Ln1−z zi = g11 g21 ⯗ gm1

where Lij =

L12 L22 ⯗ Ln2 g12 g22 ⯗ gm2

ⴢ ⴢ ⴢ L1n ⴢ ⴢ ⴢ L2n O ⯗ ⴢ ⴢ ⴢ Lnn−z ⴢ ⴢ ⴢ g1n ⴢ ⴢ ⴢ g2n O ⯗ ⴢ ⴢ ⴢ gmn

g11 g21 ⯗ g1n 0 0 ⯗ 0

g21 g22 ⯗ g2n 0 0 ⯗ 0

∂2L ∂g (X*, λ*), gij = i (X*) ∂xi xj ∂xj



ⴢ ⴢ ⴢ gm1 ⴢ ⴢ ⴢ gm2 O ⯗ ⴢ ⴢ ⴢ gmn ⴢⴢⴢ 0 =0 ⴢⴢⴢ 0 O ⯗ 0 0

(21)

(22)

So, if Q or each root of zi is positive or negative for all values of dX, X* will be a constrained minimum or maximum respectively. It must be noted that the

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point X* is not an extreme point if all of the roots of zi have not the same sign. The solution of a simple two-dimensional problem by Lagrange multipliers method is: L(x1, x2, λ) = f(x1, x2) + λ g(x1, x2)

(23)

Then, the necessary conditions for the extremum X* (x1, x2) are given by: ∂L ∂f ∂g (x1, x2) = (x1, x2) + λ (x1, x2) = 0 ∂x1 ∂x1 ∂x1

(24)

∂L ∂f ∂g (x1, x2) = (x1, x2) + λ (x1, x2) = 0 ∂x2 ∂x2 ∂x2

(25)

∂L (x1, x2, λ) = g (x1, x2) = 0 ∂λ

(26)

Optimization of Systems with Inequality Constraints. Constrained optimization subject to inequality constraints is treated as in the case of equality constrained problems after transformation of inequality constraints to equality ones by introducing the slack variables and solving the so formed constrained equations to define the feasible region and find the optimum solution. A usual problem of this category is the linear multivariable optimization with inequality constraints. For the general case of a function f (X) of n independent variables subject to m constraints gj(X) ≤ 0 (j = 1, 2, . . . , m), the constraints are transformed to equality ones as: gj(X) + yj2 = 0 (j = 1, 2, . . . , m)

(27)

where yi are non-negative slack variables. Hence, the problem becomes the finding of the extremum of f(X) subject to: Gj(X, Y) = gj(X) + yj2 = 0 (j = 1, 2, . . . , m)

(28)

where Y is the vector of slack variables and can be solved by using the previously referred Lagrange multipliers method as: m

L(X, Y, λ) = f(X) + ∑ λjGj(X,Y)

(29)

j=1

where λ is the Lagrange multipliers vector. Then, applying the necessary conditions for the stationary points, the following (n + 2m) equations of (n + 2m) unknowns result:

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Optimization

161 m

∂L ∂f ∂g (X, Y, λ) = (X) + ∑ λj j (X) = 0, i = 1, 2, . . . , n ∂xi ∂xi ∂xi j=1

3.

(30)

∂L (X, Y, λ) = Gj(X, Y) = gj(X) + yj2 = 0, j = 1, 2, . . . , m ∂λj

(31)

∂L (X, Y, λ) = 2 λj yj2 = 0, j = 1, 2, . . . , m ∂yj

(32)

Optimization of Dynamic Systems

A dynamic system is characterized by time- or space-dependent behavior and variety by time or distance. The problems are called sequential decision problems or multistage decision problems as the decisions are to be made sequentially or at a number of stages respectively. Many dynamic systems are encountered in chemical engineering and food engineering (fermentations or thermal processes of foods). For the optimization of multistage decision problems, dynamic programming techniques are used. Dynamic programming methods decompose an optimization problem into a sequence of subproblems that can be solved serially. Each of the subproblems may contain one or few decision variables. A serial multistage decision process of n stages in decreasing order is represented schematically in Fig. 5. Each single-stage i decision process can be characterized by an input (input state parameter) (Si+1) which is the output of i + 1 stage, a decision variable (Xi), an output (output state parameter) (Si) and a return or objective function (Ri). The output is obtained as a result of making the decision, while the return depends on the effectiveness of the decision made and the output that results from the decision. The state transformation functions

Figure 5 Dynamic process of n stages.

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or design equations relate input to output and the return functions are represented as: Si = ti(Si+1, xi)

(33)

Ri = ri(Si+1, xi)

(34)

where xi is the vector of decision variables at stage i. The objective of dynamic optimization is, given the values of variables of the initial state or final state or both of the stages (boundary problem), to find the continuous decision control variable(s) that optimize some function of the individual stage returns. That is to find x1, x2, . . . , xn that optimize the objective function f(R1, R2, . . . , Rn), mathematically expressed by: fn(Sn) = opt [Rn(Sn, Xn) + fn-1(Sn-1)] (35) The functions of a system may differ in nature (i.e., differential). In order for a multistage problem to be solved by dynamic programming applying the decomposition technique, the objective function must be monotonic and separable, satisfying the requirements: n

n

i=1 n

i=1 n

f = ∑ Ri = ∑ Ri(xi,Si+1) f = Π Ri = Π Ri(xi,Si+1) i=1

(36)

i=1

where xi are real and non-negative. 4. Programming Optimization Linear programming (LP) is an optimization method applicable to problems in which the objective function and the constraints (equalities or inequalities) appear as linear functions of the design/decision variables. Simplex method of Dantzig is the most efficient and popular method for solving general LP problems, although other superior methods as Karmarkar’s have been developed. It is considered suitable for complex problems solution. In case there is a large number of variables and/or constraints, the decomposition principle can be used to solve the problem, while Karmarkar’s method has been shown more efficient than the simplex method in this case. Quadratic programming deals with problems that have a quadratic objective function, linear constraints and can be solved by suitably modifying the linear programming techniques. If the objective function and the constraints are fairly simple, expressed in terms of the design/decision variables, the classical methods of optimization can be used to solve the problem. But classical analytical methods can not be

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used if the expressions are not stated as explicit functions or they are too complicated to manipulate. Several methods are available for solving an unconstrained nonlinear optimization problem. The direct search (nongradient) methods require only the objective function values but not partial derivatives of the function in finding the optimum. The direct search (zero-order) methods use zero-order derivatives of the function. Direct methods are most suitable for simple problems involving small number of variables. The descent (gradient) techniques require, in addition to the function values, only first derivatives of the function (first-order methods) or both first and second derivatives of the function (second-order methods). Descent methods are generally more efficient than direct search techniques. There are many techniques available for the solution of a constrained nonlinear programming problem. In the direct methods, the constraints are handled in an explicit manner, whereas in most of the indirect methods, the constrained problem is solved as a sequence of unconstrained optimization problems. Geometric programming (GP) is a method of solving of nonlinear programming problems, where the functions and the constraints are in the form of polynomials. It places emphasis on the relative magnitudes of the terms of the objective function rather than the variables; instead of finding optimal values of the design/decision variables first, it first finds the optimal value of the objective function. Geometric programming is especially advantageous in cases that the optimal value of the objective function is of interest, as well as in optimization of a complicated problem reducing it to one involving a set of linear algebraic equations. Many problems in plant design and operation involve variables that are not continuous but instead have integer, discrete, or fractional values. If an integer solution is desired, it is possible to use any of the common techniques and round off the optimum values of the design variables to the nearest integer values. Integer programming (IP) refers to problems where all of the design/decision variables are restricted to be integers. A special case of IP is binary integer programming (BIP) (or zero-one programming), where all variables are either 0 or 1. Discrete programming refers to cases of problems where the variables are restricted to take only discrete values. In mixed integer programming (MIP), some of the variables are restricted to be integers while others may be continuous (fractional values). Many IP problems are linear in the objective function and constraints, hence are subject to solution by linear programming (MILP). Stochastic or probabilistic programming deals with situations, where some or all optimization problem parameters are described by stochastic (or random or probabilistic) variables. The stochastic problem is converted into an equivalent deterministic one that can be solved by familiar techniques (linear, geometric, dynamic, and nonlinear programming).

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5. Experimental Optimization—Response Surface Optimization (RSM) In some problems when the behavior of the system or process is not known and a model cannot be developed or the model developed is very complex, experimental optimization methods are used. Experiments are designed with appropriate methods and empirical models based on the experimental data result. The optimum is then approached by using statistical techniques, the most known is the response surface method (RSM), which is widely used in food applications. Assuming that the response of n independent variables is a function of the levels and combinations of these variables, response surfaces that result provide insight into the overall behavior and show the existence of optimal regions.

IV. OPTIMIZATION IN FOOD EXTRACTION PROCESSING A. Application of Optimization in Food Engineering Optimization has been applied in several areas in food engineering, and various optimization methods have been used both in processing and manufacturing. Areas of such applications are individual equipment or total system design, process design, as well as design, layout, control, management, and operation of the total plant. Following, more details for all these application areas of optimization approaches are presented. A selection of practical engineering perspectives referring to plant operation and optimization has been edited (13). Mathematical programming (linear, nonlinear, integer, combinatorial) and modeling techniques for various industrial applications relating to manufacturing, operation, decision-making, production scheduling, or management have been also presented (14). Especially, optimization in design and control of chemical engineering processes approached by global optimization methods (deterministic or probabilistic) and relative applications have advanced (15). Flow-sheeting optimization during the early stages of design, applying global optimization to modular process design approaches provides many advantages (16). Genetic algorithms (GA) or evolutionary ones (EA) have been investigated in chemical engineering problems and have been proved to be powerful and robust optimizing techniques either for total processes and/or particularly for extraction processes (17, 18). Optimization resources and specific information about optimization are available on the Internet in the NEOS—Network Enabled Optimization Technology Center (OTC)—which is a joint enterprise of Argonne National Laboratory and Northwestern University (19). The design and optimization of thermal systems is of primary interest for food engineering as is strongly related to cost as well as to food quality. Thermal

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processing is commonly applied in chemical processing industries and hence thermal systems are often encountered into manufacturing plants. Thermal systems are necessary in food processing plants either in thermal processes of food (pasteurization, sterilization, blanching, cooking, cooling, freezing, etc.) or in other complementary operations as storage under cooling or freezing. Consequently, thermoeconomic optimization involving thermodynamic optimization and modification of the structure and design parameters of a thermal system, allows minimization of the total cost (optimization criterion) of the system satisfying technological or environmental constraints (20). As far as individual equipment is concerned, the design and optimization of a falling-film evaporator and the construction of a mathematical model for its operation, aiming at both minimizing the overall operating cost (by minimizing water removal cost) and maintaining product quality of a large-scale production of concentrated apple juices has been cited (21). Optimization has found application in various processes widely used in foods, as in dehydration, heat processing, and formulation of foods. Examples of such areas where optimization techniques have been successfully used are presented. In thermal processing of foods, the optimization problem generally consists in the determination of processing conditions, the combination of process time and retort temperature, ensuring the constraint of required safety level but simultaneously minimizing the associated quality degradation (or maximizing product quality) (22). In classic problems of thermal processed canned foods, kinetic models both of bacteria spores’ inactivation and nutrients (i.e., thiamine) degradation have been proposed as basis of optimization. Optimization methods require studies of time–temperature dependent kinetics for each of these factors, or studies for finding the optimum temperature profile or the optimum container geometry of the thermally processed foods. Direct search by trial-and-error, mathematical algorithms and dynamic programming approach the optimization solution. A proper optimization technique is based on the Pontryagin’s maximum principle that searches out the optimum surface temperature as a smooth, continuous function of time, which maximizes the objective function (4). Furthermore for the design and optimization of thermal processing of foods, powerful dynamic models and techniques have been developed suitable and efficient for real-time industrial applications (optimization and control), i.e., for thermal sterilization (23, 24). As far as the food dehydration is concerned, it is related to dynamic optimization problem and the scope is to find an optimum temperature profile that would result in maximum nutrients retention (i.e., ascorbic acid) achieving the desired moisture content, given a specific drying time, while constraints for the final moisture or nutrient content of the dehydrated product may exist. Mathematical models describing the drying process of foods in accordance with the

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equipment used have been developed, while optimization techniques may be similar of those applied in thermal processing optimization that consider either continuous (Pontryagin maximum principle) or discrete-step optimum temperature profile (4). Optimization is usually applied in food products formulation, as a formulation process must meet a set of specifications for nutrient levels, be costeffective, and is subject to constraints, i.e., for combined weight or certain nutrient levels. Such problems may be solved by simple mathematical optimization technique as linear programming (25). Optimization of the nutritional quality of formulated foods in respect to different nutrients proportions has been investigated by evaluating the effect of the component proportion in the final product nutritional quality. Therefore, it is feasible to maximize the nutritional quality of mixtures by obtaining optimum formulation or by substituting a component with another in the product formula. A usual problem is the formulation of protein mixtures intended to satisfy human requirements with respect to essential amino acids. The optimization in nutritional experiments is very significant and particularly in protein mixtures problems, because the protein quality is evaluated in vivo and it is possible to express synergism effects due to complementarities of essential amino acids. Hence, the maximum nutritional quality is desired during product development, either by changing the concentrations of the protein ingredients (soybean protein products, milk proteins, whey proteins, etc.) or by the substitution of an animal origin protein by another (vegetable or less costly) in a formulated mixture of known nutritional quality (26). In nutritional evaluation experiments the mixture response surface methodology proved to be an extremely useful tool for the optimization of the mixture’s protein quality. Moreover, the estimation of the nutrient values in commercial food products is very important for the human diet. They must be taken daily in certain quantities, some of them are essential for human health (amino acids, linoleic acid), others are functional components (dietary fibers). Many missing nutrients values for nutrients of interest are encountered in widely used foods’ composition data due to lack of accepted analytical methods for some nutrients, high cost of chemical analysis, and proliferation of commercial foods. The missing nutrient value estimation is performed by various methods of different accuracy. Mathematical optimization proves more accurate to estimate nutrient values of food product than other techniques used (i.e., trial-and-error); linear programming as well as quadratic programming are comparably efficient in accuracy (27). Sensory optimization of a food product is another application area where optimization methods have been used. The product ingredients affect many attributes of the product that determine its acceptability by the consumers. Hence, the best product formulation is found by evaluating multiple sensory attributes

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or responses of food, carrying different weights in the perception of the product, using graphical optimization methods (28). In product optimization problems special (classical or novel) sensory methodologies and techniques are used (29). Mathematical models with linear or quadratic equations created by experimental designs, show the relationships between independent variables (food formulas, processing conditions) and the responses (rating attributes). These models are useful for the prediction of the likely attributes, the maximization of the acceptance, or for optimization problems under certain constraints of formulation (30, 31). An example of optimizing acceptability is the case of low-calorie products containing alternative sweeteners. Following a central composite design, response surface methodology (RSM) (32) can select the optimum product formulation (optimal concentration of sweetener) that optimizes the sensory quality of the product (maximum acceptability). Also during development of new food products, combining experimental design of both product and concept generates optimal products and concept within constraints, such as cost, desired sensory profile limits, or targeted population (33). The quality optimization of thermally processed foods (i.e., cooked) and the relative optimization methods employed for these processes are of interest in food engineering. Commercial processes try to meet the requirements for microbiological safety, nutritive value, and sensory acceptance of foods. Scope of the methods is to determine the suitable processing conditions, especially the temperature profile (temperature–time of processing), which maximize quality factors of foods undergoing thermal processing (i.e., retention or degradation of nutrients, microbial inactivation, etc.) (34). Dried potato cubes are an example of thermally processed food, the quality of which is optimized. The process includes potato blanching in water containing pectin and polydextrose, followed by a two-step drying procedure by a batch-type high-temperature fluidized bed drier (HTFB) at varying conditions, and a tunnel drier at standard conditions. The objective is to find biopolymers concentration, time of blanching, as well as temperature and time of drying in HTFB that obtain the maximum of rehydration ratio, puffing and water holding capacity, and minimum of nonenzymatic browning (7). In such problems, RSM is usually the method used for manipulation of experimental data. Using the same technique, the extrusion of a high-protein snack has also been optimized. Maximum value of expansion ratio and sensory rating of extrudates and minimum value of shear strength was obtained by controlling the feed moisture and temperature at the central zone of the extruder barrel (7). The optimization framework of heat-sensitive foods’ drying process, being also essential for a food engineer, has been developed by combining the equipment and material models to attain the cost-effective product drying (with efficient energy saving), and minimizing its quality degradation. The potato slice is such a material that suffers from loss of quality relating to color, nutrients, taste

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and/or texture and therefore the optimal operation of the dryer equipment may both maximize heat recovery and minimize quality degradation (i.e., loss of ascorbic acid, nonenzymatic browning), satisfying simultaneously the imposing constraints (drying time, food moisture content, ascorbic acid content, and enzymatic browning degradation) (35). Frying of foods is another thermal process in which the frying temperature significantly affects the quality characteristics (expansion volume, expansion ratio, color, texture parameters) and the sensory ones (appearance, texture score, acceptance score) of the fried product. As continuous process is commonly used involving certain consecutive deep frying steps, the corresponding optimum temperature values can be approached based on the quality and sensory product characteristics. A popular and effective optimization method for such multivariate problems is response surface methodology (RSM) (36). Enzymatic synthesis processes of certain valuable compounds have increased interest for investigation of parameters affecting the enzyme-catalyzed synthesis. Enzymatic produced compounds are preferred over the chemical-synthesized products and are widely used in foods and beverages with many industrial applications, i.e., low-molecular-weight esters by lipase-catalyzed esterification are flavor compounds of commercial importance. Consequently the optimization of such enzymatic processes for economical syntheses can be attained by central composite rotatable experimental design (CCRD) using response surface methodology (RSM) to predict best performance conditions (37). Modeling, simulation, and optimization are nowadays particularly valuable due to widespread use of computers and are important approaches to a number of problems in the food industry. Linear programming can be used for simple problems such as the optimization of product quality determining the concentrations of certain components that affect its quality. Examples of industrial processes in which linear programming has been applied are referred. LP has been applied to optimize the fixed process arrangement in an apple juice concentration plant which includes: apple crushing, juice extraction, further pressing of the resulting pomace after rehydration, mixing of juice streams, aroma stripping (for aroma recovery), juice clarification and concentration. Optimal plant operation can maximize the net profit by arranging the operating flow rates and taking into account the effects of costs, prices, apple varieties, and the operational capacity constraints for equipment. Also, LP was utilized for quality optimization in aseptic processing, where considering the microbial and enzyme inactivation, cooking quality and component quality retention (thiamin and chlorophyll), the optimal process (holding time and temperature) was derived. Another example of optimization using LP was in potato drying, where maximizing the rate of drying and minimizing undesirable side reactions (browning, ascorbic acid oxidation) with negative effect on finished product quality, the optimal drying profile was defined. (8). In addition, LP was used to quality optimization

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of potato chips, where given the quality as a function of moisture content, oil content and color characteristics of chips, the optimum frying conditions (temperature and time) were determined considering the constraints for temperature, oil, and moisture content of chip, and temperature and time of frying (7). Optimization and control by automatic systems of various food processes has been cited both for product quality and economic reasons, i.e., white wine fermentation, sugar crystallization, extrusion cooking, pasta goods production, or for other complementary industrial operations, such as collection of raw milk, reduction of product losses, and costs of water purification during cleaning in dairy industries. Optimal design and control of individual food processing equipment, i.e., climbing film evaporator for tomato juice, multistage hyperfiltration unit for apple juice is also cited (38). Optimization of industrial operations attempts to exploit cost, time, and quality profits. For example, a methodology is proposed for optimization of a vegetable oil hydrogenation unit. A mathematical model based on data from a hydrogenation plant is developed to select the optimum operating conditions of temperature and hydrogen pressure that provide the desired product in minimum time (39). On the other hand, optimization of the overall efficiency, total operating cost, yield, total return, quality of output, and throughput of the whole food manufacturing plant is necessary when changing the raw materials or the product properties. Of course, it can be applied to large food processing plants; it is evident that routine application of optimization methods during operation is practiced only in certain parts of industrial plants. However, with the increased use of computers and computer control in food manufacturing integrated optimization may be applied to every modern processing plant. The new approach for food process and plant optimization is optimization throughout the food chain. The optimization of a complete food chain consists of product, process, and raw materials optimization, and by analyzing the system efficiency (losses, wastes) leading to optimum plant, process, and equipment design and operation (40). Finally, optimization is also applied in management and control of food processing plants. Application of optimization in management of food processing and food service industries has been presented. In addition to the above applications of optimization in food processing, similar techniques are used in optimizing food analysis methods of high accuracy such as gas and liquid chromatographic methods (41). This has increased significance when the constituents of interest are related to food safety specifications. Also, optimization of instrumental measurements related to foods have been investigated, i.e., methodologies for extracting features from sensors output responses of instrument analyses (electronic nose, near infrared analyzer) (42, 43). In conclusion, it is clear that the increased use of optimization in food manufacturing enables the efficiency, productivity, and quality to be increased

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and the energy use, product loss, and environmental pollution to be reduced. Furthermore, the optimization philosophy, ideas, and techniques should be adopted throughout the food handling system—from the harvesting of raw materials to the transportation, processing, packaging, distribution, and consumption of the final products. B. Optimization of Extraction Processes Extraction is applied to recover or remove constituents and consequently the product of interest may be the extracted materials or the extracts. The design and optimization of an extraction process entails the knowledge and the role of the technological parameters of the process. Conventional solvent extraction involves the removal of certain constituents from a mixture of solids (leaching or solid extraction) or liquids (liquid extraction) by means of a solvent in which exhibit different solubility. Supercritical fluid extraction (SFE) involves the separating of a mixture by contacting it with a fluid under conditions of temperature and pressure above its critical point. The separation of solvent from the extracted solute in conventional extraction is carried out by distillation or evaporation, while in SFE separation is accomplished either by reducing the pressure at constant temperature or by raising the temperature at constant pressure (44). Following, the most important parameters that significantly affect the extraction efficiency and determine the process design and success are presented. In leaching, the important parameters are: 1. The preparation of solid material by size reduction (crushing, grinding, flaking or cutting into pieces or cosettes); solid size must be suitable (surface area per unit volume) to make the solute more accessible to the solvent and to favor the extraction but not very fine to cause packing of solids and impede free flow of solvent 2. The selection of solvent for extraction based on a number of characteristics (capacity, selectivity, chemical inertness, thermophysical properties, flammability, toxicity, cost, availability) 3. The selection of operating temperature—temperature must be high enough to give higher solubility of solute in solvents, but not very high to cause solvent losses, extraction of undesirable constituents or damage of sensitive components 4. The equipment depending on the mode of operations (batch or continuous), the solids handled (fixed bed, percolation, full immersion, intermittent drainage or dispersed/moving contact) or the performing arrangement (one stage or multistage) In liquid extraction the important parameters are:

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1. The selection of solvent as previously referred, mainly based on high solute capacity and selectivity; particularly, solvent interfacial tension must have low value to get a good dispersion with high interfacial area for extraction, but not very low to cause emulsion formation and create problems in separation 2. The equipment depending on the contacting of the phases (by gravity or by centrifugal force) In supercritical fluid extraction the important parameters are: 1. The capacity and selectivity of the extracting supercritical fluid and their dependence on temperature and pressure 2. The ratio of solvent mass flow rate to the mass of solids treated; knowledge of the solvent, raw material and extract physical properties is necessary 3. The operating pressure and temperature, which must not cause decomposition of the raw material 4. Bulk density of the solid feed depending on the density of the solids, the form and consistency of the material and its moisture content 5. The mechanical treatment of the raw material depending on the nature of raw material (raw material purification or extract recovery); mechanical treatment should be avoided in the case of solids recovery, whereas in the production of a soluble extract may be desirable 6. The time of extraction in respect to solvents, raw material feed and operating conditions The technical and economical feasibility of an extraction process is determined knowing the equipment size, operating conditions, solvent flow rates and extraction yields. Extraction optimization is involved both in the design and the operating conditions for the equipment (separation allocation, differences in physical and/or chemical properties for separation, equipment type and sequence separators, fixation of separated phases, entire process operating conditions). Optimization of liquid–liquid extraction processes has been investigated and examples for staged and continuous models of extraction appeared in the literature (45). In staged processes treated as an integer variable, the optimization is more difficult, while in continuous processes for either cocurrent or countercurrent flow, integer variables are avoided and optimization can be carried out by other techniques. Steady-state continuous countercurrent liquid extraction has been modeled; a plug flow model was proved sufficiently accurate for a continuous pilot-scale extraction column. It can be used to determine the maximum extraction rate to various applications. A common problem in optimization of extraction processes is that operation time for the lowest cost is usually different from the time that gives the

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best yield (effectiveness), so that it is impossible to have both optimal cost and yield simultaneously. In this case the operation time becomes the independent parameter whose value determines the value of the criteria: minimum cost and maximum yield. The relative importance of the two conflicting criteria must be judged in order to obtain an optimum extraction time. So, if lowering of the operation cost is of interest, the time of extraction will be optimized, while the yield will be the primary criterion in an isolation of desired components or removal of toxic substances from food sources. Suboptimum with respect to operation cost and yield or optimum with respect to the combined criteria of cost and yield may be found. An overall criterion function can be established applying a more formalized optimization. Regional constraints on time might be imposed; a very large time above a certain value is not cost effective or the yield is not accepted at time below a certain value. After the optimum operation time has been determined other parameters not included in the original problem (extraction medium, extraction system) may be used, thus decreasing the operation cost and increasing yield by formal optimization techniques. It must be noted that formal optimization theory should be used only to find optimum parameters in well-defined systems with quantitative-dependent and independent parameters, criterion function, and functional and regional constraints. C. Application of Optimization in Food Extraction Processing Extraction is a separation process widely used in food processing industry for various applications, while recently with supercritical fluid extraction is possible to improve the quality of food products. Extraction may be a basic step in processing of many food products (i.e. sugar, edible oils, etc.), used for the recovery of active constituents or for the removal of undesirable constituents from raw materials, while important components may be separated from natural products with many applications to food industries (flavors, antioxidants, etc.). Optimization of protein isolation process by optimizing the protein extraction step has been studied from oilseeds or beans as: tomato seed, pigeon pea, peanut, and flaxseed (46, 47, 48, 49). The protein isolation process from vegetable sources includes mainly extraction or leaching of proteins by aqueous solutions and isoeletric precipitation; therefore extraction of proteins, evaluated by protein extraction yield, is determinant for the total protein yield of the whole protein isolation process. The optimization of this process should attain the maximum both of protein extraction yield or total protein yield as well as of the protein quality of the isolated product. It must be mentioned that the conditions optimizing the above responses are not the same; usually maximizing the protein extraction yield or the total protein yield decreases protein isolate content. Thus, the basic factors affecting protein solubilization during extraction such as: parti-

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cle size of proteinaceous material, temperature, extracting medium (water or diluted salt solution), solid to liquid ratio, pH (neutral or alkaline region) have been studied by proper experimental design and using response surface methodology (RSM) or steepest-ascent gradient method. Mathematical models for the protein extraction yield or total protein yield as well as for the protein content of protein isolate were obtained, while the extraction conditions that lead to the maximization of the responses were determined. Surimi usually used as raw material in processed foods (fish sausage, fish cake, or kamaboko) is water-leached, minced flesh of deboned fish. Surimi processing starts with leaching and additional processes follow, such as grinding, setting, heating, and frozen storage. Because the processing conditions significantly affect the quality characteristics of surimi products (gel strength, whiteness) the optimization of surimi processing should be conducted. An optimization study has been presented that uses a surface response method with a central composite design of surimi processing experimentation. Among the processing conditions the leaching parameters (leaching water in each leaching cycle, number of leaching cycles) were examined as well as parameters of the following processing steps. The optimal processing conditions that maximize gel strength and minimize whiteness of surimi products by canonical analysis and ridge analysis as well as fitting of a suitable model was investigated (50). Optimization has been widely applied to oil processing industry, in oil solvent extraction as well as in oil expression or mechanical extraction. Mechanical extraction is an alternative method for oil expression by crushing the seeds in a mechanical screw press. In this case, both oil yield and energy pressing cost are affected by the preprocessing conditions of the seed (dehulling, preheating, flaking or grinding, seed moisture content, seed temperature prior of pressing) as well as by the design and operation characteristics of the press (pressure, temperature, pressing time) and their interactions. Mathematical models based on expression conditions have been developed to predict the maximum sunflower oil recovery (51). Olive oil is characterized as a “natural food product” because it is the only oil produced by mechanical method that can be consumed without further processing. In addition, due to its nutritive and health properties, olive oil is recognized as a functional food product. This oil, virgin olive oil, should meet certain specifications according to regulation EC No. 2568/91 related to quality characteristics (oil acidity or oxidative status, etc.). For the above reasons, special mechanical methods have been developed. The classical method of extraction of olive oil is based on the mechanical pressing of olives. An alternative method also has been developed, which involves centrifugal separation of olive oil using horizontal decanters. Special quality levels can be attained in virgin olive oil production when all operations (olive collection and storage, extraction processing, packaging and storage of end product) are carefully carried out.

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Efficiency of expression is usually evaluated by extraction yield expressed as percent of oil extracted to the percent of oil in the olives. The expression parameters affecting oil yield have been examined: pressure compaction rate, cake thickness, rigid solid (pits) content in the olive paste, and the optimization of olive paste expression process has been studied experimentally (52). Consequently, the simultaneous variation of the parameters must be investigated to obtain the maximum olive oil extraction yield and the optimal operation of industrial olive pressing systems to be determined. The pressure raise increases oil yield, however it is important for olive oil quality the pressure to be kept under constrained values. Therefore, the effect on olive oil quality of parameters, such as olive ripening and storage prior to processing and olive paste mixing time and temperature, should be studied so extraction can be optimized for oil quality requirements (53). In recent years, extraction of vegetable oils using supercritical fluid extraction (SFE) as an alternative to the current industrial processes (expeller pressing, solvent extraction) and its optimization has been investigated (54, 55). Carbon dioxide, which is the most commonly used solvent to SFE processes, is also used in oil extraction; supercritical pentane has also been cited (54). The efficiency of SFE of oils depends on the variables: temperature, pressure, particle size of seeds, contact time between the extracting fluid and the oil-bearing material, ability of the fluid to penetrate the oil-bearing material, and solubility of the oil in the extracting fluid. Optimization of SFE of various vegetable oils such as soybean oil, canola oil, and sunflower oil, has been studied and the conditions that maximize oil recovery have been determined. In particular, the effect of extraction variables on oil recovery has been estimated by factorial designs and the optimum extraction conditions have been found using modified simplex method for the case of soybean oil. Simulation and modeling of a sunflower oil SFE plant and the optimization of the continuous process operation (extraction and CO2 recovery) was carried out and the optimal operating conditions found (55). Mathematical model of a fixed-bed extractor of canola oil has been presented giving the concentration of oil on both the solid and fluid phases and determining the overall volumetric mass transfer coefficients; a one-dimensional unsteady state model was used and the predicted results were in good agreement with the respective experimental results (56). Extractive processes used in alcoholic food/beverage products have been studied and optimized. The design, optimization, and control of extractive alcoholic fermentation in continuous mode are preferred. Combined with selective ethanol extraction, fermentation is enhanced; the combination of fermentation and selective ethanol extraction enables efficient control of the process. The dynamic behavior of the process was studied by factorial design simulating industrial process, and a dynamic matrix control (DMC) algorithm was used to

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control the extractive process. Modeling and simulation with response surface analysis was used to determine the operational conditions that maximize alcohol yield and productivity (57). Also, a countercurrent supercritical fluid extraction process on a pilot plant of distilled alcoholic drinks has been studied to optimize the efficiency of ethanol extraction yield. Thus, by factorial experimental design the extraction and fractionation conditions determined the ethanol percentage and the total yield of extracted ethanol of the residual extracted drink and of each isolated fraction, allowing the production of concentrated extracts with a rich aroma and low ethanol content (58). Other analytical extractive processes that have been optimized are the separation of certain amino acids (i.e., DL-tryptophan), which are important food components from aqueous solution using an emulsion liquid membrane. The critical extraction parameters: carrier concentration in the membrane system, pH, initial amino acids concentration were studied and using response surface methodology (RSM) the optimum separation conditions were found where the maximum amino acid extraction yield was attained (59). Extraction (or acid leaching) is a special step or pretreatment in many cases of analytical procedures of some determinations in food matrixes: vitamins, trace elements, pesticides residues, etc., in foods, industrial products, water, or soil. The analytes should be isolated and/or preconcentrated without changing their original chemical forms and subsequently analyzed using analytical or instrumental methods (i.e., atomic absorption spectrometry in trace elements, chromatographic methods such as gas chromatography (GC), highperformance liquid chromatography (HPLC) in pesticides or vitamins, spectrophotometry in tannins, etc.). The optimization of an analytical procedure often involves the study of the effects of experimental conditions particularly of time consuming and expensive ones as the extraction pretreatment processes (60–66). Critical variables in such extraction (or leaching) procedures are: type of solvent (organic solvent, saponification reagent, acid/oxidant reagent type and concentration), leaching volume, time, temperature, particle size of ground food product (matrix), extraction method. The particular variables relative to the extraction method are: (a) soxhlet: reflux frequency; (b) solid-phase extraction (SPE): type of sorbent, eluting solvent flow rate, sample pH, sample volume, elution volume, addition of modifier, water sample flow rate; (c) supercritical fluid extraction (SFE): CO2 pressure, temperature, static, and dynamic time; and (d) ultrasound or microwave assisted: sonication, frequency of ultrasound energy or microwave power, exposure time, temperature (67, 68). Experimental (central composite, Plackett-Burman—PBD, orthogonal array—OAD) design and simplex optimization method or response surface methodology (RSM) are effectively used to study the effects of extraction variables and find the optimum values of extraction variables in which maximum percent recovery of the extracted components is attained.

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NOMENCLATURE xi(i = 1, 2, . . . n)

x, (x1, x2, . . . . . , xn) X Xij or xij x* x = [x1, x2, . . . . . . , xn]T x(k), {x(k)} k = 1, 2, . . . f(x), f(X) h(x), hj(X) g(x), gj(X) C, f(x1, . . . , xn) fci = fi(x1, x2, . . . . . . . . , xn) l1 ≤ ri(x1, x2, . . . . . . . . , xn) ≤ l2 f′(x), dx f″(x) dr f(x) 䉮f(x) 䉮, (∂f/∂x1, ∂f/∂x2, . . . . . . , ∂f/∂xn)T 䉮2f(x) 䉮2 H(x), 䉮(∂f/∂xj), 䉮(䉮fT) J*x=x* = [∂2f/(∂xi ∂xj)* x=x*] dk * x=x* J

冉

f, g1, g2, . . . , gm xk, x1, x2, . . . , xm

冊

L λj Rn BIP CCD CCRD GMP

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independent parameters, variables, design variables in optimization problem vector of n variables, n-dimensional vector, design vector matrix elements of matrix X extremum (minimizer/ maximizer) column vector iterates in an iterative method objective function vector of equality constraints vector of inequality constraints optimization criterion function functional constraints regional constraints first derivative of function second derivative of function rth differential of f(x) (partial derivative of r order) first partial derivative or gradient vector first derivative operator (elements ∂xi/∂xj) second partial derivative second derivative operator (elements ∂2/ ∂xi ∂xj) Hessian matrix Hessian matrix of f(X) k-order differential of f at X* Jacobian determinants Lagrange function Lagrange multipliers n-dimensional space binary integer programming central composite design central composite rotatable design geometric programming

Optimization

IP LP MILP MIP NLP OAD PBD RSM SFE SPE

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integer programming linear programming mixed integer linear programming mixed integer programming nonlinear programming orthogonal array design Plackett-Burman design response surface optimization supercritical fluid extraction solid-phase extraction

REFERENCES 1. SS Rao. Engineering Optimization: Theory and Practice, 3rd ed. New York: John Wiley & Sons, Inc. 1996. 2. TF Edgar, DM Himmelblau. Optimization of Chemical Processes. Singapore: McGraw-Hill Book Co., 1989, Chapters 1–9, p. 3–438. 3. G Taguchi, MS Phadke. Quality engineering through design optimization. In: D Khosrow, ed. Quality Control, Robust Design, and the Taguchi Method. Pacific Grove, CA: Wadsworth & Brooks/Cole, Advanced Books & Software, 1989, pp. 77–96. 4. AA Teixeira, CF Shoemaker. Process optimization. In: Computerized Food Processing Operations. New York: AVI, Van Nostrand Reinhold, 1989, pp. 169–195. 5. J Floros. Optimization methods in food processing and engineering. In: YH Hui, ed. Encyclopedia of Food Science and Technology, Vol. 3. Toronto: John Wiley & Sons, Inc., 1992, pp. 1952–1965. 6. DA Pierre. Optimization. In: McGraw-Hill Encyclopedia of Science & Technology, Vol. 12, 9th ed. New York: McGraw-Hill, Inc., 2002, pp. 479–485. 7. E Rotstein, J Chu, IS Saguy. Simulation and optimization. In: KJ Valentas, E Rotstein, RP Sing, eds. Handbook of Food Engineering Practice. CRC Press, 1997, pp. 559–580. 8. I Saguy, L Levine, S Symes, E Rotstein. Integration of computers in food processing. In: HG Schwartberg, MA Rao eds. Biotechnology and Food Process Engineering. New York: Marcel Dekker, 1990, pp. 415–441. 9. JS Arora. Introduction to Optimum Design. Singapore: McGraw-Hill, 1989. 10. R Fletcher. Practical Methods of Optimization, 2nd ed. Chichester, UK: John Wiley & Sons, 1987. 11. G Dieter. Optimization. In: Engineering Design, 2nd ed. New York: McGraw-Hill, 1991, pp. 199–230. 12. WD Baasel. Optimization techniques. In: Preliminary Chemical Engineering Plant Design, 2nd ed. New York: Van Nostrand Reinhold, 1982, pp. 450–469. 13. GF Nalven. Plant Operation and Optimization. Practical Engineering Perspectives. New York: American Institute of Chemical Engineers, 1996. 14. TA Ciriani, RC Leachman. Optimization in Industry. Mathematical Programming and Modeling Techniques in Practice. Chichester, UK: John Wiley & Sons, 1993.

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15. CA Floudas. Global optimization in design and control of chemical process systems. J Process Control 10:125–134, 2000. 16. RP Byrne, IDL Bogle. Global optimization of modular process flowsheets. Ind. Eng. Chem. Res. 39:4296–4301, 2000. 17. B Gross, P Roosen. Total process optimization in chemical engineering with evolutionary algorithms. Comp Chem Eng 22:S229-S236, 1998. 18. MF Yeo, EO Agyei. Optimizing engineering problems using genetic algorithms. Eng Comp 15(2):268–280, 1998. 19. http://www.mcs.anl.gov/home/otc 20. A Bejan, G Tsatsaronis, M Moran. Thermoeconomic optimization. In: Thermal Design & Optimization. Toronto, Canada: John Wiley & Sons, Inc., 1996, pp. 463–513. 21. M Moresi. Design and optimization of falling-film evaporators. In: S Thorne, ed. Developments in Food Preservation—3. Essex, UK: Elsevier Applied Science Publishers, 1985, pp. 183–244. 22. A Van Loey, M Hendrickx, C Smout, T Haentjens, P Tobback. Recent advances in process assessment and optimization. Meat Sci 43:S81-S98, 1996. 23. E Balsa-Canto, AA Alonso, JR Banga. A novel, efficient and reliable method for thermal process design and optimization. Part I: Theory. J Food Eng 52:227–234, 2002. 24. E Balsa-Canto, AA Alonso, JR Banga. A novel, efficient and reliable method for thermal process design and optimization. Part II: Applications. J Food Eng 52: 235–247, 2002. 25. SK Sharma, SJ Mulvaney, SS Rizvi. Product formulation and process optimization using linear programming. In: Food Process Engineering, Theory and Laboratory Experiments. New York: John Wiley & Sons, Inc., 2000, pp. 1–19. 26. IA Castro, J Tirapegui, RSSF Silva. Protein mixtures and their nutritional properties optimized by response surface methodology. Nutr Res 20(9):1341–1353, 2000. 27. BJ Westrich, IM Buzzard, LC Gatewood, PG McGovern. Accuracy and efficiency of estimating nutrient values in commercial food products using mathematical optimization. J Food Comp Anal 7:223–239, 1994. 28. MC Gacula, Jr. Product optimization. In: Design and Analysis of Sensory Optimization. Trumbull, CT: Food and Nutrition Press, 1993, pp. 105–229. 29. C Stampanoni-Koeferli, P Piccinali-Schwegler, D Hong-Chen. Application of classical and novel sensory techniques in product optimization. Lebensm.-Wiss. u.-Technol. 31:407–417, 1998. 30. H Moskowitz. Optimizing consumer product acceptance and perceived sensory quality. In: E Graf, IS Saguy, eds. Food Product Development: From Concept to the Marketplace. New York: AVI, Chapman & Hall, 1991, pp. 157–187. 31. JL Sidel, H Stone, HA Thomas. Hitting the target: sensory and product optimization. Cereal Food World 39(11/12):826–830, 1994. 32. MV Pastor, E Costell, L Izquierdo, L Dura´n. Optimizing acceptability of a high fruit-low sugar peach nectar using aspartame and guar gum. J Food Sci 61:852– 855, 1996. 33. H Moskowitz. Sensory segmentation and the simultaneous optimization of products and concepts for development and marketing of new foods. In: E Graf, IS Saguy, eds. Food Product Development: From Concept to the Marketplace. New York: AVI, Chapman & Hall, 1991, pp. 311–326.

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Optimization

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34. SD Holdsworth. Quality optimization. In: Thermal Processing of Packaged Foods. London: Blackie Academic & Professional, 1997, pp. 183–201. 35. JC Ho, SK Chou, AS Mujumdar, MNA Hawlader, KJ Chua. An optimization framework for drying of heat-sensitive products. Appl Thermal Eng 21:1779–1798, 2001. 36. C-S Chen, J-J Chen, T-P Wu, C-Y Chang. Optimizing the frying temperature of gluten balls using response surface methodology. J Sci Food Agric 77:64–70, 1998. 37. S Hari Krishna, AP Sattur, NG Karanth. Lipase-catalyzed synthesis of isoamyl isobutyrate—optimization using a central composite rotatable design. Proc Biochem 37:9–16, 2001. 38. M Renard, JJ Bimbenet. Automatic control and optimization of food processes. Proceedings of the International Symposium on the Automatic Control and Optimization of Food Processes (ACOFP), Paris, France, 1986. Essex, UK: Elsevier Science Publishers, 1988, pp. 417–557. 39. AS Moharir, S Bhatia, DN Saraf. Optimization of industrial vegetable oil hydrogenators. JAOCS 60(8):1543–1547, 1983. 40. P Filka. Optimization of food processing plants. In: JJ Bimbenet, ed. Automatic Control and Optimization of Food Processes. Proceedings of the International Symposium on the Automatic Control and Optimization of Food Processes (ACOFP), Paris, France, 1986. Essex, UK: Elsevier Science Publishers, 1988, pp. 405–415. 41. R Romero, D Ga´zquez, MG Bagur, M Sa´nchez-Vin˜as. Optimization of chromatographic parameters for the determination of biogenic amines in wines by reversedphase high-performance liquid chromatography. J Chrom A 871:75–83, 2000. 42. S Roussel, G Forsberg, V Steinmetz, P Grenier, V Bellon-Maurel. Optimization of electronic nose measurements. Part I: Methodology of output feature selection. J Food Eng 37:207–222, 1998. 43. T Naes, H Martens. Principal component regression in NIR analysis. J Chemom 2: 155–157, 1988. 44. KK Tiwari. Extraction technologies related to food processing. In: AG Gaonkar, ed. Food Processing: Recent Developments. New York: Elsevier, 1995, pp 269–302. 45. TF Edgar, DM Himmelblau. Optimization of Chemical Processes. Singapore: McGraw-Hill, 1989, pp. 474–480. 46. GN Liadakis, C Tzia, V Oreopoulou, CD Thomopoulos. Protein isolation from tomato seed meal, extraction optimization. J Food Sci 60:477–482, 1995. 47. IY Mizubuti, OB Ju´nior, LW de Oliveira Souza, RS dos Santos Ferreira da Silva, EI Ida. Response surface methodology for extraction optimization of pigeon pea protein. Food Chem 70:259–265, 2000. 48. IYS Rustom, MH Lo´pez—Leiva, BM Nair. Optimization of extraction of peanut proteins with water by response surface methodology. J Food Sci 56:1660–1663, 1991. 49. PKJPD Wanasundara, F Shahidi. Optimization of hexamrtaphosphate-assisted extraction of flaxseed proteins using response surface methodology. J Food Sci 61: 604–607, 1996. 50. SY Hsu. Optimization of the surimi processing system with a central composite design method. J Food Eng 24:101–111, 1995. 51. MS Singh, A Farsaie, LE Stewart, LW Douglass. Development of mathematical models to predict sunflower oil expression. American Society of Agricultural Engineers. St. Joseph, MI: USA Transactions of the ASAE, 1984, pp. 1190–1194.

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52. E Pagliarini, C Peri, B Zanoni, M Ghizzardi. Optimization of olive paste expression: Maximizing yield by central composite design approach. J Sci Food Agric 71:470–474, 1996. 53. E Monteleone, G Caporale, A Carlucci, E Pagliarini. Optimization of extra virgin olive oil quality. J Sci Food Agric 77:31–37, 1998. 54. FM Lanc¸as, MEC Queiroz, ICE Silva. Simplex optimization of extraction of soybean oil by supercritical pentane. Chrom 40(7/8):421–424, 1995 55. M Bravi, R. Rubbico, F Manna, N Verdone. Process optimization in sunflower oil extraction by supercritical CO2. Chem Eng Sci 57:2753–2764, 2002. 56. AKK Lee, NR Bulley, M Fattori, A Meisen. Modelling of supercritical carbon dioxide extraction of canola oilseed in fixed beds. JAOCS 63(7):921–925, 1986. 57. AC Costa, DIP Atala, F Maugeri, R Maciel. Factorial design and simulation for the optimization and determintation of control structures for an extractive alcoholic fermentation. Process Biochem 37:125–137, 2001. 58. FJ Sen˜ora´ns, A Ruiz-Rodrı´guez, E Iban˜ez, J Tabera, G Reglero. Optimization of countercurrent supercritical fluid extraction conditions for spirits fractionation. J Super Fluids 21:41–49, 2001. 59. E Bayraktar. Response surface optimization of the separation of DL-tryptophan using an emulsion liquid membrane. Process Biochem 37:169–175, 2001. 60. H Bagheri, M Saraji, M Chitsazan, SR Mousavi, M Naderi. Mixed-level orthogonal array design for the optimization of solid-phase extraction of some pesticides from surface water. J Chrom A 888:197–208, 2000. 61. P Bermejo-Barrera, O Mun˜iz-Naveiro, A Moreda-Pin˜eiro, A Barmejo-Barrera. The multivariate optimization of ultrasonic bath-induced acid leaching for the determination of trace elements in seafood products by atomic absorption spectrometry. Anal Clin Acta 439:211–227, 2001. 62. MC Garrigo´s, F Reche, ML Marı´n, K Pernı´as, A Jime´nez. Optimization of the extraction of azo colorants used in toy products. J Chrom A 963:427–433, 2002. 63. Z Jo´kai, J Hegoczki, P Fodor. Stability and optimization of extraction of four arsenic species. Microch J 59:117–124, 1998. 64. J Lee, L Ye, WO Landen, Jr., RR Eitenmiller. Optimization of an extraction procedure for the quantification of vitamin E in tomato and broccoli using response surface methodology. J Food Comp Anal 13,45–57, 2000. 65. B Martı´nez, F Rinco´n, MV Iba´n˜ez. Optimization of tannin extraction from infant foods. J Agric Food Chem 48:2097–2100, 2000. 66. CC Nascentes, M Korn, MAZ Arruda. A fast ultrasound-assisted extraction of Ca, Mg, Mn and Zn from vegetables. Microch J 69:37–43, 2001. 67. GA Csiktusna´di-Kiss, E Forga´cs, T Cserha´ti, T Mota, H Morais, A Ramos. Optimization of the microwave-assisted extraction of pigments from paprika (Capsium annuum L.) powders. J Chrom A 889:41–49, 2000. 68. P Larra´yoz, M Carbonell, F Iba´n˜ez, P Torre, Y Barcina. Optimization of indirect parameters which affect the extractability of volatile aroma compounds from Idiaza´bal cheese using analytical supercritical fluid extractions (SFE). Food Chem 64: 123–127, 1999.

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6 Fats and Oils from Plant Materials Lei Xu and Levente L. Diosady University of Toronto Toronto, Ontario, Canada

I. INTRODUCTION Although “extraction” often describes the mass transfer process between two liquid phases, in the following the term “extraction” refers to the preferential dissolution of one or more constituents of a solid by contact with a liquid solvent. It is one of the oldest and most commonly used unit operations in the food industry. “Leaching” is another term often used to describe the operation, although originally referring only to percolation of a liquid solvent through a fixed bed of the solid. Sometimes the term “washing” is employed, as it is intimately associated with extraction as a complementary step. The fundamental purpose of extraction is the removal of a soluble fraction from an insoluble, permeable solid phase with which it is associated. The soluble fraction may be solid or liquid, and may be physically held within, chemically bound with, adsorbed on, or mechanically held in the pore structure of the insoluble matrix. Although the mining and metallurgical industries are the largest users of extraction technology, extraction is also a critical unit operation in the food industry. Many important foods and food ingredients are obtained by extraction from materials of life origin, including microbes, plants, and animals. For instance, sugar is extracted from sugar beets with hot water, tannin is dissolved out of various tree barks, and beverages such as tea and coffee are prepared both domestically and industrially by leaching operations. In terms of technological sophistication and production scale, vegetable oils are among the most important products recovered from seeds such as soybeans and rapeseed by extraction; therefore, these are discussed in greater detail in this chapter. Although in some cases mechanical pressing methods can be used, solvent extraction is a more

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efficient method for the recovery of oil from oil-bearing materials. It is the only efficient technique to recover oil from seeds or other materials low in oil (1). Since heat treatment can be minimized, oil produced by solvent extraction is of high quality, and thermal damage to meal, the residue after oil removal, is also minimal. The effectiveness and efficiency of extraction depend on many factors, including the solvent used, solid preparation, extraction temperature, modes of operation, and equipment. The theories of extraction have been well developed and are presented in detail in texts such as Treybal’s Mass Transfer Operations (2). Commercial processes of edible oil extraction are extensively reviewed in Bailey’s Industrial Oil and Fat Products (3), while essential oils are reviewed in Guenther’s The Essential Oils (4). The choice of solvent for extraction is largely determined by the solubility characteristics of the constituent of concern and the solid. Ideally the smaller component of interest should be highly soluble in the selected solvent, whereas the matrix or other major components should have little or no solubility in it. “Infinite” selectivity is never achieved in practice, and the process must be manipulated to obtain high yield and high purity of the desired components with minimal coextraction of undesired impurities. In addition to solubility, cost and safety are also taken into consideration when a solvent is chosen for extraction. To achieve complete extraction, a combination of several unit operations, such as successive extractions with two or more solvents, may be required. The pretreatment given to the oil-bearing solid frequently plays a very important role in the efficiency of the extraction process. In many instances of extraction, small particles of the soluble material are completely surrounded by a matrix of insoluble materials. The solvent must then diffuse into the mass, and the resulting solution must diffuse out before a separation can result. Particle size reduction by crushing and grinding of such solids greatly accelerates the extraction action, since the soluble portions are then made more accessible to the solvent. The extraction process must find a suitable compromise between the increased extraction rate obtained by reducing the particle size and increased difficulty in separating the small solid particles from the liquid solvent. Plants, microbes, animals, and their parts are cellular in structure, and the desired components are usually found in the cells. If the cell walls remain intact upon exposure to a solvent, the extraction depends solely on the diffusion of the solute through the cell walls, often controlled by osmotic pressure. Pretreatment techniques can be used to disrupt cell walls chemically or thermally. Alternatively, the aspect ratio is changed in a mechanical process. This allows the reduction of the penetration distance of the solvent while maintaining sufficiently large particle size to permit efficient separation from the solvent. As an example, cellular material may be cut into thin slices before extraction so as to reduce the penetration distance required for the solvent to reach the individual plant

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cells. In oil extraction, oilseeds are usually rolled to form flattened flakes with thickness ranging from 0.15 to 0.5 mm, while retaining major dimensions of up to 15 mm. The rolling and flaking evidently crushes some of the cell walls and opens up passageways for penetration of the solvent by capillary action (5, 6). It is impractical and sometimes even undesirable to grind the material small enough to release the contents of individual cells because of the difficulties excessively fine particles may cause in subsequent separation. Seed preparation includes heat and moisture adjustment to facilitate cell disruption and solvent penetration. It is usually desirable to extract at as high a temperature as possible because higher temperatures result in higher solubility of the solute in the solvent; thus, higher concentrations in the extraction liquor are possible. Moreover, the viscosity of most liquids is lower and the diffusivity greater at higher temperatures, leading to increased rates of extraction. Higher operating temperatures also reduce energy requirements in solvent recovery. However, the solvent selection and temperature must take into account that in some natural products high temperatures may cause deterioration and denaturation. Extraction operations can be conducted in batch (unsteady state) or continuous (steady state) mode. In each case, both stage-wise and continuous-contact types of equipment can be used. Extraction equipment uses two techniques for solid–liquid contact. The liquid can be sprayed and percolated over the solid, or, alternatively, the solid can be entirely immersed in the solvent. The choice of equipment depends on the physical and chemical characteristics of the matrix, the value of the products, and the desired throughput.

II. THEORIES OF EXTRACTION A. Rate of Extraction The performance of an extraction process is governed by both mass transfer and equilibrium phenomena. It can be affected by many physical and chemical factors, some of which are difficult to evaluate quantitatively. It is, therefore, not a simple matter to theoretically predict extraction rates, and quite frequently, such data have to be determined experimentally. If extraction only involves simple washing of a solute from the surface of a solid, it may be very fast, comprising merely the blending of solution and solvent, and its efficiency is thus determined almost entirely by the effectiveness of the mechanical separation of liquid from solid. Extracting a solute from the internal parts of a solid, on the other hand, will be much slower, in which case solids are made of a skeletal structure of insoluble substances with pores impregnated with the solute. Diffusion theory can be used to describe extraction processes that fall into this category. Beginning with Fick’s law, the following equation is derived (7):

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NA =

冉冊

NA DAB ρ NA + N B z M

ln

av

NA/(NA + NB) − xA2 NA/(NA + NB) − xA1

where NA and NB are molar fluxes of components A and B, respectively, DAB is the diffusivity of A in B in solution, ρ and M are the solution density and molecular weight, respectively, xA1 and xA2 are mole-fraction concentrations of component A at the beginning and end of the diffusion path, respectively, and z is the distance in the direction of diffusion. For most common cases where component A diffuses through nondiffusing B, NB = 0, where NA =

冉冊

DAB ρ z M

(xA1 − xA2)

av

The diffusivity varies significantly among liquids. In the absence of experimental data, it can be estimated by the following empirical correlation for nonelectrolytes: DAB =

(117.3 × 10−18(ϕMB)0.5T µνA0.6

where D AB is diffusivity of A in a very dilute solution in solvent B, MB the molecular weight of solvent, T the temperature, µ the solution viscosity, νA the solute molar volume at normal boiling point (0.0756 m3/kmol for water), and ϕ the association factor for solvent (2.26 for water, 1.9 for methanol, 1.5 for ethanol, 1.0 for unassociated solvents such as benzene and hexane). The complexity of the structures of natural products may make the application of these models difficult. In addition to the above attributive factors, the rate of diffusion can also be affected by the degree of cell rupture, the diversity of structures in the matrix, and the rate of dissolution of each constituent. All of these phenomena make it impossible to apply a single simple equation to model extraction. Although methods of dealing with solvent extraction of vegetable oils have been suggested (8, 9), very little has been published on modeling the extraction of other natural products. In oil extraction, the term “miscella” refers to the mixture of oil and solvent outside seed particles, and “raffinate” the mixture of the two within the particles. The design of large-scale solvent extraction apparatus for oilseeds extraction requires the knowledge of the rate at which equilibrium is attained between the miscella and raffinate. In most oilseeds, oil is stored in small, 1- to 10-µm oil bodies, which are enclosed by protein-embedded membranes. The membrane protein oleosin has high affinity for both oil and water on opposite ends of the molecule. The extraction proceeds first by washing out of free oil, which is held in large intercellular spaces and is not bound to the flake structure; it is followed by a diffusion process. Assuming that the cell walls are broken,

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the oil must still diffuse through the oleosin membrane of the oil droplets. The next step is the release of bound oil into the solvent, which may involve the breaking of hydrogen bonds or hydrophobic attractive forces. The solution then must diffuse back out through the solid matrix to the bulk miscella, which involves diffusion first through the solid itself, then through the laminar boundary layer on the surface of the solid. Therefore, oil extraction may be quite slow, particularly when the oil content of the seed is reduced to low levels (≤1.0%). Studies have shown that the oil extraction rate is influenced by a number of factors, including the thickness, size, shape, and internal structure of the seed particles; the intrinsic capacity for diffusion of solvent and oil, which is determined primarily by the viscosities of both; and, especially at low oil levels, the rate of extraction of other substances that are less readily soluble than glycerides, which are the primary components of interest in vegetable oils (1). Among these factors, the effect of the thickness of oilseed flakes is most evident. Othmer and Agarwar (5) correlated the rate of hexane extraction of soybean oil to flake thickness and remaining oil concentration as follows: −

dC = kF −3.97C 3.5 dt

where C is the concentration of oil in the flake, t the time, k the proportionality constant, and F the flake thickness. Since F has an exponent of almost 4, a small increase in flake thickness would result in a great decrease in the oil extraction rate. When using a homogeneous oil-impregnated material made of thin platelets of uniform thickness, approach of estimation of extraction rate becomes simpler as the application of diffusion theories is possible. The calculation has been thus developed by Boucher et al. (8) as follows: E=

8

n=∞

1

冋

冉 冊 冉 冊册 2

π Dt

exp −(2n + 1)2 2 ∑ (2n + 1) 2 π n=0

R2

where E is the fraction of total oil unextracted at the end of time t, R is one–half the platelet thickness, and D is the diffusivity of the oil within the platelets. Except for the initial stage where the free oil is washed out quickly, the above equation can be reduced to: E=

8 π

冉 冊

exp − 2

π2Dt 4R2

or logE = −0.091 − 1.07

Dt R2

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Thus, when the extraction proceeds past its initial stage, a plot of log E against t gives a straight line. The slope is a function of the diffusivity of the oil and the platelet thickness. The turbulence in the solvent, indicated by the Reynolds number, was found to have no significant effect on the extraction rate, showing that the limiting step in extraction is diffusion through the cell wall within the platelets, and the liquid-film resistance to the transfer of oil to the solvent was inconsequential (8, 10). The rapid release of oil during the initial stage of extraction was thought to be caused by the rupture of some oil bodies in slicing the seeds, plus the occurrence of void spaces in the seeds after drying (9). The diffusivity of the oil in the platelets was correlated to the viscosities of both solvent and oil by Boucher et al. (8) under their test conditions: D = 12.96 × 10−6(µ 0 µ s)−0.46 where µ o and µ s refer to the viscosities of oil and solvent, respectively. The numerical values in the formula are obviously specific to the structure of the platelets and therefore are applicable only to the lot used in their tests. B. Calculations for Steady-State Operation In steady-state operation, it is assumed that the solid is contacted with more than enough solvent to dissolve all the soluble solute and there is no preferential adsorption of either solvent or solute by the solid. Adequate contact time permitting, all the solute will be dissolved, and the oil concentration in the miscella will be equal to that in the solvent inside the seed matrix; thus, there is an equilibrium between the effluent solution and the solid. The insoluble solid is then separated by settling, filtration, drainage, or centrifugation. This is usually not achieved in practice as the solute may be incompletely dissolved due to inadequate contact time and some solute is usually adsorbed by the solid. Therefore, without corrections for these effects, the actual stage efficiencies are always lower than the calculated ones. Similarly to other unit operations, two typical calculations are performed in extraction: characterization and design. In the former case the initial solute content of the solid, and the number and amount of washings are known, and the final amount or concentration of solute in the extract is sought. This yields an estimate of the extent to which an extraction process proceeds. In case of design calculations, the final solute content of the solid is specified, with a known amount and concentration of solute in the solvent; whereas the number of washings, or the number of stages, is calculated in order to design an extraction process. Steady-state operation can be achieved in either a single-stage or a multistage manner. Single-stage extraction involves only one cycle of mixing the solid with the solvent and separating the resulting insoluble solid by physical means. Figure 1 shows all the streams involved in a complete process of single-

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187

Figure 1 Single-stage extraction (11).

stage extraction. Mass balance is performed for all components in the system to give (11): FyyF + R0 x0 = E1y1 + R1x1 F(1 − yF) + R0(1 − x0) = E1(1 − y1) + R1(1 − x1) F + R 0 = E 1 + R 1 = M1

for solute C for solvent A for solution A + C

M1 is the mass of the mixture, or the slurry, on an insoluble solid–free basis. The solid and solute concentrations in the slurry are, therefore: NM1 =

B B = F + R 0 M1

yM1 =

yFF + R0 x0 F + R0

respectively. By continuously contacting the solids with fresh batches of solvent, more solute can be extracted than a single batch extraction. This is called multistage cross-current extraction. All of the above equations are applicable to the calculations for addition stages, in which the already extracted solids from any stage are the feed solids to the next; thus, the procedure for a single stage is repeated with the changes in the subscripts to indicate different stages. Multistage crosscurrent extraction can be used to extract additional solute from the solids, but a much larger amount of solvent is required for this operation, and as a result, dilute solutions of the solute will inevitably be produced as it proceeds to the last stages of extraction. Although this is uneconomical use of solvent, the procedure allows experimental determination of the performance of countercurrent systems, using simple laboratory equipment. Countercurrent extraction minimizes solvent use and operating costs. In

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this approach, the final solution is withdrawn from contact with the fresh solid and the fresh solvent is mixed with the solid with most of the solute already extracted, keeping the driving force at the highest level throughout the process. A flowsheet of multistage countercurrent extraction is shown in Fig. 2. The process can be operated with any number of extraction and separation steps. An array of these steps is called an extraction battery. Like in single-stage extraction, calculations of multistage cross-current extraction are based on stage-wise mass balance. The solution (A + C) balance gives: F + RNp+1 = R1 + ENp = M The solute (C) balance gives: FyF + RNp+1xNp+1 = R1x1 + ENpyNp = MyM where M is total mass of the mixture (slurry) on an insoluble solid–free basis. The slurry is produced by mixing the solids and solvent such that: NM = yM =

B F + RNp+1 FyF + RNp+1xNp+1 F + RNp+1

where NM and yM are, respectively, the concentrations of solid and solute in the slurry on insoluble solid–free basis. The extraction process can be graphically modeled using a diagram as shown in Fig. 3, to indicate the approach to equilibrium obtained theoretically or experimentally, and the number of theoretical stages required for the extraction. The number of stages is dependent on the average stage efficiencies, which are affected by the mass transfer properties of each stage. In the special case where there is a constant value of N for slurry of all stages, the system approximates a constant R/E. When, in addition, the equilibrium between the solute concentration in the effluent solution, x, and that in the solid (insoluble solidfree basis), y, is of a linear relationship, i.e., m = y/x = constant, the final concentration in the solid, yNP can be calculated by the following equation: yF − yNp yF − mxNp+1

=

冉 冊 冉 冊 R mE

Np+1

R mE

−

Np+1

R mE

−1

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Fats and Oils from Plant Materials

Figure 2

Multistage countercurrent extraction (11).

189

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Figure 3 Stage construction diagram (11).

Similarly, the number of stages, Np, can be estimated when the final concentration is specified. If no preferential adsorption of the solute occurs, the solute concentration is the same in both the solution and the solid, so that m = 1. To calculate the steady-state performance of a multistage countercurrent extraction, the concentrations in the intermediate stages can be experimentally estimated through batch simulation, using cross-current procedure as indicated earlier. The scheme for such a simulation was proposed by Scheibel (12) and Treybal (13). Fig. 4 shows an example of batch simulation of two-stage countercurrent extraction. Each circle in the figure represents a batch extraction. Starting from stage A, F amount of feed solid is contacted with S amount of fresh solvent. The mixture is then separated into a clear solution R′ and a solid EA. At stage B, the solid from stage A, EA, is again contacted with the same amount of solvent S before the separation to yield a solid E′. The whole simulation process is followed through as illustrated in Fig. 3. It consists of several runs.

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191

Figure 4 Batch simulation of a countercurrent cascade (13).

From each run R amount of solution and E amount of solid are produced. Although R and E at the beginning are clearly different in solute concentration from those produced by a countercurrent extraction, after several runs in this manner, they approach very closely the values obtained in a truly continuous multistage countercurrent extraction.

III. SOLVENTS FOR EXTRACTION For oil extraction, nonpolar solvents are usually used, as triglyceride-based oils are typically miscible with these solvents. The most common solvents in solvent

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extraction of oil are light paraffinic petroleum fractions. Both hexane (bp 66– 69°C) and heptane (bp 89–98°C) mixtures are widely used; sometimes cyclic hydrocarbons, such as cyclohexane (bp 71–85°C), are also used (16). While these solvents are efficient in oil extraction, the major disadvantage to their use is their flammability. Strict safety measures must be taken to avoid fire and reduce explosion hazard in the plants where they are used (17). Much effort has been taken to find alternative solvents, particularly since the Clean Air Act of 1990 designated hexane as a hazardous air pollutant. A methylpentane-type naphtha (bp 55–61°C) is now commercially available (18). Trichloroethylene (bp 86.7°C), due to its nonflammability, is a much safer solvent to handle in terms of prevention of fires and explosions (19, 20), but all chlorinated organic solvents are increasingly perceived as too toxic to be used in the production of food materials. There is pressure on the industry to switch to solvents that are perceived to be more benign: alcohols and even aqueous solutions. Both isopropanol (21) and ethanol (22, 23) have been used commercially to extract soybean and cottonseed oil. Recently, high-concentration isopropanol (⬃96%), recovered by pervaporation techniques, has been used instead of the commonly distilled azeotropic isopropanol (87.8%) for oil extraction in order to increase the oil solubility (24). Both cottonseed and soybean oils thus extracted, after refining, bleaching, and deodorization, well met commercial standards. However, although improving the flavor, the alcohols tend to lower functionality of soybean protein products, and due to low oil solubility and higher latent heat than hexane, they require significantly more energy for solvent recovery. Supercritical fluids have been tested for oil extraction (25, 26), but due to the extremely high pressures required, most equipment has low capacity, which currently limits the application of this technology to the production of high-value products such as coffee, hops, and flavor concentrates.

IV. SOLVENT EXTRACTORS Two types of commercial units are currently used for extraction: immersion and percolation. In the immersion type of extractor, solids to be extracted are conveyed through a pool of solvent. The particle size of the solid material must be reduced as much as possible to facilitate maximal contact between the solvent and the solid. However, in this method the separation of fine particles and the miscella is difficult, thus limiting its use to low-volume operations. Most largevolume units are percolation-type extractors that are considerably more efficient in terms of energy consumption and space requirements. In the percolation unit, the solid material (flakes or prepressed cake) forms a fixed bed, and the solvent is pumped and sprayed over the material, and drained through the bed, washing the oil down with it. Clearly the flakes or

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pieces of prepressed cake should be large enough to permit a reasonable flow rate of solvent through the solid material. Fine material can prevent the drainage, causing the bed to be flooded with solvent and resulting in improper washing and poor extraction. One of the first designs based on percolation was the basket-type extractor (Fig. 5), wherein the seed flakes are carried in baskets supported on endless chains. The baskets are loaded at the top of the descending leg, and the flakes are sprayed with half miscella that percolate through the beds of the flakes in these baskets as they go down. The miscella collected at bottom on the descending side is full miscella and thus is removed for oil and solvent recovery. On the ascent, the flakes are contacted with another stream of liquid that starts as pure solvent; thus, oil in the flakes is extracted in a countercurrent manner as the baskets move up. A short drainage time is provided at the top before the baskets are automatically flipped to unload the extracted and drained flakes into a discharging hopper, and they are then conveyed to the desolventizer. This early type of solvent extractor is bulky and requires intensive maintenance due to frequent chain breakdown. It has been largely superseded by rotary and horizontal extractors. A rotary extractor is, in fact, a vertical cylindrical shell with radially divided chambers called baskets or cells, either rotating or stationary, around a central shaft. Figure 6 shows a cut-away view of a rotary extractor manufactured by De Smet Process and Technology, Inc., the reflex extractor. Its liquid spraying nozzles and flakes hopper are fixed while the baskets in the shell rotate. The material is mixed with miscella as it enters the extractor and is thus slurry-fed to the rotating baskets. The miscella leaves the basket through the perforated bottom, and is collected and pumped to the previous basket. Thus, a countercurrent flow is set up between the moving seed beds and the stationary miscella or solvent headers. The most exhausted meal is contacted with fresh solvent, while the fresh seed is first contacted with nearly oil-saturated miscella. The totally sealed basket dividers ensure that each miscella stage flows vertically through the basket of material, and these dividers also allow the entire bed of material in the extraction zone to be thoroughly soaked in miscella. It is claimed that, since the miscella has more contact time to penetrate the flakes than in shallow-bed designs, even thicker flakes can be used to achieve desired residual oil content. After the extraction process is complete, a basket passes over an open portion of the screen, allowing the material to discharge into the dump hopper at the base of the extractor by gravity. A number of similar designs were developed based on the earlier Rotocel extractor by the Dravo Engineers and Contractors. Another variant of this design, developed by the French Oil Mill Machinery Company, called the stationary basket extractor, maintains the seed-holding baskets or cells immobile while rotating the solvent headers and bottom connector systems. Horizontal extractors feature the horizontal movement of the baskets while

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Figure 5 Basket solvent extractor (1).

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Figure 6 Reflex solvent extractor. (Courtesy of De Smet Process & Technology, Inc.)

the solvent is percolated through the moving seed bed. One typical example of this type is the Crown Model III continuous loop extractor made by the Crown Iron Works Company (Fig. 7), in which the solid material is deposited on veebar screen as a shallow bed (0.8 m) and travels a distance about 50 times the bed depth in a closed chamber shaped like a loop. The fresh material is fed to the extractor through an inlet hopper on the top, and moves through four stages on the upper level and another three on the lower level. In the first stage on the upper level it is washed with already concentrated miscella. The solids then move to the second stage concurrent to the miscella. The rich miscella drained from the second stage is pumped through a hydrocyclone before leaving the extractor as full miscella containing approximately 25% oil. The next five stages

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Crown loop extractor Model III. (Courtesy of Crown Iron Works Company.)

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Figure 7

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are all operated in countercurrent manner. As the solids move from the upper level to the lower level between stages 4 and 5, the bed is completely turned over in the “tail” section so that both sides of the solids are washed with the miscella. Before being discharged, the extracted solids are drained of residual miscella in a drainage section. The mechanical features of this type of extractor promote solvent contact, rapid drainage, and complete extraction with a variety of products and allow utilization of fragile flakes with high content of fines. Extractors with capacities of 50–4000 t/d are in commercial service. Crown Iron Works Company also developed a Model IV extractor for countercurrent extraction of granular materials that are heavier than solvent (Fig. 8). The material is carried through a pool of solvent by a series of inclined drag conveyors. The solids pulled out of the solvent pool by the last conveyor is sprayed with fresh solvent and then are drained of residual miscella before being discharged. Full miscella is drawn off where solids are fed to the extractor. During the transfer of solids from one conveyor to another, the beds are completely turned over, thus ensuring good solvent–product contact.

V. EXTRACTION OF OILSEEDS The recovery of oil from oil-bearing plant materials has been a vital industry for thousands of years. Hence, its technology evolved from primitive manual operations in the early times to continuous processing by automated machinery in the modern days. This evolution was obviously driven by the need to maximize the oil yields, improve the oil quality, and produce a valuable residue after oil extraction. Oils from plants are usually contained in seeds, such as soybeans, sunflower seed, cottonseed, and rapeseed/canola, or fruits such as palm (Table 1). They contain 15–50% of oil and are the major source of vegetable oils for human consumption (Table 2). The oil extraction process is often followed by further processing to yield more refined oil products. Many processes have been developed to recover oil from oilseeds, but the most common ones are hydraulic pressing, expeller pressing, and solvent extraction. Developed in late 1700s, hydraulic pressing, also known as batch pressing, applies pressure using a hydraulic ram to batches of oilseeds confined in a barrel that allows oil to escape while retaining solids. Because it is a laborintensive, batch operation, its industrial use is uneconomical and thus declined drastically over the last two decades. Currently continuous expellers or screw presses are used for the mechanical extraction of oilseeds. These machines are able to create the pressures generated by a hydraulic press ram but require minimal labor because they operate continuously. The reduced labor cost and increased throughput and yield more than make up for their higher power requirement and maintenance cost (27).

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Model IV Extractor. (Courtesy of Crown Iron Works Company.)

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Table 1 World Production of Major Oil-Bearing Seeds, Legumes, and Fruits in 1998–1999 Crop Year (in 106t) Soybeans Rapeseed/canola Cottonseed Sunflower seed Groundnuts, shelled Palm Linseed Sesame seed

154.70 35.78 33.63 27.26 20.88 16.71 2.62 2.59

Source: Ref. 14.

It is impossible to obtain complete oil removal by mechanical pressing, since the solids retain significant quantities of oil inside the matrix. In addition, as the oil content is reduced, the friction between particles escalates rapidly, and the heat generated by expeller causes very high temperatures that can degrade both the oil and the residual meal. To recover the last 20% of the oil, solvent extraction is the preferred operation. Solvent extraction as a batch process first appeared in 1870. Continuous solvent extraction was developed shortly after World War I. Since then, its use has expanded rapidly in oilseeds processing due to its high efficiency in oil removal and recovery. As solvent extraction is energy intensive, in oilseeds with high oil contents much of the easily removable oil is recovered by expelling prior to solvent extraction. Thus sunflower, rapeseed, and cottonseed are processed by a two-stage technique consisting of a prepress followed by solvent extraction, resulting in more complete removal of oil with a lower usage of solvent.

Table 2 Oil Content of a Number of Oil-Bearing Vegetable Materials Cottonseed Palm kernels Peanuts Rapeseed/canola Sesame seed Soybeans Sunflower seed

18–20 45–50 45–50 45–50 50–55 18–20 35–45

Source: Ref. 15.

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A. Canola (Rapeseed) The technology to process canola or rapeseed is typical of high-oil-content seed processing. Although expeller pressing or direct solvent extraction may be used, these techniques either leave a fairly high residual oil content in the meal or are not energy efficient. The prevailing process in developed countries is prepresssolvent extraction, which is a combination of mechanical and solvent extraction, with the final portion of oil being removed with a solvent, usually hexane. In this process, much less residual oil is left in the meal. 1. Pretreatment and Prepressing of Oilseeds A typical oil extraction process begins with seed preparation (Fig. 9). The seeds usually carry foreign materials such as unwanted seeds, sticks, leaves, and even metal shreds. They are typically cleaned by passing through a magnetic separator and seed cleaners using a combination of air aspiration, agitation, and screens. In rapeseed, the enzyme myrosinase catalyzes hydrolysis of glucosinolates. Among the hydrolysis products are undesirable isothiocyanates, oxazolidinethiones, and nitriles (28), which are soluble in oil. In order to maintain the oil quality, it is necessary to keep the glucosinolates intact by rapidly inactivating myrosinase by “cooking” in a multistage cooker. The seed is first cracked to increase the area available for heat transfer. To achieve rapid myrosinase destruction and to minimize the time spent at the active temperatures of the enzyme (40–65°C), the cracked seed is contacted with live steam, which quickly raises the temperature by releasing the latent heat, and also adds water to aid in the denaturation of myrosinase. The excess water is removed by indirect heating in subsequent stages of the cooker employing stacked steam-jacketed trays with bottoms heated by steam. Since canola has much lower levels of glucosinolates than conventional rapeseed varieties, the cooking temperature for canola has been decreased from ⬃120°C to less than 100°C. The cooked seeds are then immediately pressed to separate oil by screw presses. A typical screw press consists of a hard-surfaced, horizontal worm shaft within a barrel composed of cages containing spaced lining bars. As the worm shaft rotates inside the cage assembly, it moves the seed through and out. Pressure on the seed increases as it moves through the barrel, expelling the oil between the lining bars. The pressure at the discharge end of the shaft forms the partially extracted seed into a cake. Important operating factors in prepressing are the speed of shaft rotation, the shaft assembly configuration, and setting of the choking mechanism at the discharge. Pressing cannot remove all of the oil without significant thermal damage to the meal. Typical residual oil content in fully pressed cake is 6–10%, which

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Preparation and prepressing of canola.

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makes this approach noncompetitive. Thus, in modern plants seeds are pressed only to the point where the meal protein is not seriously damaged, leaving 15– 22% oil in the prepressed material. This removes two-thirds of the oil, uses much lower pressures and considerably less energy than the equivalent fullpress operation, and results in better oil and meal quality. As the oil in the seed is contained in structures shaped like fibrous capillaries, application of too high pressures may seal these channels again and make further extraction difficult. Too much pressure may also cause the cake to become hard and impermeable to the solvent. It is, therefore, critical to control the pressure in the press and limit the amount of oil removed from the cake. To further free the oil, and to stabilize and strengthen the seed, another thermal step, usually referred to as “conditioning,” is required after prepressing. Conditioning involves heating the seed to reduce the viscosity of the oil and to make it easier for the oil to separate from the ruptured seed cell. As the seed is heated by the expeller, external heating may not be needed for this step. During conditioning, it is important to control the moisture content of the seed, as well as the temperature, because once the seed is crushed and flaked, microbial deterioration can take place rapidly, particularly if the material has a high moisture content or is of poor quality. Therefore, it is crucial to have a continuous and rapid flow of material from one piece of equipment to another. The flaked seeds are usually conditioned to a moisture content of 10–12% at a temperature of 80–90°C. As indicated earlier, oilseeds consist of oil bodies that contain liquid oil enclosed by a protein membrane. A typical oil body is 2–10 µm in diameter. The walls of oil bodies are essentially nonpermeable; therefore, for subsequent solvent extraction these walls must be broken to free the oil and facilitate the penetration of solvent into the cells. This is usually accomplished by flaking the prepressed seed in a roller mill after carefully adjusting the temperature and humidity. Flakes of 0.15–0.5 mm in thickness and up to 10 mm in diameter allow rapid solvent penetration with high bed permeability. 2. Solvent Extraction A simplified flow sheet of a typical solvent extraction process is shown in Fig. 10. The seed flakes after preparation or the cakes from prepressing are conveyed to the solvent extractor. As described in detail in the section II.D, every extractor performs two primary functions. First, it must provide retention time required to allow large volumes of solvent to penetrate and wash the solid material (flakes or cake). Second, it must separate the solids from the miscella to minimize solvent carry-over in the solids to the next phase of the operation, which is desolventizing of the extracted solid material.

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Solvent extraction of prepressed canola.

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Figure 10

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3. Posttreatment After oil extraction, the solid material (meal) is transported in a closed conveyer system to the desolventizer-toaster (DT), where the residual solvent is removed by heating the meal enough to evaporate the solvent, but not so much as to deteriorate its nutritional value (Fig. 11). This unit is made of steam-heated trays mounted, one on top of the other, with a vertical shaft and sweep arms attached to move the meal around and down to the level through automatic openings in the trays. The top four trays make up the desolventizer and the bottom two trays

Figure 11 Desolventizer-toaster. (Courtesy of De Smet Process & Technology, Inc.)

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the toaster. The desolventizer may be divided into a pre-desolventizing and a desolventizing zone. By the time the meal reaches the toaster, most of the solvent has been evaporated and moved out the top of the desolventizer. Some moisture addition through steam injection is required to facilitate further evaporation by dissociation of bonding between the meal protein and hexane through hydrophobic interactions. The meal discharged from the DT may contain considerable moisture; thus, it must be further dried for final moisture adjustment and cooled. In some cases, all of these steps are accomplished in a single unit, dubbed DTDC for desolventizer-toaster-dryer-cooler. The hot solvent vapor from the desolventizer is used in most instances to heat other evaporation equipment before being condensed and sent back to the solvent tank for reuse. Finemeal dust is recovered prior to condensation and solvent recovery. To recover the crude oil from miscella, the solvent must be stripped from the oil in typical distillation columns. As mentioned above, the hot desolventized vapor is used to indirectly heat the vessel to evaporate some of the solvent. An additional stripping unit indirectly heated by steam is used to remove most of the remaining solvent and water. The final traces of solvent are removed by direct steam injection. The solvent vapors are then condensed in either watercooled or air-cooled condensers. During solvent extraction, all equipment is maintained under slightly negative pressure to minimize solvent losses. The air leaked into the system is passed through a vent condenser and a mineral oil absorption column before being vented to the atmosphere, both for explosion safety and solvent economy. After the solvent is stripped from miscella, the crude oil can be further processed before storage, to remove entrained compounds such as phospholipids or gums. B. Sunflower Seed Sunflower seed processing is similar to that of other high oil content oilseeds including canola and rapeseed. Therefore, this section, concentrates on differences in the operations. 1. Dehulling Sunflower seed contains 23–25% fibrous hull, which can interfere with the extraction process. Thus, the processor must decide whether to dehull prior to processing, which in turn determines the nature of the meal produced. Processing with the hulls produces a meal with a fiber level of about 18–20% and protein content of 28–30%. Removal of about 75% of the hulls produces a meal with a fiber level of 11–13% and a protein level of 40–42%. The market determines if dehulling is required.

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However, if the hulls are not removed, they tend to reduce the total yield of oil by absorbing and retaining oil in the press cake. To accommodate the additional volume of the hulls, larger equipment is required to process the extra tonnage. Moreover, wear on the prepress lining bars and worm shafts is increased, thus increasing the cost of the already expensive prepressing operation. Therefore, dehulling is often desirable. Two types of dehulling equipment are usually used: knife and impact, although a disk type is also available. Such machines break the hull fraction away from the seed, and the mixture is then separated using shaking screens and aspiration. The dehulling process must be finely tuned to leave a minimum of meats in the hull fraction to minimize oil losses. 2. Preparation The preparation of sunflower seed is similar to that of other high oil content seeds such as rapeseed. The seeds, or meats, are rolled to rupture and expose the oil cells. They are then conditioned, by heating, to reduce the viscosity of the oil and to make it easier to separate the oil from the meats or seeds. Proper temperature and moisture adjustments are critical to maximize the effectiveness of the prepressing stage that follows. This adjustment is usually termed as “tempering.” Cooking at high temperature is not required for sunflower seed. 3. Prepressing and Solvent Extraction These steps vary little from canola and rapeseed processing except for the adjustments made on the prepressing equipment including the shaft arrangement, spacing of the bars in the cage surrounding the shaft, the rotation speed, and the choking mechanism. After prepressing, the cake containing 16–18% oil is broken and granulated. To make sure that the residual oil content is less than 2% after solvent extraction, some manufacturers condition and flake the broken cake pieces again before conveying them to the solvent extractor. Others simply convey the material directly to the solvent extraction process. In both cases, production of fine particles must be minimized because large amounts of fine material may cause the solvent to flood the extractor bed instead of percolating through the material for proper extraction. C. Soybean Oil removal from soybeans is normally carried out by solvent extraction alone since soybeans have a relatively low oil content of 18–20%, as compared with that of rapeseed or sunflower seed. The clean, dry, and tempered beans are broken into four to eight pieces in cracking mills, usually composed of two pairs

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of fluted rolls turning at different speeds, cutting beans as they are fed through the two passes. The hulls of the dry beans are then easily separated by aspiration. The cracked and dehulled beans are conditioned by raising their temperature to 65°C and adjusting their moisture content using live steam. Conditioners are normally rotating drums with an internal steam coil. Conditioning enhances the performance of the subsequent flaking and extraction operations. The conditioned beans are fed to flaking mills to produce consistent flakes with an ideal thickness of 0.3 mm. Good flaking is essential to maximize oil extraction in the solvent extractor. Extruders are becoming the preferred means of pretreating soybeans. Water is added to the cracked seed, which is then heated and pressurized in the extruder. As the pressure is released at the exit, the absorbed water is rapidly vaporized, resulting in an expanded, porous structure where many of the cells are ruptured. This porous microstructure improves oil and solvent mobility. The solvent extraction of soybean oil is similar to that of other seeds (Fig. 12). During the process, the heat reduces the natural trypsin inhibitor levels in soybeans, thus increasing the nutritive value of the meal. D. Corn Since the corn kernel contains only about 5% oil, it is uneconomical to process corn simply to obtain the oil. Corn oil is obtained by processing the corn germ, which is a byproduct of the wet or dry milling of corn, a major industry. Corn germ contains about 50% oil. This oil is recovered by expelling or direct solvent extraction (dry-milled germ), or by their combination in prepress-solvent extraction (wet-milled germ). The actual processes involved are almost identical to those described above for other oilseeds.

VI. EXTRACTION OF ESSENTIAL OILS While edible oils are triglyceride-based oils mainly from oilseeds, essential oils are the volatile chemical components from various parts of plants, which are responsible for the characteristic aromas of these plants. To name a few, oils extracted from bitter almond, lemon and orange peels, rose and jasmine flowers, peppermint, and lovage roots are all essential oils. Although lipid-soluble, essential oils do not contain triglycerides, and they are complex mixtures of compounds that usually fall into the following four classes (29): 1. Terpenes 2. Straight-chain hydrocarbons and their oxygen derivatives

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Solvent extraction of soybean oil.

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Figure 12

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3. Benzene derivatives 4. Volatile sulfur and nitrogen compounds Essential oils are widely used in the food and fragrance industries. Their active components contribute a significant portion of the flavor or aroma of a spice. They are often diluted with oil or emulsified to make them dispersible before addition to a flavor or fragrance system. To recover essential oils, distillation has always been most commonly used to take advantage of their volatility (30). The components in essential oils, however, have much higher boiling points than water; therefore, they are actually codistilled with steam or water to avoid thermal damage. Codistillation with water or steam distillation takes advantage of the fact that in the vapor phase the oil, water, and air do not interact. The composition of the vapor phase is directly proportional to the vapor pressures of the vapor components, i.e., water/ steam and the volatile oils. The steam acts as a carrier and removes the oil vapors, which have been evaporated well below their boiling point. This is especially important because many of the essential oil components have high boiling points and would break down thermally well below their normal boiling points. After condensation, the oils and water are immiscible and thus are easily separated. Like oilseeds processing, the production of essential oils also begins with the size reduction of the raw material, the comminution of the plant parts. As the essential oils are enclosed in “oil glands” or “oil cells” of the plant, the rate of oil vaporization will be entirely determined by the rate of hydrodiffusion if the plant parts are left intact, which is obviously a very slow process. Consequently, the plant material must be cut to some extent to rupture enough oil glands and meanwhile reduce the thickness of the material for the steam to diffuse, thus increasing the vaporization rate of the essential oils. The extent of comminution required varies with different plant parts. Seeds or fruits must be thoroughly crushed in order to disrupt as many of the cell wall as possible so as to make the oil directly accessible to the steam. This can be achieved by passing them through a roller as in oilseeds processing. Root, stalks, and all woody materials should be cut into small pieces to expose a large number of oil glands. They can be processed by a hay or ensilage cutter, which reduces the long natural parts of the plant to short lengths to be more easily handled for distillation. On the other hand, flowers, leaves, and other thin parts of the plant can be distilled without comminution since the structures in these parts are thin and permeable enough to allow rapid evaporation of the oil. The crushed or chopped material is ready for distillation. Three types of hydrodistillation are used in the essential oil industry: water distillation, water and steam distillation, and direct steam distillation. In water distillation, the material makes direct contact with boiling water by floating on the water or

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being immersed. The water can be brought to boil by fire, steam jacket, or coil, or even perforated steam coil. Some plant materials, such as almonds and rose flowers, can only be distilled in this manner since upon direct contact with steam they tend to agglomerate and form compact lumps, making it hard for the steam to penetrate. In water and steam distillation, the plant material is supported on a perforated screen placed above the boiling water, so that it is contacted by the steam instead of water. The steam remains saturated in this system (Fig. 13). In steam distillation, live steam (saturated or superheated) is applied through open or perforated steam coil. The choice among these distillation meth-

Figure 13 Retort for water and steam distillation of essential oils (30).

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ods is usually based on both their efficiency and their effect on the essential oils. Although higher steam pressures result in increased vaporization rate of the oils, the higher temperatures of the pressurized steam may cause thermal decomposition of the oils. The equipment for distillation of essential oils consists of three parts: the retort, the condenser, and the oil separator. The retort serves as a vessel where the steam or boiling water contacts the plant material to vaporize its essential oils. It can be as simple as a cylindrical container with a removable cover that can be clamped on the cylindrical section. A pipe is attached to the top of the retort to lead the vapor to the condenser. In the case of steam distillation, trays are placed close to the bottom of the container, and the steam is introduced through a open steam line. In multitray retorts, each tray is only filled with a relatively shallow layer of the material to ensure a uniform distribution of both the material and the steam. The height of a typical retort is 1.5–1.8 m. It is insulated to minimize heat loss. The oil vapor is converted to liquid and cooled down using a condenser. The most common condensers, used in the production of essential oils are coil condensers with the oil vapor and the steam passing inside the coil while cold water enters from the bottom of the condenser and flows outside the coil against the steam and oil vapor as shown in Fig. 14. The third important part of the distillation equipment is the oil separator, which, as indicated by its name, separates the essential oil from the condensed water. Many separators are built based on the principle of the ancient Florentine flask (Fig. 15). The condensate flows from the condenser into the separator where the water and the oil separate into two layers since they are immiscible and different in specific gravity. The oil is usually lighter and floats on top of the water. Both layers are then removed continuously from the separator through outlets at different levels. In addition to distillation, essential oils can also be processed by extraction. Extraction with cold fat is practiced with jasmine and tuberose flowers for their essential oils. Although this method gives a much greater yield than other methods, it is a lengthy and labor-intensive process. Extraction with volatile solvents is considered as the most technically advanced process, producing concentrates and alcohol soluble “absolutes,” the aromas of which truly represent the oils in their natural forms. Citrus oils, for instance, are often extracted with alcohol, from which two layers are obtained: one hydrophobic layer containing the terpene hydrocarbons and one aqueous alcohol layer with the polar compounds. They are separated and the polar compounds recovered. The essential oils may be used as collected or subjected to further purification, concentration, or separation. Vacuum distillation is the preferred method for refining most essential oils. With this technique the oil components can be

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Figure 14 Coil condenser (30).

fractionated according to their boiling points. Rectified, folded, and terpeneless oils are made through the selection of the various fractions obtained from redistillation. Extraction with supercritical carbon dioxide or propane is now practiced on small scales for highly valued components of plants in order to produce either flavor or aroma concentrates in the form of oils or resins.

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Figure 15 Florentine flask for oils lighter than water (30).

REFERENCES 1. FA Norris. Extraction of fats and oils. In: D Swern, ed. Bailey’s Industrial Oil and Fat Products, 4th ed., Vol. 2. New York: John Wiley & Sons, 1979, pp. 175–252. 2. RE Treybal. Mass Transfer Operations, 3rd ed. New York: McGraw-Hill, 1980. 3. YH Hui, ed. Bailey’s Industrial Oil and Fat Products, 5th ed., Vol. 4. New York: John Wiley & Sons, 1996. 4. E Guenther. The Essential Oils, Vol. 1. New York: D. Van Nostrand, 1948. 5. DF Othmer, JC Agarwal. Extraction of soybeans—theory and mechanism. Chem Eng Progr 51:372–378, 1955. 6. DF Othmer, WA Jaatinen. Extraction of soybeans. Mechanism with various solvents. Ind Eng Chem 51:543–546, 1959. 7. RE Treybal. Mass Transfer Operations, 2nd ed. New York: McGraw-Hill, 1968, pp. 15–37.

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8. DF Boucher, JC Brier, JO Osburn. Extraction of oil from a porous solid. Trans Am Inst Chem Eng 38:967–993, 1942. 9. HP Fan, JC Morris, H Wakeham. Diffusion phenomena in solvent extraction of peanut oil. Effect of cellular structure. Ind Eng Chem 40:195–199, 1948. 10. JO Osburn, DL Katz. Structure as a variable in the application of diffusion theory to extraction. Trans Am Inst Chem Eng 40:533–556, 1944. 11. RE Treybal. Mass Transfer Operations. 2nd ed. New York: McGraw-Hill, 1968, pp. 628–675. 12. EG Shreibel. Calculation of liquid-liquid extraction process. Ind Eng Chem 46: 16–24, 1954. 13. RE Treybal. Mass Transfer Operations. 2nd ed. New York: McGraw-Hill, 1968, pp. 408–488. 14. ISTA. Oil World Statistics Update. Hamburg, Germany: Mielke, 1998. 15. NOV Sonntag. Sources, utilization, and classification of oils and fats. In: D Swern, ed. Bailey’s Industrial Oil and Fat Products, 4th ed., Vol. 1. New York: John Wiley & Sons, 1979, pp. 271–288. 16. AL Ayers, JJ Dooley. Laboratory extraction of cottonseed with various petroleum hydrocarbons. J Am Oil Chem Soc 25:372–379, 1948. 17. NFPA 36. Solvent Extraction Plants. Quincy, MA: National Fire Protection Association, 1993. 18. MA Williams, RJ Hron Sr. Obtaining oils and fats from source materials. In: YH Hui, ed. Bailey’s Industrial Oil and Fat Products, 5th ed., Vol. 4. New York: John Wiley & Sons, 1996, pp. 61–157. 19. IJ Duncan. Commercial extraction of vegetable oils by means of trichloroethylene. J Am Oil Chem Soc 25:277–278, 1948. 20. OR Sweeney, LK Arnold. A new solvent extraction process for soybean oil. J Am Oil Chem Soc 26:697–700, 1949. 21. FC Magne, EL Skau. Phase equilibrium data pertaining to the extraction of cottonseed oil with ethanol and 2-propanol. J Am Oil Chem Soc 30:288–291, 1953. 22. RK Rao, LK Arnold. Alcoholic extraction of vegetable oils. IV. Solubilities of vegetable oils in aqueous 2-propanol. J Am Oil Chem Soc 34:401–404, 1957. 23. RK Rao, LK Arnold. Alcoholic extraction of vegetable oils. V. Pilot-plant extraction of cottonseed by aqueous ethanol. J Am Oil Chem Soc 35:277–281, 1958. 24. EW Lusas, LR Watkins, SS Koseoglu. Research in replacing hexane with isopropyl alcohol as an extraction solvent. Annual Meeting of American Oil Chemists’ Society, Chicago, 1991. 25. GR List, JP Friedrich, DD Christianson. Properties and processing of corn oils obtained by extraction with supercritical carbon dioxide. J Am Oil Chem Soc 61: 1849–1851, 1984. 26. GR List, JP Friedrich. Processing characteristics and oxidative stability of soybean oil extracted with supercritical carbon dioxide at 50°C and 8,000 psi. J Am Oil Chem Soc 62:82–84, 1985. 27. GC Mustakas. Recovery of oil from soybeans. In: DR Erickson, EH Pryde, OL Brekke, TL Mounts, RA Falb, eds. Handbook of Soy Oil Processing and Utilization. St. Louis: American Soybean Association, 1980, pp. 49–66.

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28. CH VanEtten, ME Daxenbichler, IA Wolff. Natural glucosinolates in foods and feeds. J Agric Food Chem 17:483–491, 1969. 29. AJ Haagen-Smit. The chemistry, origin, and function of essential oils in plant life. In: E Guenther, ed. The Essential Oils, Vol. 1. New York: D. Van Nostrand, 1948, pp. 15–83. 30. E Guenther. A. Distillation of essential oils. In: E Guenther, ed. The Essential Oils, Vol. 1. New York: D. Van Nostrand, 1948, pp. 85–187.

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7 Proteins from Plant Materials Lei Xu and Levente L. Diosady University of Toronto Toronto, Ontario, Canada

I. INTRODUCTION As the world population continues to grow exponentially, the demand for food proteins will inevitably increase substantially. It has been forecast that within the next decade, in order to just maintain the present level of human nutrition, the supply of vegetable proteins must be doubled together with a fourfold increase in the production of animal-based proteins (1). Since such a dramatic growth in meat production is hard to achieve, human diet will likely rely more on vegetables and cereals as protein sources. In addition to continuing to sustain of human life as they have done for thousands of years, plant-based proteins (or vegetable proteins) offer great potential as functional food ingredients. In the past decades, many processes have been developed to produce proteins to be used as meat extenders in processed meat products or as protein-enriching agents in beverages. In the processing of vegetable proteins, much attention is paid to the preservation or improvement of their nutritional value as well as organoleptic and functional properties. Storage proteins in oilseeds, legumes, and cereals have been the focus of process and product development for food and industrial protein products. Protein products can be produced by the preferential extraction of nonprotein seed components, as is the case with soy protein concentrate, or by the dissolution, purification, and recovery of proteins in the form of protein isolates. For protein isolate production a number of aqueous and some nonaqueous organic solvents have been investigated (1). The storage proteins include a number of albumins, globulins, prolamines, and glutelins with a variety of molecular weights and a range of solubility characteristics. Albumins are typically soluble

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in water (pH 6–8), globulins in dilute salt solutions, prolamines in ethanol, and glutelins in dilute acid or alkali. Most of these proteins are highly soluble in strong alkali, although their functional properties may be altered by partial or complete denaturation at these extreme conditions. Soy proteins have been used as traditional foods in the orient. Accordingly, they have been studied extensively, and there are numerous commercial processes for the production of a wide range of commercial soy protein products. Many other protein sources were investigated in attempts to find alternatives to soy proteins. This was driven by the functional, nutritional, and allergenic problems of soy proteins, as well as the desire to develop value-added products from a number of underutilized plants or plant products, such as oilseed meals after oil extraction. Significant research advances were made in the production of wheat gluten, corn zein, and a number of oilseed proteins from peanuts, cottonseed, canola, and others. In cereal grains the main storage proteins are usually the alcohol soluble prolamines, while the salt-soluble globulins are abundant in oilseeds and legumes (2, 3). Regardless of their origins, these globulins all belong to two classes of different molecular sizes, usually denoted in the literature by their sedimentation coefficients as 7S and 11S (1). Most protein extraction processes are based on aqueous extraction using single- or multistage batch operation. The latest advances made in the methods for protein extraction feature the combinations of solvents with optimized extraction parameters and advanced separation technologies such as membrane processing to increase both the yield and quality of the protein products. In addition to their main use as functional ingredients in many food systems, proteins from vegetable sources are also used in some industrial processes as size, adhesive, or dispersive agents.

II. SOY PROTEINS The processing of soybean proteins has evolved since ancient times. For nearly 15 centuries people in Asia have combined the use of rice and the products of soybeans to produce an inexpensive diet that is reasonably well balanced in terms of essential amino acids. They learned that the greatest nutritional value from soybeans is obtained through a water extraction followed by coagulation of the resulting milky liquid into a curd. Crude as this ancient method is, it has provided the basis for the development of many modern processing methods. Nutritional studies show that soy proteins are an excellent source of food proteins. The amino acid composition of soy protein isolate nearly meets the essential amino acid pattern set by FAO/WHO/UNU as shown in Table 1 (3, 4). In combination with rice, the low methionine level of soy and the low lysine

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Table 1 Comparison of Essential Amino Acid Content of Soy Protein Isolates to WHO/FAO/UNU Requirements WHO/FAO/UNU suggested pattern (mg/g protein) Essential amino acid Histidine Isoleucine Leucine Lysine Methionine and cysteine Phenylalanine and tyrosine Threonine Tryptophan Valine

2–5 years

10–12 years

Adult

Essential amino acid content of isolated soy protein (mg/g protein)

19 28 66 58 25 63 34 11 35

19 28 44 44 22 22 28 9 25

16 13 19 16 17 19 9 5 13

26 49 82 63 26 90 38 14 50

Source: From Ref. 3.

level of rice are both overcome. The high digestibility of soy proteins has been demonstrated by extensive nutritional studies on both animals and humans. Currently, four major classes of soy protein products are in commercial production: flour, protein concentrates, protein isolates, and textured products. Their total production exceeds one million tonnes. Soy flour can be made from either full-fat dehulled soybeans or dehulled defatted flakes. Full-fat soy flour contains fat in excess of 18%, while the oil content of defatted flour is usually less than 1%; thus, the protein content is much higher than that of full-fat flour. To process full-fat flour, high-quality, sound, clean, yellow soybeans are selected. Conventionally, these beans are first conditioned to a desirable moisture level and then cracked in roll mills. The cracked beans are dehulled by aspiration and screening. The meats are cooked at temperatures above 93°C by steam in another conditioner to inactivate enzymes such as lipoxygenase. After being dried, they are ground in two steps using a hammer mill to particle sizes small enough to pass through a 0.149-mm screen (No. 100 U.S. Standard Screen) (5). The production of defatted soy flour is directly linked to oil extraction from soybeans, which involves solvent extraction and desolventizing. The defatted and desolventized soy flakes with an appropriate moisture content are then ground in a hammer mill until 97% passes through a 0.149-mm screen (No. 100 U.S. Standard Screen). The term protein concentrate refers to products containing at least 70% protein produced by alcoholic extraction of carbohydrates, leaving behind the

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insoluble proteins, whereas protein isolate is usually used to describe products with more than 90% protein produced by the extraction, purification, and recovery of soy proteins. Although both protein concentrates and isolates can be made from full-fat soybeans, the products find limited use due to their appreciable fat content. Therefore, manufacturing of these protein products usually start with defatted soy flour after the grinding of the defatted flakes to facilitate subsequent extraction. The dehulled flour has a protein content of about 50%. Protein concentrates are produced by washing the meal to remove soluble carbohydrates with dilute acid, aqueous ethanol, or hot water, as none of these dissolves significant amounts of soy protein. With dilute acid, the pH of the extraction solution is adjusted to 4.5, the observed isoelectric point of soy proteins, where the solubility of soy proteins is minimal (6). The insoluble material containing most protein is thus separated from the soluble impurities in a batch operation that is repeated several times. The effect of ethanol is similar as most proteins are insoluble in 60–80% aqueous ethanol; thus, soluble oligosaccharides are extracted and separated (7). Aqueous alcoholic extraction also causes denaturation of the trypsin inhibitors present in the seed and removes some of the undesirable phenolic components, thus improving the quality of the product. The ethanol is removed from the protein concentrate and recovered by flash desolventizing. For hot water extraction, the starting material is first toasted to denature the proteins and decrease their nitrogen solubility index (NSI), and thus decrease protein solubility, while maximizing carbohydrate dissolution (8). In the treatment, the soy flakes are extracted with hot water at 66–93°C in either a batch or a continuous countercurrent manner, and pH is maintained in the range of 5.3–7.5. The isolation of soy proteins takes an approach opposite to protein concentration (5, 9–11), wherein the protein is dissolved while most of the impurities are left behind in the solids. A generic process is outlined in Fig. 1. The extraction is carried out with dilute alkali, pH 8–10, usually at elevated temperatures of 50–55°C. Although more protein could be extracted in more alkaline solutions, protein tends to deteriorate faster at high pH values. Extraction conditions, including pH, temperature, liquid-to-solids ratio, and additional reagents, vary among different manufacturers, and information about most of the detailed operating conditions remains proprietary. After extraction, separation of the aqueous extract from the solids is achieved by screening, centrifugation or decanting, and polish-filtration. Optionally, the extract may be further clarified by chemical purification by adsorbents, depending on the process adopted by the manufacturer. A food grade acid is then used to acidify the extract containing the soluble protein portion to pH 4–5. The most commonly used acids are acetic, hydrochloric, phosphoric, and sulfuric. Upon acidification a protein precipitate is formed, collected by centrifugation or filtration, and washed. The precipitate is usually neutralized with dilute food grade alkali to form sodium proteinates, which can

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Figure 1 Commercial production of soy protein isolates (42).

be modified by jet cooking at 141–160°C, followed by treatment with a proteolytic enzyme for 1–30 min to improve the wettability and dispersibility of the product (10). Although spray drying is the preferred drying method, the product may be dried by other means such as a forced draft oven or a range drier. In the end, about one-third of the starting mass is recovered as a protein isolate containing more than 90% protein on a moisture-free basis (N × 6.25), one-third remaining as the insoluble residue, and one-third in the whey solution. After drum drying, the insoluble residue is used as a feed ingredient. The whey solution contains several undesired flavor components and trypsin inhibitors that must be heat inactivated before it can be used even for feed. Despite the generally desirable nutritional properties of soy protein concentrates and isolates, these products are typically powdery and do not exhibit the consistency required for palatable foods. In order to expand their use, novel processes have been developed to obtain soy protein products with the more desirable meat-like textures. Technologies involved in these processes include extrusion and spinning. In the 1960s, an extrusion cooking process was developed for the production of a fully toasted, full-fat, and texturized soy flour (12). The extrusion of soy protein is similar to other food extrusion processes. The extruders for texturizing are designed to handle high-moisture materials. In the process, the dehulled flakes are first cooked with steam at 93–100°C and preconditioned to a moisture content of 18%. The cooked material is then passed

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through a high-speed mixer where more steam is mixed with the flakes before entering a Wenger extruder, wherein the material is pushed along inside a barrel by a rotating screw while being heated by friction or indirect steam. High temperatures of 121–143°C are reached for 1–1.5 min within the barrel. At these high temperatures some hydrogen bonds break, unfolding the structured proteins, which are then elongated and aligned by shear forces. When the melt is forced through the die at the end of the barrel, it is rapidly expanded by the release of steam upon leaving the extruder, producing a porous matrix. As the proteins cool, new hydrogen bonds form to give the product a fibrous, meatlike texture. The cooled product is ground in a pin mill to desirable particle sizes and used in foods such as “bacon bits.” The spinning process was originally developed in the synthetic textile industry for the production of nylon fibers. It has been adapted to the commercial production of soy protein fibers (10, 11). A typical process for soy protein spinning is shown in Fig. 2. A soy protein isolate is first dissolved in strong alkali to make a “dope,” which is a solution with a protein concentration of about 20% at a pH of 12–13. After filtration, the dope is pumped through a spinneret made of a platinum plate with thousands of small holes less than 1 mm in diameter into a coagulating bath. As the dope contacts the acid and salt in the coagulating bath, the protein precipitates, forming protein fibers. Phosphoric acid and NaCl are usually used as the precipitating agents in the bath. To continue the texture-forming process, the fibers are combined to make tows about 0.5 cm in diameter to be stretched. The stretched tows are then washed to remove the acid and salt, and heated to be hardened. Binders such as egg albumin are added to improve the cohesiveness of the product, followed by the addition of fat, flavor, color, and supplementary nutrients in order to obtain a palatable food. Because the final product has a texture similar to that of processed meat, it is used as a meat analogue. The functional properties of these soy protein products play a critical role in determining their use in food systems. These include solubility, hydration and water absorption, viscosity, gelation, emulsification, foaming properties, and organoleptic properties such as color and flavor. Commercially available soy proteins exhibit a wide range of solubilities at pH 7 from 25% to 95% (13), and the solubility is dependent on pH and salt concentration. While higher pH results in high solubility, the effect of elevated salt level is to reduce solubility. Commercial soy proteins also have a wide range of water binding ability (14). Insoluble protein granules bind much more water than soluble proteins. Slurries of soy proteins have relatively low apparent viscosity and do not form strong gel networks; however, heat or alkali treatments can increase viscosity and improve gelation functionality (15). As with other functional properties, commercial soy proteins display a range of emulsion and foam properties. Kolar et al. (10) reported a range from 10 to 40 mL oil/100 mg protein for oil binding. Unlike

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Figure 2 Spinning process for production of soy protein fibers (41).

other functional properties, color and flavor are primarily due to small nonprotein compounds and their interactions with proteins. A number of phenolic compounds and their oxidization products have been linked to the yellowish brown color of soy protein products (16). The products of lipid oxidation catalyzed by lipoxygenase are responsible for the undesirable beany flavor of most commercial soy proteins (17). Once formed, these compounds bind to proteins and thus are hard to remove. However, for commercial food applications these organoleptic problems can be avoided through careful control of food processing conditions. Presently a large variety of soy protein products are manufactured commercially worldwide. These are used in processed meat products, imitation cheese, whipped toppings, soy milk, nutritional beverages, and baked products. There is also a significant market for the industrial use of soy proteins in paper products, adhesives, and polymers.

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III. GLUTEN “Gluten” refers to the insoluble portion of proteins in cereal grains, first identified more than 200 years ago in dough from wheat flour, and isolated by simply washing the dough with water. Much research has been done on gluten and the flour proteins ever since. The wet impure gluten carries 65–70% water, has a creamy color, and feels like rubber. When dried, the crude gluten contains 75–80% protein and 5–15% carbohydrates, the remaining being lipids. Gluten can be divided into two fractions. One fraction is soluble in 60% aqueous alcohol, known as gliadin or prolamine. The other fraction, although insoluble in neutral water, is soluble in acidic or alkaline solutions, and is known as glutelin (18). The nutritional and functional properties of gluten have been well documented. Gluten has acceptable levels of most essential amino acids but is low in lysine. Its functional properties can be changed by reducing and oxidizing agents, and heat treatment. Oxidizing agents make gluten lose its extensibility and become brittle, whereas reducing agents increase its extensibility. Gluten may lose some or all of its “vitality” when subjected to heat treatment. In early industrial processes gluten was only a byproduct of wheat starch processing. The Alsatian method (19) is among the first methods to recover gluten, wherein whole wheat is steeped in water for 1–2 days. The softened wheat grain is then placed in bags of mesh to permit dissolution of starch while retaining the gluten. The bags are passed between a series of rolls with progressively smaller gaps, thus squeezing out starch but leaving gluten in the bags. The further separation of gluten from the hulls is long, and the yield of gluten is usually low. The Martin process uses wheat flour to make both starch and gluten (20). It starts with making a stiff dough containing about 40% water. After hydration for 1 h, the dough is rolled between fluted rolls under a spray of water to wash away the starch and leave the gluten in a single, coherent mass. This method produces good-quality gluten with high yield. Another process, called the batter process, involves making a batter, which is broken up while water is added to wash away the starch from the gluten (21). The gluten in this method is recovered as fine curds with more than 80% protein. Although developed decades ago, both the Martin and the batter processes are still used in separating starch and gluten from wheat flour. Figure 3 shows the major steps involved in either method. The gluten is separated from starch milk by a series of centrifuges. In the refining step, the lighter low-grade starch is separated from the heavier prime starch. The gluten is either flash or spray dried. The mass distribution among products is 59% prime starch, 8% tailing starch, and 15% gluten with a protein content of 80%. Based on the batter process, a continuous process has been developed (22). First a slack dough is made by mixing flour with water at a ratio of 0.7 : 1. Then the dough is heated to 48–57°C, followed by breaking up of the dough in the presence of additional water. The starch is

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Figure 3 Manufacture of wheat starch (41).

thus largely washed out. The gluten is recovered as curds and separated from the starch by screening the slurry. Other processes include the alkali process (23), in which the wheat protein is dissolved in 0.03 N NaOH solution by mixing 1 part of flour with 18 parts of the alkaline solution, and after centrifugation to separate starch, the supernatant is acidified with H2SO4 to produce a precipitate containing 70–90% protein, which has no vitality. This method was modified with NH4OH to replace NaOH (24). As a result, the gluten is recovered with vitality. The vital gluten is used extensively as an ingredient in yeast-raised baked goods, particularly bread. Besides increasing the final protein content in the bread, it also results in a greater volume of better crumb texture and a longer shelf life of the bread. Canada has been a major producer and exporter of wheat gluten and contributed significantly to the commercial development of gluten extraction processes.

IV. RAPESEED AND CANOLA PROTEIN PREPARATION Although its oil has been consumed as food by humans for thousands of years, the idea of using rapeseed protein in food is relatively recent. One obvious

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reason for the delay is directly related to the organoleptic characteristics of rapeseed meal after oil removal. It has an unappealing dark green color and an unpleasant taste that may be characterized by both bitterness and astringency. According to modern studies, both problems are caused by phenolic compounds in rapeseed (16). Nevertheless, due to increasing demand for vegetable oils, the production of rapeseed has grown drastically in the last three decades. It is now ranked second in world production, exceeded only by soybeans (24a). The meal is used in animal feed but commands a significantly lower price than soy meal. Nutritional studies have shown that rapeseed proteins have well-balanced amino acid composition (25, 26), and are particularly high in lysine (27), which is a limiting amino acid in most cereal and oilseed proteins. Due to its abundance and high-quality proteins, rapeseed should play an important role in meeting the need of the world’s fast increasing population for food proteins. Consequently, there has been continued research interest in extracting rapeseed proteins for food. Rapeseed meal has a protein content of up to 40% after oil extraction; however, its direct use as a food protein source is prevented by its undesirable components. In addition to phenolic compounds, it contains high levels of glucosinolates (more than 100 µmol/g defatted meal), which upon hydrolysis release toxic compounds such as isothiocyanates and oxazolidinethiones (27a). Although the genetically improved “canola” varieties have much lower amounts of glucosinolates (20–30 µmol/g defatted meal), their meals are still unacceptable for incorporation into any food formulations. Canola meal also has a much higher phytate content of 3–5% than many other cereals and oilseeds (28, 29). Phytates are undesirable in foods as they tend to strongly bind minerals such as Fe and Zn, and make them unavailable for metabolism (30, 31). It is obvious that before rapeseed or canola protein can be used for food, much or all of these compounds must be removed. Although much process research has been directed to treating the meal, a variety of methods have been developed to isolate high-quality canola proteins. Most of the published methods were based on soy protein extraction technology: aqueous extraction followed by protein precipitation. Unlike soybeans, canola contains a wide range of proteins, with a broad band of isoelectric points and a wide range of molecular weights. For rapeseed protein extraction, alkaline aqueous solutions have been suggested as solvent. While over 80% extractabilities with dilute NaOH solutions have been reported by several researchers (32–34), these values cannot be attained using commercial meal as starting meal. To improve extraction, researchers used consecutive extractions at differing pH values. Blaicher et al. (35) extracted rapeseed protein at pH 9.5 and 12.0 in two consecutive stages, respectively, and achieved a 92% extractability. Although higher pH results in increased protein extractability, it could cause chemical modification of the proteins such as

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the formation of harmful lysinoalanine (36) under extremely alkaline conditions (pH > 12). Since rapeseed proteins are predominantly globulins (37), dilute sodium chloride solution has been explored as a protein solvent. NaCl concentrations from 0.2 N to 2 N have been reported (25, 38–40). Extractabilities obtained with NaCl solutions were typically lower than those obtained with NaOH media. The recovery of salt-extracted protein is complicated by the need to remove the salt by dialysis. Following the example of soy technology, rapeseed proteins are usually recovered by isoelectric precipitation. The precipitate, which is usually washed and dried, constitutes the protein isolate. A wide range of isoelectric points from pH 2.6 to 10.0 are observed with rapeseed proteins (32, 34, 41, 42), due to their complex compositions as well as varietal difference among rapeseed strains. Consequently, protein recovery of single-step isoelectric precipitation is low, and the highest ever reported is 65.7% of the amount of protein extracted at pH 11, achieved at pH 3.6 by El Nockrashy et al. (43). Multistage precipitation at different pH values does not significantly increase the yield of protein precipitate (34, 44). Despite extensive studies in the past, no process has been commercialized due to low protein recovery, poor product quality, and high cost of the processing methods. To reduce the product losses and improve product quality, a membrane-based process was developed for rapeseed and canola protein isolation starting with defatted meals (45, 46). It consists of five main steps: alkaline extraction, isoelectric precipitation, ultrafiltration followed by diafiltration, and drying (Fig. 4). Two protein isolates are produced: soluble and precipitated, with a combined protein recovery of more than 70% of the protein in the meal. Both products are high in protein (>90%), low in phytates (<1%), essentially free of glucosinolates (<2 µmol/g), and have desirable functional properties comparable to those of soy protein. This process is simple, economical, and seems commercially viable. Recently, it has been modified with additional membrane processing to remove phenolic compounds (47). As a result, both color and taste of the products have been significantly improved, thus making the process more attractive. The high quality of both canola protein isolates generated much commercial interest, but commercial production has not yet been initiated.

V. OTHER EDIBLE PROTEIN SOURCES AND PROCESSES A. Cottonseed Cottonseed kernels contain up to 50% protein. The presence of toxic gossypol in the pigment glands of cottonseeds makes the protein unacceptable for food use. Hence, all of the processes for production of cottonseed protein for human

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Figure 4 Membrane-based canola protein isolation process (66).

consumption involve removal of the pigment glands. A mixed solvent extraction process was developed for the simultaneous removal of the oil and pigment (48). In this process, cottonseeds are dehulled and flaked to a thickness of 0.07– 0.24 mm in the conventional manner. The moisture content is adjusted to 7– 15% by drying of the flakes at 60°C. The pigment glands in the meats remain intact under these conditions, so the gossypol generally does not mix with the protein of the meats. The flakes are then extracted and washed with a solvent mixture made of 53% acetone, 44% hexane or petroleum ether, and 3% water (v/v) at ambient temperature. Both oil and pigments are dissolved in the solvent mixture, and the extracted meats could have a protein content up to 90% or higher, and are essentially free of gossypol.

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High protein concentrates for human consumption are also produced by a liquid cyclone process (49). Preparation of the seeds entails drying, flaking, disintegrating, and separating by screen and gravity. The cottonseed flakes are mixed with hexane to form a slurry fed to a liquid cyclone for classification and separation. The underflow from the cyclone contains essentially all of the intact pigment glands whereas the overflow carries the fine solids with the desirable protein. After filtration and drying, a protein concentrate with a protein content of 73% is obtained. There has been limited commercial development of this process. B. Sunflower Seed Defatted sunflower meal can also be a valuable source of protein for incorporation into food products such as breakfast cereals, processed meat products, and snack foods, as both a protein supplement and a functional ingredient. Processes for the isolation of sunflower protein have also been based on the soy protein isolation process, involving alkaline extraction of the meal to extract protein at pH 9–11, followed by acidic precipitation of protein from the extract at pH 3.5–6 (50). The recovered precipitates contain more than 90% protein. However, the products thus obtained readily turn to an undesirable green color. This is caused by chlorogenic acid, a phenolic compound in sunflower meal. Once it appears, the green color cannot be removed from the product by any conventional technique. It is, therefore, necessary to remove the color-causing compound prior to protein isolation. This can be achieved by multistage acid washing of the meal with water adjusted to acidic pH close to that for precipitation (51). Another method is ultrafiltration of the alkaline extract of sunflower meal to remove chlorogenic acid before acid precipitation of protein (52). Both treatments lead to a much lighter colored protein isolate, but the complexity and expense of these processes have thus far prevented commercialization of sunflower protein isolates. C. Sesame Seed Sesame seed is an excellent ancient food source. Its oil content is greater than 50%. After oil removal the meal contains about 60% protein, and its dehulled flour is even richer in protein. In commercial processing, sesame oil is extracted by expelling and/or solvent extraction before the recovery of the protein materials. However, in a patented aqueous process both sesame oil and protein were recovered simultaneously (53). The dehulled cracked seed was extracted with calcium hydroxide solution. The mixture of oil and protein was then either (a) sent to a precipitation tank where the pH was brought down to 4–5, and the resulting precipitate was separated by centrifuge and spray dried; or (b) passed

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to an oil separator where the oil was removed by centrifuge, and the dissolved protein was then recovered as calcium proteinate by spray drying. D. Peanuts In peanut processing, it is essential to use the dehulled kernels in good condition in order to avoid aflatoxin contamination. Skins may also be removed by blanching followed by air aspiration to produce clean cotyledons. Prepress solvent extraction is commercially used for oil removal (54). Peanut kernels are ground in a hammer mill, and conditioned to desirable moisture content in a stack cooker. The cooked meats are expelled to prepress the oil. The prepressed meal is ground again and reconditioned before being extracted with hexane. The defatted flakes are desolventized in steam-jacketed tubes at temperatures increasing from 65°C to 107°C during the process. The desolventized flakes are cooled and ground to a flour that contains about 60% protein. Processes for making full-fat and partially defatted flakes from peanut kernels involve first grinding the low-moisture kernels to a flour consistency and mixing the flour with water to form an emulsion-suspension, which is heated to high temperatures between 93°C and 116°C to inactivate lipoxygenase. The solids are drum dried as flakes. Full-fat flakes contain 50% oil and 30% protein, whereas partially defatted flakes have 30% oil and 40% protein (55–57). E. Lupin Lupin is a legume with a high protein content ranging from 25% to 44%, depending on the variety. In the past its use has been limited to the feed for ruminant animals due to the presence of toxic alkaloids. The emergence of genetically improved “sweet” varieties low in alkaloids are now making it possible to use lupin as a protein source in food. Lupin is attractive particularly in areas where soybeans do not grow well whereas lupin is abundant, such as in northern Europe. Like soy and rapeseed proteins, lupin protein can also be isolated by a process based on alkaline extraction followed by acidic precipitation (58, 59). The protein in defatted lupin flour is approximately 70% soluble under slightly alkaline conditions such as pH 8.6. Lowering the pH to 4–5 permits precipitation of about 80% of the soluble protein. The obtained precipitates contain more than 90% protein. Although studies show that the protein efficiency of lupin protein alone is low, it is significantly improved when used in combination with additional proteins or amino acids (58). Due to their high solubility, lupin protein isolates can be used in protein-enriched drink formulations. In Chile, lupin protein products have been incorporated into milk-based formulas for school breakfast and lunch programs since the mid-1980s (58). Extensive development work is proceeding in Denmark and Germany. However, there are several hurdles due to the high toxicity of the untreated seed.

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REFERENCES 1 . MF Marcone. Biochemical and biophysical properties of plant storage proteins: a current understanding with emphasis on 11S seed globulins. Food Res Int 32: 79–92, 1999. 2. VA Portyanko, NR Sharopova, AA Sozinov. Genetic control and polymorphism of some subunits of storage globulin in cultivated oat. Cytol Gene 31:20–23, 1997. 3. DH Waggle, FH Steinke, JL Shen. Isolated soy proteins. In: RH Matthews, ed. Legumes: Chemistry, Technology and Human Nutrition. New York: Marcel Dekker, 1989, pp. 99–138. 4. WHO/FAO/UNU. Energy and protein requirements. Report of a joint WHO/FAO/ UNU meeting. Technical Report Series 724. Geneva: WHO, 1985. 5. SJ Circle, AK Smith. Processing soy flours, protein concentrates, and protein isolates. In: SJ Circle, AK Smith, eds. Soybeans: Chemistry and Technology, Vol. 1, Proteins. Westport, CT: AVI, 1972, pp. 294–338. 6. L Sair. Proteinaceous soy composition and method of preparing. US Patent 2,881,076 (1959). 7. GC Mustakas, LD Kirk, EL Griffin Jr. Flash desolventizing defatted soybean meals washed with aqueous alcohols to yield a high protein product. J Am Oil Chem Soc 39:222–226, 1962. 8. FA Norris. Protein recovery from seed meal. Canadian Patent 697,264 (1964). 9. CW Kolar, SH Richert, CD Decker, FH Steinke, RJ Vander Zanden. Isolated soy protein. In: AM Altschul, HL Wilcke, eds. New Protein Foods, Vol. 5, Seed Storage Proteins. New York: Academic Press, 1985, pp. 260–299. 10. WJ Wolf, JC Cowan. Soybeans as A Food Source. Boca Raton, FL: CRC Press, 1975. 11. HE Snyder, TW Kwon. Soybean Utilization. New York: Van Nostrand Reinhold, 1987, pp. 74–144. 12. GC Mustakas, EL Griffin Jr, VE Sohns. Full fat soybean flours by continuous extrusion cooking. World Protein Resources. Am Chem Soc Adv Chem Ser 57: 101–108, 1966. 13. AM Nash, WJ Wolf. Solubility and ultracentrifugal studies on soybean globulins. Cereal Chem 44:183–192, 1967. 14. CW Hutton, AM Campell. Water and fat absorption. In: JP Cherry, ed. Protein Functionality in Foods. Washington, DC: American Chemical Society, 1972. 15. N Catsimpoolas, EW Meyer. Gelation phenomena of soybean globulins. I. Protein–protein interactions. Cereal Chem 47:559–570, 1970. 16. FW Sosulski. Organoleptic and nutritional effects of phenolic compounds on oilseeds protein products: a review. J Am Oil Chem Soc 56:711–714, 1979. 17. JE Kinsella. Functional properties of soy proteins. J Am Oil Chem Soc 56:242– 258, 1979. 18. JW Knight. Wheat Starch and Gluten. London: Leonard Hill, 1965, pp. 84–105. 19. DA Fellers. Fractionation of wheat into major components. In: Y. Pomeranz, ed. Industrial Uses of Cereals. St. Paul, MN: American Association of Cereal Chemists, 1973, pp. 207–228. 20. RA Anderson. Manufacture of wheat starch. In: RL Whistler, EF Paschall, eds. Starch: Chemistry and Technology. New York: Academic Press, 1967, pp. 53–63.

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CE Rist. Batter process makes wheat flour a source of gluten and starch. Sugar J 11(9):26–27, 1949. 22. RA Anderson, VF Pfeifer, EB Lancaster, C Vojnovich, EL Griffin, Jr. Pilot-plant studies on the continuous batter process to recover gluten from wheat flour. Cereal Chem 37:180–188, 1960. 23. RJ Dimler, HA Davis, CE Rist, GE Hilbert. Production of starch from wheat and other cereal flours. Cereal Chem 21:430–446, 1944. 24. KL Phillips, HR Sallans. Ammonia process for separating starch and gluten from wheat flour. Cereal Sci Today 11(2):61–64, 1966. 24a. ISTA. Oil World Statistics Update. Hamburg: Mielke, 1998. 25. MT Lo, DC Hill. Evaluation of protein concentrates prepared from rapeseed meal. J Sci Food Agric 22:128–130, 1971. 26. FW Solsulski, G Sarwar. Amino acid composition of oilseed meals and protein isolates. Can Inst Food Sci Technol J 6:1–5, 1973. 27. R Ohlson, K Anjou. Rapeseed protein products. J Am Oil Chem Soc 56:431–437, 1979. 27a. CH VanEtten, ME Daxenbichler, IA Wolff. Natural glucosinolates in foods and feeds. J Agric Food Chem 17:483–491, 1969. 28. B Uppstro¨m, R Svensson. Determination of phytic acid in rapeseed meal. J Sci Food Agric 31:651–656, 1980. 29. M Naczk, LL Diosady, LJ Rubin. The phytate and complex phenol content of meals produced by alkanol- ammonia extraction of canola. Lebensm Wiss Technol 19:13–16, 1986. 30. A Rutkowski, H Kozlowska. Chemical constituents and protein food processing of rapeseed. J Am Oil Chem Soc 56:475–477, 1979. 31. M Cheryan. Phytic acid interaction in food systems. CRC Crit Rev Food Sci Nutr 15:297–335, 1980. 32. A Girault. The study of some properties of rapeseed protein with a view to protein concentrate production. J Sci Food Agric 24:509–518, 1973. 33. AJ Finlayson, RS Bhatty, SL McKenzie. The effects of extraction solvents on the yields and structures of Brassica sp. meal proteins. Can Inst Food Sci Technol J 9:212–215, 1976. 34. L Gillberg, B To¨rnell. Preparation of rapeseed protein isolates: Dissolution and precipitation behavior of rapeseed proteins. J Food Sci 41:1063–1069, 1976. 35. FM Blaicher, F Elstner, W Stein, KD Mukherjee. Rapeseed protein isolates: effect of processing on yield and composition of protein. J Agric Food Chem 31:358– 362, 1983. 36. P Pfaender. Lysinoalanine—a toxic compound in processed proteinaceous food. World Rev Nutr Diet 41:97–109, 1983. 37. RS Bhatty, SL McKenzie, AJ Finlayson. The proteins of rapeseed (Brassica napus L) soluble in salt solutions. Can J Biochem 46:1191–1197, 1968. 38. DF Owen. Detoxification and isolation of rapeseed protein by aqueous saline extraction and isoelectric protein precipitation. US Patent 3,758,452 (1973). 39. RD Siy, FDF Talbot. Preparation of low-phytate rapeseed protein by ultrafiltration: II. Membrane development and testing. J Am Oil Chem Soc 58:1021–1023, 1981.

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41. 42.

43.

44. 45.

46. 47. 48. 49.

50. 51. 52.

53. 54.

55. 56. 57. 58. 59.

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TJA Finnigan, MJ Lewis. Nitrogen extraction from defatted rapeseed, with particular reference to United Kingdom commercial rapeseed meal. J Sci Food Agric 36:520–530, 1985. FW Sosulski, A Bakal. Isolated proteins from rapeseed, flax and sunflower meals. Can Inst Food Sci Technol J 2:28–32, 1969. E Keshavarz, RK Cheung, RCM Lui, S Nakai. Adaptation of the three stage extraction process to rapeseed meal for preparation of colorless protein extracts. Can Inst Food Sci Technol J 10:73–77, 1977. AS El Nochrashy, KD Mukherjee, HK Mangold. Rapeseed protein isolates by countercurrent extraction and isoelectric precipitation. J Agric Food Chem 25: 193–197, 1977. LP Kodagoda, S Nakia, WD Powrie. Preparation of protein isolates from rapeseed flour. Can Inst Food Sci Technol J 6:135–141, 1973. YM Tzeng, LL Diosady, LJ Rubin. Production of canola protein materials by alkaline extraction, precipitation, and membrane processing. J Food Sci 55:1147– 1151, 1990. L Xu, LL Diosady. The production of Chinese rapeseed protein isolates by membrane processing. J Am Oil Chem Soc 71:935–939, 1994. L Xu, LL Diosady. Removal of phenolic compounds in the production of highquality canola protein isolates. Food Res Int 35:23–30, 2002. WH King, VL Frampton. Cottonseed meal of improved quality. US Patent 2,950,198 (1960). EA Gastrock, EL D’Aquin, PH Eaves. High-protein oilseed, particularly cottonseed, concentrate production by a continuous process. US Patent 3,615,657 (1971). LP Hanson. Vegetable Protein Processing. Park Ridge, NJ: Noyes Data, 1974, pp. 186–214. DE O’Connor. Light-colored protein isolate from sunflower meal by acid washing prior to alkaline extraction. US Patent 3,586,662 (1971). DE O’Connor. Isolating light-colored protein from sunflower meal by alkali extraction under an inert gas blanket followed by membrane ultrafiltration. US Patent 3,622,556 (1971). JJ Liggett. Recovery of protein and oil from oilseeds with a saturated calcium hydroxide solution. US Patent 3,476,739 (1969). JG Endres. Legume protein flour and concentrates. In: RH Matthews, ed. Legumes: Chemistry, Technology, and Human Nutrition. New York: Marcel Dekker, 1989, pp. 139–160. JH Mitchell. Process for making peanut flakes. US Patent 3,689,287 (1972). JH Mitchell. Peanut flakes. US Patent 3,800,056 (1974). JH Mitchell. Process for making flavorless food extenders derived from peanuts, and a method of recovering peanut oil. US Patent 3,947,599 (1976). J King, C Aguirre, S de Pablo. Functional properties of lupin protein isolates (Lupinu albus cv Multolupa). J Food Sci 50:82–87, 1985. FS Taha, SS Mohamed, AS, El-Nockrashy. The use of soya bean, sunflower and lupin seeds in the preparation of protein bases for nutritious beverages. J Sci Food Agric 37:1209–1216, 1986.

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8 Sugars and Carbohydrates Pascal Christodoulou Hellenic Sugar Industry S.A. Thessaloniki, Greece

I. INTRODUCTION: THE AIM OF EXTRACTION Solid–liquid extraction is one of the unit operations of process engineering, the objective of which is to effect the migration of a substance enclosed in a solid insoluble matrix to a surrounding solvent, analogous to a desorption. In the case of sugar beet extraction (for sugar cane extraction see Secs. X–XV, and for starch extraction see Sec. XVI and XVII), sucrose is present in the form of an aqueous solution (juice) in the cellular structure of the sugar beet the solvent within the solid matrix is identical to the external extraction medium. To allow the sucrose to pass through the tissue, the semipermeability of the cell membranes must be overcome by thermal denaturation. The exit surface for the diffusion is enlarged by slicing the beet. The transport of substance by diffusion (release of sugar and entry of water into the cossettes) results from the concentration gradient existing in the cossettes. The driving force of diffusion appears to be not only the concentration gradient but rather the chemical potential. As the extraction proceeds, not only does this gradient diminish, but the concentration of nonsugar substances in the extract increases, so that the whole process must be stopped at the appropriate point. Extraction is counted as one of the major stages of manufacturing process in the sugar industry (Fig. 1). The change over time of the mean sucrose concentration in the cossettes is determined by a number of process parameters. To a large extent these can be influenced by operational measures (draft, temperature, shape and surface of the cossettes, as well as load factor and design of the diffuser). Uncontrolled, on the other hand, are properties inherent in the cellular structure of the beet, which depend on the variety, time of harvesting and storage conditions, as well as possible diseases and frost damage.

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Figure 1 Block diagram of process units from beet or cane. 236

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Over the years numerous researchers have contributed to investigate the theoretical understanding of the course of cossette extraction and to analyze the effect of changes in process engineering and design. Pursuit of the theory of sugar extraction ultimately aims at allowing the most exact simulation of the process possible within the context of process control and to achieve its optimization, not only in terms of extraction yield and juice purity, but also in the design of the installations (1). During the extraction of sugar from the sugar beet or sugar cane the physical processes are prevailing. The soluble substances of beet cossettes and cane particles are extracted, and remaining are the exhausted pulp and bagasse, respectively. The aim of extraction should be to win an extract that contains as low as possible of nonsugars, has a high concentration, and can be further processed in an economical way. During extraction the destruction of sugar through thermal or microbiological activity should be avoided (2). The operating parameters of extraction should ensure good pressability of the pulp (3). The issue of optimization of sugar extraction is a dynamic problem. The subject is not simple because with the parameters numbering more than four their interdependence is quite complex (4). More information about optimization of beet extraction is given in Sec. VIII. In the technical application of sugar extraction (raw material sugar beet or sugar cane) and of starch and carbohydrates (raw material corn or potatoes) there are some similarities and some differences. One similarity is that for the extraction of sugars warm water of pH 5.5–6.0 (50–55°C in the case of starch extraction from corn, 70–80°C in the case of sugar extraction from sugar beet or sugar cane) in countercurrent direction to the high concentration is used. A difference is that the solid insoluble matrix varies in every case and the techniques of extraction (diffusion, milling, wet milling) are modified accordingly. The similarities and the differences in the process of extraction are mirrored also in the technological equipment applied in each case. In the case of sugar beet and sugar cane, the equipment used for the preparation of the raw material before the main process of extraction is different, i.e., slicing machines in the case of sugar beet (Fig. 9) and shredders in the case of sugar cane (Fig. 28), but the main extractors are similar (Figs. 16, 20, 31). The equipment used for the extraction of starch from corn presents some similarities to the old discontinuous Robert diffusion batteries, which were used in the beet sugar industry until the end of the 1950s and are now obsolete.

II. COMPOSITION OF SUGAR BEET AND CHEMICAL BEHAVIOR OF CONSTITUENTS DURING EXTRACTION Advances in analytical chemistry in recent decades, particularly the development of chromatographic and enzymatic methods, have considerably expanded

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the understanding of the nonsugar substances in sugar beet. A typical chemical composition of sugar beet is presented in Table 1. By the marc content of sugar beet is meant the total beet components remaining after complete aqueous extraction of the soluble constituents under industrial processing conditions of temperature, pH value, and duration (6). The marc content affects the amount of pressed pulp produced and thus is relevant to the mass balance of extraction. Knowledge of the marc content or the volume of juice derived from it is necessary for the analytical determination of the sugar content in beet. Cellulose 0.9–1.2%, hemicellulose 1.1–1.5%, pectin substances 0.9–2.4%, and lignin 0.1–0.3% are the principal constituents of sugar beet marc and of the skeletal substance of exhausted pulp. However, not to be overlooked are significant quantities of proteins (0.1– 0.4%), lipids (0.05–0.1%), saponins (0.05–0.1%), and ash constituents (0.1%). More information about the behavior of the beet constituents during extraction is given in the literature (5, 7). Invert sugar, apart from respiration of sugar, normally accumulates in the beet during storage at a rate of 10–20 g/(t.d) beet. Invert sugar can be considered as a quality criterion of beet handling and sucrose loss, i.e., treatment of the beet in the period after harvesting and during storage. It is important to note that the organic acids increase in the thin juice owing to invert sugar degradation during juice purification (8). A. Morphology of Sugar Beet Sugar beets, as grown for sugar production, are the storage organs of the biennial Beta vulgaris saccharifera in its first vegetative year. The tap roots are spindle shaped or coniform, and have white flesh with a white or bone-colored

Table 1 Chemical Composition of Sugar Beet (5) Component Water Dry substance Sucrose Nonsucrose substances Water-insoluble compounds (marc) Soluble compounds Nitrogen-free organic compounds Nitrogenous compounds Inorganic compounds

g/100 g beet 73.0–76.5 23.5–27.0 14.0–20.0 7.0–9.5 4.5–5.0 ⬃2.5 0.9–1.1 1.0–1.2 0.4–0.5

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rind. All sugar beets currently grown in the world have their origins in the socalled white Silesian beet, selected as new cultivar out of a multitude of folder beet strains by Franz Achard at the end of 18th century. The body of the beet, after elimination of the leaves, is divided into three zones: the epicotyl or crown, the hypocotyl or neck, and the tap root (Fig. 2) (9). B. Ultrastructure of the Native Sugar Beet Root The inner structure of the sugar beet tissue is similar to that of other dicot plants. The following cell types can be distinguished among others: - Meristem cells are young cells that divide. - Parenchyma cells are capable of dividing only in a limited way, with an average diameter of 40–60 µm. Their main function is sucrose storage, and they account for about two-thirds of all cells in the sugar beet root. - Phloem cells are involved in the transport of dissolved organic substances.

Figure 2 Beet root showing morphological and technologically important zones (40).

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- Treachery elements, of 20–40 µm diameter, serve for the transport of water and dissolved ions; dead cells, the walls of which are stiffened by lignin incrustations, together form a tubular system, the xylem (Fig. 3). - Epidermal cells cover the outer beet surface. The vacuole accounts for about 95% of the total volume of the parenchymatous cells in sugarbeet. Bounded by a membrane, the vacuole contains almost exclusively dissolved sucrose. The remaining 5% of the inner cell space is taken up by the cytoplasm that contains the cell nucleus and other organelles (Fig. 4). Sucrose is synthesized in the leaves of the living, unharvested sugar beet plant and transported via the vascular system along the phloem (Fig. 3) to the storage tissue of the root. Whereas the diffusion of sucrose from the cell wall through the adjacent cell membrane into the cytoplasm is only moderately impeded, the vacuolar membrane presents a barrier of extremely low permeability for sucrose. In the living cell, this barrier is overcome by active, energy-consuming transport, which produces the high sucrose concentration of about 0.5 mol/ L in the vacuole. In contrast, the sucrose concentration in the cell wall amounts to only about 0.06 mol/L. The resulting osmotic pressure of 0.4–0.8 MPa, depending on the metabolic activity, pushes the cell walls together and gives them their mechanical stability (10). In Figs. 37–40 are given computer simulations, showing the electrostatic potential profiles (Fig. 38) and the hydrophilic and hydrophobic regions of the sucrose molecule. In Fig. 41 is given the hydrophobic topographies for the amylose fraction of starch (46, 47). These computer simulations contain a tremendous amount of information. The complexity of the molecule surface of sucrose and amylose can explain the complexity of their extraction process. C. Physical Properties of Beet The technological value of the beet is affected not only by its components but also by its morphological and physical characteristics (11). The elastic and plastic properties of beet affect slicing and the behavior of the cossettes during the extraction process. These properties are identified above all by the elasticity modulus, the breaking stress, and the bending capacity (12). D. Physical Properties of Beet After Denaturation Denaturation means killing the living plant tissue. This can be done by heat, freezing, chemical action, electroplasmolysis, or ultrasound. The technological objective is to improve the material transport of the substances immobilized in the living cell through the tissue and into the extraction liquid. In industrial processing, denaturation takes place by heat (70–78°C).

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Sugars and Carbohydrates

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241

Figure 3 Vascular system of beet (40). Py, Parencyma tissue; Sc, sclerenchyma tissue; pX, protoxylem; X, xylem; C, cambium (area between phloem and xylem); P, phloem; s, sieve tubes; g, companion cell.

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Figure 4 Parenchyma cell from a young sugar beet (4 months old), 1200 times enlarged (40). A, Amyloplast; C, cytoplasm; I, intercellular space; L, lipid vesicle; M, mitochondrion; N, cell nucleus; T, pit; Pl, plasmalemma; V, vacuole.

In principle this leads to continuous alteration of the cell tissue. Not only are the cell membranes destroyed but the cell walls also change their inner chemical structure through hydrolytic degradation reactions (molecular chain breakage and detachment of polysaccharide fragments). Despite the chemical changes caused by temperature, the cell retains its physical integrity. Thus, it continues to be a barrier to mass transport throughout the subsequent pressing process. Following denaturation the tissue has lost its strength and the cell liquid can be squeezed out because the osmotic pressure exerted on the cell wall through the membrane is absent (13). Conventional sugar technology uses the minimal temperature stability of the cell membranes during the heating of the sugar beet cossettes to greater than 70°C in the extraction apparatus. Only remaining is the cell wall with a thickness of 2 µm, which can be considered from its porous texture as an ultrafiltration of this physical membrane. Figure 5 shows the inner structure of this physical membrane. In this scheme it is possible to recognize the cellulose fibrils and other soft polymers, between them also the pectin, where these fibrils combine in chain formations. In an electron microscope it is possible to see clearly the pores that are formed through the cellulose fibrils (Fig. 6) (14). E. Fractal Structure of the Beet Tissue Examination of Figs. 3, 5, and 6 clearly reveals the self-similarity in different scales that is the fractal structure of the beet tissue. The majority of the biological

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Sugars and Carbohydrates 243

Figure 5

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Inner structure of the physical membrane in flowering plants (14, 41).

244

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Christodoulou

Figure 6 Electronic microscopic photo of the cellulose microfibrils in the cell wall of onions, free from pectin, beam length 0.2 µm (14, 41).

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tissues have a fractal structure, and beet tissue belongs in this category (15). In a biological tissue the fractal chaotic structure ensures the following advantages: - In very small volumes huge surfaces are found. - In an usual Euclidean solid or surface the diffusion grade of one substance under the influence of random fluctuations is directly proportional to time. In the fractal structure the diffusion grade increases very quickly at a power of time t n (n > 1). This phenomenon is called overdiffusivity. - One fractal structure can be constructed in a short time from a simple algorithm, which is repeated in different scales without significant changes (16). The phenomenon of overdiffusivity of beet tissue during extraction by hot water can explain the observed acceleration of the diffusion rate, especially in the beginning of the extraction process.

III. MATHEMATICS OF EXTRACTION A considerable advance was made in the mathematical theory of extraction by Silin (17) when he developed equations relating the various factors affecting extraction. He started with Fick’s law, a special case of Fourier’s general diffusion law: ds = D ⴢ A ⴢ

dc ⴢ dt dr

(1)

where ds is the weight of the dissolved substance diffusing through the area A in time dt and dc/dr is the concentration gradient of the dissolved substance. D is the diffusion coefficient, which depends on temperature according to the Einstein correlation D=

kⴢT η

(2)

where k is a constant for the dissolved substance, T the absolute temperature, and η the viscosity of the solution (18). The diffusion coefficient D is a measure of the mass transport velocity in the cossettes. In Table 2 diffusion coefficients of sucrose from beet tissues are given according to different authors. It must be noted, however, that the values for sugar beet shown in Table 2 do not refer to identical material, as would be desirable (13). Schliephake and Wolf (19) investigated the mechanism of sugar extraction, especially during the initial stages. They defined three basic phases: osmo-

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Table 2 Diffusion Coefficients of Sucrose from Beet Tissues (40) Diffusion coefficient [cm2/(s.10−5)] 0.6–1.13 1.0–1.17 0.5–1.0 0.8–1.2 0.82–0.92 0.8–0.88 0.91 1.0

Temp. (°C) 75 75 75 70 70 75 65–75 75

sis, denaturation, and diffusion (Fig. 33). During the initial phase, water enters the cell by osmosis. Denaturation increases the permeability of the cell wall, and the juice–water mixture is forced to flow out of the cell by the sudden change of the cell pressure. Only after the completion of those two initial reactions can diffusion be considered the rate-determining process. The duration of the initial phase depends on the extraction temperature and corresponds to the time needed for denaturation. During that time, less than 5 min under normal operating conditions, more than 20% of the sugar is extracted. Figure 7 shows the three basic phases, as indicated by the change of the relative concentration of sugar and water in the beet slices, during the extraction with water, at two temperatures. The straight line represents the theoretical extraction, during which the water and sugar concentration should remain nearly constant relative to each other throughout the process, if diffusion is the only rate-determining factor. The maxima and the minima of the curves indicate the completion of the first two stages: osmosis and denaturation (19). Only in the ideal case does the extraction liquid pass by the cossettes in laminar flow. In reality, there is some turbulence that is superimposed on the desired direction of the material transport along the extraction route and is characterized by the axial dispersion coefficient Dax,2. Superimposed on this micromixing of the liquid phase is a macromixing caused by the churning of the essentially uniformly moving cossettes and which, analogous to the behavior of the liquid, can be represented by an axial dispersion coefficient Dax,1. According to Buttersack and Schliephake (1), the extraction is described by the following partial differential equation: ∂c2 ∂c¯ 1 ∂c¯ ∂2c¯ = + u ⴢ Φ ⴢ 1 − Dax ⴢ Φ2 ⴢ 21 ∂t ∂t ∂z ∂z

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(3)

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Figure 7 Change of water, with change of sugar concentration in the beet during extraction, with water at 60°C and 70°C (18, 19).

where c1 and c2, respectively, represent the sucrose concentration in the cossettes and in the liquid along the extraction route z, while Dax is the axial dispersion coefficient comprising the effective contribution of macro and micromixing (Dax,1 or Dax,2, respectively). The mean cossette velocity is u1 and Φ is the ratio of the difference velocity u (between cossettes and liquid) and the mean cossette velocity u1. Φ=

u u1

(4)

Formula (3), being a partial differential equation, has only chaotic solutions (15, 45). Buttersack and Schliephake (1) and Christodoulou (15) emphasized the complexity of the extraction process. Other theoretical handling of extraction in beet sugar factory is given by Claasen (6), Dubourg (24), Bru¨niche Olsen (25), Silin (17), Ebell and Storz (18), and Schneider and Reinefeld (22). The principles of extraction was the priority subject of the 15th General Assembly of the International Commission of Sugar Technology (CITS) in Vienna 1975 (26).

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IV. TECHNICAL EXTRACTION OF BEET In all extraction systems, fresh cossettes are heated and subsequently extracted in a countercurrent of liquid and solid phases. The extraction equipment is fed at one end with fresh cossettes, and exhausted cossettes are removed at the other end. At this end, feed water is introduced and passes through the extractor countercurrent to the cossettes. An extraction plant is made up of (Fig. 8): -

Cossette production, i.e., slicing Cossettes heating (denaturation or scalding) Transport of cossettes to the extractor and removal A countercurrent exchange section Fresh water supply and separation of exhausted cossettes Pulp-pressing equipment Press water return (20)

V. SLICING MACHINES Only two types of machine out of those which came into being more than 100 years ago were successful, namely, the disk- and drum-slicing machines. A. Disk-Slicing Machines The disk-slicing machine is characterized by its vertical configuration and horizontally rotating slicing disk. The diameter of the disk has been increased over the years to 2780 mm. With 32 knife blocks in total and a slicing length of 400 mm per knife box, the slicing capacity can be raised to about 4000 t/d depending on the knives used and the cossette thickness. B. Drum-Slicing Machines The drum-slicing machine consists of a horizontally rotating drum in which the knife blocks are carried on supporting members (Fig. 9). The knife blocks are depending on the manufacturer, fitted with two or three row boxes. The box length is 600 mm, which means that there are three knives per row, grouped side by side. With a drum diameter of 2200 mm and 22 knife blocks each with 3 rows, the maximum output of the machine is 8000 t/d, according to the manufacturer’s claim.

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Figure 8 Block diagram of beet extraction process (40).

C. Control of Throughput The slicing capacity of the machine is proportional to the rotational speed of the disk or drum. Control of slicing output is achieved via a belt-weighing scale that is situated in the conveyor leading to the extractor. This picks up the deviation from the preset value and transmits a signal to the rotational speed controller on the slicing machine that raises or lowers the slicing rate to the correct level. Drum and disk slicers are fitted with frequency inverters. The optimal slicing speed lies in the range of 2.0–5.0 m/s, which corresponds to a drum rotational speed of 25–50 min−1. D. Cleaning Both disk- and drum-slicing machine are fitted with special cleaning arrangements. These consist of a blow-out device using compressed air, alone or in combination with rotating brushes or high-pressure steam. These arrangements

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250 Figure 9

Drum slicing machine 2000-600-60 (Maguin) (40).

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should ensure that while the machine is operating, fibers and other small fragments are removed from the face of the knives. In this way, the slicing throughout can be maintained over a longer period and the quality of the cossettes can be kept consistent. E. Comparison of Types of Slicing Machines Considering the situation today, it can be said with certainty not only that the drum-slicing machine yields a greater output but that its technological advantages enhance its future prospects. F. Knives The quality and shape of cossettes will be determined by the choice of the beet knife. Of the many types of beet knives the Ko¨ nigsfelder knife has been successful, mainly as a semi-cutting knife. Based on the requirements of the extraction, beet knives with different numbers of divisions are used as a corrugated splitter chevron or square cossettes as wanted; then A or B knives synchronized or offset are installed in the knife blocks (Fig. 10). The same beet knives are suitable for disk-slicing machines and drumslicing machines. The length of the knife can be 137, 167, or 200 mm (Fig. 11). New slicing machines, independent of the manufacturer, are presently being equipped with 200-mm knives. There are four steps in the preparation of the knives: -

Cleaning and dismounting of the knife blocks Straightening and dressing of the knives Routing of the knives and finishing by filing Reassembly and adjustment of the knife blocks

The first of these steps will be mainly manual. The other steps are performed either semiautomatically or automatically. The consumption of the knives depends principally on the preparation of the beet and the treatment of the knives themselves. The consumption should be of the order of 3–5 knives per 1000 t of beet processed. G. Ventilation of Slicing Machine Operating Position Inadequate space can cause aerosols to appear in the vicinity of the machine. Adequate ventilation should be introduced to avoid impairing the health of the operator (20).

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Figure 10 Cutting sequence for corrugated splitting knives (40). Top: synchronized. Bottom: offset.

VI. PROCESS PARAMETERS OF EXTRACTION A. Quality of Cossettes The size and physical condition of the cossettes are of great importance for extractor performance. Since time is required for sugar to diffuse through a given distance, advantage is gained by shortening the distance. The Silin number and the Swedish number serve as indicators of cossette quality. 1. Silin Number Length of 100 g of cossettes in meters, thus giving a standard for the fitness and surface area of the cossettes. This parameter depends on the number of

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Figure 11 New knife designation (Putsch), new designation, e.g., Ko¨nigsfelder knife, model 1050: 200 × 87 with 28 divisions at 7.2 mm (19 in number) (40). Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved.

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knife divisions, type of knife, and clearance to the fore layer; it ranges between 10 and 18. 2. Swedish Number Standard for the permeability of the cossettes’ bed. Measured by the quotient: Mass of cossettes >5 cm long mass of cossettes <1 cm long The ratio of the portion of the cossettes >5 cm long to that <1 cm long should be greater than 10. 3. Mush Content The mush content is the mass of cossettes <1 cm long in relation to the total cossettes’ mass. This should not exceed 5%. 4. Draft The ratio between the beet processed and the raw juice extracted expressed in percentage, either in terms of mass or volume (normally not used), is referred to as the “draft” (21). Draft =

weight of diffusion juice × 100 weight of beets

Drafts used vary between 100 and 135; sugar lost in pulp and draft varies inversely. Steam cost and evaporator capacity have to be balanced against the additional sugar obtained (18). Their quality must be adapted to the extractor in use. Percolation rate across a cylinder packed with cossettes decreases rapidly if the Swedish number falls below 10. Maximal percolation rates are obtained at values >15. The rigidity (turgidity) of cossettes has a considerable influence on cossette permeability. This depends on the quality of beet and can deteriorate as a result of storage or frost. Process parameters that can modify cossette rigidity are the extraction temperature and the pH value, as well as the feed water pH value and the cation composition. Foaming of the liquid phase and the presence of fines can also reduce the permeability (21). 5. pH Value The cell juice of sound beets varies between 6.3 and 6.6. In the absence of a diffuser infection the pH of the diffusion juice is about 0.1 unit lower than the pH of the cell juice. Within the diffuser a lower pH may have beneficial effects. At a pH of 5.5 the diffusion of impurities appears to be at a lower level. This

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might be a result of degradation of the pectins at higher pH. The exhausted pulp is firmer and permits more efficient removal of the water in the pulp presses. Considering corrosion, a pH of 6.0 or slightly below 6.0 within the diffuser appears to be a good compromise. To obtain this, not only the supply water but also the press water may have to be acidified (18). Acids used for pH control in extraction are sulfuric, sulfurous, or rarely hydrochloric acid (a highly corrosion-resistant static mixer is needed); 95% of the anions of these acids are found in the raw juice. Sulfuric and sulfurous acids are precipitated to an extent of about 50% in the juice purification (21). 6. Temperature The temperature levels in the extractor are generally between 68°C and 75°C. The target temperature in extraction is dictated by several variables: - Denaturation of the beet cells. - Protection against microbiological development, which is achieved above 73°C. - Rate of extraction. - Limiting of the thermal degradation of the beet marc, especially the cellulopectic skeleton. To denature cossettes should be heated as quickly as possible to 70–75°C. Extraction accelerates as temperature rises because the diffusion rates increase with increasing temperature; for example, a fall in the sucrose losses from 0.25 to 0.20 g/100 g beet at equal draft can be mathematically forecasted for a rise in temperature from 69°C to 73°C. Sugar losses from microbiological activities also decline with rising temperatures. Extraction above 75°C is impossible because of the inherently labile nature of beet at elevated temperature. This is due to the breakdown of the cell wall substances, especially through pectin degradation. Targeted average temperatures for normal beet material are about 70– 73°C. Under technical conditions, taking into account the extraction losses, mechanical properties of the cossettes during their transport through the extractor, pressability of the cossettes, as well as the filtration properties in juice purification, one will gradually approach the optimal operating temperature. Excessive operating temperatures affect the processing of healthy beet negatively. Much more attention has to be paid to the operational temperatures in the case of deteriorated beet (21). Schneider and Reinefeld proposed a temperature profile as displayed in Fig. 12 for healthy mature sugar beet (22). 7. Time The time during which the cossettes are in contact with the surrounding liquid is considered the extraction time. The average extraction time should not exceed

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Figure 12 Optimal temperature profile for juice extraction using normal, healthy beet (40).

75 min. The breakdown of cell wall substances (e.g., pectin) increases with increasing average extraction time and poor retention time distribution. This leads to deteriorating juice and pulp quality. Short extraction times lead to improved utilization of the equipment (21). 8. Bacterial Control The very broad range of sugar losses due to microbial activity found in practice, i.e., from 0.05% to 0.2% on beet, is certainly due to operating conditions. Minimization of sucrose losses or formation of acids, improved pulp dewatering, and avoidance of the addition of bacteriostats are arguments either in favor or not in favor of bacterial control. Bacterial control in extraction equipment is common practice with formaldehyde application as a standard procedure in countries, where its use is allowed. However, other substances, such as SO2, carbamates, quaternary ammonium compounds, and hydrogen peroxide, have also been tested and applied. Avoiding the use of any bacteriostat has been practice in the Japanese and Austrian sugar industries. With deteriorated beet that has undergone freezing, significant activity by mesophilic microorganisms can occur. Notably, lactic acid bacteria grow under such conditions when no appropriate control is possible (23).

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Hein and Pollach proposed the use of products from hops extracts to inhibit thermophilic microorganisms while avoiding the potential health hazards usually associated with the application of bacteriostats (44). 9. Extraction Water Historically the first Roberts discontinuous batteries were fed with fresh pure water. All the extracted water from pulp straining and pressing was discarded as waste water. Nowadays for environmental as well as economic reasons the press water is reintroduced into the extraction system. The mass balance in Fig. 21 gives an example of the press water and fresh water flows. The more effective the pressing of the pulp, the greater is the mass of press water and the lower the necessary complement of fresh water. When the dry substance content of the pulp increases, its mass on beet decreases. With increasing pressed pulp dry substance content, for the same sugar losses on beet, the sugar contents of the pressed pulp and the press water increase. It is possible to introduce fresh water and press water separately. The fresh water is fed at the tail (top) of the extraction equipment and the press water at the point of the extractor where the liquid phase in the extraction equipment has the same sugar content as the press water. It is also possible to mix the two water feeds and to introduce the mixture at the tail (top) of the extraction equipment. Condensates, used as fresh water, are alkaline and contain ammonia. It is necessary to acidify them so that the pH value at the tail (top) of the extractor is below 6. The desired temperature is obtained by mixing cold and warm condensates. In the case of feeding the extraction with mixed water, it is possible to control the temperature of the mixture by adjusting the amount of the different kinds of water without reheating (21). 10. Pulp Press Water Treatment Pulp press water contains colloids and small pulp particles, which in some cases are eliminated by a pulp screen before passing a heat exchanger and reintroduction into the extractor. Because of the heat losses during pulp transport and pressing, press water is generally obtained at below the extraction temperature. At press water temperatures thermophilic bacteria can develop. To fight this cause of infection, heating the press water before its reintroduction into the extraction equipment is desirable. Complete pasteurization requires heating to about 90°C for the required time. In fact, this is rarely done, but some heating is always done (21). 11. Pressing Aids To improve the pulp pressing, pressing aids are used. Generally these pressing aids are di- or trivalent cations—Ca2+ or Al3+ —which are mostly added as sul-

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fates. These polyvalent cations increase the rigidity of the pulp. Aluminum sulfate is added as a water solution. Calcium sulfate can be produced in the factory by reacting sulfuric acid with milk of lime or carbonation lime. Natural gypsum is also used. The effect on the dry substance content of the pulp is maximal for a consumption of aluminum sulfate of about 350 g/t beet (i.e., 6 eq Al3+ /t beet) and for a consumption of calcium sulfate of about 1000 g/t beet (i.e., 13 eq Ca2+ /t beet). The addition of such pressing aids modifies the ionic composition of the liquid phase by yielding higher concentration of Ca2+ or Al3+ throughout the extractor. Fixation of these ions begins when the cossettes mix with the juice so that the rigidity of the cossettes as well as the permeability of their mass are both improved (21). 12. Antifoaming Agents The beet root contains surfactive components that pass into the liquid phase in the extractor. Production of persistent foam occurs by air inclusion within the cossettes’ mass. Foam inhibits circulation of the liquid in the extractor, so that it is necessary to use antifoaming agents. There are many kinds of antifoaming agents, including fatty acids, that are used in the extraction equipment at levels between 0 and 150 g/t of beet (21). 13. Presence of Oxygen During Extraction Extractor design has an influence on the aeration of the juice–cossette mixture. In this respect there are essential differences between the drum (RT) and belt (De Smet) extractors, as well as between trough (DDS) and tower extraction equipment. Oxidation of polyphenols to melanins, may be different in different extraction equipment. Polyphenols are readily oxidized to melanins, which are removed in juice purification. The total polyphenol contents in DDS and RT extractors were about 11 mg/L. The oxygen content of the cossette–juice mixture influences the composition of the metabolites in the event of developing bacterial activity (21). 14. Extractor Operation with Deteriorated Beet The impact of alternating freeze and thaw cycles can lead to considerable degradation of the cell structure in sugar beet. Such sugar beets are not suitable for normal extractor operations. The extraction procedures must therefore be adapted to the substantially altered beet material even if extraction loss increases. Deteriorated sugar beets are unsuitable for producing desirable cossettes because the mush content increases. Percolation of the extraction fluids through cossettes having a high mush content is considerably impaired. Operation with coarser cossettes is therefore necessary to avoid mechanical problems (plugged screens,

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extractor plugs). Extractor temperatures are generally lowered (70°C maximum) to minimize further breakdown of the cell wall substances. It could be desirable to reduce the retention time of the cossettes in the extraction fluid, consequently reducing the average extraction time. Operating capacity is substantially reduced; 5% of the beet material is partly degraded. Periodic shock treatment with bacteriostats (formaldehyde among others, depending on approval for this application) is recommended to reduce the bacteriological activity. Without such measures being taken, the microorganisms present in the deteriorated beet, being associated with the decomposition, will increase substantially (21). 15. Diffusion Juice The juice drawn from the extractor contains a considerable amount of colloidal matter besides a large number of fine pulp particles, which are difficult to remove by screening. It is desirable to eliminate most of the suspended matter before the juice is sent to the purification process. The color of the juice is usually gray and changes to a dark gray or almost black on contract with the air. This darkening is caused by enzymatic reactions and is accelerated in the presence of iron. However, the color of the diffusion juice is of little importance as it is removed without difficulty during purification. The nonsucrose content of the diffusion juice is related to the quality of the beets and the conditions under which the sugar is extracted in the factory. In some beet-growing areas, storage after harvesting has more effect on the quality of the diffusion juice than normal variations of the growing conditions as experienced from year to year. The following percentages of the mineral components are extracted from the beet: 80–90% of K and Na, 10–30% of Ca, 60–80% of Mg, and about 80% PO43− and SO42−. Amino acids and betaine are almost completely extracted, and the protein and pectin content of the juice is a function of pH and temperature. At higher temperatures more pectins are dissolved, but the protein content appears to be at its lowest level at about 70°C. The following organic non–amino acids are present in larger quantities: citric, oxalic, malic, acetic, and lactic acid. There are only minute amounts of lactic acid found in healthy beets’ acid; its presence in the diffusion juice usually indicates a thermophilic infection in the extractor. The invert sugar content in the juice normally ranges from 0.4% to 0.8% on solids. Higher concentrations are found during processing of stored beets, up to 2.1% on solids (18).

VII. BEET EXTRACTORS In the first sugar beet factories, the sugar juice was obtained from the beet by pressing (the procedure followed for centuries with sugar cane). While the structure of sugar cane is extremely fibrous and therefore suitable for pressing, beet

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tissue is soft, and not particularly permeable, which makes difficult even a moderate exhaustion of the sugar juice contained in the beets by pressing. In the efforts of finding a method other than the uneconomical one of pressing, the first to succeed were Florent Robert and his son Julius Robert. After a long period of experiment, they succeeded for the first time, in the 1864 campaign at the Seelowitz factory in Ma¨hren, in producing juice on a factory scale by what they called “osmotic maceration” (25). Historically, the Robert battery, also called a “batch diffuser” and now obsolete, first realized the countercurrent principle with fixed cells where the solid phase was enclosed and transport of the liquid phases was controlled through pipes connected to the cells (21). Schneider and Reinefeld categorized the continuous extraction systems, which are all working countercurrently, in three classes (22): 1. Controlled transport of both juice and cossettes: cell–extractor (RT extractor), cross-flow filtering belt conveyor extractor (De Smet extractor). 2. Controlled transport of cossettes and uncontrolled transport of juice. This class comprises several chain-type extractors: Silver chain-type, Olier, Oppermann & Deichmann, J-diffuser, and Oliver–Morton. 3. Uncontrolled transport of juice and cossettes: tower extractors [BMA, Buckau Wolf, sloping through extractors (DDS and Silver slope type)]. A. RT extractors (RT from Raffinerie Tirlemontoise, Belgium) RT extractors are large revolving drums, separated into “cells” by a helix attached to the interior surface. As the drum with its helix revolves, the juice, which stays at the bottom, is transported from the tail to the head end. Thus, the cell actually moves, but it is more convenient to consider the cell the location of one turn of the drum. Fixed to the cylinder are grids that, revolving with the drum, sweep the cossettes up until they slide off and fall into the next cell. Thus, the cossettes and juice travel in opposite directions through the cylinder (35). The first extractors of this type were called Berge´ extractors. The RT2 extractor was developed from the Berge´ extractor. It has a double-helix, or double-threaded, screw, with each screw having twice the pitch of the single-helix screw (21). The diameter of the drum is enlarged at the cossette inlet side. The enlarged shell is perforated and rotates inside a fixed head, which is not connected to the drum. The cossettes are flushed through the scalding pipe into the mobile head. Two grids are mounted inside the mobile head and carry the cossettes into the drum with each turn. Scalding juice, together with juice from the diffuser, flows through the perforated head into the

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circulating juice tank. Fresh water is introduced through the hollow shaft into the second compartment and pulp press water is added to the fourth compartment, counting from the tail end. Diffusion juice, corresponding to the draft, is pumped from the circulating juice tank to the process. The remaining juice is heated and used for scalding and flashing the cossettes to the extractor (18). Figures 13 to 15 show the general diagram of an RT2 extractor and its associated equipment. The largest RT2 extractors (7 m diameter and about 45 m long) have a nominal capacity of 5000 t/d beet, which can be exceeded by more than 50%. It is impossible to enlarge this model because of geometrical difficulties with the slope of the cossettes passages and also because of the important empty volume inside the drum, which is expensive (21). The RT4 extractor is directly derived from the RT2 model. In the RT4 extractor the redesigned internal structure produces a more continuous movement of the cossettes during the rotation of the drum. The capacity of such a new drum is larger and the retention times of both juice and cossettes are

Figure 13 Constitutive elements of an RT2 extractor (40). 1 and 2, Helicoidal plates forming two separate juice channels; 3, transversal plate; 4, slopping passages for cossettes; 5, transversal screens.

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Figure 14 Juice end of an RT2 extractor (40). 1, Transversal screens; 2, front casing; 3, juice and cossettes mixture inlet; 4, frontal and peripheral screens; 5, juice outlet to the circulation tank.

Figure 15 RT2 extractor (40). 1, Slicer; 2, cossettes belt and belt weighing scale; 3, scalder; 4, circulation juice heater; 5, circulation juice tank; 6, fine pulp separator; 7, RT2 drum extractor; 8, press water tank; 9, fresh water tank; 10, raw juice to prelimer; 11, exhausted cossettes.

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shorter. Even more simplified is the RT5 extractor, in which the idle times are shorter and the mixing phase is increased by about 7.5% (27). The most common RT5 extractor has a diameter of 6.25 m and a length of 49 m. Its nominal capacity is 7500 t/d with sugar losses of 0.20% on beet at a draft of 115 kg/100 kg beet. It is capable of processing (with higher losses and draft) up to 12,000 t/d. The largest model, of diameter 7.6 m and length 61 m, has a nominal capacity of 12,000 t/d. It is important that the quality of the cossettes gives a good juice separation, which is obtained with a high Swedish number (⬃15) and a small number of fines (21). B. Plate Conveyor De Smet Extractor This Belgian equipment is the adaptation by the industry of an extractor of vegetable oil from seeds using solvents (Fig. 16). It consists of a horizontal perforated plate belt conveyor about 30 m long, which may reach 7 m. On this conveyor, a 1-m-thick layer of cossettes is placed. The conveyor moves forward at low speed so that the cossettes are transported from the inlet to the outlet in about 75 min. Above this layer of cossettes 18 juice distributors are installed. At the tail of the extractor, the last distributors are fed with press water and fresh water (21). The juice distributors are fed from pumps that are arranged in groups of three in one housing on a common shaft and are driven by one motor. Each pump receives the juice from the preceding slopped collecting hopper, which is installed between the upper and the lower belt. Floats are located in every hopper to prevent the intrusion of air and the excess juice is allowed to overflow into the next hopper. The cossettes are heated before they are deposited on the diffuser belt. They pass through an inclined trough and are preheated countercurrently with juice from the diffuser. The cooled juice is withdrawn from this trough and sent to the process. Heated circulation flumes the cossettes through a specially designed pipe to the diffuser belt, where the thickness of the cossette layer can be regulated by an adjustable dampening device (18). An equipment is used that is similar to the cane extractor. If the pH value and calcium concentration of the water are at the right levels, the temperature inside the extractor can reach 75–77°C without juice percolation problems. The retention time of the cossettes is about 75 min. The average juice retention time is between 30 and 40 min. The main operating difficulty with this kind of extractor comes from foam formation. Antifoaming agents (derivatives of fatty acid and products) are generally used at levels of 100–150 g/t of beet (21). C. Chain-Type Extractors The first continuous beet extractors, which were constructed in around 1930, were the chain-type Olier extractor in France and the Silver chain extractor in

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Figure 16 De Smet extractor (40). 1, Cossette layer leveler; 2, cossettes layer; 3, plate conveyor; 4, juice pumps; 5, heat exchangers; 6, flushing device; 7, slicing machine; 8, belt weighing scale; 9, raw juice/cossettes heat exchanger (scalder); 10, cossette inlet tube; 11, extractor; 12, circulation juice tank; 13, fine-pulp separator; 14, circulation juice heat exchanger; 15, exhausted cossettes elevator; 16, pulp presses; 17, press water fine-pulp separator; 18, press water heat exchangers; 19, press water tank; 20, fresh water tank; 21, raw juice to prelimer. raw juice; ⴢⴢⴢ ⴢⴢⴢ raw juice fine pulp. — --- — Press water; ----- fresh water; — - —- circulation juice;

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the United States (Fig. 17). They consisted of a series of U-shaped cells forming a serpentine tube. Cossettes were transported in the tube by transverse screens moved by two strong chains. The necessary heat was supplied by steam jackets fitted around the head end of the extractor. Some other systems of chain extractors were also developed (Oppermann & Deichmann, J-diffusion). These extractors are now generally out of use, chiefly because it is difficult to build for large throughputs and also because of chain wear (21). D. Tower Extractors Two models of tower extractors have been developed simultaneously in Germany by BMA and Buckau-Wolf (Fig. 18). The models are comparable in their principle of two main and distinct parts: the countercurrent mixer and the extraction tower. The tower is a 14- to 20-m-high cylinder. Inside the tower, a tubular shaft rotates slowly at 0.2–0.8 min−1. Special steel pieces of helicoidal shape or flights are fitted on the shaft and give the cossettes their upward movement (Fig. 19). The juice and the cossettes move countercurrently in the approximately 2m-wide space between the outer casing and the inner shaft. Steel elements (stationery flights) are fitted on the internal side of the tower wall to prevent the whole of the cossette mass rotating with the shaft.

Figure 17 Silver chain extractor (40). 1, Cossettes inlet; 2, fresh water inlet; 3, exhausted cossettes outlet; 4, raw juice outlet; 5, juice transfer.

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Figure 18 Tower extractor with countercurrent mixer (40). 1, Countercurrent mixer; 2, bottom screens; 3, side screens; 4, cossette/juice mixture pump; 5, sand separator; 6, tower shaft with attached flights; 7, stationary flights (wings); 8, tower shaft drive; 9, foam separator; 10, circulation juice pump; 11, circulation juice heat exchanger; 12, tower shaft ballast (water).

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Figure 19 BMA extraction equipment—central shaft and flights (40). 1, Flights; 2, wall arms.

The juice–cossettes mixture at a temperature of 70–73°C is pumped from the countercurrent mixer by the variable-speed pump to the tower wall. The circulation juice leaves the tower through the bottom screens occupying the whole bottom section area. The BMA tower has additional side screens in the lower tower wall. A revolving “scraper” device cleans the bottom screen from the cossettes. The juice then passes a cyclone sand separator and heat exchanger and enters the countercurrent mixer. The press water and fresh water enter through two (radial header) pipes with several inlets. The control of water feed consists of a level sensor in the tower that actuates the fresh water inlet valve. The cossettes are extracted by horizontal screws placed at a higher level than the water inlet, so that the cossettes are drained before being removed from the tower (21). The packing of the juice–cossettes mixture depends on the rotational speed of the tower shaft. An increase in the speed reduces the packing, consequently decreasing the cossettes retention time and increasing the retention time dispersion. The consequence of these two phenomena is an increase in the sugar content of the exhausted cossettes. A lower rotating speed increases the packing in the tower and consequently the retention time, which gives better exhaustion, but it also reduces the permeability of the cossette mass and hence the throughput of the bottom screen. The necessary power for the drive of the tower shaft depends on the packing of the cossettes in the tower. The power uptake is a measure of that, as is the torque on the center shaft. The manufacturer recom-

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mends a packing of 650–700 kg of cossettes per cubic millimeter. Blocking of the bottom screens is controlled by pressure measurement above and below the bottom screens. The Buckau Wolf tower extractors differ mainly in the following aspects: - Position of extraction water inlets, which are placed in rotating arms attached to the tower shaft - Configuration of the bottom screens without side screens - Shape of the bottom scraper inside which the cossettes–juice mixture inlet is fitted - Slope (form) and size of the rotating transport elements (flights) - Absence of side screens - Heating arrangement; the necessary, flow of juice is picked up at the outlet of the tower and passes through the heater before being introduced in the countercurrent mixer Tower extractors work with cossettes of Silin numbers between 5 and 7 m/100 g. They also need a low content of fine particles to guarantee good juice transport. As in the case with DDS extractors, the rigidity of cossettes is improved by extraction water treatments such as acidification and calcium sulfate addition. Thus, it is now possible to operate these extractors at temperatures of up to 72–73°C. The biggest extractors now constructed by BMA and Buckau have a nominal throughput of 10,000 t/d. The BMA tower has a diameter of 11 m and a height of 21.6 m. The maximal practicable throughput can exceed the nominal value by more than about 15%. The BMA tower extractor of Hohenau Sugana sugar factory has a daily slice capacity of 10,000 t. At a draft value of 109%, sugar losses of 0.23% on beet are realized with beet containing an average of 20% sugar (21). E. Trough (DDS) Extractor This extractor was invented by Bru¨niche–Olsen and developed by DDS (De Danske Sukkerfabrikker) in the early 1950s (Fig. 20). In the United States, it is known as the Silver DDS slope extractor. It consists essentially of a U-shaped sloping vessel in which two overlapping screws with opposite pitches rotate. Fresh cossettes fall from a conveyor belt to the lower end. The cossettes are transported upward by the two screws to a paddle wheel, which lifts the exhausted cossettes out of the extractor. The raw juice leaves the extractor through a screen at the bottom end. The lower section of the extractor works in a similar way to the juice-cossettes heat exchangers: the juice is cooled and the cossettes are heated. The temperature difference between raw juice and fresh cossettes is

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Figure 20 DDS extractor—longitudinal and cross-sectional view (40). 1, Cossettes conveyor and scale; 2, screw elements; 3, press water inlet; 4, fresh water inlet; 5, exhausted cossettes; 6, heating jackets; 7, juice screen; 8, raw juice pump; 9, cossettes level; 10, juice level.

about 15°C. The necessary heat is supplied by 12 steam jackets having temperature controls. There is no circulation juice. At the nominal capacity of the extractor, the average retention time of the cossettes is between 125 and 140 min. The packing of the cossettes is high at about 700 to 730 kg of cossettes per cubic millimeter. The mean juice retention time is about 55 min. For a regular juice flow, it is very important that the permeability of cossettes remain high. Figure 20 shows respectively the normal levels of juice and cossettes when the permeability of the cossettes is correct. In this case, the entrainment of the mass of cossettes by the screws gives a higher level of cossettes and juice in the middle of the extractor. If the mush content is low, i.e., less than 5, it is possible to process cossettes with a Silin number of 10 m/100 g. The DDS extractors are, however, normally fed with cossettes having a Silin number of 7–8. The maximal nominal daily capacity of this type of extractor is 3600 t/d for equipment that is 28 m

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long and 8.5 m wide (21). This extractor can work with 40% overcapacity (28). In Fig. 34 is given a general automation and control diagram of a beet extraction tower and in Fig. 35 the same diagram of a DDS beet extraction trough. In Fig. 36 are explained the symbols of automation and control used in Figs. 34 and 35.

VIII. MASS AND HEAT BALANCE OF THE EXTRACTION All of the industrial extractors follow the general procedure in Fig. 8. Exhausted cossettes are pressed and the press water is reintroduced into the extractor. This flow of recycled water is supplemented by the addition of fresh water. Figure 21 gives an example of a mass balance calculated according to the following theoretical conditions: Sugar content of cossettes Marc content of fresh cossettes Dry substance content of exhausted cossettes Dry substance content of pressed pulp Draft Press water purity Raw juice purity Known sugar losses Unknown sugar losses

17% 4.5% 10.55% 25% 120% on beet 80% 89.5% 0.27% on beet 0.10% on beet

The balance shows that the mass of exhausted cossettes extracted from the extractor is significantly less than the mass of fresh cossettes. This is mainly a consequence of the leaching and pressing effect that takes place in most extractors (21). A mass and heat balance of a DDS extractor are given in Fig. 22 (28). Initially the DDS extractor, which produced cold diffusion juice 15°C above ambient temperature, needed less heat (approximately 1% beets standard steam with 550 kcal/kg or 2300 kJ/kg heat of evaporation), whereas the other extractors (RT, tower, etc.) needed more heat (approximately 5% beets standard steam) because their diffusion juice was hot (50°C). This changed in the 1980s. By modification inside the countercurrent mixer in tower extractors the diffusion juice produced was also cold; subsequently, the heat consumption was less (1% beets standard steam) (29). See also Fig. 23 where, according to Baloh, the correlation between the steam consumption in the extraction, the draft, and the temperature of raw juice is given. The lowest steam consumption is achieved by low draft 105% on beet and low temperature of raw juice 15°C (29).

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Figure 21 Example of an extraction mass balance (40).

IX. OPTIMIZING THE ECONOMIC PERFORMANCE OF BEET EXTRACTOR Beet sugar manufacture yields three products: sugar, beet pulp, and molasses. The relative value of these products and the costs of processing materials and aids (i.e., mainly cost of fuel), as well as the processing of any additional juice product, determines the optimum draft. Van der Poel et al. demonstrated how the final fractions of sucrose extracted can be divided between white sugar and molasses by establishing the relationship between the extraction of sucrose to that of potassium (30). Anderson assumes that 60% of the decrease in sugar loss to the pulp will be recovered

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Figure 22 Mass and heat balance of a DDS extractor (28).

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Steam consumption for extraction

kg/100kg beets

Sugars and Carbohydrates

ets

g

0k

be

0 /1

g

tk

D

f ra

Temperature ºC of Raw Juice Figure 23 Correlation of steam consumption in the extraction, the draft, and the temperature of raw juice (29).

as white sugar for use in the DDS Isocost chart. Thus, the ratio of pulp, sugar, and molasses can be estimated (31). Production costs can then be assigned to the changing level of draft and impurities, such as in purification, electrical costs for pumping, processing aids, and evaporation. The primary influence on cost is typically draft vs. evaporation cost. Combination of the values of the three products vs. the costs of processing allows the calculation of the overall economic impact of changing the extraction parameters, from which an optimum can be derived. A typical example of an optimization calculation as used at CSM was given by Van der Poel et al. (32). The figures show the optimum value at a sugar loss in pressed pulp of 0.352% on beet and draft 120% on beet. The absolute levels of the yields are not right because many processing costs, such as personnel and maintenance, are not included. However, the differences in

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relation to the sugar losses in pressed pulp as well as the position of the optimum (draft and sugar loss in pulp) are correct. Adamopoulos et al. studied the optimization of the extraction process in DDS extractor of 3000 t/d on the basis of economical criteria (cost of sugar production), concluding that the optimal values for draft are 107–111% on beets, the losses 0.11–0.24% on beets, the temperature 70–75°C (4). Gudmundson described computer programs developed by the Swedish Sugar Company, where an energy balance can be run to judge the impact across the factory. A second program calculates the optimum between the draft and the overall factory operation (33). Christodoulou described the result of trial test runs of pulp presses in two different factories of Hellenic Sugar Industry, Greece. Not only do the operating parameters of extraction influence the good pressability of the pulp; the type of extraction equipment does so as well (3). Budicek and Hladikova` presented an econometric model of a sugar factory that is not limited to the extraction stage. Decision making is based on calculation of the sugar factory’s annual gross profit. The model presents mass flows as well as cost–benefit calculations. Mass balances consist of processed beet, accompanying impurities, and processing aids, thus providing an overall view of the main products and byproducts, including their production costs (34).

X. COMPOSITION OF SUGAR CANE Sugar cane is a type of giant grass belonging to the family, botanically known as “Saccharum,” a generic name for sugar cane. One of the cultivated species of Saccharum is known as Saccharum officinarum. The wild form of the sugar cane belongs to the family, botanically known as “Saccharum spontaneum.” The cultivated wild varieties of sugar cane have played a very important role in the evolution of new commercial varieties of sugar cane, now grown by hybridization and selection techniques. Thus, all the sugar cane varieties grown in different countries have the parents belonging to the Saccharum officinarum and Saccharum spontaneum and, to some extent, other species (Fig. 24) (35). The chemical composition of sugar cane varies widely depending on many factors. Therefore, only indicative average figures in percentage are given below (36): 75 water 25 solids 13 fiber 12 soluble solids 10.5 sugar 9.8 sucrose

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0.7 invert sugar 1.5 nonsugars 0.8 organic nonsugars 0.2 nitrogenous substances 0.06 proteins 0.14 amino acids 0.1 nitrogen-free substances 0.03 carboxylic acids 0.02 starch, wax, fats, phosphatides 0.5 unidentified substances 0.7 mineral water

XI. EXTERNAL STRUCTURE OF THE SUGAR CANE PLANT A. Stalk The sugar cane plant consists of a number of unbranched stalks that store the sucrose. The stalks are tall and slender, roughly circular in cross-section, and bear two rows of leaves. The stalks are divided by the nodes, which are distinctive areas where the leaves are attached (one leaf at each node) (Fig. 24). Situated within the root band is the bud. There is normally one bud at each node. The buds are situated on alternate sides of the stem. The bud is an embryonic shoot consisting of a small stem bearing miniature leaves, the outer ones of which are scales. The size and shape of the buds as well as the form of the outer scales or flange vary considerably with variety (Fig. 25). B. Leaf The leaves are arranged alternately a single leaf arising from each node (Fig. 26). They increase in size as the plant develops. Trash is formed when the leaf ages and dies. C. Root System There are three main types of sugar cane root: sett roots, shoot roots, and mature roots. The sett roots develop from the primordia in the root band of the cutting. The shoot roots develop from the root primordia on the lower nodes of young shoots (Fig. 27). The mature roots arise from root bands of shoots after the initial flash of shoot roots (37).

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Figure 24 Parts of sugar cane stem (40).

XII. TECHNICAL EXTRACTION OF SUGAR FROM CANE A. Cane Unloading Since cane unloading is a materials handling problem, the methods adopted depend on the methods used for harvesting the cane and transporting from field to factory. Large factories use automated cane handling to a great extent. Where cane payment or factory control systems demand, the cane unloading station must often include weighing the cane, recording its origins and sampling for the

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Figure 25 Buds are located in root bands on nodes of sugar cane stems (40).

Figure 26 Structure of a sugar cane leaf (40).

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Figure 27 Young cane plant showing two kinds of roots: sett roots and shoot roots (40).

determination of cane quality. In the end the cane handling system must supply the cane to the factory cane preparation equipment in as uniform a stream as possible and at the appropriate rate. In some places—notably Hawaii—the cane is washed, but this operation can be avoided by good harvesting and handling methods. B. Cane Preparation For best efficiency of the extraction plant (crushing or diffusion) the cane must be as finely divided as is economically feasible. Modern factories use heavy-

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duty, swing hammer “shredders” to comminute the cane to a fibrous mass where the largest particles are pieces of rind of about 100 mm or shorter in length and of cross-section 4 × 2 mm or smaller. The rind is the toughest part of the stalk where the “fibrovascular bundles” are most tightly packed together. In the interior of the stalk and particularly in the “internode“ sections, the fibrovascular bundles are less densely packed and there are more juice cells and juice. C. Description of Heavy-Duty Shredders Figure 28 is a diagrammatic representation of the cross-section of a swing hammer shredder. The swept diameter of the hammers ranges from 1500 to 1800 mm. The rotor is typically 2100 mm long, rotates at about 1000 min−1, and is driven via a reduction gear box by a steam turbine of up to 4500 kW. The billets of chopped cane enter at the top [1] and slide down or are thrown against the feed plate [2]. Below the feed plate is a series of heavy grid bars [3] that retard the progress of the cane, retaining it in the range of the hammers to be broken into fine fibrous particles. The clearance between the extended hammer tips and the grid bars is often 2 mm or less. Shredded cane is discharged at [4]. A set of hammers numbers between 100 and 200, and each hammer weighs 15–20 kg.

Figure 28 Schematic cross-section drawing of heavy-duty cane shredder (40). 1, Cane feed; 2, feed plate; 3, grid; 4, prepared cane discharge; 5, hammer.

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Static and dynamic balancing of the rotor is essential. The hammers are hard faced by welding or have hard replaceable inserts, and a refurbished set of hammers must often be provided at weekly intervals. A high amount of soil in the cane supply requires more frequent replacement if throughput and extraction efficiency are to be maintained at acceptable levels (38).

XIII. CANE MILLS A. Crushing Trains Extraction of sugar from cane by “crushing” or “grinding” is carried out in a series of roller mills. The separation is carried out by volumetric reduction aided by dilution of residual juice by counterflow washing. The normal “compound imbibition” system is illustrated for a train of four mills in Fig. 29. For best results the imbibition liquid applied to the feed to the final mill is hot water— usually condensate at up to 85°C. The juice expressed from the first and second mills is sent to process and is called “mixed juice.” The whole milling process is usually completed in about 20 min. Uniform application of imbibition liquid across the bagasse “blanket” is important, although the whole blanket is satu-

Figure 29 Schematic arrangement of modern six-roll crushing mill showing diameters and depths (40). 1, Delivery roll; 2, top roll; 3, feed roll; 4, top pressure feeder roll; 5, bottom pressure feeder roll; 6, underfeed roll; 7, feed chute; 8, pressure feeder chute; 9, trash plate; 10, feed cane or bagasse; 11, delivery bagasse.

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rated with juice between the first pair of rolls at which juice is expressed and there is no evidence that premixing of imbibition liquid and bagasse improves extraction efficiency. A modern crushing unit (mill) consists of up to six rolls arranged as shown in Fig. 30. The three main rolls (delivery–1, top–2, feed–3) are of nearly equal diameter (except for differing wear). In the best modern practice the pressure feed rolls 4 and 5 are of the same diameter as is the underfeed roll 6. Many other combinations of auxiliary rolls and other “feeding devices” have been used, but the six-roll mill arrangement illustrated gives high throughput and extraction efficiency (38).

XIV. CANE DIFFUSERS There are a number of reports of cane diffusers (the common term diffuser is used here, although technically speaking this is an extractor) installed before 1900, and at various other times and in different countries. However, it was only with the successful introduction of beet extractors that cane extraction (diffusion) became a practical scheme. A batch cane diffusion system operated in Egypt for more than 50 years, but since the first successful continuous cane diffusers were installed in the early 1960s, diffusers have been operated as continuous countercurrent solidliquid systems. Early designs evolved from beet diffusers, and even the commonly used name “diffuser” came from the beet industry, although the terminology is not really appropriate in the context of extraction of juice from cane.

Figure 30 Diagrammatic representation of a four-mill crushing train with ordinary compound imbibition (40). 1, 2, 3, 4, Crushing units—mills 1 to 4; 5, shredder; 6, drag conveyor or rubber belt—prepared cane elevator; 7, 8, 9, drag conveyors—intermediate carriers; 10, cane feed to shredder; 11, final bagasse to boilers, storage, or otherwise; 12, imbibition water; 13, imbibition recirculation system; 14, mixed juice to process.

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Differences between cane and beet extraction hinge around the considerable differences in raw material to be extracted. Although beet is conveniently cut into cossettes, it was only when adequate preparation of cane for extraction was achieved that cane diffuser installations became successful. A large number of published reports on cane diffusion from 1965 to 1975 reflected the interest generated at the adoption of cane diffusion. Since that time its adoption has been rapid in some countries, particularly in southern Africa, where roughly 80% of all cane are processed in diffusers. However, milling is still the predominant extraction process in a large number of cane sugar–producing areas. There are essentially two variants of the process, termed bagasse diffusion and cane diffusion. The former involves a single mill ahead of the diffuser, and in the later prepared cane is accepted directly into the diffuser. Early installations favored bagasse diffusers because they represented a smaller step-change from milling, and are still required in countries where payment for cane is based on an analysis of first expressed juice. However, cane diffusers have generally shown themselves to be considerably more cost effective and are almost exclusively favored over bagasse diffusers in new installations. Therefore, this chapter covers only cane diffusers. Diffusers must be provided with well-prepared cane. A dewatering stage, usually one or more mills, is required to dewater the very wet bagasse leaving a diffuser. In some installations a French screw press was used as a dewatering device but proved to be unreliable and subject to considerable wear. Conventional mills are now used universally for this duty. Because of the large amount of liquid to be removed, dewatering is generally done in two stages if four toll mills are used, or else in a single stage using pressure-fed mills (Fig. 30). The types of cane diffuser that have been used can be categorized as follows: Uncontrolled transport of juice and cossettes (also called true countercurrent diffusers), e.g., DDS, Saturne. Controlled transport of juice and cossettes (also called moving-bed diffusers). There are either cross-flow moving belt extractors, such as B.M.A., De Smet (Fig. 31), Silver ring, Huletts-type extractors, or other types such as F&S/Van Hengel or Rotocel (39). Moving bed diffusers are also countercurrent extraction devices but operate on a staged basis. Juice is pumped onto a moving bed of prepared cane or bagasse, about 50–60 m long, in 10–18 stages. A schematic diagram of a cane diffuser is shown in Fig. 31. The De Smet cane diffuser (Sec. VII) is essentially the same as the De Smet beet extractor. The cane or bagasse bed forms on a horizontal slow-mov-

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Figure 31 Schematic diagram of a moving-bed cane diffuser (40). 1, Direct injection vapor; 2, heating vapor; 3, heat exchanger; 4, prepared cane; 5, raw juice; 6, diffuser; 7, press water; 8, lifting screws; 9, imbibition water; 10, final bagasse; 11, dewatering mills; 12, stage trays. Figures in brackets: Mass flow rate referred to the fiber rate in bagasse.

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ing screen. The Silver ring diffuser is essentially similar, but the screens move in a circle instead of along a straight line. The BMA and Hullets diffusers differ from the De Smet in having a fixed screen, with a series of chains that transport the cane bed across the screen. This generally results in a cheaper diffuser for the same screen area (39). The dragging of cane by chains across the fixedscreen diffuser generally results in the formation of a more compact cane layer at the screen, which affects percolation. The processing capacity of a cane diffuser–type BMA lies between 2000 (the smallest unit) to 16,000 t/d (the biggest unit), while the same diffuser-type BMA by bagasse processing has a capacity between 3000 (the smallest unit) to 24,000 t/d (the biggest unit).

XV. FACTORS AFFECTING EXTRACTION EFFICIENCY A. Cane Preparation The process of breaking down the cane into small pieces is referred to as cane preparation. This is the most important variable affecting extraction in diffusers. If high extraction efficiencies are to be achieved it is essential that the cane be prepared in a heavy-duty shredder (Fig. 28) so that most of the sugar-containing cells of the cane stalk are ruptured. Laboratory and pilot plant work showed very clearly that more intensive preparation of cane makes more of the sucrose containing juice readily accessible to the extracting liquid, minimizing the amount of sucrose that has to be extracted by a much slower diffusion mechanism. Unfortunately, measurement of the degree of cane preparation is difficult and existing measures are not always reliable. The way in which the cane is prepared is also important. Ideally the type of preparation should result in material where all cells are ruptured but where long fibers are still evident, resulting in a cane bed that is stable and open enough to allow high percolation rates to be achieved. In practice it has been found that this is best achieved in heavy-duty shredders with a minimum of knifing, since intensive knifing reduces the average fiber length. B. Imbibition Rate As with any solid-liquid extraction process, the more extracting liquor that is added the easier is the extraction. So it is with cane diffusion, where higher imbibition rates invariably result in higher extraction efficiencies. The amount of imbibition water added is generally related to the quantity of fiber being processed, since it is the fiber that removes with it juices in final bagasse. There is no maximal or minimal imbibition rate for diffusion. Since high

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imbibition rates enable a smaller diffuser to be utilized to achieve a given extraction efficiency, the reduction in the cost of the diffuser would have to be balanced against the cost of additional evaporator capacity and/or the cost of steam. Therefore, the optimal imbibition rate for any factory is contingent on local factors at that factory. C. Effect of Number of Stages The use of a number of stages rather than a single big mixed tank enables higher concentration difference between sucrose in cane and true countercurrent flow is approached more closely. However, the benefit drops off as the number of stages increases and marginal improvement becomes very small. D. Effect of Percolation Rate Although preparation is the most important variable affecting extraction efficiency in a cane diffuser, the percolation rate is probably the next most important variable. This is the rate at which liquid percolates down through the bed of prepared cane. Laboratory and pilot plant studies have shown that an increase in percolation rate promotes the rate of mass transfer and increases the proportion of the juice in open cells that is accessible to the extracting liquid. E. Effect of Cane Retention Time The longer the time the prepared cane spends in the diffuser the higher will be the extraction efficiency. Provision of adequate retention time is probably one of the most important design specifications. Since the raw juice offtake from the diffuser is roughly equal to the mass of cane entering the diffuser, juice retention time in the diffuser is nearly twice the retention time of fiber, particularly if the juice trays below the diffuser are not kept empty. F. Effect of Temperature High temperatures are advantageous because they increase the rate of extraction. However, this effect is not as important as the effect of preparation and liquid flow rate. Nonetheless it was estimated that an increase in temperature from 75°C to 80°C would lead to an increase in extraction efficiency of about 0.2% sugar on cane. The most important reason for keeping the temperature above 75°C is to control microbiological activity. Generally, diffusers are operated at about 85°C, allowing low-pressure steam to be used for heating purposes (39).

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XVI. STRUCTURE AND CHEMICAL COMPOSITION OF CORN The gross structural features, physical properties, structural details, and composition of corn are described in the literature together with very nice photographs and microphotographs of corn particles (42). A proximate analysis of corn grain is given in Table 3 (42).

XVII. THE WET MILLING PROCESS OF CORN FOR EXTRACTION OF STARCH Corn is abundant and relatively inexpensive; it has a high starch content and protein of acceptable quantity and quality. Thus, its primary use is for animal feed. It is also processed into valuable food and industrial products, such as ethyl alcohol by fermentation, corn meal by dry milling, and highly refined starch by wet milling. The greatest volume is processed by wet milling to produce starch products and sweetener products for foods. Nonfood products, such as industrial starches, corn gluten feed, and corn gluten meal, are also manufactured. The wet milling process involves an initial water soak under carefully controlled conditions to soften the kernels. The corn is then milled and its components separated by screening, centrifuging, and washing (Fig. 32), to produce starch, oil, feed byproducts, and sweeteners (by starch hydrolysis). Applications for these products have shown steady growth, which has necessitated major investment to expand production facilities in recent years (43).

Table 3 Proximate Composition of Corn Grain (42) Component (%, wet basis)

Range

Average

Moisture Starch Protein Fat Ash (oxide) Pentosans (as xylose) Fiber (neutral detergent residue) Cellulose + lignin (acid detergent residue) Sugars, total (as glucose)

7–23 61–78 6–12 3.1–5.7 1.1–3.9 5.8–6.6 8.3–11.9 3.3–4.3 1.0–3.0

16.0 71.7 9.5 4.3 1.4 6.2 9.5 3.3 2.6

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Figure 32 Wet-milling process flow diagram, showing equipment arrangement for the separation of the major components: steepwater, germ, fiber, gluten, and cornstarch (43).

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Figure 33 Direction of material transport in correlation to the basic process (schematic), W, water; Z, sugar (19).

A. Steeping The first critical step in the wet milling of corn is steeping—the soaking of corn in water under controlled processing conditions of temperature, time, sulfur dioxide (SO2) concentration, lactic acid content, and so forth. These conditions have been found necessary to promote diffusion of the water through the tip cap of the kernel into the germ, endosperm, and their cellular components. Steeping softens the kernels, facilitating separation of components. Corn is shipped in bulk to the wet milling plants by truck, hopper car, and barge. It is then cleaned on vibrating screens to remove coarse material (retained on 12.7-mm × 1/2-in. openings) and fine material (through 3.18-mm × 1/8-in. openings). These screenings are diverted to animal feed. If they are allowed to remain with the corn, they cause processing problems such as restricted water flow through steeps and screens, increased steep liquor viscosity, and quality problems with the finished starch. Steeping is accomplished by putting corn into tanks (steeps) that have a capacity of 50–330 t each. The corn is then covered with steepwater, heated to 52°C, and held for 22–50 h. Steeps have cone bottoms with screens so that the water can be separated from the corn and pumped elsewhere or recirculated back to the top of the steep. To maintain steeping temperature, the recirculated flow is heated directly by steam injection or indirectly by heat exchanger. The

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Figure 34

General automation diagram with extraction tower.

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General automation diagram of extraction with DDS trough extraction.

Christodoulou

Figure 35

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Figure 36 Symbols of automation and control.

water should not exceed 55°C to avoid destroying the bacteria needed to produce lactic acid. Steeping is a countercurrent system, utilizing a battery of 6–12 or more steep tanks. Steeps are filled one at a time as they become empty. The corn does not move—just the water, which is transferred from one steep to the next. However, steeping is accomplished in one plant by continuously adding dry corn at the top of the steep while continuously withdrawing steeped corn from the bottom. Water for the steeps originates in the wet milling process, where it accumulates corn solubles. It is treated with SO2 to a concentration of 0.12–0.20%. The SO2 is purchased as a liquid or manufactured on site by burning of elemental sulfur. The SO2 increases the rate of water diffusion into the kernel and assists in breaking down the protein-starch matrix, which is necessary for high starch yield quality. The SO2-treated water is added to the steep containing the oldest corn. As the water is advanced from steep to steep, the SO2 content decreases and bacterial action increases, resulting in the growth of lactic acid bacteria. The desired lactic acid concentration is 16–20% (dry basis) after the water has advanced

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Fully relaxed energy potential surface of sucrose as a function of the two intersaccharidic torsion angles (46).

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Figure 37

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Figure 38 Representation of the molecular electrostatic potential (MEP) profiles of the two relevant sucrose conformers. The MEPs are depicted on the corresponding contact surfaces in 16-color code ranging from violet (most electronegative potential) to red (most electropositive potential) (in original) in relative terms (46).

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Figure 39 Molecular lipophilicity profiles for the two sucrose conformers of Fig. 37, with blue (in original) corresponding to hydrophilic surface areas and yellow (in original) to most hydrophobic regions. For both sucrose conformers, the entire “backside” of the fructose moiety is decisively hydrophobic (46).

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Figure 40 Snapshot of an MD simulation of sucrose surrounded by 571 water molecules (47).

through the system and been withdrawn as light steepwater. Meanwhile, the SO2 content drops to 0.01% or less. The volume of water available for steeping is normally 1.2–1.4 m3/t of corn. About 0.5 m3/t is absorbed by the corn to increase its moisture from 16% to 45% during the steeping. The remaining 0.7–0.8 m3/t is the quantity withdrawn from the steeping system. This water contains the solubles soaked out of the corn, which is 0.05–0.06 t solids/t corn processed. It is evaporated to 40– 50% solids, mixed with corn fiber, dried, and sold as corn gluten feed (42). B. Separations The next step of the process is the separation of kernel components (germ, fiber, gluten, and corn starch) by means of degerminating rotating mills, hydrocyclone separators, wedge-bar screens, disk-nozzle centrifuges, etc. (42). Solubles extracted from the corn during steeping are routed to evaporators where 0.6–0.7 m3 of water per ton of corn is removed to increase the solids from 5–10% to 40–50%. The solids are then mixed with corn fiber and processed into corn gluten feed. Falling film recirculating evaporator systems are used to increase the light steepwater solids to 30%, using steam at a pressure of 1.2 kg/cm2 or less (42). Multiple effect design, in which 1 kg of steam can evaporate several

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Figure 41 Hydrophobic topographies for the amylose fraction of starch (46).

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kilograms of water, is necessary to minimize energy costs. Energy can be further reduced with mechanical recompression, which completely recycles the vapors, compresses them, and discharges them to the evaporator steam chest (42). A relatively pure starch slurry from the wet milling operation contains 40% solids. C. Trends, Automation The wet milling process and associated equipment have matured sufficiently to permit consistent, reliable separation and product quality on a 24-h basis with minimal operating labor. The most notable achievements are attained in the new large plants where television-like displays and control systems distributed by cathode ray tubes are utilized to start process equipment sequentially on demand and then to monitor and control the system. Furthermore, the technician can be alerted, by rate-of-change or trendin-measurement functions, to variables that are about to get out of control. This allows time to make the necessary adjustments to avoid spills and off-quality. These computer systems are the latest development in the search for reduced costs and better product quality. Better computer applications are probable as improved on-line measuring devices for protein, starch, fiber, and soluble content are perfected. D. Utilities Fresh water usage for a typical large sweetener plant is about 4.5–6.0 m3/t of corn processed, of which one-fourth is consumed in wet milling. Reduction in the requirements for the wet milling portion is limited because the usage equals the need for properly steeping the corn. Greater water consumption for the finishing processes is likely because the trend is toward more sophisticated products that require more water. Wet milling operations are high-energy users at 1.48 × 106 kcal/t of corn, even for the efficient large sweetener plants. About 20% of the total are for electricity, which indicates that most of the energy goes into fuel to make steam. As products continue to become more sophisticated, the use of energy will increase unless new technology reverses the trend. The use of mechanical vapor recompression evaporators in new facilities and in the replacement of old equipment is reducing energy usage because it is five to six times more efficient than quadruple-effect steam evaporation. More imaginative use of heat exchangers can recover heat now being lost to the atmosphere via cooling towers. Steepwater evaporators now in service can utilize feed dryer exhausts as their source of energy. Superheated steam is showing promise for use in drying feed materials. Most (60–80%) of the energy for the primary heating of steam is recovered in

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low-pressure steam for use elsewhere in the process. Not only is 20–40% energy saving possible, but there is no discharge to the atmosphere, eliminating odor and particulate problems associated with feed drying. Reverse osmosis is improving due to breakthroughs in membrane technology. When operating with a pressure differential of 35–70 kg/cm2 they may replace steepwater and sweetener evaporators, achieving a major reduction in energy requirements. New steeping technology may be in the offing that improves the possibility of success in concentrating the solids content of steepwater; it works by reducing the low molecular weight components such as alcohol that reverse osmosis, up to now, has had difficulty separating. Another benefit is reduced steeping time. A technology being introduced to the industry to treat wastes relies on anaerobic digestion, which has the advantage of using less energyintensive processes and giving energy credits via recovery of methane gas, which is utilized as a fuel source. The cost of energy is being reduced by installing cogeneration. New coal-fired boilers operating at 40–80 atm drive electric generators. Steam for processing is then extracted in the 10 kg/cm2 range, and steam for low-temperature heating (of water, for evaporation, etc.) is extracted at exactly 2 kg/cm2 absolute (42).

GLOSSARY (7, 40) Bagasse Beet Beet brei Beet knife Beet tail Beet tops

Beet washer Cossettes Cultivar

Denaturation

Cane fiber leaving cane mill/diffuser after extraction of juice, e.g., first mill bagasse, etc., to final bagasse Sugar beet root, botanically the thick main root with hypocotyl in which sugar is stored Beet sample prepared for analysis is the form of fine particles Rectangular piece of steel designed to slice beet into cossettes Elongated lower part of the beet Beet leaves and petioles, which may or may not be accompanied by crowns or pieces of crowns that are removed in the field at the time of harvest Installation for cleaning beet (e.g., jet washer, revolving-arm washer, drum washer, cyclone washer) Beet slices produced by a beet slicer A horticulturally or agriculturally derived variety of a plant, as distinguished from a natural variety [cultivated + variety) Deliberate alteration of beet cells, often by heat, in preparation for extraction

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Draft Deteriorated beet Diffusion Diffusivity

Dry substance Exhausted cossettes Extraction

Extraction fresh water Extraction losses Flume water Imbibition Invert sugar

Juice purification Knife block Molasses

Mush content Nonsucrose content Nonsugar

Osmosis

299

The ratio percent weight of raw juice produced to the weight of cossettes introduced into the extractor Beet of reduced suitability for processing due to external causes, e.g., frost Gradual mixing of the molecules of two or more substances, as a result of random thermal motion Rate at which a substance diffuses between the opposite sides of a unit cube when there is unit concentration difference between them. Also called “diffusion coefficient.” In most cases a moisture-free substance Cossettes leaving the extraction plant Process of obtaining juice from sugar beet or sugar cane; the term “diffusion” should be used only for the physicochemical process Water introduced into the extraction in addition to press water Quantity of sugar entered but not contained in the raw juice as a percentage of the beet or cane mass Water used to transport beet Mixing of water or juice with bagasse during cane milling Mixture of (close to) equal parts of glucose and fructose resulting from the hydrolysis of sucrose (inversion) Partial removal of nonsugar substances from the raw juice while producing a thin juice Box-like device used to hold a number of knives in the disk or drum of a slicing machine The sugar-bearing product of the sugar whose purity has been reduced to the point that further crystallization of sugar is not economical feasible without special treatment of molasses Portion of cossettes shorter than 1 cm Difference between dry substance content and its sucrose content Common overall term for substances contained in the raw materials and products of the sugar industry except sucrose (sugar) and water Passage of a solvent through a semipermeable membrane into a more concentrated solution (Fig. 33)

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Polarization

Press water Press water pulp Pressed pulp Purity Raw juice Raw juice draft Silin number Slicer (beet) Sucrose Swedish number Wet pulp

Christodoulou

Term customarily used in sugar analysis for the optical rotation of a sugar industry product measured under defined conditions (ICUMSA*), as a percentage of the rotation of pure sucrose, measured under the same conditions Liquid effluent from the pulp presses Particles of exhausted cossettes contained in the press water Pressed, exhausted cossettes, leaving the pulp presses Sugar content as percentage of dry substance content Juice obtained from beet or cane after extraction, pressing, or milling, mixed juice (cane) Mass of juice drawn from the extraction plant as percentage of mass of cossettes introduced Length in meters of 100 grams of cossettes Machine designed to hold knives in knife blocks to produce cossettes Common term for the disaccharide α-D-glucopyranosyl-β-D-fructofuranoside Ratio of the mass of cossettes Commercial term for partially dewatered, exhausted cossettes

REFERENCES 1. C Buttersack, D Schliephake. Extraction theory. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin, Germany: Verlag Bartens KG, 1998, pp. 310–327. 2. F Bonacker. Extraction. In: Die Zuckerherstellung, 3rd ed. Leipzig: Fachbuchverlag, 1984, pp. 153–206. 3. P Christodoulou. Operating parameters of extraction systems for optimum pulp pressing, Zuckerindustrie 119:1004–1007, 1994. 4. K Adamoupoulos, P Christodoulou, H Petropakis. Optimization of the process of sugar extraction from beets. Hellenic Sugar Industry Quart Bull 50–51:57–79, 1982. 5. K Bohn. Composition of sugarbeet and chemical behavior of constituents in processing. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 115–151. 6. H Claassen. Die Zuckerfabrikation. In: Die Saftgewinnung. Magdeburg: Schallehn & Wollbruck, 1930, pp. 45–85.

*ICUMSA International Committee for Uniform Methods in Sugar Analysis.

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7. RA McGinnis, RN Sequeira. Chemistry of the beet and processing materials. Glossary of technical terms in common use in the beet sugar factory. In: RA McGinnis, ed. Beet Sugar Technology, 3rd ed. Fort Collins, CO: Beet Sugar Development Foundation, 1982, pp. 25–63, 759–782. 8. RE Watkins, PV Schmidt. Changes in the chemical composition of beet. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 276–277. 9. E Junghans. Morphology and physical properties of sugarbeet. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, p. 209. 10. C Buttersack. Ultrastructure of the native sugarbeet root. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin, Germany: Verlag Bartens KG, 1998, pp. 211–212. 11. K Vukov. Physics and Chemistry of Sugar Beet Manufacture. Budapest: Akade´miai Kiado, 1977, p. 595. 12. E Junghans. Physical properties of beet. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 212–214. 13. C Buttersack. Physical properties of beet after denaturation. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 215–216. 14. C Buttersack. Separation by adsorption, chromatography or membranes. Zuckerindustrie 124:846–858, 1999. 15. P Christodoulou. Possibilities of application of the chaos and complexity theory in sugar technology. Proceedings of 21st General Assembly of CITS, Antwerp, May 24–28, 1999. Berlin: Verlag Bartens KG, 1999, pp. 331–349. 16. SI Nikolis. Chaos and Information Processing. Singapore: World Scientific, 1991. 17. PM Silin. Technology of Beet Sugar Production and Refining. Jerusalem: Israel Program for Scientific Translations, 1964, pp. 88–178. 18. HO Ebell, M Storz. Diffusion. In: RA McGinnis, ed. Beet Sugar Technology, 3rd ed. Fort Collins, CO: Beet Sugar Development Foundation, 1982, pp. 119–153. 19. D Schliephake, A Wolf. Mass transport processes in the initial phase of aqueous sugar extraction from cossettes. Zucker 21:484–493, 1968. 20. H Van Malland. Technical extraction of beet. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 328–334. 21. J Vetter. Process parameters of extraction. Equipment for cell denaturation and the cooling of raw juice. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 334–348. 22. F Schneider, E Reinefeld. Ruebenzerkleinerung. In: F Schneider ed. Technologie des Zuckers, 2nd ed. Hannover: M&H Schaper, 1968, pp. 152–157. 23. K Bucholz, KM Bliesener, R Buczys, K Thieleckte, D Miehe. Bacterial control. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 169–170. 24. J Dubourg. Sucrerie de Betteraves. Paris: Librairie JB Bailliere & Fils, 1952, pp. 79–150. 25. H Bru¨niche-Olsen. Solid–Liquid Extraction. Copenhagen: Nyt Nordisk Forlag, Arnold Busck, 1962, p. 463.

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26. D Schliephake, B Mechias, E Reinefeld. Retention time distribution and movement mechanisms in extraction equipment without controlled transport of cossettes. Proceedings of 15th General Assembly of CITS, Tienen, Belgium, Principles of Extraction, Vienna, Austria, 1975, pp. 31–123. 27. G Mantovani. Sugar technology–development since 50 years. In: Proceedings of 21st General Assembly of CITS, Antwerp, May 24–28, 1999. Berlin: Verlag Bartens KG, 1999, pp. 47–51. 28. Anonymous. The Danish Sugar and Sweetener Engineering Beet Diffuser, 1999, Birkorood, Denmark. 29. A Baloh. Energiewirtshaft in der Zuckerindustrie. Berlin: Verlag Bartens KG, 1991, pp. 90–110. 30. JA Richmond, D Noble. Technical accounting and process control. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 1060–1061. 31. K Anderson. The DDS beet extraction. Zuckerindustrie 114:136–138, 1989. 32. PW Van der Poel, NHM de Visser, CC Bleyenberg. Sugar Techn Rev 9:15–18, 1982. 33. C Gudmundson. 18th Meeting of the CITS Subcommittee Measurement and Process Control, Peterborough, 1993. 34. L Budicek, L Hladikova`. Econometric model of a sugar factory. Proceedings of 20th General Assembly of CITS, Munich. Berlin: Bartens KG, 1995, pp. 34–41. 35. J Berge´. Sucr Belge 72, 1930. 36. PJM Rao, Industrial Utilization of Sugarcane and Its Co-products, New Delhi: ISPCK, 1997, pp. 5, 12–13. 37. GM Thomson. External structure of the sugar cane plant. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 225–230. 38. CR Murry. Technical extraction of sugar from cane. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 354–381. 39. PW Rein. Control of cane diffusers. In: RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998, pp. 381–383. 40. RW Van der Poel, H Schiweck, T Schwartz, eds. Sugar Technology. Berlin: Verlag Bartens KG, 1998. 41. NC Carpita, DM Gibeaut. Structural models primary cell walls in flowering plants: consistency of molecular structure with the physical properties of the walls during growth. Plant J 3:1–30, 1993. 42. SA Watson. Structure and composition. In: SA Watson, PE Ramstad, eds. Corn: Chemistry and Technology. St. Paul, MN: American Association of Cereal Chemists, 1994, pp. 53–82. 43. JB May. Wet milling process and products. In: SA Watson, PE Ramstad, eds. Corn: Chemistry and Technology. St. Paul, MN: American Association of Cereal Chemists, 1994, pp. 377–397. 44. W Hein, G Pollach. New findings with the use of hop products in the sugar industry. Zuckerindustrie 122:940–949, 1997.

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45. WE Boyce, RC Diprima. Elementary Differential Equations and Boundary Value Problems, 6th ed. New York: John Wiley & Sons, 1997, pp. 129, 605. 46. WF Lichtenthaler. Computer simulation of chemical and biological properties of sucrose, the cyclodextrine and amylose. Int Sugar J 97(1153):12–22, 1995. 47. WF Lichtenthaler, S Immel. The conformation of sucrose in water. A molecular dynamic approach. Liebigs Ann, 1995:1925–1937.

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9 Flavor and Aroma Substances Youn-Woo Lee and Youn Yong Lee Korea Institute of Science and Technology Seoul, Korea

Extraction of flavor and aroma substances from naturally occurring raw materials is important because the compositions of flavors are often too complex to be synthesized economically. Raw materials are isolated from balsam, bark, berry, blossom, bud, fruit, grass, gum, heartwood, leaves, peel, root, seed, twig, wood, and resinous exudation. Extracts of flavor and aroma substances prepared from raw materials are termed pomade, concrete, absolute, resinoid, or tincture according to their production technique (1). In processing essential oils and natural extracts, steam distillation, solvent extraction, supercritical fluid extraction, and expression are the major methods. Steam distillation provides the essential oil, whereas solvent extraction provides both the essential oil and the oleoresins. Oleoresins are concentrated natural liquid flavorings that contain both volatile and nonvolatile flavor components. Essential oils are obtained from plant materials by steam distillation or water distillation. After condensation of the vapor phase, the oil separates from the aqueous phase and is removed. Essential oils consist of volatile, lipophilic substances that are mainly hydrocarbons or monofunctional compounds derived from the metabolism of mono- and sesquiterpenes, phenylpropanoids, amino acids, and fatty acids. Pomades consist of fats that contain fragrance substances and are produced by the enfleurage technique. Concretes are prepared by evaporation of residue substances that are extracted from fresh plant material with nonpolar solvents such as benzene, toluene, hexane, and petroleum ether, as shown in Fig. 1. Concretes contain viscous waxy compounds with volatile fragrance materials. Concretes and pomades are not completely soluble in ethanol. Absolutes are prepared by taking up concretes in ethanol. Compounds that precipitate on cooling are then removed by filtration. After evaporation of the ethanol, a wax-free residue, the

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Figure 1

Solvent extraction process for concrete and absolute.

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absolute, remains. Absolutes are completely soluble in ethanol. Resinoids are prepared by extracting plant exudates (balsams, oleo gum resins, natural oleoresins, and resinous products) with solvents such as methanol, ethanol, or toluene. The products are usually highly viscous to improve their flow and processing properties. Resinoids mainly consist of nonvolatile, resinous compounds and are primarily used for their excellent fixative properties. Tinctures are alcoholic solutions that are prepared by treating natural raw materials with ethanol or ethanol-water solution. They can also be obtained by dissolving other extracts in these solvents. Extraction is still one of the most important methods for producing flavor and aroma substances and is the selective extraction of a soluble constituent from solid (solid-liquid extraction) or liquid mixtures (liquid-liquid extraction) by liquid solvents (2). In the production of natural extracts, the most impressive progress is the introduction of supercritical fluid extraction technology (Chapter 3). Characteristic applications are the extraction of caffeine from coffee or tea, or hops extraction.

I. SEPARATION PROCESSES (3) A. Distillation The most widely method used for producing flavor and aroma substances is distillation. There are three different processes used: steam distillation, simple boiling water distillation, and water and steam distillation, as shown in Fig. 2. In all cases, hot steam carries the most volatile compounds of the aromatic material with it and is then condensed. The resulting distillate is composed of the essential oil and water. The mixture of condensed oil and water runs into the layer separator where the lighter insoluble oil floats on the surface and accumulates slowly and from where it is drawn off periodically, as shown in Fig. 3. Some of soluble components may be lost in the water. Steam distillation uses an outside source of steam and no water is allowed in the still, as shown in Fig. 2a. The steam passes through the aromatic material in the distillation unit and exits into the condenser. In simple boiling water distillation, the plant materials are fully submerged in water and water is heated to produce steam, which contains flavor and aroma substances, as shown in Fig. 2c. This is the oldest method of distillation and the most versatile. This process works best for powders and very tough materials like seeds or roots. Production of steam by direct heating of plant material with water will result in hydrolysis and loss of fragrant esters and at least some pyrolysis of plant material. The result is low-grade oil. In water and steam distillation, a basket (or a perforated grid) is used for supporting the plant materials; thus, only rising steam contacts with the plant materials, while water is boiling in the still, as shown in Fig. 2b. This is the best method for distilling leafy materials, but it doesn’t work well for woods, roots, seeds, and so forth.

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Figure 2

The different types of steam distillation.

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Flavor and Aroma Substances

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Steam distillation unit.

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Figure 3

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B. Solvent Extraction Solvent extraction of the plant materials is a well-known method for the production of valuable oleoresins as well as essential oils. A solvent extracted oleoresin includes essential oil, organic soluble resins, nonvolatile fatty acids, and other plant materials, and can yield a more potent flavor profile. To capture delicate aromas like jasmine, linden blossom, and so forth, without thermal degradation, a process of solvent extraction is used. Plant materials such as leaves, flowers, roots, or stems are dissolved into a solvent such as hexane. They become wax-like concretes. Absolute is obtained by alcohol extraction of concretes where most of the alcohol is later removed. Plant materials such as seeds are first crushed and dissolved into solvents. Hexane is the most commonly used solvent. The solvent is evaporated under vacuum to give the oil. Resinoids are resins dissolved in a solvent such as benzene or alcohol. Resins are naturally occurring solid or semisolid substances produced by plants or trees. In general, the highest extraction yields are normally obtained compared to the other extraction methods. The solid to be extracted is placed in a basket suspended in the extraction vessel, as shown in Fig. 4. Solvent fed to the boiler, is heated and distilled off. It runs from the condenser, via the reflux head, into the extraction vessel where it percolates through the solid to be extracted. Solvent loaded with extract returns, via a seal loop, to the boiler, where it evaporates again. This cycle continues until the extraction is complete. C. Carbon Dioxide Extraction Carbon dioxide extraction is another type of solvent extraction in which carbon dioxide is used under high pressure to extract both essential oils and oleoresins. Supercritical and liquid CO2 can both be used as very inert, safe solvents. Liquid CO2 fits the requirements of solvent ability and low boiling point. It is also nonflammable, inert, inexpensive, and has low toxicity. As a solvent, liquid CO2 is highly selective, particularly toward those esters, aldehydes, ketones, and alcohols representative of aroma constituents (4). Low-temperature extraction using liquid CO2 produces extracts that more closely resemble the aroma of the original plant materials than any process involving heating of plant materials. Some examples of commercial extraction with liquid CO2 are as follows: anise star, cardamon, celery seed, clove bud, coffee, coriander seed, ginger Jamaican, ginger Nigerian, ginger terpeneless, hop oil, hoparome absolute, juniper berry 20%, mace, nutmeg, pepper oil black, pimento berry, and vanilla absolute. In contrast to liquid CO2, the solvent power of the supercritical fluid is highly dependent on its temperature and pressure. The advantage of the method, of course, is that no solvent residue remains, since at normal pressure and tempera-

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Figure 4 Single-stage extractor with solvent recycle.

ture the CO2 simply reverts to a gas and evaporates. However, when supercritical CO2 is used to produce flavors and aromas, some unwanted waxes, resins, and pigments are usually present simultaneously. Some examples of commercial extraction with supercritical CO2 are as follows: celery, ginger oil, jasmine absolute, massoia oil, paprika flavor and color, pepper oil, rosemary, sage, and vanilla absolute (5, 6).

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D. Cold Pressing Cold pressing is also known as expression. In this method, the oil-containing outer layer of the fruit is pressed and filtered to yield pure essential oil. A heating stage is used to help release the oil, usually not higher than 60–80°C. The pressed citrus oils are commercially produced by expression with either hydraulic press or screw-type press, as shown in Fig. 5. A drawback to cold pressing is that recovery of oil is lower than from solvent extraction. The yields to make an essential oil from raw materials are about 2 wt% in eucalyptus, 0.7 wt% in lavender, 0.2 wt% in rosemary, 0.1 wt% in jasmine, and less than 0.05 wt% in rose.

Figure 5 Plate press.

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E. Enfleurage In this technique, the flavors released by flowers are absorbed with fats for a long time. The petals of flower are spread over large sheets of glass that are covered with lard. The sheets are then pressed together. The flowers are refilled until the lard becomes fully charged. The oil is extracted by an alcohol solvent. Carrot oil is produced by using a carrot root extract macerated in vegetable oil. Carrot oil is rich in β-carotene, vitamins B, C, D, and E. It is useful as a skin rejuvenator and is recommended for treatment of dry and aging skin. F. Maceration (Hot Enfleurage) Maceration is one of the oldest techniques for preserving plant flavor. In this method, flowers are directly dipped in hot liquid fats that absorb essential oils. This produces pomades which consist of fats with flavor.

II. EXTRACTION SOLVENTS The choice of solvent is the primary consideration for an extraction process, and the selectivity of the solvent is of special importance. The required solvent can be selected based on the similar polarity with the solute. According to the Part I of the Annex of the Council Directive on the approximation of the laws of the member states on extraction solvents used in the production of foodstuffs and food ingredients (7), propane, butane, butyl acetate, ethyl acetate, ethanol, carbon dioxide, acetone, and nitrous oxide are the solvents to be used in compliance with good manufacturing practice (GMP) for all use. An extraction solvent is considered as being used in compliance with good manufacturing practice if its use results only in the presence of residues or derivatives in technically unavoidable quantities presenting no danger to human health. In Parts II and III, extraction solvent for which conditions of use are specified (Table 1).

III. APPLICATION OF SOLVENT EXTRACTION OF FLAVORS AND AROMAS (1, 3–12) A. Anise Star Anise star (Illicium verum) is so named from the star shape of its fruit. In Asia, it is often chewed in small quantities after meals to promote digestion and freshen the breath. The plant, which is cultivated in China, Indo-China, Japan,

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Table 1 Parts II and III of the Annex of the Council Directive PART II Extraction solvents for which conditions of use are specified

Name Hexanea

Methyl acetate

Ethylmethylketonea

Dichloromethane

Methanol Propan-2-ol

Condition of use (summary description of extraction)

Max. residue limits in extracted foodstuff or food ingredient

Production or fraction of fats 5 mg/kg in the fat or oil or coand oils and production of cocoa butter coa butter Preparation of protein products 10 mg/kg in the food containing and defatted flours the protein products and defatted flours Preparation of defatted cereal 5 mg/kg on the defatted cereal germs germ Defatted soya products 30 mg/kg in the soya product as sold to the final consumer Decaffeination of, or removal of 20 mg/kg in the coffee or tea irritants and bittering from coffee and tea Production of sugar from mo1 mg/kg in the sugar lasses Fractionation of fats and oils 5 mg/kg in the far or oil Decaffeination of, or removal of 20 mg/kg in the coffee or tea irritants and bittering from coffee and tea Decaffeination of, removal of ir- 2 mg/kg in the roasted coffee ritants and bittering from cofand 5 mg/kg in the tea fee and tea For all uses 10 mg/kg For all uses 10 mg/kg

PART III Extraction solvents for which conditions of use are specified

Name

Max. residue limits in foodstuff due to use of extraction solvents for preparation of flavorings from natural flavoring materials (mg/kg)

Diethyl ether Hexane Methyl acetate Butan-1-ol Butan-2-ol Ethylmethylketone Dichloromethane

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2 1c 1 1 1 1c 0.02

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Table 1 Continued

Name

Max. residue limits in foodstuff due to use of extraction solvents preparation of flavorings from natural flavoring materials (mg/kg)

Methlypropan-1-ol Propan-1-ol Cyclohexane

1 1 1

a

Hexane means a commercial product consisting essentially of acyclic saturated hydrocarbons containing six carbon atoms and distilling between 64°C and 70°C. The combined use of hexane and ethylmethylketone is forbidden. b The presence of n-Hexane in this solvent should not exceed 50 mg/kg. This solvent may not be used in combinatoin with hexane. c The combined use of these two solvents is forbidden. Source: Ref. 7.

and Philippines, is an evergreen tree up to 12 m high with a tall, slender, white trunk. Its fruit consists of 5–13 seed-shaped follicles attached to a central axis in the shape of a star. The best time to collect the seeds when they begin to turn from green to grayish brown is in the morning while the dew is still on them. Store the seeds in a tightly sealed opaque container to preserve the volatile oil. The essential oil is traditionally obtained from seeds and dry flowering tops by steam distillation, which is replaced by liquid CO2 extraction (8% yield) or supercritical CO2 extraction (10% yield). The essential oil is a pale yellow liquid with a warm, spicy, extremely sweet, licorice-like scent. Major components of the essential oil are trans-anethol, methyl chavicol, anisaldehyde, and cis-ocimene. It is mainly used in beverages and confectionery. B. Beeswax Beeswax absolute is a solid, waxy mass of pale yellow color and a very mild, sweet, oily odor, with a hay-like bodynote and a soft waxy backnote. Extraction is usually performed as a direct solvent washing of the beeswax. The most common solvents used for commercial extraction are ethanol, ether, glycol, and water. Selection of the solvent depends on the final use of the extract and on technical feasibility. The extraction yield is generally less than 1 wt%. Most active ingredients seem to be soluble in propylene glycol and ethanol. Fewer ingredients are soluble in water, but even water extracts show at least some bactericidal and fungicidal effects, as well as wound healing properties. Acetone extracts have been used for production of shampoos and lotions.

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C. Calendula Calendula (Calendula officianalis) oil is a herbaceous green sweet and dark brown liquid. It is obtained by solvent extraction from the flowers and contains calendulin (a yellow resin), waxes, and a small amount of volatile oil. The CO2 extracted total of the calendula blossom is much thicker and richer than the selective extract because it contains the plant waxes and heavier phytochemicals. Calendula oil may be produced using cold pressed olive oil and infused with calendula flowers that have been dried for 24–36 h to reduce the moisture content. D. Cardamon Cardamon is cultivated in Guatemala, India, and Sri Lanka. It has a large, fleshy rhizome, and the alternate, lanceolate leaves are blades from 0.3 to 0.75 m long, smooth and dark green above, pale, glaucous green, and finely silky beneath. The oblong, gray cardamon (Elettaria cardamomum) fruit has many seeds and they are gathered just before they are ripe. The dried ripe fruits (seeds) are hand crushed rather than machine treated so as to guarantee that the precious volatile oils are never subjected to heat. Then liquid CO2 extraction is used for avoiding heat. Traditionally it was extracted by steam distillation or solvent extraction with ethanol or acetone. Cardamon seed essential oil is a colorless or very pale yellow liquid with a sweet-spicy, warm fragrance and a woody-balsamic note. Major components are 1,8-cineole and terpinyl acetate. It is used as a spice (especially in Arab countries) for baked gingerbread, fruits, and marinades and as an ingredient of curry. E. Chamomile German Chamomile German (Matricaria chamomilla), sometimes called wild chamomile, has flower heads about 1.9 cm broad, with about 15 white, strap-shaped, reflected ray florets and numerous tubular yellow, perfect florets. Chamomile German is now cultivated extensively in Hungary, Egypt, eastern Europe, and France. The simple daisy-like white flower head that is smaller than chamomile Roman is used to extract the essential oil. Chamomile German, produced by supercritical carbon dioxide extraction, is very different from the traditional steam-distilled blue oil. The original color of the steam distilled essential oil is blue, but it becomes moderate green and finally dark brown when exposed to light and air. Since CO2 extraction contains all the water-soluble parts of the plant, the oil is much thicker. The CO2 extracted oil has even stronger antiinflammatory qualities than the steam-distilled essential oil, and has a more natural, fruity odor.

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F. Coffee In decaffeinating green coffee, a solvent must be selected that extracts caffeine with high selectivity. Thus, the solvent does not extract other coffee ingredients. Moreover, the solvent employed must be completely removable from the treated coffee. Most known processes for decaffeinating green coffee employ benzene (C6H6), trichloroethylene (CCl2=CHCl), 1,2-dichloroethane (CH2Cl−CH2Cl), methylene chloride (CH2Cl2), or chloroform (CHCl3) on account of the selective dissolving capability of these solvents for caffeine. Successful extraction of caffeine from coffee bean was achieved by the German chemist Friedrich Ferdinand Runge in 1820 following the suggestion of his friend, the poet Johann W. von Goethe. Runge investigated the components of coffee that caused his insomnia and isolated caffeine for the first time. The first commercial decaffeination process was invented by Ludwig Roselius, a German coffee importer, who founded a company, Kaffee Hag in Bremen, in 1906 with the brand Sanka. His success was based on the accidental steam treatment of ruined coffee beans. Steaming elevates the moisture content and swells the bean to facilitate the extraction of caffeine. This discovery made it possible to extract caffeine by benzene and produce decaffeinated coffee on a large scale. In the first half of the 20th century, trichloroethylene was used until it was shown to cause liver tumors in mice by the U.S. National Cancer Institute (NCI) in 1976. Since 1970s, most coffee makers have switched to other solvents, such as methylene chloride and ethyl acetate, or other types of processing to decaffeinate coffee. The U.S. Food and Drug Administration (FDA) has authorized by regulation the use of both methylene chloride and ethyl acetate for coffee decaffeination. According to an FDA report in the Federal Register, most decaffeinated coffee contains less than 0.1 ppm of residual methylene chloride, 100 times less than the maximal level of 10 ppm allowed by the FDA. Under European law, the level of methylene chloride residue in decaffeinated beans must be less than 2 ppm. Since the use of the solvents has given rise to objections about health, most coffee producers no longer use methylene chloride. In other processes, esters, ketones, light hydrocarbons, and ethers have been employed. However, esters are readily saponified, and it is difficult to sufficiently remove saponification products, i.e., acids and alcohols, from coffee. Due to their relatively high polarity ketones have insufficient selectivity for removal of caffeine. Hydrocarbons and ethers dissolve caffeine only sparingly, so that relatively high temperatures and long treating periods must be employed when these solvents are used. One of the newest decaffeination methods uses an orange peel extract. The green beans are soaked in pure water and washed with an orange peel extract to remove the caffeine. There are four main methods to extract caffeine from coffee beans:

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1. In the decaffeination process with methylene chloride, the green beans are placed in a rotating drum and soaked by steam for approximately 30 min. This treatment swells the beans, increasing their surface area and making the caffeine easier to remove. The next stage is extraction of the caffeine by methylene chloride at a temperature close to the boiling point of the solvent. Methylene chloride (CH2Cl2) extracts the caffeine from the coffee beans by bonding to the caffeine molecules (13). The caffeine-laden methylene chloride is drained away, and the beans are steamed for 9–12 h, evaporating the remaining solvent off the beans. Since methylene chloride actually bonds to the caffeine instead of just dissolving it, the caffeine evaporates with the solvent and does not remain in the coffee. At the final stage, air or vacuum drying removes excess moisture from the decaffeinated beans. In the indirect-contact method, sometimes called as the “water process,” the green beans soak for several hours in a water-coffee solution at a temperature near the boiling point. Gradually the solution draws the caffeine, as well as other flavors and aroma components, from the beans. The caffeine-water mixture solution is drained off and the caffeine is removed from water using methylene chloride, which absorbs the caffeine. The caffeinefree water, with all the remaining desirable flavor components, is returned to the beans. After drying the beans, most of the coffee oils and flavor elements are regained on to them. Methylene chloride itself never come in contact with the beans. The roasting and grinding process further evaporates any minute residues of methylene chloride. Although methylene chloride levels in the coffee bean are reduced to residual levels that are “legally safe," health-conscious consumers consider these residues unacceptable. Most decaffeinated coffee producers no longer use methylene chloride because it is now strongly suspected to cause cancer in humans. 2. Ethyl acetate decaffeination is often referred to as “naturally decaffeinated” because ethyl acetate is compound found in apples, peaches, pears, orange peel, and other fruits (14). Many processes employed natural ethyl acetate obtained from the fermentation of sugar cane. However, in decaffeination a synthetic chemical is used due to high cost of natural ethyl acetate. The process is the same as in conventional methylene chloride decaffeination (direct-contact method), except that ethyl acetate replaces methylene chloride as the solvent. There are still many commercial operations using ethyl acetate extraction in practice in the world. 3. Water decaffeination is, for most people, synonymous with Swiss Water Process, which is a trademarked name. The green coffee beans are first soaked thoroughly in pure water or steam under pressure, making the caffeine soluble so that it can be drawn out easily (12). The aqueous solution is full of many desirable flavors and aroma components as well as caffeine. The solution is drained off, and the coffee thrown away, because it is now flavorless. The caffeinerich solution is then passed through an activated carbon bed, which selectively removes the caffeine but not the flavor. This flavor-saturated solution is then

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poured onto a new batch of coffee. Because the liquid is already full of flavor, this flavor-charged water doesn’t extract any additional flavor from the coffee beans. It does, however, extract the caffeine. Advantage of this method is that no chemicals are used, so that no residual chemicals are left in the beans. This method successfully removes approximately 95% of the caffeine while retaining more of the flavor compounds present in the essential oils than the chemical solvent extraction. However, this method results in the loss of some other water-soluble components of coffee, such as carbohydrates. To overcome this problem, the activated carbon is pretreated with a carbohydrate, typically sucrose. This pretreatment process helps it to absorb caffeine without removing other compounds. 4. Carbon dioxide (CO2) decaffeination uses pressurized CO2 (a dense fluid) as a solvent to dissolve and draw the caffeine from the coffee beans, leaving the larger-molecule flavor components behind (15). Green beans are first softened by steam for higher selectivity of caffeine before loading them into an extraction vessel. Supercritical CO2 is introduced into the vessel to dissolve the caffeine. The caffeine-laden CO2 is then transferred to a separate scrubbing vessel where the caffeine is absorbed by water. Carbon dioxide is circulated until the caffeine content of the beans is reduced to 0.02% or less. The supercritical CO2 is then drawn off, leaving the beans free of caffeine. In this extraction process, no residual chemicals are left in the beans but it produces the most flavorful decaffeinated coffee. The CO2 decaffeination process is superior to other decaffeination methods because it removes caffeine while leaving the coffee’s flavor compounds intact. CO2 is a gas at normal atmospheric pressure and temperature, so it is not usually thought of as a solvent. However, under pressure it becomes a dense, liquid-like fluid. While in this liquid state, CO2 has the ability to selectively bond with and dissolve other materials. It is this selectivity that makes CO2 decaffeination unique. While CO2 removes 99.9% of caffeine from coffee, it does not affect the carbohydrates (sugars, starch) and peptides (protein), which are ultimately responsible for the flavor and aroma of brewed coffee. Since carbohydrates and peptides are large polar molecules, nonpolar CO2 extracts only the caffeine molecules, which are small nonpolar molecules. CO2 decaffeination technology was discovered and developed at the Max Planck Institute in Germany in the 1970s. Yet this process is not widely used because of the high costs of opening a CO2 decaffeination plant. However, as competition for sales of decaffeinated coffee increases, consumers will demand decaffeinated coffee that is processed without harmful chemicals but is full-flavored and aromatic. G. Coriander Coriander (Coriandrum sativum) is an annual herb that belongs to the carrot family (Umbelliferae). It is cultivated in East Asia, Hungary, the Mediterranean, Morocco, North America, Poland, and Russia. Coriander brown seeds, which

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are sweet and vaguely reminiscent of orange peel, are used to obtain a sweetsmelling, spicy essential oil. The essential oil obtained through steam distillation of the fruits is a colorless or pale yellow liquid, and the flavor is described as mild, sweet, and spicy-aromatic yet somewhat warm and slightly burning. Coriander oil would be prepared by steam distillation of dried fruits (16). To obtain the maximal yield of essential oil and to reduce the processing time, it is necessary to crush the spice prior to distillation. The extraction yield of partially dried ripe coriander seeds by steam distillation or hydrodistillation is up to 1.7% essential and up to 20% fatty acid. Liquid CO2 extract of seed has 3% essential oil with 50% of low molecular weight saturated solid lipids. Supercritical CO2 oleoresin extract after separating the lipids has 1.3% volatile oil. Essential oil of coriander is used in perfumes, alcoholic beverages, baked goods, candies, ice creams, chewing gums, meats, sauces, curries, and tobacco. Demand is expected to grow due to increased consumption of ethnic foods and growth in the meatprocessing industry. H. Elemi Elemi (Canarium luzonicum ) is a tropical tree up to 30 m high grown in Philippines and Indonesia. It yields a resinous pathological exudation with a flesh, citrus-like, peppery odor. Although it is called a gum, it is almost entirely made up of resin and essential oil. Steam distillation has been used to prepare essential oils. Nowadays elemi essential oil is extracted by liquid CO2 from its resin and essential oil is a colorless to pale yellow liquid with a light, fresh, balsamic-spicy, lemon-like odor. The major components of elemi oil are limonene, α-phellandrene, and sesquiterpene alcohol elemol. I. Frankincense Frankincense (Boswellia carterii), originated from Oman, Ethiopia, Somalia, South Arabia, and China, yields a natural oleo gum resin that is collected by making incisions in the bark. At first, a milky white liquid appears which then solidifies into amber or orange-brown crystals of resin. It is prepared by CO2 extraction or steam distillation of selected oleo gum resin, and essential oil is a warm, woody, sweet balsamic, spicy fragrance with a hint of lemon. J. Galbanum Galbanum (Ferula galbaniflus), originated from the Middle East, is a large perennial herb with a smooth stem, shiny leaflets, and small flowers. It contains

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resin products that exude a milky juice, a natural oleoresin. The dried resinous exudate is collected by cutting at the base of the stem or upper part of the uncovered roots. Galbanum resinoid is produced by extraction of gum with a nonpolar solvent. The essential oil is obtained from resinoid and the extraction method is turning from steam distillation to CO2 extraction. The essential oil is a yellow liquid with a green, slightly spicy odor. Both resinoid and essential oil are used for creating green top notes. K. Ginger Ginger (Zingiber officinale), originated from China, Fiji Islands, Indonesia, Jamaica, Malaysia, Nigeria, Taiwan, and the West Indies, is a perennial herb up to 1 m high with thick, spreading, tuberous roots, which are very pungent. Ginger oil is produced by steam distillation of dried crushed rhizomes, whereas ginger oleoresin is prepared by extracting ginger rhizomes with acetone or alcohol. Ginger oil is a light yellow liquid and its major components are zingiberene (35–40%), Ar-curcumene (18%), and β-sesquiphellandrene. Ginger CO2 extract is much more aromatic than the steam-distilled version. Like other CO2-produced oils, ginger CO2 seems much closer to the scent and taste of a freshly grated root of fresh ginger, rather than the scent of the dried ginger found in the steamdistilled oil. Ginger concentrates are used in beverages like ginger ale, sweet baked goods, confectionery, curry powder, meats, and cordials. L. Heliocarrot Heliocarrot (Radix daucus carota) essential oil prepared by CO2-extracted dried carrot roots into pure jojoba oil is extremely high in β-carotene, as well as vitamins A and E. CO2 extract is much more concentrated than many of the infused carrot root oils available on the commercial market. M. Hop Oils Hop oils consist principally of hydrocarbons, oxygenated compounds, and small amounts of sulfur-containing compounds. The hydrocarbons typically make up 80–90% of the total oil; the terpenes myrcene and β-pinene, and the sesquiterpenes β-caryophyllene and α-humulene are found in the largest quantities. Two of these, β-caryophyllene and α-humulene, can be easily oxidized in air, thus contributing to the oxygenated fraction of the oil as well. Other oxygenated compounds include alcohols such as linalool and geraniol and esters such as geranyl isobutyrate and methyl dec-4-enoate. Although many brewers think that esters are all fermentation byproducts, hops can contribute a number of fruity aromas, e.g., grapefruit and pineapple.

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For years, oils and the hop aroma have been extracted with steam distillation. However, the heat of steam distillation changes the hop aroma profile. Both oils and α acids have been extracted with solvents such as ethanol, ether, hexane, and methylene chloride. During the extraction with hexane, highly concentrated soft resins in solution can result in a considerable uptake of hard resins. In ethanol extraction, a large amount of water-soluble components, such as inorganic salts, nitrates, and tannins, will be extracted. Heating of the extract to remove solvent markedly modifies its aroma profile. There are also concerns that some solvent remains. Recently, CO2 has been used to extract oils and α acids without these problems (17, 18). Liquid CO2 (typically at 6 MPa pressure and 5–10°C in extraction plants) is a relatively mild, nonpolar solvent that is highly specific for hop soft resins and oils. The low temperature and selectivity associated with liquid CO2 extraction allows recovery of the hop oils in undamaged condition. Therefore, liquid CO2 becomes the most selective solvent used commercially for hops and hence produces the purest whole resin and oil extract. It extracts none of the hard resins or tannins, much lower levels of plant waxes, no plant pigments, and less water and water-soluble materials. However, the yield of α acids with liquid CO2 (89–93%) is lower than that of supercritical CO2 (91–94%) or the organic solvents (93–96%). Above the critical point (typically at 30 MPa pressure and at 60°C in extraction plants), CO2 has the properties of both a gas and a liquid and is a much stronger solvent. Supercritical CO2 is more selective than the organic solvents and extracts less of the tannins and waxes and less water and water-soluble components. It does extract some of the plant pigments like chlorophyll but rather less than the organic solvents do. This modification of the hop aroma profile also applies to some extent to supercritical CO2 extracts, which are produced at about 60°C. Supercritical CO2 also extracts less oil than does liquid CO2 but more than the organic solvent extracts. As mentioned, liquid CO2 extracts are the most pure whole-resin extracts; moreover, the low-temperature extraction (5–10°C) results in an aroma profile most closely resembling that of the leaf hops from which they were prepared. The only real disadvantage of liquid CO2 extract in comparison with the others is its higher cost resulting from the lower extraction efficiency. N. Jasmine Jasmine extraction is one of the most important businesses nowadays. The exotic scent of jasmine comes from the flowers of Jasminum officinale, which grows in Algeria, Egypt, France, India, Italy, Morocco, South Africa, Spain, and Turkey. The star-shaped, very fragrant white flowers are used to obtain jasmine absolute via the concrete. Hexane is normally used as solvent in solvent extraction. Sometimes jasmine is extracted with benzene. Multistage extraction is carried out with hexane getting richer in perfumery content with each stage. Evapo-

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ration is then carried out in the falling-film evaporator until 90% of the hexane of the mother liquor is evaporated. Concentrated liquor is distilled under vacuum to obtain the final jasmine concrete. The three-stage extraction of jasmine blossoms with hexane gives a 0.3 wt% yield of concrete and the extraction of the concrete with ethanol gives a 60 wt% yield of absolute. Jasmine is the most exquisite of scents; the absolute is a deep, reddish brown liquid with a sweet, floral and exotic, slightly heady fragrance, whereas the concrete is a brown waxy mass. Even though benzyl acetate is major volatile component of jasmine oil, indole, cis-jasmine, and methyl jasmonate contribute strongly to the typical jasmine fragrance. O. Linden Linden is a deciduous tree growing to 30 m high with wide, heart-shaped leaves with a whitish down on the underside, particularly on the veins. The small, extremely fragrant white flowers grow in drooping clusters on long stalks. Linden (Tilia vulgaris) blossom absolute is extracted from concretes or pomades, which are produced by steam. Other methods include solvent extraction, infusion, water distillation, and CO2 extraction. It is greenish, viscous, light, floral, and sweet. P. Myrrh Myrrh oil is obtained by either CO2 extraction or steam distillation of handpicked and selected Burseraceae oleo gum resins produced by Balsamodendron myrrha. CO2-extracted myrrh (Commiphora myrrha grows in Northeast Africa and Arabia), a rich golden resinous liquid, contains more components of the actual myrrh beads than does the steam distilled essential oil, which is a pale yellow to amber viscid clear oil with a characteristic warm, sweet balsamic, slightly spicy-medicinal odor. The most active ingredient of the myrrh flavor is lindestrene. Myrrh resinoid is prepared by solvent extraction of the gum with hexane. The oil, resinoid, and tincture are used in pharmaceutical products, including mouthwashes, gargles, and toothpaste; they are also used in dentistry. The oil and resinoid are used as fixatives and fragrance components in soaps, detergents, cosmetics, and perfumes, especially oriental types and heavy floral. The essential oil is also used as a flavoring in most major food categories, alcoholic and soft drinks. Q. Nutmeg Nutmeg (Myristica fragrans), which is an apricot-like fruit of an evergreen tree up to 20 m high with grayish brown smooth bark, dense foliage and small dull-yellow flowers, is cultivated in Madagascar, Indonesia, Sri Lanka,

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and the West Indies. Nutmeg essential oil is obtained from the dried nutmeg seed by solvent (alcohol) extraction, steam distillation (6–16% yield), or liquid CO2 extraction (16% yield) (19). Most active ingredients of the nutmeg flavor are sabinene, α-pinene, β-pinene, and myristicin. It is a pale yellow liquid with a pleasant, warm, spicy, aromatic odor. Nutmeg oil is used as seasonings for curries, vegetables, baked goods, and processed meats. R. Rose Rose concrete and rose absolute can be obtained by solvent (hexane) extraction, whereas rose essential oil is derived from steam distillation. The extraction of rose flower with steam distillation gives about 0.025 wt% yield of rose oil. The premium rose perfume is the otto or attar, which is obtained by water and steam distillation from the rose blossom (Rosa x damascena). A perforated grid is used for supporting the rose flowers; thus, only rising steam contacts with the plant materials while water boils in the still. The steam, which contains flavor and aroma, is condensed and collected. The waxy constituents of the rose oil that float on the water are decanted off. The lower layer water is distilled again to recover any remaining oil. Both are combined to make rose oil. Since a lot of phenyl ethyl alcohol is dissolved in the distillation water, the otto does not accurately represent the rose flower flavor. The extraction yield is generally less than 0.02 wt%. Solvent extraction is now more frequently used to extract the aroma of rose blossoms. The product of solvent (hexane) extraction is a waxy, light brown, semisolid material known as a concrete. The concrete has phenylethyl alcohol in the same ratio as the blossoms, so that it represents the rose flower flavor intimately. Rose absolute is obtained from the concrete by extraction with a polar solvent like alcohol. The absolute is a reddish liquid with a deep, rich, sweet, rosy-spicy, honey-like fragrance. The phenylethyl alcohol content of its volatile portion is 60–70%. Typically, solvent extraction yields about 10 times that obtained by steam distillation, in the order of 0.1–0.2% (1–2 mL concrete/kg flowers). Rose oil is used primarily as a fragrance component in pharmaceutical preparations (e.g., ointments and lotions) and is extensively used as a fragrance ingredient in perfumes, creams, and soaps. Rose oil and absolute are also used extensively as flavor ingredients in fruit-type flavors. Food products in which they are used include beverages, frozen dairy desserts, sweets, baked goods, gelatins, and puddings. S. Rosehip A soft golden red oil of rosehip (Rosa rubignosa Germany; originated southern Andes) is obtained from its red berry-like fruits or hips by CO2 extraction or

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cold pressing process. Rosehip is a rich source of vitamin C that is made into a drink for babies and young children. Those are contained in the rosehip shell. The oil extracted from the seeds contains essential polyunsaturated fatty acids, and trans-retinoic acid, which is now known to promote scar healing and more youthful-looking skin. T. Tuberose Tuberose (Polianthes tuberosa) is cultivated in Egypt, India, and Morocco. Large, very fragrant, white lily–like tuberous blossoms are used to extract absolute. The solvent extracted absolute is a dark orange or brown soft paste, with a heavy, sweet-floral, sometimes slightly spicy, tenacious fragrance. U. Vanilla Vanilla (Vanilla planifolia) has been one of the most important and most publicized flavors since its discovery in Mexico. The vanilla beans of commerce are the cured, unripe fruit of Vanilla planifolia, Mexican or Bourbon vanilla, which is native to Mexico, Central America, and northern South America; or Vanilla tahitensis, Tahiti vanilla, which is native to Oceania. The principal sources of vanilla are Madagascar, the Comoros, and Reunion, which together furnish about 70–75% of the world’s supply, and Mexico, Uganda, and French Polynesia. Vanilla extracts are used extensively in chocolate, dairy products (milk, ice cream, etc.), beverages (colas), baked goods, and confections. Vanilla is also used as a background note or flavor enhancer (or sweetness enhancer) to round out the flavor profiles of many food products. The type of vanilla used depends on the product, the ingredients in the base formulation, and the desired flavor profile. In solvent extraction, extracts are obtained by extracting crushed vanilla beans with a polar solvent (methanol, ethanol, acetone, or different concentrations of water/alcohol). Sometimes countercurrent extraction technique is used to obtain vanilla essential oil. Concentrated vanilla extract is made by vacuum distillation, which is often used to separate materials that might suffer thermal degradation at the higher temperatures encountered in conventional distillation. However, distillation destroys some of the aromatic substances of vanilla flavor. Solvent is removed until the desired concentration is reached. Vanilla essential oil is a viscous dark brown liquid with a rich, sweet, balsamic, vanilla-like odor. Liquid CO2 extracted absolutes (4.5% yield) have been tried with success in high-quality ice cream and desserts. Supercritical CO2 extraction with ethanol as a cosolvent gives essential oil (10% yield) rich in natural vanilla aroma and fine crystals of vanillin. The basic key ingredients of the vanilla extract are vanillin and phenol derivatives.

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V. Violet Violet is extracted by solvent extraction. A small, tender, perennial plant with dark green, heart-shaped leaves and fragrant violet-blue flowers are the parts of the plant (Viola odorata) used for preparing absolutes. The leaf absolute is an intense dark green viscous liquid with a strong green-leaf odor and a delicate floral undertone. The flower absolute is yellowish green viscous liquid with a sweet, rich, floral fragrance, characteristic of fresh flowers. Most active ingredient of the violet leaf absolute is 2-trans-6-cis-nonadienal, which is responsible for the flavor. W. Tolu Balsam Tolu balsam is formed after injuries in the trunk of Myroxylon balsamum tree, native of the jungles of northern South America, particularly, in Colombia, Honduras, Peru, and Venezuela. The balsam is a brown, orange-brown, or dark yellowish brown mass. It is brittle when cold, and the fracture is glass-like or flint-like. Its odor is sweet balsamic, cinnamic in type, faintly floral, and with a backnote of vanillin. The fresh balsam is soft and sticky, but exposure to the air makes it hard and brittle, more like resin, with a crystalline appearance. The resinoid is prepared by extraction of the balsam, while essential oil is obtained by distillation of the balsam. Tolu balsam absolute is used mainly as a fixative in citrus colognes, oriental perfumes, chypres, and floral bases.

REFERENCES 1. JA Rogers Jr, F Fischetti Jr. Flavors and spices. In: M Grrayson , ed. Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., Vol. 10. New York: John Wiley & Sons, 1980, pp. 456–488. 2. W Hamm. Liquid-liquid extraction in the food industry. In: TC Lo, MHI Baird, C Hanson, eds. Handbook of Solvent Extraction. New York: John Wiley & Sons, 1983, pp. 593–603. 3. B Meyer-Warnod. Natural essential oils: Extraction processes and application to some major oils. Perfumer and Flavorist 9:93–104, 1984. 4. WG Schultz, JM Randall. Liquid carbon dioxide for selective aroma extraction. Food Tech 24:1282–1286, 1970. 5. DA Moyler. Extraction of flavours and fragrances with compressed CO2. In: MB King, TR Bott eds. Extraction of Natural Products Using Near-Critical Solvents. Glasgow: Chapman and Hall, 1993, pp. 140–183. 6. KU Sankar. Supercritical fluid carbon dioxide technology for extraction of spices and other high value bio-active compounds. In: SSH Rizvi, ed. Supercritical Fluid Processing of Food and Biomaterials. London: Chapman and Hall, 1994, pp. 155– 166. 7. Official Journal L 157, 24/06/1988 p. 0028–0033

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8. K Bauer, D Garbe, H Surburg. Flavors and fragrances. In: W Gerhartz, ed. Ulmann’s Encyclopedia of Industrial Chemistry, 5th ed., Vol. A11. Weinheim, Germany: VCH, 1988, pp. 141–250. 9. C Karl. Herbs, spices and essential oils. In: E Ziegler, H Ziegler, eds. Flavourings. Weinheim: Wiley-VCH, 1998, pp. 184–218. 10. M Grieve, CF Leyel, eds. A Modern Herbal. New York: Dover Pubns, 1978 11. http://www.aromabar.com.au/database/essentialoils.htm 12. http://www.darwinsgarden.com 13. L Nutting, GS Chong. Continuous process for producing decaffeinated beverage extract. US Patent 3,361,571 (1968). 14. JM Patel, AB Wolfson, B Lawrence. Decaffeination process. US Patent No. 3,671,263 (1972). 15. E Lack, H Seidlitz. Commercial scale decaffeination of coffee and tea using supercritical CO2. In: MB King, TR Bott, eds. Extraction of Natural Products Using Near-Critical Solvents. Glasgow: Chapman and Hall, 1993, pp. 101–139. 16. T Hirvi, I Salovaara, H Oksanen, E Honkanen. Progress in essential oil research. In: EJ Brunke, ed. Proceedings of International Symposium on Essential Oils. Berlin: Walter de Gruyter, 1986, pp. 111–116. 17. DS Gardner. Commercial scale extraction of alpha-acid and hop oils with compressed CO2. In: MB King, TR Bott, eds. Extraction of Natural Products Using Near-Critical Solvents. Glasgow: Chapman and Hall, 1993, pp. 84–100. 18. DS Gardner. Industrial scale hop extraction with liquid CO2. Chem Ind 19:402– 405, 1982. 19. BM Lawrence. Major tropical series on nutmeg and mace. Essential Oils 1976– 1978:51–103, 1978.

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10 Extraction of Natural Antioxidants Vassiliki Oreopoulou National Technical University of Athens Athens, Greece

I. INTRODUCTION Natural antioxidants are compounds from plant or animal sources that retard oxidative rancidity of oils, fats, and fat-soluble components, thus protecting them while delaying the development of unpleasant flavors and odors resulting from oxidation. Antioxidants are present naturally in most raw food sources. Processing can remove or degrade some of these antioxidants. Therefore, a supplementation with suitable antioxidant compounds is needed to maintain acceptable quality of the products. Especially oils, fats, and products with a high fat content are susceptible to oxidation and require the addition of antioxidants. The most widely used antioxidants are synthetic ones, such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertiary butylhydroquinone (TBHQ) and propyl gallate (PG). Doubts about the safety of synthetic antioxidants arose first in the 1960s and led to an increased interest and a broad research on natural antioxidants (1, 2). Natural antioxidants are primarily phenolic compounds that may occur in all parts of a plant. They are multifunctional and can act as free radical terminators, metal chelators, and singlet oxygen quenchers. The common plant phenolic antioxidants are tocopherols, flavonoids, and related compounds like coumarins, cinnamic acid derivatives and chalcones, phenolic diterpenes, and phenolic acids. Another widespread antioxidant in nature is ascorbic acid, but today its extraction from natural sources is not significant as all ascorbic acid used for food is chemically synthesized. To be considered as an antioxidant for practical use, a compound extracted from a natural source must meet several criteria, including (a) absence of any

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toxic or physiological effect, (b) no impartation of any strong odor, flavor, or color to the product, and (c) considerable antioxidant activity at small concentrations in the product. In fact, antioxidants present in or added to foods are functional at very small concentrations, usually up to 0.02%. Usually, in literature surveys on antioxidants, compounds that have no antioxidant activity themselves but enhance the activity of antioxidants, called synergists, are included. This chapter covers the main sources and extraction processes of natural antioxidants and synergists.

II. SOURCES The main sources for the extraction of natural antioxidants are of plant origin. Some animal sources, such as shrimp, have also been reported (3). The most common plant sources are (a) cereals, (b) citrus fruits, (c) cocoa and coffee beans, (d) herbs and spices, (e) oilseeds, (f) olives, (g) onion and garlic, (h) tea, as well as some miscellaneous products. A. Cereals Several cereals have been reported to possess antioxidant activity. Oat has been the most widely investigated because it has greater activity than the others and a bland flavor. Oat flour has been incorporated in dairy products, potato crisps, sausages, mayonnaise, etc., and improved their stability. Malt and barley flour or meal acted as antioxidants on fats too (2). Extracts of groats and hulls from oat were more active than synthetic phenolic antioxidants in the protection of oils during frying and of emulsions (4, 5). The antioxidant activity of oat hulls and groats is attributed to several phenolic compounds, mainly ferulic, p-coumaric, caffeic, vanillic, p-hydroxybenzoic acids and vanillin (6). The extract of defatted oat kernels also showed antioxidant activity similar to BHT and PG that was attributed to caffeic and ferulic acid and their derivatives (7, 8). Oat hulls contain more phenolic acids than oat flour; therefore, oat hulls are more efficacious for antioxidant extraction (9). Byproducts of other cereals could be used as raw materials for antioxidant extraction, too. Buckwheat hulls extracts showed a good antioxidant activity, which was attributed to the presence of dihydroxy phenolic components, flavonols, and flavone glucosides (10, 11). B. Citrus Fruits Citrus fruits and especially oranges contain antioxidant components (12, 13). These antioxidants are mainly concentrated in the outer part of the peel, called

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flavedo. Extracts of orange and grapefruit flavedo had significant activity against d-limonene oxidation, but extracts of flavedo from lemon and lime had very little activity (13). On the contrary, extracts of lemon peel offer effective protection from peroxidative damage in living systems (14). Antioxidant properties of extracts from orange flavedo were attributed to tocopherols (2, 13). Also citrus essential oils and terpenes showed antioxidant activity in several food products (2). However, flavonoids isolated from various citrus extracts seem to be the most important antioxidant components present in citrus fruits. More than 60 individual flavonoids have been identified in citrus fruits (15). Among them, flavanones are the most abundant, and especially hesperetin and naringenin are presented in the pulp of all citrus fruits (16). Highly methoxylated flavones occur in much lower concentration but exhibit the greatest antioxidant activity. Two isoflavones are referred as the main antioxidant components of osage orange peel (12), while a flavanone, eriocitrin and its aglycon, eriodictyol, are referred as the main antioxidant components of lemon peel (14). A review on citrus flavonoids and on the correlation of their structure to antioxidant activity has been presented by Benavente-Garcia et al. (17). C. Cocoa and Coffee Bean Cocoa bean husk powder and especially water and alcohol extracts were reported as potential antioxidants (18, 19). Fractions with antioxidant activity were isolated from the extracts, but the active substances were not identified. The brown pigment was suggested as the antioxidant factor (2), though it was not active to all substrates (20). Roasted coffee powder showed antioxidant properties in oils. However, the antioxidant activity of the coffee powder was lower than that of its constituents: caffeic acid and quinic acid. Another potent antioxidant component in coffee is chlorogenic acid (2). D. Herbs and Spices Herbs and spices are the most broadly examined raw materials for the occurrence and extraction of antioxidants. Since the pioneer work of Chipault et al. (21), many researchers have reported results on the antioxidant efficiency of various herbs and spices and on the identification of active components. Though the results are sometimes contradictory, the extracts have in most cases higher activity than the relevant ground herbs or spices (2). Rosemary has exhibited the greatest antioxidant activity of all spices and herbs. The extracts of rosemary were more effective than BHA and BHT in the protection of fat and of products with a high fat content (22–24). Bleached, odorless, and tasteless antioxidants have been produced from rosemary and com-

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mercially exploited since the early 1980s (25, 26). The most active components of rosemary, identified in the earlier studies, were carnosol, rosemaridiphenol, and rosemariquinone (27–29). Nowadays several commercial rosemary extracts are available and their most active compounds are reported to be carnosol, carnosic acid, and rosemarinic acid (30–32). Sage belongs to the same family as rosemary—the Labiatae family—and possesses almost equivalent antioxidant activity. Many investigations on rosemary antioxidant efficiency also include data on sage efficiency (2), and the same procedures for the extraction and purification of the antioxidant fraction from both spices have been suggested (25, 26). The main antioxidant constituents in sage were also found to be carnosol, carnosic acid, and rosemarinic acid, followed by other related phenolics (31, 33). In addition to the main sage species, i.e., Salvia officinalis, several other varieties have also shown remarkable antioxidant efficiency (34–36). Other plants of the Labiatae family are potent sources of natural antioxidants (37). Oregano is one of the most promising. Extracts of oregano retarded lipid oxidation (37–40), while the essential oil of the spice also contained antioxidant components (41). Thyme seems to be another promising spice: extracts of the plant showed appreciable antioxidant activity (37, 42) and its essential oil was effective (43). Dittany and marjoram extracts exhibited antioxidant activity too (2, 37). Several other herbs, spices, and medicinal plants are rich in antioxidant components (2, 42, 44–50). E. Oilseeds Oilseeds contain several antioxidant compounds that protect them against rancidity. Therefore, whole seeds or the byproducts obtained after oil extraction could be potential sources of antioxidants. Methanolic extracts of cottonseed exhibited antioxidant activity attributed mainly to flavonoids (51). Flavonoids are also claimed to be the major antioxidants of soybean methanolic extracts (52–54). Most of these flavonoids remain in the waste during soybean curd processing and could be recovered (54). According to Rhee et al. (55), cottonseed meal had a higher phenolic content and antioxidant activity than soybean meal. Canola meal is another potent source of antioxidants with a total phenolic content remarkably higher than both cottonseed and soybean meals (56). Other oilseeds studied with positive results include sunflower seed (57) and sesame seed (58). F. Olives Olives and olive oil are rich in antioxidant compounds, especially tocopherols. In addition to tocopherols, several other phenolic compounds, i.e., hydroxytyro-

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sol, caffeic, protocatechuic, ferulic, syringic, and vanillic acids, were identified in the extracts of virgin olive oil and were found effective in prolonging the shelf life of the refined olive oil (59, 60). Rape, a major byproduct of olive oil production, was used for the extraction of antioxidants. The extracts contained various phenolic acids, catechol, and tyrosol, and inhibited oxidative deterioration of refined vegetable oils (61). Also, olive leaves contain phenolic antioxidant compounds, and some of them were identified (62–64). G. Onion and Garlic Onion contains appreciable amounts of the well-known flavonoid quercetin (16) and exhibits a remarkable antioxidant activity (65, 66). Onion skins are also rich in quercetin and quercetin derivatives, and their methanolic extracts were effective antioxidants (67, 68). Garlic was found effective for some substrates, i.e., linoleic acid or minced pork (65), but ineffective in lard (44). H. Tea Tea leaves are a well-known source of natural antioxidants, especially catechins. Fresh or mildly processed commercial tea has been found to contain large quantities of these polyphenols, up to 30% of its dry mass (69). Comparable concentration has not been reported in any other foodstuff (70). Green tea extracts were 21–24% more effective at radical quenching than black tea extracts in both water and lipid-soluble media (71). Aqueous extracts of green tea retarded the oxidation of oils and fats more than commercial rosemary extract (72), and were also effective in emulsions (73) and fish meat (74). Commercial tea extracts are available with various catechin contents (74). I. Miscellaneous Products Various other fruits, vegetables, and grains contain considerable amounts of antioxidants (16, 48, 75). Among them those that could serve as potential sources for the extraction of antioxidants are the byproducts of agrofood industries or crops with a limited utilization due to damage or to low commercial profit. Grape skins and seeds are a byproduct of the wine making industry, with a high phenolic content and a remarkable free radical scavenging capacity and antioxidant activity in oils (76–80). The antioxidant activity of citrus peel and pulp was mentioned above, and similar byproducts from other fruit processing industries can be used as well (81). Many other potential sources are referred, including bean hulls (82, 83), lupin derivatives (84), leaves of some agricultural products (67, 85), potato peel waste (86), and so forth.

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III. EXTRACTION PROCESSES AND PARAMETERS Although some of the raw materials mentioned above have been incorporated as dry powders to certain foods or food systems in order to protect them from oxidative deterioration, the use of extracts of the raw materials is suggested in most cases. The aim of extraction is to concentrate the antioxidant components of the raw material, apart from inert substances, so that the product of the extraction could be added to the food in smaller quantities. The extraction process involves a more or less vigorous agitation of the ground raw material with the extraction solvent at ambient or elevated temperature and subsequent separation of the extract from the residue by filtration. Repeated extraction steps may be accomplished to increase the extract yield. Alternatively, a packed bed of the ground material can be used which is leached by the extraction solvent under refluxing conditions.

A. Solvent Selection The solvent used for the extraction is of major importance for the recovery of the antioxidant components, the coextraction of undesirable substances, and the process yield. For the extraction of antioxidants from the majority of the sources mentioned above organic solvents have been used. Water has been used to a lesser extent, mainly for extracting antioxidants from tea and for the recovery of essential oils through steam distillation. In addition, water is used in mixtures with alcohols. Edible vegetable oils have been also reported and patented as extraction solvents. Several studies focused on the efficiency of different organic solvents in extracting antioxidant components. The antioxidant components are of a phenolic nature; therefore, organic solvents of higher polarity are more effective in quantitative recovery of these substances than nonpolar solvents. For example, extracts obtained with methanol or ethanol from flour or hulls of cereals had higher antioxidant activity than extracts obtained with hexane, petroleum ether, ethyl ether or its mixtures with chlorophorm, acetone, and ethyl acetate (2, 4, 10). Also, the extraction of various aromatic herbs by hexane, ethyl acetate, or ethanol showed that the ethanol extracts were the most active in retarding the autoxidation process in lipid substrates (50). Similarly, in extraction of cocoa bean husks with various organic solvents, the alcoholic extracts were the most effective (18). In fact, methanol or ethanol are widely used for the extraction of antioxidants from rosemary, sage, and other herbs and spices, as well as from oilseed and other agroindustrial byproducts. Aqueous solutions of methanol are also used.

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Although methanol and ethanol are suitable for quantitative extraction of total phenolics, many undesirable substances are coextracted, and purification is necessary to isolate the antioxidant fraction. Meanwhile, solvents of lower polarity may be used to recover and isolate special groups of phenolics. Thus ethyl ether yields a very low recovery of total phenols compared with 80% aqueous methanol, but it can extract the low molecular weight phenolic acids (87) which are good antioxidants. Both ethyl ether and hexane are suitable for the extraction of tocopherols and terpenoids with appreciable activity (40, 49). On the other hand, pure methanol or aqueous solutions of methanol may cause some decomposition to certain phenolic glucosides, which has not been detected with acetone, aqueous acetone, or ethyl acetate (87). Results obtained by successive extractions of various raw materials with different solvents further indicated that the choice of the appropriate solvent depends on the raw material and the nature of the antioxidants it contains. Thus, in a sequential extraction of olive rape with hexane acetone and ethanol, the hexane extract, despite amounting to 9% of the dried rape, had very few polyphenols and lower activity than either the acetone or the ethanol extract, which had similar behavior and amounted to 4% and 7% of the dried rape, respectively (61). A similar result was obtained when cocoa byproducts were extracted with petroleum ether, or methanol, or successively extracted by petroleum ether followed by methanol. The methanol extract showed the highest activity which was further increased for the methanol extract obtained from the residue of petroleum ether extraction (88). Also, pretreatment of cocoa husks with petroleum ether and chloroform before alcoholic extraction improved the antioxidant properties of the alcohol extract (19). In contrast in a sequential extraction of turmeric with hexane, benzene, and 80% aqueous methanol, all of the extracts had very good antioxidant properties but the benzene extract showed the highest activity. The yield of the hexane, benzene, and 80% aqueous methanol extracts were 1.98%, 4.15%, and 5.24% on a dry basis, respectively (89). When another spice, oregano, was successively extracted with hexane, ethyl ether, ethyl acetate, and ethanol, the hexane and ethyl ether extracts showed higher activity than the following extracts (40). The yield of hexane extraction amounted to 8.8% on a dry basis and the main antioxidant components were terpene derivatives. Yields of following extracts with ethyl ether and ethyl acetate were rather low (1.5% and 2.8% d.b., respectively), while the ethanol extract yield was 12.3% d.b. The antioxidant components of the ethanol extracts were mainly flavonoids, but a lot of other substances were coextracted; therefore, this extract had to be purified (90). A comparative study of extraction yields and antioxidant activities of extracts obtained by different solvents and sequential extraction steps, from several aromatic herbs, was conducted by Dapkevicius et al. (46). Hydrodistillation was

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used to isolate the essential oils and the yields were greatly dependent on the plant, (as high as 6.9%). Except for thyme essential oil, the rest showed no significant antioxidant activity. Extracts obtained by acetone after hydrodistillation or without previous hydrodistillation had good antioxidant activity. The yield was higher when the acetone extraction was conducted without a previous hydrodistillation step and amounted to 11.3% and 6.6% for the rosemary and sage extract, respectively. Methanol-water (1:1) extraction of the residues obtained from the acetone extraction resulted in extracts with yields higher than the acetone extraction (except for rosemary), but with lower antioxidant activity. In the same study, the water extract of the herbs obtained after hydrodistillation was tested and showed no antioxidant activity. B. Effect of Extraction Parameters The parameters that affect antioxidant recovery by aqueous ethanol extraction were investigated by Wettasinghe and Shahidi (92) during the extraction of an oilseed meal. The concentration of ethanol in the extraction medium affected markedly the recovery of antioxidants and best values lied in the range of 50– 60%. The extraction temperature and time affected also the antioxidant activity of the extract. In particular, the activity increased with temperature and time up to 70°C and 60 min, respectively, and declined afterward. The optimal extraction conditions were determined through response surface methodology as 52% ethanol in water, 74°C, and 62 min. In the same study, aqueous methanol or aqueous acetone were tested as extraction media with slightly lower antioxidant activity than the ethanolic extracts. Alcoholic extraction of other raw materials, like grape byproducts, is most effectively accomplished under acidic conditions. HCl acid is used and the pH is arranged around 3–3.5 (76). To avoid acid without decreasing phenolic recovery several sequential extraction steps with alcohols and alcohol-water mixtures were suggested (93). However, the hydrolysis of glucosidic bonds of polyphenols, induced by acid, is not necessarily undesirable as it might even increase the antioxidant activity of the relevant compounds. Solvent extraction parameters for the isolation of phenolic antioxidants from wood hydrolysates were studied by Cruz et al. (94). They used ethyl acetate or ethyl ether as extraction solvents and found that ethyl acetate was more effective in quantitative removal of phenolics. A single-stage extraction at a solvent/hydrolysate volume ratio of 3:1 or a two-stage extraction at a solvent/ hydrolysate volume ratio of 1:1, and a contact time of 30 min were adequate for the removal of 84% of the phenolics. Acidic pH conditions, i.e., pH 3, favored phenolic recovery, while temperature in the range of 10–40°C had no effect. The antioxidant activity coefficient of the extract obtained under the best conditions was equal to 64% of the relevant value of BHT.

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Water extraction has been studied mainly for the extraction of various components from tea (95). The same kinetic model was fitted to the extraction of catechins, which are the main natural antioxidants present in tea, and of caffeine. The smaller catechin molecules (i.e., ungallated catechins) presented a higher rate of extraction, supporting the hypothesis that the rate-determining step is a diffusion one through the leaf matrix to the surface (69). Also the percent recovery after 20 min at 80°C of the gallated catechins decreased with increasing concentration of the tea, while the recovery of the ungallated catechins was not affected (96). Another parameter that affected extraction efficiency was the pH of the water. Increase of the pH from 6 to 7.6 resulted in decrease of catechin recovery due to epimerization (96). Temperature dependence of the rate of extraction of individual catechins was different (69). Therefore, extraction parameters may be selected to allow preferential extraction of some catechins instead of others. Caffeine is coextracted with catechins by water, and its rate of extraction is higher than the corresponding catechin value for green tea and lower for black tea. Also, the effect of temperature on the rate is greater for green than for black tea (69, 97). Other parameters, which were proposed for the recovery of tea polyphenols, were extraction with water at 80°C, at a liquid-to-solid ratio of 14:1 (v/w), applying three successive extractions (98). For the recovery of flavonoids, ethanol-water mixtures (7:3), at a liquid-to-solid ratio of 5:1 (v/w), at ambient temperature were used with success (99). C. Extraction Procedures Several extraction procedures have been proposed and some of them have been patented. In most cases purification of the extracts is included. An extraction and purification process for rosemary and sage was patented by Chang et al. (25). The solvent initially used was ethyl ether at a liquid-tosolid ratio of 2.4:1 and the extraction was conducted under reflux for 2 h. The residue obtained after filtration of the mixture was reextracted with ethyl ether under the same conditions, and the filtrates were combined and freed of solvent to yield up to 26% of crude antioxidant depending on the number of extractions. Methanol was also proposed as the extraction solvent in two reports (27, 91). To produce a bland, odorless, and tasteless antioxidant the crude extract was subjected to molecular distillation or vacuum steam distillation to yield approximately 10% of purified antioxidant. Kalsec Inc., which has the exclusive license to use this patent, modified the procedure to reduce color in its rosemary extract and manufactures two types of rosemary extracts with antioxidant inhibitors: an oil-soluble extract and a water-dispersible extract. Sequential extraction of oregano with solvents of increasing polarity is a promising process. Hexane, ethyl acetate, and ethanol were used and the extrac-

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tion was conducted under reflux through a fixed bed of the raw material. The extracts obtained by hexane and ethyl acetate were light colored and could be bleached with active carbon, if desired. The methanol extract had to be further purified in order to remove substances with no antioxidant activity. Final antioxidant solutions in oil were prepared by thorough mixing with refined vegetable oils and removal of the solvent under vacuum. Hexane extraction could be replaced by petroleum ether extraction with equal results. The process was also applied successfully to a sage species (Salvia triloba L.). Except of organic solvents, solutions of potassium or sodium bicarbonate or disodium phosphate were also used in an extraction process conducted at 40–90°C under nitrogen. The extracts were subsequently decolorized, deodorized, and demineralized. The process was patented by Nestle SA (100) for the extraction of antioxidants from rosemary, sage, and parsley. Oil extraction of ground spices with an edible animal or vegetable oil at 120–125°C was patented by Campbell Soup Company (101). The extract was separated from the spice solids by centrifugation and filtration, and deodorized by heating under vacuum while sparging with steam. Micronization of the ground spice in an edible oil was later proposed by Bracco et al. (26). The antioxidant components were separated from the lipid phase by molecular distillation either on falling film or on a centrifugal system. Several other processes have been patented for the extraction of natural antioxidants (102, 103). These extracts are mainly characterized as flavorings and are usually approved as such by the legal organizations. However, they also have the ability to manage oxidation. Rosemary extracts are by far the most commercially exploited products and have been reported to represent about 40– 50% of the antioxidant market in Europe (102). D. Extraction with Supercritical CO2 A relatively new process for the extraction of natural antioxidants involves supercritical CO2 extraction. Extracts obtained by CO2 extraction of various herbs at 30 MPa and 40°C had similar or higher antioxidant activity than the relevant acetone extracts (46). The yields obtained by supercritical CO2 extraction were in most cases lower than the ones obtained by acetone extraction. Green tea was extracted by CO2 at 31 MPa and 60°C, and the total yield as well as the yield of individual catechins was considerably lower than that obtained by ethanol or water extraction in a Soxhlet apparatus (104). The addition of aqueous ethanol solutions as cosolvents in the supercritical extraction increased the yield of catechins and the ratio of catechins to caffeic and galic acid. The increase was higher as the concentration of ethanol in the solutions increased up to 95%. The effect of extraction conditions on total yield of solutes and on the antioxidant activity of the extracts was investigated. Esquivel et al. (105) used

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summer savory and found that an increase of pressure above 12 MPa did not increase significantly the extraction yield that amounted approximately to 75% of that obtained by hexane. Extraction was conducted at 40°C and the other conditions determined for maximal solute recovery was the solvent-to-solid ratio, equal to 120 kg CO2/h kg solid, and the duration of extraction, equal to 1 h. The extracts obtained from summer savory under the studied conditions did not show any antioxidant activity in oils at 120°C, as they consisted mainly of essential oils, while the residue of extraction was effective. Extraction of rosemary was accomplished on a pilot plant scale at 30–35 MPa and 40–60°C (106). The extracts were separated in two fractions with different antioxidant activity by arranging the conditions of pressure and temperature in two separator vessels. Methanol was also added as a modifier. The highest antioxidant activity was demonstrated by the extract obtained at 35 MPa and 50°C with no modifier added and separated in the first separator by a pressure reduction to 20 MPa, resulting in a density reduction from 0.9 to 0.78 g/mL. Supercritical CO2 extraction has also been proposed for the isolation of tocopherols from soy sludge (107). The most important raw material for the extraction of tocopherols is the deodorizer sludge obtained in the deodorization of vegetable oils and fats. The processing parameters (temperature, vacuum, quantity of injected steam) are critical for the yield of tocopherols. Besides the various tocopherols these distillates also contain sterols, hydrocarbons, flavor components, free fatty acids, and neutral triglycerides. The separation of tocopherols from other compounds is possible by esterification with a lower alcohol, followed by washing and vacuum distillation, by saponification, or by fractional liquid-liquid extraction. Additional purification steps may include molecular distillation, extraction, crystallization, or a combination of these processes (108). Due to the similar volatility of sterols, tocopherols, and fatty acids, it is quite difficult to separate them by fractional distillation and steam stripping at high vacuum. Recently, enzymatic processes have been used for hydrolysis of the neutral triglycerides to fatty acids (109) and esterification of free fatty acids to methyl esters (110) or butyl esters (109). The esterified products were then fractionated by distillation at high vacuum to isolate tocopherols and sterols together. E. Purification Purification of the crude extracts is essential, especially for the alcoholic extracts, in order to improve the antioxidant properties and to create a product with a light odor, taste, and color, suitable as a food additive. Vacuum steam distillation (25) and molecular distillation (26) are efficient methods, although they may affect the heat-sensitive natural products. Removal of undesirable components, such as lipids, carotenoids, and other fat-soluble materials, can be

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done by washing the concentrated alcoholic extracts with an appropriate solvent, e.g., hexane (68). Isolation of an antioxidant fraction rich in flavonoids, from alcoholic extracts, was achieved through solvent evaporation under vacuum, dissolution of the solid residue in water, and liquid-liquid extraction with ethyl ether (90). Chloroform has been also suggested to remove caffeine or other pigments from aqueous extracts of flavonoids, such as extracts of tea. Subsequently an enriched flavonoid fraction was obtained by repeated extraction of the aqueous layer with equal volume of ethyl acetate (98, 99). Fractionation with column liquid chromatography is suitable (27, 111) for the purification of solvent extracts, but a limitation for large-scale production arises from economic factors. A more convenient process proposed involves high-speed countercurrent chromatography with a multilayer coil separator-extractor. It permits the direct efficient separation of undesirable constituents, such as chlorophylls and carotenoids, from the crude spice extracts, in a single step, without the need of a clean-up step (112). Another approach is membrane separation of the polyphenolic fraction, which was applied to green tea extracts in aqueous ethanol (113). The membrane separation focused on the removal of caffeine from the polyphenolic group of catechins, which are the main antioxidant components of green tea. Ethanol concentration affected the performance of the nanofiltration, and higher concentration (e.g., 80%) gave higher retention of catechins and better separation from caffeine. Supercritical CO2 fractionation of alcoholic extracts to isolate the antioxidant fraction has been also reported (114). The alcoholic extract was bleached through an active carbon column, mixed with thermally treated aluminosilicate, and resulting suspension evaporated to dryness under reduced pressure. The solid residue was extracted with CO2 at 60°C at 10–40 MPa and at 100°C at 50 MPa, for 6 h, at CO2 flow of 20 kg/h to give five fractions with higher antioxidant activity than the original extract. A total yield of 2.7% (calculated on the basis of air-dried plant material) was obtained.

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61. FZ Sheabar, I Neeman. Separation and concentration of natural antioxidants from the rape of olives. J Am Oil Chem Soc 65:990–993, 1988. 62. E Notte, N Romito. Autoxidation of olive oil: influence of polyphenols 1. Ind Agrarie 9:325–331, 1971. 63. FM Vela, AV Roncero. Antioxidantes naturales de las hojas de olivo II. Grasas Aceites 13:124–127, 1962. 64. O Benavente Garcia, J Castillo, J Lorente, A Ortuno, JA Del Rio. Antioxidant activity of phenolics extracted from Olea europaea L. leaves. Food Chem 68: 457–462, 2000. 65. S Naito, N Yamaguchi, Y Yokoo. Studies on natural antioxidants. II Antioxidative activities of vegetables of Allium species. J Jap Soc Food Sci Technol 28:291– 296, 1981. 66. C Karastogiannidou. Effect of onion quercetin on oxidative stability of cook-chill chicken in vacuum-sealed containers. J Food Sci 64:978–981, 1999. 67. Y-H Chu, C-L Chang, H-F Hsu. Flavonoid content of several vegetables and their antioxidant activity. J Sci Food Agric 80:561–566, 2000. 68. HJ Suh, JM Lee, JS Cho, YS Kim, SH Chung. Radical scavenging compounds in onion skin. Food Res Int 32:659–664, 1999. 69. WE Price, JC Spitzer. The kinetics of extraction of individual flavonols and caffeine from a Japanese green tea (Sen Cha Uji Tsuyu) as a function of temperature. Food Chem 50:19–23, 1994. 70. WE Bronner, GR Beecher. Method for determining the content of catechins in tea infusions by high-performance liquid chromatography. J Chromatogr 805:137– 142, 1998. 71. PT Gardner, DB McPhail, GG Duthie. Electron spin resonance spectroscopic assessment of the antioxidant potential of teas in aqueous and organic media. J Sci Food Agric 76:257–262, 1998. 72. Z-Y Chen, L-Y Wang, PT Chan, Z Zhang, HY Chung, C Liang. Antioxidative activity of green tea catechin extract compared with that of rosemary extract. J Am Oil Chem Soc 75:1141–1145, 1998 73. A Roedig-Penman, MH Gordon. Antioxidant properties of catechins and green tea extracts in model food emulsions. J Agric Food Chem 45:4267–4270, 1997. 74. Y He, F Shahidi. Antioxidant activity of green tea and its catechins in a fish meat model system. J Agric Food Chem 45:4262–4266, 1997. 75. CA Rice-Evans, NJ Miller, G Paganga. Antioxidant properties of phenolic compounds. Trends Plant Sci 2(4):152–159, 1997. 76. N Saint-Cricq de Gaulejac, Y Glories, N Vivas. Free radical scavenging effects of anthocyanins in red wine. Food Res Int 32:27–333, 1999. 77. N Saint-Cricq de Gaulejac, Y Glories, N Vivas.Comparative study of polyphenol scavenging activities assessed by different methods. J Agric Food Chem 47:425– 431, 1999. 78. V Katalinic. Grape catechins-natural antioxidants. J Wine Res 10:15–23,1999. 79. N Gamez-Meza, JA Noriega-Rodriges, LA Medina-Juarez, J Ortega-Garcia, R Cazarez-Casanova, O Angulo-Guerrero. Antioxidant activity in soybean oil of extracts from Thomson grape bagasse. J Am Oil Chem Soc 76:1445–1447, 1999.

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80. J Castillo, O Benavente-Garcia, J Lorente, M Alcaraz, A Redondo, A Ortuno, JA Del Rio. Antioxidant activity and radioprotective effects against chromosomal damage induced in vivo by X-rays of flavan-3-ols (procyanidins)from grape seeds (Vitis vinifera): comparative study versus other phenolic organic compounds. J Agric Food Chem 48:1738–1745, 2000. 81. Z Ju, WJ Bramlage. Phenolics and lipid-soluble antioxidants in fruit cuticle of apples and their antioxidant activities in model systems. Postharvest Biol Technol 16:107–118, 1999. 82. P-D Dur, WY Yen, P-C Du, G-C Yen. Antioxidant activity of Mung bean hulls. J Am Oil Chem Soc 74:1059–1063, 1997. 83. SN Onyenebo, NS Hettriarachchy. Effect of Navy bean hull extract on the oxidative stability of soy and sunflower oils. J Agric Food Chem 39:1701–1704, 1991. 84. E Tsaliki, V Lagouri, G Doxastakis. Evaluation of the antioxidant activity of lupin seed flour and derivatives (Lupinus albus ssp. Graecus). Food Chem 65:71–75, 1999. 85. BJF Hudson, SEO Mahgoub. Naturally-occuring antioxidants in leaf lipids. J Sci Food Agric 31:646–650, 1980. 86. D Rodrigues de Sotillo, M Hadley, ET Holm. Potato peel waste: stability and antioxidant activity of a freeze-dried extract. J Food Sci 59:1031–1033, 1994. 87. R Julkunen-Tiitto. Phenolic constituents in leaves of Nothern willows: Methods for the analysis of certain phenolics. J Agric Food Chem 33:213–217, 1985. 88. AH Azizah, NM Nik Ruslawati, T Swee Tee. Extraction and characterization of antioxidant from cocoa by-products. Food Chem 64:199–202, 1999. 89. S Chatterjee, SR Padwal Desai, P Thomas. Effect of γ-irradiation on the antioxidant activity of turmeric (Curcuma longa L.) extracts. Food Res Int 32:487–490, 1999. 90. SA Vekiari, V Oreopoulou, C Tzia, CD Thomopoulos. Oregano flavonoids as lipid antioxidants. J Am Oil Chem Soc 70:483–487, 1993. 91. SS Chang, Y Bao. U.S. Patent US 5,043,100 (1991). 92. M Wettasinghe, F Shahidi. Antioxidant and free radical-scavenging properties of ethanolic extracts of defatted borage (Borago officinalis L.) seeds. Food Chem 67: 399–414, 1999. 93. E. Revilla, J-M Ryan, G Martin-Ortega. Comparison of several procedures used for the extraction of anthocyanins from red grapes. J Agric Food Chem 46:4592– 4597, 1998. 94. JM Cruz, JM Dominguez, H Dominguez, JC Parajo. Solvent extraction of hemicellulosic wood hydrolysates: a procedure useful for obtaining both detoxified fermentation media and polyphenols with antioxidant activity. Food Chem 67: 147–153, 1999. 95. M Spiro, D Jago. Kinetics and equilibria of tea infusion. Part III: Rotating disc experiment interprated by a steady-state model. J Chem Soc Faraday Trans I 78: 295–305, 1982. 96. Y Yoshida, M Kiso, T Goto. Efficiency of extraction of catechins from green tea. Food Chem 67:429–433, 1999. 97. M Spiro, D. Jaganyi, MC Broom. Kinetics and equilibria of tea infusion. Part IX:

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Oreopoulou The rates and temperature coefficients of caffeine extraction from green Chun Mee and Black Bukial teas. Food Chem 45:333–335, 1992. A Zhang, PT Chan, YS Luk, WKK Ho, ZY Chen. Inhibitory effect of jasmine green tea epicatechin isomers on LDL-oxidation. J Nutr Biochem 8:334–340, 1997. M Milovanovic, K Picuric-Jovanovic, B Vucelic-Radovic, Z Vrbaski. Antioxidants effects of flavonoids of Anthriscus sylvestris in lard. J Am Oil Chem Soc 73:773–776, 1996. Nestle SA. Process for extracting antioxidants. British Patent 1,468,271 (1977). Campbell Soup Company. Spice antioxidant principle and process for the extraction of the antioxidant principle from spice. British Patent 1,331,050, U.S. Patent 3,732,111 (1973). BF Haumann. Antioxidants: Firms seeking products they can label as “natural” Inform 1(12):1002–1013, 1990. J Giese. Antioxidants: Tools for preventing lipid oxidation. Food Technol 50(11): 73–81, 1996. CJ Chang, K-L Chiu, Y-L Chen, C-Y Chang. Separation of catechins from green tea using carbon dioxide extraction. Food Chem 68:109–113, 2000. MM Esquivel, MA Ribeiro, MG Bernardo-Gil. Supercritical extraction of savory oil: study of antioxidant activity and extract characterization. J Supercritical Fluids 14:129–138, 1999. FJ Senorans, E Ibanez, S Cavero, J Tabera, G Reglero. Liquid chromatographicmass spectrometric analysis of supercritical-fluid extracts of rosemary plants. J Chromatogr 870:491–499, 2000. H Lee, BH Chung, YH Park. Concentration of tocopherols from soybean sludge by supercritical carbon dioxide. J Am Oil Chem Soc 68:571–573, 1991. P Schuler. Natural antioxidants exploited commercially. In: BJF Hudson, ed. Food Antioxidants. London: Elsevier Applied Science, 1990, p 111. S Ghosh, DK Bhattacharyya. Isolation of tocopherol and sterol concentrate from sunflower oil deodorizer distillate. J Am Oil Chem Soc 73:1271–1274, 1996. S Ramamurthi, AR McCurdy. Enzymatic pretreatment of deodorizer distillate for concentration of sterols and tocopherols. J Am Oil Chem Soc 70:287–295, 1993. S Nieto, A Garrido, J Sanhueza, LA Loyola, G Morales, F Leighton, A Valenzuela. Flavonoids as stabilizers in fish oil: an alternative to synthetic antioxidants. J Am Oil Chem Soc 70:773–778, 1993. N Fischer, B Weinreich, S Nitz, F Drawert. Application of high-speed countercurrent chromatography for the separation and isolation of natural products. J Chromatogr 538:193–202, 1991. V Nwuha. Novel studies on membrane extraction of bioactive components of green tea in organic solvents: Part I. J Food Eng 44:233–238, 2000. Z Djarmati, RM Jankov, E Schwirtlich, B Djulinac, A Djordjevic. High antioxidant activity of extracts obtained from sage by supercritical CO2 extraction. J Am Oil Chem Soc 68:731–734, 1991.

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11 Extraction of Alkaloids from Natural Plants Using Supercritical Fluids Marleny D. A. Saldan˜a and Rahoma S. Mohamed The State University of Campinas–Unicamp Campinas, Sa˜o Paulo, Brazil

I. INTRODUCTION Alkaloids are nitrogen-based organic compounds commonly found in leaves, seeds, and roots of plants, particularly the Papaveraceae (poppy and opium), Papilonaceae (lupins), Ranunculaceae (aconitum), and Solanaceae (tobacco) families (1). Some alkaloids have bitter taste and are highly toxic, but can have therapeutic applications when used in moderate amounts. Almost all alkaloids have pharmacological activity together with pronounced physiological actions of interest to chemists, biochemists, and pharmacologists. Alkaloids such as caffeine, morphine, cocaine, nicotine, quinine, codeine, emetine, pilocarpine, among others, form the active components in a variety of stimulants and medication drugs. Among the most consumed alkaloid-containing products are coffee, tea, and guarana´, which contain caffeine, tea and cocoa nuts, which contain theobromine; and tobacco, which contains nicotine. The alkaloids found in these products have stimulant and/or sedative effects. Until recently, most of research on alkaloids has focused on the chemical aspects of their structure and isolation techniques (2). Increased attention is currently devoted to the identification of the pharmacological effects of these compounds on animals and the extraction and characterization of new alkaloids. The extraction of the very small quantities of alkaloids in natural plants required the development of new and improved experimental equipment and instrumental techniques. Alkaloids are normally removed from natural products using solvent extraction. For the purification of these alkaloids in the extracts, conventional

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separation processes such as fractional distillation, fractional crystallization, and fractional salt precipitation (3) as well as the new and promising technology of extraction with supercritical carbon dioxide (4, 5) have been employed. The potential of each of these processes is determined by a compromise between cost and selectivity. For large production volumes, the tendency is to choose the conventional processes mentioned above, as they only require conventional equipment. However, these processes bring with them the risk of contamination of extracted products with residual chemical solvents used in extraction and possible alteration in the quality of thermally labile products due to the high temperatures used in distillation. Extraction with supercritical carbon dioxide requires higher capital costs but is highly selective; therefore, it is recommended for the extraction and purification of valuable products such as alkaloids. With this technology, extraction and purification are carried out simultaneously since solvent power and selectivity of the supercritical fluid is easily modified with changes in thermodynamic variables of temperature and pressure and the use of a suitable cosolvent. In supercritical carbon dioxide extraction, there is no risk of thermal degradation as the extraction is carried out at temperatures close to the critical temperature of carbon dioxide (31°C). Few experimental data on the solubility and extractability of alkaloids such as emetine, cephaeline, reserpine, vincristine (4), caffeine, theophylline, theobromine (6), codeine, thebaine, papaverine (7), quinine, morphine (8), pyrrolizidine (9), trigonelline (10), and ephedrine (11) in various supercritical solvents such as carbon dioxide, ethylene, nitrous oxide, and fluoroform have been obtained over the past few years. While there is a growing market for decaffeinated products (12), extraction of the many alkaloids encountered in natural plants with supercritical carbon dioxide is yet to reach a desirable level of industrial application (5). In this chapter, we present information on the potential application of this promising technology: extraction using supercritical fluids in the isolation and recovery of alkaloids from natural products. This presentation includes a brief summary of the physiological effects and applications of alkaloids, a description of commercial decaffeination applications of coffee and black tea, followed by experimental laboratory data on the potential use of carbon dioxide for the isolation of a number of alkaloids from natural products, and future prospects in the isolation of important alkaloids.

II. PHYSIOLOGICAL EFFECTS AND APPLICATIONS OF PURINE ALKALOIDS The principal purine alkaloids—caffeine, theobromine, and theophylline—are consumed in large scale through the daily ingestion of coffee, tea, herbal mate´, cola drinks, and chocolate as well as in analgesics, diuretics, and other pharma-

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ceutical products. Physiological effects of methylxanthines on humans have received substantial attention, due largely to the high coffee consumption worldwide. All three purine alkaloids do, however, exhibit stimulant and psychomotor effects, with caffeine and theophylline having highest activity. Theophylline is also known to exhibit more diuretic effects as it inhibits the tubular reabsorption of water and sodium in kidneys (13). It is also used as a vascular as well as a bronchial dilator (14). Theobromine is found to have toxic effects on virtually all animals tested, including chickens (15), dogs (16), mice (17), rats (18), rabbits (19), and humans (20). Recent interest in caffeine and theobromine is also motivated principally by their reproductive toxicities and their potential to induce fetal malformation through effects on embryo development. Furthermore, teratogen effects in animals are also known to occur as a result of biotransformation when consuming products containing caffeine, theophylline, and theobromine (21). Finally, it is important to note that the presence of these alkaloids in products such as cocoa beans could prove to be a limiting and restrictive factor in its potential as a nourishing food in developing countries (22).

III. COMMERCIAL SCALE APPLICATIONS Large commercial scale operations for the extraction and isolation of valuable alkaloids from natural plants for the pharmaceutical and cosmetic industries have been developed and operated for many years (23–25). Among the best known operations using supercritical carbon dioxide are the decaffeination processes of coffee and black tea and the removal of nicotine from tobacco. With the increase of consumption of decaffeinated coffee worldwide, caffeine is now a major byproduct of coffee decaffeination with significant contribution in plant amortization (26–29). There are several patents and commercial processes for the decaffeination of coffee beans (30–32) and black tea (33, 34). These processes employ a number of organic, nonorganic, and supercritical solvents. Except when supercritical carbon dioxide is used, the extraction process either is nonselective for caffeine or brings with it the potential risk of toxic residues in the decaffeinated products that could prove to be harmful to the consumer. Extraction with supercritical carbon dioxide as a substitute for chemical solvents eliminates the risks of potential toxic residues due to the inert nature of CO2 (35, 36) and thermal degradation of the decaffeinated product due to the mild operating temperatures (near the critical temperature of CO2 of 31°C), besides being highly selective for caffeine. These characteristics result in the preservation of most of the qualities of the original non-decaffeinated product. In what follows, a more detailed description of decaffeination operations of coffee beans and black tea using conventional and supercritical fluid solvents is presented.

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A. Decaffeination of Coffee Beans Coffee, one of the most widely known natural products, contains 0.8–2.5% weight of caffeine, depending on the species (Coffea arabica, C. canephora, C. liberica, C. racemosa, etc.). It is used as a daily beverage in many parts of the world. Green coffee beans are cultivated mainly in South America and Africa. 1. Decaffeination Using Conventional Solvents Commercial coffee decaffeination is carried out on green coffee beans before roasting in order to minimize loss of flavor and aroma. These processes involve the swelling of raw beans with water, caffeine removal with water-insoluble organic solvents, steam stripping or distillation to remove residual solvent from extracted beans, and drying of decaffeinated coffee beans to their initial humidity. Organic solvents commonly used include toxic and flammable substances such as benzene, incombustible and highly volatile chlorinated methylene chloride (37), ethyl acetate (38), methyl acetate, ethylmethylketone, and trichloroethane (39, 40). Water (40–42) is an ideal solvent for decaffeination except for its high nonselectivity for caffeine, which results in the extraction of other watersoluble substances. Caffeine is subsequently recovered by washing the aqueous extracted solutions with organic immiscible solvents such as methyl chloride or adsorption of caffeine on activated carbon. As noted earlier, these conventional processes suffer from two serious limitations: the risk of toxic residues in the decaffeinated products when using organic solvents and/or the nonselective extraction when using water, which results in the removal of important constituents together with caffeine from the original products. 2. Decaffeination Using Supercritical Carbon Dioxide Supercritical carbon dioxide (SC-CO2) is an excellent and selective solvent for caffeine as it solubilizes caffeine and does not remove other important watersoluble components from coffee beans. The commercial decaffeination of coffee is the first important industrial application of supercritical extraction in the food industry with plants constructed and operated in Germany and in the United States (Table 1). The plant owned and operated by General Foods has a yearly production capacity of 50 × 106 kg of coffee beans (43). The German plant processes nearly 30 × 106 kg of green coffee beans per year (44). In this decaffeination process, color and odor of the extracted beans are totally preserved, which is not always achieved when using conventional extraction processes (45). Extracted caffeine is reported to be of a very high purity and is easily isolated and refined to provide a salable product (45). Zosel’s patents on the decaffeination of green coffee beans using SC-CO2 (30, 31, 46, 47) served as the basis for the development of these commercial

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Table 1 Commercial Plants Using SC- CO2 Extraction Process/alkaloid extraction Coffee decaffeination

Tea decaffeination Nicotine extraction

Manufacturer Kaffee HAG AG, Bremen, Germany General Foods, Texas, USA Hermsen, Bremen, Germany SKW-Trostberg, Poszzillo, Italy SKW-Trostberg, Munchmuenster, Germany Philip Morris, Virginia, USA

Source: Data from Ref. 23.

decaffeination plants. These patents propose, basically, three possible processes. In the first process, moistened green coffee beans placed in the extractor are contacted with CO2 at 16–22 MPa and 70–90°C, where caffeine diffuses from the beans into the CO2 stream. This caffeine-saturated stream is passed into a water-washing tower operated at 70–90°C where caffeine is retained and the water-saturated CO2 is recycled to the extractor. Caffeine retained in the wash water is separated by distillation. Decaffeinated coffee beans have a residual caffeine content of only 0.02%. In the second process, caffeine is extracted from coffee beans under the same conditions as described in the first process while the caffeine-saturated CO2 stream is passed through a bed of activated carbon where caffeine is retained. Adsorbed caffeine is subsequently recovered from activated carbon. In a third alternative presented in these patents, a mixture of moistened green coffee beans and activated carbon pellets is charged into the extractor (1 kg of activated carbon/3 kg of coffee beans). CO2 is subsequently introduced into the extractor and maintained at 22 MPa and 90oC. Caffeine diffuses from coffee beans into carbon dioxide until reaching the activated carbon. After the extraction, coffee beans are separated from the activated carbon pellets using vibrating sieves. Modifications of Zosel’s processes as well as new decaffeination processes were also suggested in U.S. and German patents as reported by Lack and Seidlitz (12) and McHugh and Krukonis (48). These new processes and modifications involve (12, 48, 49) the following: (a) incorporation of two smaller pressure vessels that are periodically charged and discharged with coffee beans allowing a continuous extraction and using less quantity of CO2 than a discontinuous process; (b) recovery of extracted caffeine by washing the CO2 stream with water and subsequent evaporation or crystallization; (c) use of other solvents such as NO2, NH3, and CHF3 for caffeine extraction; (d) use of an ion exchange resin with temperature change to retain caffeine instead of activated carbon; (e) use of liquid CO2 as a solvent; (f) addition of cosolvents such as acetone, methanol, and ethanol to increase solubility of caffeine in SC-CO2 and decrease

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the operating pressures. Other proposed modifications included the use of mixed solvents such as carbon dioxide and propane, butane, or ethane (48), and the utilization of roasted coffee as described in the patents of Vitzthum and Hubert (32, 50) who employed a multistage process in which flavor and aroma were first extracted in the form of coffee oil. The oil-free coffee was subsequently moistened and decaffeinated using a stream of water-saturated SC-CO2. The decaffeinated roasted coffee is then spray dried and aromatized with the coffee oil extracted earlier. Pilot plants in operation using the methods proposed by Zosel are also described in the literature (12, 45). These commercial and pilot plants consist, basically, of extractors, pumps, washing columns, and heat exchangers. An interesting economic projection of a supercritical plant with daily feed rates of 32,000 and 64,000 kg of green coffee containing 12% of moisture operated during 330 days was presented (51). Decaffeinated whole green coffee beans with 3% of the original caffeine content and an aqueous caffeine solution were the main products. The extraction conditions considered were 14–35 MPa and 70–130°C and the separation conditions of 5–10 MPa and 15–50°C. Processing costs were estimated by considering (a) electric power, steam, cooling and process water, and carbon dioxide; (b) operation labor and supervision; and (c) maintenance, taxes, insurance, and plant overhead. Estimated costs were found to be $0.83 and $0.68 per kg of coffee produced for plant capacities of 32,000 and 64,000 kg, respectively. 3. Decaffeination with SC-CO2 and Conventional Solvents: Main Advantages and Disadvantages Economical studies of Lack and Seidlitz (12) have shown that although the initial investment cost of the SC-CO2 plant is higher, this process provides a higher profit per ton of coffee processed than does the ethyl acetate process, due to the excellent quality of both decaffeinated coffee and caffeine produced and negligible losses in the SC-CO2 process. Carbon dioxide extracts caffeine without affecting the reduced sugar and amino acid contents. These compounds need to be preserved because they are converted to flavor and aroma during the roasting process (45). The flavor and appearance of decaffeinated coffee beans obtained with the carbon dioxide process are very close to those in the original un-decaffeinated coffee and superior to the product obtained from other conventional decaffeination methods. This process is recognized by the consumer as a “natural” decaffeination technique for the high qualities of both products and byproducts. The total quantity of caffeine recovered is much higher using compressed carbon dioxide than ethyl acetate. Supercritical carbon dioxide decaffeination is also considered a clean and environmentally acceptable technology. Steam cost is another factor that contributes to the higher process cost when using ethyl acetate. For the pretreatment of beans, both processes require

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steam to swell the beans but only the ethyl acetate process requires almost four times more steam to strip residual solvent from the final product. The limiting factor for the use of the CO2 process is, however, the capital cost due mainly to the highly specialized equipment needed for a safe plant operation. Necessary equipment includes compressors for recirculation of CO2, separators and extractors with baskets that support high pressure and allow fast operation, adequate piping, as well as safety and control systems to avoid interruptions in production and precaution against explosion. B. From Black Tea (Camellia sinensis) Tea, a natural product, contains approximately 2.0–3.5% caffeine (52). The most common types of tea are green and black. It is used as a daily beverage worldwide, mainly in China and India. Most decaffeinated tea is produced and consumed in North America and Europe. 1. Tea Decaffeination Using Conventional and SCCO2 Processes These processes are similar to those used for coffee decaffeination and normally employ organic solvents, water, or supercritical fluids. Organic solvents such as methylene chloride and ethyl acetate are employed in a similar manner to that used in the decaffeination of coffee beans (45). As pointed out earlier, the operations using organic solvents bring with them the risk of leaving toxic residues in decaffeinated products, whereas the loss of valuable water-soluble tea constituents is markedly increased when using water as the solvent due to its nonselectivity for caffeine. Extraction with supercritical carbon dioxide is selective for caffeine and eliminates the risk of possible toxic residues in the decaffeinated products. Vitzthum and Hubert (34) describe a multistage procedure that avoids the loss of flavor and aroma when decaffeinating black tea. Aroma components are removed from tea by extraction with dry SC-CO2 at 25 MPa and 50°C. The dearomatized leaves are subsequently moistened and decaffeinated with watersaturated CO2. The decaffeinated leaves are vacuum dried at 50°C and rearomatized by contact with the expanded CO2 solution containing the aroma components. This procedure is also used for the production of caffeine-free instant tea. In this case the decaffeinated tea is extracted with hot water and the extract is freeze dried. The powder produced is impregnated with tea aromas using a solvent that is subsequently evaporated. An SC-CO2 tea decaffeination plant is currently operated by SKW/Trostberg in Meunschmeunster, Germany (53). Decaffeinated ice tea with tropical flavors, where decaffeination is obtained using SC-CO2, was reported to have been introduced into the market by the Hansen Company (54).

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From a laboratory study, Klima et al. (33) reported the extraction of black tea using wet carbon dioxide at 25.5–35 MPa and 50–80°C. The moistened tea with 15–50 wt% water content is packed into the extractor in alternate layers with activated carbon. Before leaving the extractor, the CO2 stream passes through the bed of pure adsorbent placed inside of gas-permeable bags or tubes where extracted caffeine is adsorbed. C. Potential Applications: Extraction of Methylxanthines from Natural Products The potential extraction of methylxanthines—caffeine, theophylline, and theobromine—a major group of purine alkaloids, from coffee beans (55–57), cocoa nibs (58), and shells (59) and guarana´ seeds (60) has also been investigated over the past few years. In what follows, we present a brief review of laboratory experiments and results obtained. 1. From Mate´ Tea (Ilex paraguariensis) Leaves Mate´ tea is a beverage prepared by the infusion of dry mate´ leaves and is traditionally consumed in southern Brazil, Argentine, Paraguay, and Uruguay (61, 62). A careful inspection of the chemical constituents of Ilex species (Table 2) reveals the reason for current and successful use of this natural product as a stimulant, an antirheumatic, and a diuretic. Nevertheless, a high intake of herbal mate´ tea could provoke irritability and insomnia, and the possibility of developing cerebral depression, nervous tremor, and numbness. Several laboratory studies (63–66), on the other hand, have shown that polyphenols found in this tea could inhibit the formation and growth of tumors. Similar to the decaffeination of coffee beans, removal of caffeine from mate´ tea leaves can be performed using organic solvents or water. While there are problems associated with the use of chemical solvents (potential toxic residues in extracted products) and water (nonselectivity, which results in loss of valuable flavor components) (68, 69), the use of carbon dioxide at supercritical conditions proved to be potentially convenient in the extraction of methylxanthines from natural products (44, 68). While there are many patents for the use of SC-CO2 as a solvent to extract caffeine from coffee beans and Camellia sinensis tea leaves (44, 56, 59), little is known about the extraction of methylxanthines from mate´ tea. During the past 2 years, our group has conducted an investigation on the extractability of methylxanthines from Ilex paraguariensis (70) where we used a semicontinuous-flow, high-pressure system purchased from Autoclave Engineers (Erie, PA). The major components of the apparatus included positive liquid displacement pumps for solvent delivery, high-pressure extraction vessels,

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Table 2 Taxonomy, Chemical Compoisition, and Other Characteristics of Ilex paraguariensis Taxonomy

Other characteristics

Division: Anthophyta Type/Subtype: Magnoliopsida/Rosidae Order: Celastrales Family: Aquifoliaceae Gendus: Ilex Specie: paraguariensis, paraguayiensis Common names: yerba mate´, mate´, erva mate´, Paraguay Cayi, Paraguai Tea, South American Holly Chemical composition/quantity (100 g) Protein: 10.89 Carbohydrate: 12.04 Starch: 4.55 Glucose: 3.84 Fiber: 16.96

Consumed part: leaves and some barks Color: green Taste: astringent and sour Humidity - “in natura”: 50–60 wt% - commercial: 8–10 wt%

Alkaloids (mg/kg) Theophylline: 142 ± 6,a 768 ± 3b Theobromine: 340 ± 7,a 209 ± 5b Caffeine: 5371 ± 161,a 8375 ± 251b

a

Old leaves New leaves from branches with fruit Source: Data from Refs. 61 and 67.

b

and three separator flasks in series (Fig. 1). Flow rates and accumulated gas volumes passing through the apparatus were controlled with micrometering valves and measured with a flow computer-measuring device. Heating tapes were used to maintain constant temperature in the extraction section and in valves to prevent freezing of solvents or solid solute precipitation following depressurization. Pressure in both extractors was monitored with a digital transducer system. Mate´ leaves were extracted with liquid CO2 at 25.5 MPa and 70°C; samples were collected and analyzed for purine alkaloids. A supercritical extraction curve for mate´ tea revealed that high caffeine removal rates were obtained in the early stages of the extraction with extraction rates diminishing at later stages (Fig. 2). A similar qualitative behavior was observed for theophylline and theobromine extraction (Fig. 3). These results reveal the higher selectivities of CO2 for caffeine followed by theobromine and theophylline. The cumulative amounts of methylxanthines extracted from mate´ tea amounted to 4308, 348, and 47 mg of caffeine, theobromine, and theophylline per kg of dry mate´ tea, respectively. Considering that different mate´ leaves were used, these values are in good agreement with alkaloid contents in mate´ tea reported by Mazzafera (67). After about 7 h of extraction, 94%, 68%, and 57% of extracted caffeine,

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Figure 1 Experimental apparatus. BH(1-2), solvent; BH(3-4), cosolvent; G(1-2-3-4), pressure indicator; P(1-2-3-4), pumps; F(1-2-3-4), filters; V. microm., micrometrical valve; CV(1-2-3-4-5-6-7-8), valves; TC(1-23-4-5-6-7), thermocouples; Extractor 2, extractor with stirring and a window; Separ., separator flasks; Amst., sample; FM, flow measurement (68, 70).

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Figure 2 Caffeine extraction curve for the fractionation of mate´ tea at 25.5 MPa, 70oC, and a CO2 flow of 0.9–1.2 g/min.

theobromine, and theophylline in the plant matrix, respectively, had been recovered. By the last fractions, 99.9% of caffeine, 96% of theobromine, and 95% of theophylline had been removed. Fractions obtained at the late stages became each time richer in theobromine and theophylline and could provide an interesting approach to the separation of extractable methylxanthines into fractions of varying concentrations. 2. From Cocoa (Theobroma cacao) Beans Cocoa beans are the seeds of the tropical cocoa tree, Theobroma cacao. These beans are used as raw material for producing chocolate and cocoa powder, which are subsequently used in the manufacturing of other products such as cake fillings, pudding powders, ice cream, and cocoa beverages (chocolate), among others. A theobromine content of about 1–2% and a caffeine content of 0.1% of dry nut are reportedly found in the species Theobroma cacao (71, 72). During fermentation, theobromine and caffeine migrate to the shell. Theobromine contents of about 2% of the dry shell were reported (73). The shells represent about

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Figure 3 Dimethylxanthine extraction curves for fractionation of mate´ tea at 25.5 MPa, 70oC, and a CO2 flow of 0.9–1.2 g/min.

10–12% of the nut’s weight. Once theobromine is extracted, shells are nowadays used as animal ration. For example, considering the production of 150 × 106 kg of cocoa nuts [Brazilian production 1980–1981 according to CEPLAC (74)] results in 13 × 106 kg of shells and potentially 30,000 kg of theobromine available for use in food, pharmaceutical, and cosmetic industries (72). Cocoa beans are also a potential source of high-quality cocoa butter (Table 3) that is used in a variety of applications in the cosmetic and pharmaceutical industries. Furthermore, the residue material obtained in cocoa bean processing is fermented to produce organic fertilizers. Cocoa butter with the characteristic and pleasant flavor and odor of cocoa beans contains many different types of triacylglycerols. Hexane is one of the conventional solvents used for the extraction of cocoa butter. Due to the toxic nature of hexane, a maximal residue of 5 mg/kg is allowed in the final cocoa butter. Methylxanthines could be extracted from cocoa beans conventionally in a four-step process (75): extraction with warm water, purification using an adsorbent, recovery of the xanthines from the adsorbent, and, finally, addition of

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Table 3 Composition of Cocoa Beans and Cocoa Shells

Component

Cocoa bean (wt%)

Cocoa shell (wt%)

5 54 1.2 0.2 6 11.5 1 6 1.5 9 1.5 0.5 2.6

4.5 1.5 1.4 — — 10.9 0.1 — 7 26.5 — — 8

Moisture Fat Theobromine Caffeine Polyhydroxyphenols Crude protein Mono- and oligosacharides Starch Pentosans Cellulose Carboxylic acids Other compounds Ash Source: Data from Refs. 71 and 72.

residual concentrated aqueous extract to the cocoa beans to reestablish the solid content of the bean. Extraction of theobromine and caffeine from cocoa seeds (58, 76) and shells (59) can be also done using supercritical carbon dioxide. Margolis et al. (77) described a process in which stimulants are removed from cocoa by SCCO2 extraction of water-swollen nibs at 30 MPa and 90°C. It was earlier indicated that water acted as a chemical agent that freed caffeine linked to coffee substrate allowing the caffeine to be extracted by SC-CO2 (56). As caffeine is similar to theobromine, one could assume that the same effect would be observed in the supercritical extraction of theobromine from cocoa beans. A characterization of cocoa extracts obtained with SC-CO2 as a function of temperature and pressure was presented by Rossi et al. (78). This work pointed to improved extraction yields at pressures exceeding 40 MPa but resulted in a nonselective process. For a better extraction of all cocoa fat, fine milling of the lipid bearing material was required. Experimental data on the extraction of cocoa butter from three substrates—nibs, shells, and liquor in which approximately 20%, 60%, and 100%, respectively, was obtained at 40 MPa and 80oC in 5 h—were also presented (79). Li and Hartland (58) used ethanol as a cosolvent for the SC-CO2 extraction of methylxanthines from cocoa nibs. The solubility of theobromine increased with ethanol concentration in the supercritical solvent. However, extraction with

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ethanol as a co-solvent proved to be more selective for cocoa butter than for theobromine, contrary to what was observed when using an aqueous cosolvent (77). Brunner (59) investigated the influence of water content on the extraction of theobromine from cocoa seed shells using supercritical carbon dioxide at 30 MPa and 80°C and reported that the solubility of theobromine in water-saturated SCCO2 was found to be much less than that of caffeine. The extraction yield was also found to increase with pressure and moisture content. Furthermore, higher extraction temperatures favored the removal of xanthines and cocoa butter, whereas grinding and swelling of the starting material with water did not improve the extraction (80). Recently our group explored the extractability of theobromine from Brazilian cocoa beans (81). Results revealed the extraction of almost 89% of the original theobromine at 20 MPa and 70°C using 10% ethanol as a cosolvent of supercritical carbon dioxide. When extraction was performed with dry or watersaturated carbon dioxide or at lower pressures or temperatures, additional time and larger amounts of carbon dioxide were needed to achieve the same yield. Furthermore, the extraction was coupled to an adsorption step using activated carbon where extracted theobromine and cocoa butter were simultaneously adsorbed and separated from the CO2 solvent. 3. From Guarana´ (Paullinia cupuana) Seeds Guarana´ is a bush plant native to Brazil, with the seeds being the only part used for human consumption. The seeds are commonly used in concentrated and soft drinks and as ingredients of a variety of pharmaceutical products. They are the richest source of caffeine, with 3–6% weight on a dry basis (68), and have shown aphrodisiac and stimulant effects, acting on the cardiovascular and nervous systems and kidneys. These effects are attributed to the rich caffeine content encountered in guarana´ seeds (82, 83). South American Indians have traditionally toasted, ground, and mixed the seeds with water to form a paste that can be molded into different configurations (rods and sticks) to be used in special rituals. The toasted seeds can be ground to a powder and added to syrups as an essence or to water to form consumable drinks. Similar to coffee and tea decaffeination, caffeine removal of guarana´ seeds could be performed using organic solvents such as dimethyl chloride and water (84). The use of supercritical carbon dioxide eliminates the risks of toxic residues in the extracted products and the long nonselective extraction presented with water as a solvent. As observed with coffee beans, water can act as a valuable cosolvent leading to a substantially improved extraction yield (56, 60, 68). The literature contains the limited data of Mehr et al. (60) who reported extractions for pressures of 13.74–27.47 MPa and temperatures of 35°C, 45°C,

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and 55°C. Our recent findings (85) revealed some interesting information on the ability of supercritical fluids in the decaffeination of widely consumed caffeinerich natural guarana´ seeds with water-saturated supercritical carbon dioxide. The extraction was performed using a semicontinuous-flow high-pressure microextraction apparatus at 40°C and 70°C and pressures of 10, 20 and 40 MPa. Carbon dioxide flow rates of 3 and 5 L/min were used. Extraction curves showed the existence of a thermodynamic solubility– dependent, an intermediate, and a diffusion controlled regions. Extraction at 40 MPa and 70oC using water-saturated supercritical carbon dioxide at a flow rate of 3 L/min allowed the removal of almost 99% of initial caffeine content in wet ground guarana´ seeds in 240 min. When extractions were performed at lower pressures or temperatures, additional time and larger amounts of carbon dioxide were needed to achieve the same yield. Increasing the carbon dioxide flow rate did not present any economic advantages unless the extraction was limited to the thermodynamic solubility region. For total extraction of caffeine, the use of low flow rates resulted in similar final product yield but at lower solvent consumption. A retrograde behavior for the extraction of caffeine from guarana´ seeds was also observed at 10 MPa for the 40–70°C isotherms. 4. From Other Natural Matrices Many alkaloids encountered in natural plant species are of interest as anticancer agents. Supercritical carbon dioxide extraction has provided a solution for the extraction of active components from some natural matrices without the risk of degradation or contamination with toxic solvents. Besides the applications already cited, the extraction and isolation of chemotherapeutic pyrrolizidine alkaloids (monocrotaline) from the seeds of Crotalaria spectabilis using supercritical carbon dioxide with ethanol and water as cosolvents has also been reported (9). In this operation, 94–100% of the monocrotaline was obtained using an ion exchange resin column, demonstrating the potential and effective extraction and isolation of other alkaloids of this class. Nicotine and nornicotines are alkaloids encountered in Nicotiana (86, 87) and are used as a powerful insecticide as well as in epidermal patches that help ease the difficulties associated with cigarette addiction. Nicotine has been extracted from tobacco using carbon dioxide–water mixtures (88, 89), and nowadays there is even an industrial plant for its extraction operating in the United States. Supercritical carbon dioxide has also been employed for the extraction of alkaloids such as quinine and morphine (8). Morphine is one of the major alkaloids isolated from plants by conventional methods for industrial use. Annually, approximately 160,000 kg of morphine (24) is purified and 90–95% of that is methylated to codeine (25), which is then either used directly or chemically converted to a variety of derivatives that find use as analgesics. The illicit pro-

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duction of morphine for acetylation to heroin reaches almost 10 times that amount, totaling more than 1.2 × 106 kg/a (90). Few alkaloids were also extracted with other supercritical fluids such as fluoroform (CHF3), which is an attractive solvent with a good polarity (1.6D) and accessible critical parameters. The solubilities of opium alkaloids—codeine, thebaine, papaverine, and noscapine—in supercritical CO2, N2O, and CHF3 have been studied by Stahl et al. (7, 91) at 15 MPa and 40°C. With the exception of codeine, all other alkaloids tested exhibited higher solubility in CHF3 than N2O and CO2. Sanders (39) has also presented some production of alkaloid extracts from vegetable matter using supercritical carbon dioxide. D. Future Prospects During the past 20 years, the use of supercritical fluid solvent technology has advanced rapidly. Supercritical fluids have been applied to areas as diverse as food industry, pharmaceutical, polymer, oils, petroleum, textile, biotechnology, among others. Some alkaloids that offer future prospective for pharmaceutical applications include emetine, an active ingredient of Ipeca (Cephaelis ipecacuanha Rubiaceae) used by South American Indians for the treatment of amoebic dysentery and other rubiaceous species. Quinine, medically used as in the treatment of malaria, and quinidine, used as an antiarrhythmia drug (92), are also alkaloids that could potentially be extracted from natural plants like Cinchona shells. Physostigmine, also called eserine, is another alkaloid that could be extracted from the calabar bean of West Africa (Physostigma venenosum Balf) and used clinically in the management of glaucoma by reduction of intraocular tension (93). Other potential alkaloids include colchicine, an ancient and well-known drug used for the management of gout (94), from the plant Colchicum autumnale, the autumn crocus or meadow saffron, and the glory lily Gloriosa superba; bark extracts from the Yuzuriha tree (Daphniphyllum macropodum), which have been used for centuries as a folk remedy for asthma (95), and the tropane alkaloids in solanaceous plants that have been traditionally used for their medicinal, hallucinogenic, and poisonous properties (96, 97). The narcotic, anesthetic, and psychostimulant cocaine (98) is a tropane alkaloid found outside of the Solanaceae in Erythroxylum coca (Erythroxylaceae). The initial source of taxol was the bark of Taxus brevifolia, which proved to be efficient for refractory ovarian cancer in 1992 and since 1994 being tested for breast cancer and other tumors (99). Current efforts to apply metabolic engineering to increase the production of economically important alkaloids continue and should expand to include more plant species and compounds. The enormous worldwide effort to screen plants for new biologically active compounds is expected to bring new drugs,

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some of which will probably be alkaloids, to the market. In the coming years, considerable interest may thus be expected in the field of plant cell biotechnology and metabolic engineering of secondary metabolism by the pharmaceutical industry. Recent data obtained by this research group at the State University of Campinas (UNICAMP) in Brazil, in collaboration with Professor Brunner’s group at the Technical University of Hamburgh-Harburg (TUHH) in Germany, have revealed interesting and very useful information on the extraction of methylxanthines from natural products such as guarana´ seeds, mate´ leaves, and cocoa beans using supercritical carbon dioxide and ethanol, and on the extraction of caffeine and theobromine along with cocoa butter from Brazilian cocoa beans (85, 100, 101). These findings point to new opportunities in the extraction of other important active principles in natural plants of interest to pharmaceutical, food, and cosmetic industries. For research on alkaloids, the future looks very exciting leading to a number of interesting applications in the production of specialty chemicals and pharmaceuticals (92). Environmental and ecological factors can have a considerable impact on future processes.

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87. W Bauer, MH Zenk. 2 methylenedioxy bridge forming cytochrome P-450 dependent enzymes are involved in (s)-stylopine biosynthesis. Phytochemistry 30 (9): 2953–2961, 1991. 88. O Vitzthum, P Hubert. Fluid extraction of hops, spices and tobacco with supercritical gases. Angew Chem Int Ed Engl 17:710–715, 1978. 89. R Roselius, O Vitzhum, P Hubert. Nicotine recovery from tobacco. German Patent 2,043,537 (1972). 90. M Amann, N Nagakura, MH Zenk. Purification and properties of (s)-tetrahydroprotoberberine oxidase from suspension-cultured cells of berberis-Wilsoniae. Eur J Biochem 175 (1):17–25, 1988. 91. A Stahl, KW Quirin, D Gerard. Dense Gases for Extraction and Refining. New York: Springer-Verlag, 1988, p. 176. 92. R Verpoorte, R van de Heijden, J Memelink. Plant biotechnology and the production of alkaloid: prospects of metabolic engineering. In: GA Cordell, ed. The Alkaloids: Chemistry and Biology, Vol 50. San Diego: Academic Press, 1998, pp. 453–508. 93. B Witkop. From the “ordeal bean” (Physostigma venenosum) to the ordeal of Alzheimer’s disease—some of the legacy of Percy Lavon Julian (1899–1975). Heterocycles 49:9–27, 1998. 94. M Levy, M Spino, SE Read. Colchicine—state of the art review. Pharmacotheraphy 11(3):196–211, 1991. 95. J Leonard. Alkaloids. In: RH Thomson, ed. The Chemistry of Natural Products. London: Chapman and Hall, 1993, pp. 218–258. 96. N Takao, M Kamigauchi, M Suguira, I Ninomija, O Miyata, T Natio. The structure of the alkaloid macarpine. Heterocycles 16 (2):221–225, 1981. 97. T Tanahashi, MH Zenk. J New hydroxylated benzo[c]phenanthridine alkaloids from eschscholtzia-Californica cell-suspension cultures. J Nat Prod 53 (3):579– 586, 1990. 98. J Berlin, E Forche, V Wray, J Hammer, W Hosel. Formation of benzophenanthridine alkaloids by suspension cultures of eschscholtzia-californica. Z Naturforsch C 38(5–6):346–352, 1983. 99. ME Wall, MC Wani. History and future prospects of camptothecin and taxol. In: GA Cordell, ed. The Alkaloids: Chemistry and Biology, Vol 50. San Diego: Academic Press. 1998, pp. 509–536. 100. MDA Saldan˜a, C Zetzl, RS Mohamed, G Brunner. Extraction of methylxanthines from guarana´ seeds, mate´ leaves and cocoa beans using supercritical carbon dioxide and ethanol. J Agri Food Chem 50:4820–4826, 2002. 101. MDA Saldan˜a, RS Mohamed, P Mazzafera. Extraction of cocoa butter from Brazilian cocoa beans using supercritical CO2 and ethane. Fluid Phase Equilibria 194– 197, 885–894, 2002.

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12 Removal of Cholesterol from Food Products Using Supercritical Fluids Rahoma S. Mohamed, Marleny D. A. Saldan˜a, Alvaro B. A. de Azevedo, and Uiram Kopcak The State University of Campinas–Unicamp Campinas, Sa˜o Paulo, Brazil

I. INTRODUCTION The necessity of some consumers to reduce or control weight due to health problems or for private reasons, as well as the general concerns of the population as a whole of the effects of diet on health are the major driving forces to produce foods with low fat and cholesterol contents (1). When consumers in the United States were asked in a recent survey about their concerns of food products, 61% were worried about fat and 35% about cholesterol (2, 3). The recommendation is that total fat calories in a daily diet should be limited to 30% with the saturated fatty acids contribution not to exceed 10% (4). A high cholesterol and fat concentration in the blood can lead to their precipitation in the circulatory system and cause arteriosclerosis. Worries and concerns about cholesterol and fat on the part of the worldwide population can be appreciated when observing the decrease in the consumption of whole milk (5) and egg yolk–containing food products (6), and the resulting increase in the production of low-fat, skim milk, and cholesterol-free egg yolk products to satisfy growing consumer demands for healthier products. Butter and butter oil have, traditionally, been the main derivatives of milk processing, with skim milk being a subproduct. The shift in consumer behavior in search for light products has generated a large surplus of butter and butter oil. The surplus in butter oil reached 155 × 106 kg in 1990 and was expected to reach the 550 × 106 kg mark in 2000 (7).

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Cholesterol removal and fractionation of fat can be an attractive solution to this surplus problem and a viable alternative in the production of new fat products with characteristics that meet new consumer concerns and demands. Cholesterol, a byproduct, can be used for a variety of applications, including the production of sterols, emollients, skin creams, and so forth (8). Various technologies have been suggested for the removal of cholesterol from food products, including enzymatic conversion of cholesterol to a nonabsorbable steroid (2), steam stripping (2, 9), distillation (10), complex formations with or without adsorption (11), molecular distillation (12), melt crystallization (13, 14), complexation with β-cyclodextrins (15, 16), use of cholesterol-degrading bacteria (17), and supercritical extraction using CO2 (18–21) and ethane as solvent (22). Rizvi and Bhaskar (19) made a comparison of the methods used for the extraction of cholesterol from milk fat and concluded that fat fractions produced with supercritical CO2 had favorably distinct and different physical and chemical properties than those obtained by other methods. Fractionation techniques have been reviewed by Hamm (23), who divided these processes into dry crystallization, solvent fractionation, and detergent fractionation, together with the new, attractive, and promising supercritical fluid fractionation technology as an alternative. The technique to complex cholesterol with β-cyclodextrin is applied on an industrial scale to produce low-cholesterol milk with over 90% reduction in cholesterol content (16). A low cholesterol liquid egg product (80% less in cholesterol content) has also been introduced in North America but with little success due to the high cost of the β-cyclodextrin complexation process (24). Other processes have also been suggested for the reduction of cholesterol content in food products, but they seem to be somewhat inconvenient because they introduce significant chemical changes in the protein and triglyceride contents in the raw material (24). Consumer concern over chemical residues in foods and increasing demand for high-quality healthy food products have rendered supercritical fluid extraction processes with solvents that are nontoxic, inexpensive, and having a relatively low critical temperature, such as carbon dioxide, one of the most promising alternatives to the use of chemical solvents in the removal of cholesterol from food products. The solubility of cholesterol in supercritical fluids has been widely investigated (25–31). Furthermore, experimental data on the removal of cholesterol from dehydrated meat (32), chicken (33), pork (34), fish (35), eggs and derivatives (36–39), and butter oil (2, 8, 14, 40, 41) using supercritical carbon dioxide have also been reported. Cully et al. (18) registered a patent on the process for the removal of cholesterol or cholesterol esters from egg yolk powder and butter fat by extraction with compressed CO2. The possibility of producing milk fat with 90% less cholesterol while maintaining the original color and flavor was

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described by Bradley (8). The removal of cholesterol and fractionation of butter oil with supercritical fluids have been reported by Shishikura et al. (42), Rizvi et al. (19), and Mohamed et al. (22). The use of cosolvents to improve the removal efficiency in the extraction of cholesterol with supercritical CO2 was evaluated by Saldan˜a et al. (21) and Singh et al. (30). In any extraction process, it is often difficult to reconcile conditions that lead to high recovery yields with those that result in high selectivity. A possible approach that could allow this reconciliation is through the coupling of the extraction process to a highly selective separation step. Shishikura (42) obtained butter oil fractions with only 5% of the cholesterol in the initial butter using supercritical extraction followed by adsorption on silica gel. However, in this process it was only possible to obtain a maximal extracted oil yield of 50% of the original oil. Huber et al. (43) investigated selective removal of cholesterol from anhydrous milk fat using supercritical carbon dioxide in a multiseparator process in which selective removal was only possible with the use of silica gel as an adsorbent. Mohamed et al. (22) recently presented data on the simultaneous removal of cholesterol and fractionation of butter oil through a coupled extraction/adsorption process that uses carbon dioxide or ethane as the supercritical solvent and alumina as the adsorbent. The extraction of cholesterol from animal fats with supercritical CO2 using a descending pressure profile with fractions collected in several separators connected in series was described by Chao et al. (44). There seems to be a substantial lack of detailed study in the literature, however, on the changes in the sensory, nutritional, and physiological properties of foodstuffs with the loss of important lipophilic components other from cholesterol during the extraction. In what follows, we present a brief summary of the physiological effects of cholesterol on health; cholesterol analysis and solubilities in supercritical fluids; a description of extraction and fractionation of cholesterol from different food products, followed by some experimental laboratory data on the potential use of supercritical fluids for the removal of cholesterol, and future prospects in the reduction of cholesterol levels from important meals.

II. PHYSIOLOGICAL EFFECTS OF CHOLESTEROL AND IMPLICATIONS ON HUMAN HEALTH Cholesterol (Fig. 1) is a sterol, differentiated from the main steroid ring structure by an aliphatic side chain, methyl groups, and a hydroxyl (OH) group. The presence of the OH group makes cholesterol a steroid alcohol or sterol (45). Cholesterol is an extremely important biological molecule with a crucial role in membrane structures as well as being a precursor for the synthesis of

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Figure 1 Cholesterol molecular structure.

steroid hormones, bile acids, and sexual hormones (46). The cholesterol molecule and the challenges faced by researchers to elucidate its role is illustrated in the work of Vance and Van den Bosch (47). In a general manner, it is well established that the human body not only synthesizes cholesterol but also absorbs it through the intestine, as cholesterol is eventually transported to the liver (48, 49). Some of the body cholesterol is produced by hepatocytes in the liver; therefore, every cell in the body is capable of producing cholesterol (50). In the cell, cholesterol is made available through synthesis in the endoplasm reticulum and by endocytosis of cholesterol-containing ligands (46). The complex mechanisms that govern the synthesis and the distribution of cholesterol in the cells are well described in the work of Blanchette-Mackie (46). The accumulation of cholesterol in the body depends on several factors such as nutrition, stress, and obesity (51). It is well established that a high cholesterol concentration in the blood can lead to its precipitation in the circulatory system and arteriosclerosis (52). The low-density lipoprotein (LDL) cholesterol, called “bad cholesterol,” circulating in the blood can slowly precipitate and accumulate in the walls of the arteries that feed the heart and brain. Together with other substances it can form plaque, a thick, hard deposit that clogs the arteries. This condition is known as arteriosclerosis. On the other hand, one-third to one-fourth of total blood cholesterol is what is known as high-density lipoprotein (HDL), also called “good cholesterol.” A high level of HDL cholesterol is believed to prevent the precipitation of the LDL part and therefore arteriosclerosis that could lead to serious health problems such as heart attacks and strokes. The association of high blood cholesterol level with heart diseases or cancer is the motivating factor in recent research on the reduction of cholesterol levels in consumed meals (53, 54). The American Heart Association recommends that there be no more than 300 mg/d of cholesterol in daily diet. The cholesterol contents in some foods of

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animal origin are presented in Table 1. The removal of cholesterol from food products without altering the composition or properties of the other constituents would be necessary for these products to be accepted by consumers in the form of low-cholesterol butter, cheese, ice cream, and others (2).

III. DETERMINATION OF CHOLESTEROL CONTENT IN FOOD PRODUCTS AND EVALUATION OF ITS SOLUBILITY IN SUPERCRITICAL FLUIDS A. Cholesterol Compositional Analysis Several different techniques are applied for the compositional analysis of cholesterol (56). Jime´nez-Carmona and Luque de Castro (57) reported some cholesterol and cholesterol oxides analysis using gas chromatography (GC) and high performance liquid chromatography (HPLC). When using GC, an appropriate capillary column and a flame ionization detector are normally used, and before the analysis samples must be pretreated according to the standard AOAC 933.08 method (58). Recent HPLC techniques use a nonaqueous reversed-phase system with saponified or esterified derivatives (59–62). The AOAC also reported different methods for cholesterol analysis in food components such as eggs or oils and fats. The method 941.09 (63) is a titrimetric method for determination of cholesterol in eggs as a whole, while the method 43.290 (64) is recommended for determination in egg yolks. For oils and animal fats with low levels of unsaponifiable matter the AOCS official method Ca 6a-40 (65) is recommended. As reported by Pasin et al. (66), most of the methods used to determine the cholesterol content in foods were developed with serum cholesterol procedures as the starting point. These methods can be divided into three groups: colorimetric, chromatographic, and enzymatic. The colorimetric methods were based on the Liebermann-Burchard color reaction or the Zlatkis procedures (67, 68). Some researchers have questioned the accuracy and the limitations of these Table 1 Cholesterol Contents in Foods (mg/100 g) Foods Pork Chicken Chicken with skin Beef Eggs

Raw

Boiled

49–54 58–80 104 51–52 33–190

56–97 75–124 139 66–67 1000–1019

Source: Adapted from Ref. 55.

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methods (66). The chromatographic methods, which include gas chromatography (69), gas–liquid chromatography (70), HPLC (71), or capillary supercritical fluid chromatography (37), require a pretreatment saponification step of samples prior to analysis. These pretreatment steps are time consuming, cumbersome, and seldom environmentally friendly, all of which place strong limitations on sample throughput (66). Pasin et al. (66) also reported on an enzymatic method for determination of total cholesterol in fresh, frozen, and dried egg yolk using a diagnostic cholesterol reagent. The results revealed that this enzymatic method could be used for the quantitative determination of cholesterol without the need for saponification. Cholesterol determination is made by dye absorbance analysis. The intensity of the color produced is directly proportional to the total cholesterol in the sample (66). B. Cholesterol Solubility in Supercritical Fluids In order to explore the ability of supercritical fluid extraction in the reduction of cholesterol content in foods and products of animal origin (fats, meats, egg yolk, etc.), the solubility behavior of cholesterol in supercritical solvents must be determined. Some experimental data on the solubility of cholesterol in supercritical fluids, particularly CO2, have been presented in the literature. Some of these data, when compared with each other, show some disagreements and inconsistencies that are believed to be associated with the method used and the purity of substances employed. Yun et al. (29) used a continuous high-pressure flow apparatus to obtain solubility data of cholesterol and triglycerides in supercritical CO2. A similar apparatus was used by Neves (20) for the same measurements, with the data obtained found in good agreement with those of Yun et al. (29). When Yun et al. (29) compared their data with those of Chrastil (25) (who used a batch technique) and also with those of Wong and Johnston (27) (who used a micro sampling apparatus), some discrepancies were observed; these were attributed to differences in the experimental techniques used. Chrastil (25) studied the cholesterol solubility in carbon dioxide at pressures ranging from 10 to 25 MPa and temperatures ranging from 40°C to 80°C, with 20° intervals. An increase in solubility with increase in pressure at constant temperature was observed, as expected by the resultant increase in the fluid density and consequent increase in the solvent power of the supercritical fluid. A decrease in solubility with increase in temperature at constant pressure was also observed. This is the commonly encountered phenomenon known as retrograde condensation, a characteristic of all supercritical extraction systems (72). A similar result was observed by Neves (20) at the same temperature and pressure ranges. This phenomenon is often attributed to the fact that the increase in vapor pressure of the solute with increase in temperature cannot compensate the loss of solvent power caused by the resultant decrease in solvent density (72).

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Neves (20) reported that the change from retrograde to nonretrograde behavior occurred at a pressure of 16 MPa. This pressure is known as the upper crossover pressure below which the effect of vapor pressure prevails over the effect of solvent density. This behavior can be explored among the many other possibilities in determining the optimal conditions for the extraction, separation, and fractionation of solutes present in a mixture. To correlate the solubility data, Yun et al. (29) tested several models, including a model proposed by Chrastil based on association between solute and solvent molecules to form a solvation complex. This model had successfully correlated the solubility of lipids (25). Another model proposed by Kumar and Johnston (73) provides a more realistic description of the solvent–solute molecular interactions. The data available in the literature for the system of cholesterol-carbon dioxide (25, 28, 29, 31) revealed low solubility values at relatively low pressures, with substantial increase in solubility at high pressures and relatively high temperatures. Therefore, a viable extraction process of cholesterol with supercritical CO2 implies high energy costs. An increase in solubility without the use of substantially high pressure and temperatures could be brought about with the use cosolvents or solvents with better affinities for cholesterol. The cholesterol molecule has a large nonpolar tail (similar to a hydrocarbon). For this reason, the use of a hydrocarbon as a solvent or a cosolvent could be an attractive alternative. The use of hydrocarbon solvents such as supercritical ethane, which has a critical temperature very close to that of carbon dioxide, a critical pressure that is lower than CO2, and is an acceptable solvent in food processing (30), has been explored. Singh et al. (30) presented solubility data of cholesterol in supercritical ethane and in the binary solvent systems: ethanepropane (3.5 and 14 mol % propane) and ethane-carbon dioxide (3.5 and 14.5 mol % ethane) at pressures ranging from 7 to 29 MPa and temperatures ranging from 308 to 338 K. Saldan˜a et al. (21) presented solubility data of cholesterol in CO2/ethane mixed solvent systems at 55°C and pressures from 12 to 20 MPa and concentrations of ethane ranging from 8 to 96 mol %. These investigations revealed the same effects of pressure and temperature on cholesterol solubility obtained earlier with CO2 but with cholesterol solubilities that are 3 to 15 times higher than in supercritical CO2 (25, 27, 28, 31). Results reported by Mohamed et al. (22) and Saldan˜a et al. (74) are presented in Figs. 2 and 3, respectively. This higher solubility of cholesterol in ethane was attributed to the larger ethanecholesterol dispersion forces in comparison with the weaker polar interaction forces between CO2 and cholesterol molecule. Mendes (75) and Mohamed et al. (22) reported similar observations for the β-carotene-ethane and cholesterolethane-butter oil systems, respectively. As observed in Figs. 2 and 3, solubilities in mixed solvents are intermediate to those obtained with pure solvents but not linear in behavior, as the increase in cholesterol solubility is not proportional to the amount of ethane added

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Figure 2 Cholesterol solubility in CO2 and ethane.

Figure 3 Solubility of cholesterol in mixtures of CO2 and ethane at 328.15 K.

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to CO2 (21). A similar increase in solubility was observed when using propane as a cosolvent (30). Moreover, Foster et al. (76) also observed the increase of cholesterol solubility with the increase in the concentration of acetone and hexane when added as cosolvents to supercritical CO2 and supercritical ethane.

IV. POTENTIAL APPLICATIONS OF CHOLESTEROL REMOVAL FROM FOOD PRODUCTS The removal of cholesterol from milk fat (77), eggs (38, 78, 79), chicken (33), pork (34), fish (35), beef (80), and others using supercritical carbon dioxide is among the extraction and fractionation operations tested in laboratory with several patents reported in the literature (9, 12, 17). While the removal of cholesterol from these products can employ organic, nonorganic, or supercritical solvents, extraction with supercritical carbon dioxide eliminates the risks of potential toxic residues due to the inert nature of CO2 (19, 72), and thermal degradation of extracted products due to mild operating temperatures (near the CO2 critical temperature of 31°C). In what follows, a more detailed description of cholesterol extraction of food products using conventional and supercritical fluid solvents is presented. A. From Milk Fat Milk fat, a major product in the dairy industry, has been primarily used as butter. It is a good source of essential fatty acids and contains a high proportion of short-chain fatty acids, which contributes to its easy digestibility. Milk fat contains a mixture of triglycerides with a wide range of molecular weights. The most desirable property of milk fat is its pleasant flavor not found in other fats. However, the relatively high saturated fatty acids content and the presence of cholesterol in milk fat has raised nutritional and health concerns that have resulted in a substantial decrease in the direct consumption and utilization of milk fat as an ingredient in many industrial products. In order to reduce its cholesterol and saturated fatty acids contents, Arul et al. (14), Bradley (2, 8), and others suggested the fractionation of milk fat to obtain fractions with desirable physical, functional, and nutritional properties. Milk fat fractionation is a potential technology for the development of novel ingredients with varied functional properties for use in numerous food formulations. Hamm (23) reports the possibility of three types of fractionation: (a) dry fractionation, also known as melt crystallization; (b) solvent fractionation using such solvents as acetone or hexane; and (c) detergent fractionation, wherein a surfactant solution is used to transfer the crystallized material from

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the oil phase to the aqueous phase and facilitate its subsequent separation. Dry fractionation of milk fat is performed using the Tirtiaux process (81). Norris et al. (13) described the fractionation by crystallization at different temperatures (melt crystallization) with or without the use of solvents. Although the use of solvents or surfactants produces a good separation of triglycerides, these techniques are not environmentally friendly due to problems related to solvent removal and disposal. The commercial value of the secondary fraction produced in any fractionation process plays an important part in determining the viability of the overall process, and upgrading of this secondary fraction is an important part of the total production and marketing process (23). Modification of milk fat can also be carried out using chemical methods such as interesterification (82) and hydrogenation (83), but these methods cause losses of many desirable characteristics and destroy the natural flavor of it (13). The fact that none of the conventional methods could provide an adequate flavor concentrate has motivated the urgent search for another method for the removal of cholesterol from milk fat. Several research groups have suggested the use of supercritical CO2 to fractionate and modify milk fat (2, 8, 14, 42, 84–87) and to remove milk cholesterol (88). Using supercritical carbon dioxide at 20 MPa and 80°C, Kaufmann et al. (40) fractionated butter oil into a liquid fraction and a solid fraction with different cholesterol contents. Arul et al. (14) also studied the distribution of cholesterol in milk fat fractions obtained with supercritical carbon dioxide at temperatures of 50°C and 70°C and pressures varying from 10 to 35 MPa, and compared the cholesterol contents in these fractions with those in fractions obtained by distillation and crystallization. Cholesterol removal efficiency was highest with distillation followed by supercritical fluid fractionation. Bradley (2) reported that 90% cholesterol removal efficiency from milk fat at 80°C and pressures from 15.8 to 41.4 MPa is quite feasible for a broad range of dairy foods. A redistribution of cholesterol in butter oil was also presented by Chen et al. (41) in which cholesterol concentrations were increased in fractions extracted at 40°C and 10.3, 13.8, 24.1, and 27.6 MPa. A similar cholesterol redistribution in butter oil fractions extracted with supercritical CO2 was reported by Bradley (8). Hammam et al. (89) characterized the supercritical carbon dioxide fractionation products, including the redistribution of cholesterol in such products according to their physical properties. Bhaskar et al. (90) fractionated milk fat into fractions of short chain (C4 –C8), medium chain (C10 –C12) and long chain (C14 –C18) fatty acids and concluded that fractions collected at higher pressures were richer in the higher molecular weight triglycerides and that carbon dioxide displays low selectivity for cholesterol in relation to triglycerides. Shukla et al. (91) presented physicochemical and rheological properties of butter fractions obtained when using supercritical fluids for the fractionation of milk fat. The butter obtained with supercritical fluids showed a potential use at ambient and higher tempera-

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tures in contrast with the oiling off and leakage problems exhibited by normally encountered market butter. Furthermore, the extracted butter was reported to be richer in unsaturated fatty acids and lower in cholesterol content (117.6 mg/100 g) than those found in commercial butter (240.6 mg cholesterol/100 g). Rizvi and Bhaskar (19) also separated milk fat into saturated and unsaturated fatty acids using supercritical CO2, identified the physical properties, and quantified the cholesterol content. They concluded that the fractions obtained with supercritical CO2 were unique exhibiting different characteristic physicochemical properties. They also presented a summary of all recent research on the fractionation of butter oil with supercritical carbon dioxide and a basic study for scaleup of the process. Solubility and fractionation data of cholesterol in supercritical carbon dioxide and ethane were obtained by Mohamed et al. (22) using a high-pressure experimental extraction apparatus (92). The amounts of cholesterol in butter oil extracted using supercritical ethane were found to be much higher than when using supercritical CO2 (Fig. 4). Both supercritical CO2 and melt crystallization processes produce fractions with varying physical properties (93). The advantage of using supercritical CO2 is that the flavor components are simultaneously concentrated during the fractionation. Furthermore, solid fractions obtained with supercritical CO2 have

Figure 4 Cholesterol contents in fractions obtained with the extraction of butter oil using supercritical CO2 and ethane at different pressures and temperatures.

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lower cholesterol content than those obtained by melt crystallization. However, process cost favors melt crystallization (0.02–0.05 $/kg) over supercritical CO2 extraction (0.10–0.15 $/kg) as reported in a detailed study by Singh and Rizvi (7). The lower cholesterol content and consequently higher quality of milk fat fractions obtained with supercritical CO2 could still make this process a very viable option. Shishikura et al. (42) concluded that the preparation of a low-cholesterol butter oil by simple extraction with supercritical CO2 is not practical due to the observed relatively low butter oil capacity and cholesterol selectivity. They proposed the use of supercritical extraction in conjunction with an added adsorbent. Cholesterol levels were substantially reduced when passing the extract through a silica or alumina adsorbent (22, 42). Adsorption and desorption of cholesterol in continuous supercritical fluid processing of anhydrous milk fat were also reported by Lim and Rizvi (87), with magnesium silicate as the adsorbent. The extraction/fractionation operation was also coupled with an adsorption step that uses alumina as the adsorbent (22). The combined extraction/adsorption operation resulted in the removal of more than 97% of the cholesterol in the original butter oil (Fig. 5). The increase in cholesterol content in fractions obtained in late stages is attributed to the desorption of cholesterol from the alumina bed to the supercritical fluid stream. The operation has also resulted in

Figure 5 Cholesterol extraction curves of butter oil for the fractionation/adsorption process with supercritical ethane.

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the generation of butter oil fractions with characteristic properties that are distinctly different from those of the original oil. The detailed economic analysis for the continuous supercritical CO2 processing of milk fat performed by Singh and Rizvi (7) and referred to earlier shows this process to be economically viable. The final processing cost can be offset by the value of the byproduct, cholesterol. Some economic analysis for continuous countercurrent processing of milk fat fractionation was also carried out by Ko¨nig-Schreer (93). Assuming a plant capacity ranging from 800 to 10,000 t/a for a continuous milk fat fractionation by supercritical CO2, a payback period of about 5 years was estimated. These calculations confirm that an optimized milk fat fractionation process could be carried out on an industrial scale. The addition of an adsorption step for cholesterol would improve the economics of this process. Semicontinuous systems are not as economically favorable as continuous large-scale systems. B. From Eggs Eggs are one of the most complete foods consumed by humans, being an important source of high-quality protein, vitamins, and minerals (38). However, egg yolk has a high concentration of cholesterol, and concerns about cholesterol concentration in blood serums and potential health problems are the principal causes of the worldwide decrease in egg consumption. Products containing white eggs, low or no egg yolk are already available in the market, resulting in a surplus of egg yolk (78). Removal of the cholesterol would be a promising solution for this surplus problem and the return to normal whole-egg consumption. Extraction of lipids with organic solvents for the purpose of reducing the cholesterol content in egg yolks is no longer acceptable because organic solvents are nonselective, and such extractions result in the loss of other valuable components such as phospholipids which carry out important functional properties including emulsion stabilization (38). In these processes, proteins are denatured, which impedes the use of the resultant protein concentrate (38), and toxic residues could potentially contaminate both final product and environment (79). Supercritical fluid extraction using carbon dioxide as a solvent has presented good results in cholesterol selective extraction, resulting in the extraction of approximately 70% of the cholesterol content of dried egg yolk (38) and the total removal of cholesterol from egg yolk powder (79). Therefore, the works of Paraskevopoulou et al. (94) extracted about 45% of the lipids and 75% of the cholesterol content in dried egg yolk using a continuous supercritical fluid apparatus at 31.4 MPa and 35–45°C. Phospholipids were not coextracted with cholesterol and other lipids, which is highly desirable due to their important functional and organoleptic properties. Egg yolk, both dry and degreased, exhibited

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similar stability, and the concentrate had a satisfactory performance in the cake preparation. Wu and Hou (79) extracted egg yolk powder using supercritical carbon dioxide as a solvent in a pilot plant at pressures and temperatures ranging from 25 to 36 MPa and 35 to 70°C, respectively. A complete extraction of all egg yolk oil was obtained. The egg yolk powder particle size had no influence on the extraction rate, and increasing the flow rate resulted in a faster extraction. A kinetic model developed to describe the extraction egg yolk presented results in good agreement with experimental pilot plant data. C. From Beef, Chicken, Fish and Pork Meats, important sources of protein in the human diet, generally have high contents of fat and cholesterol. Reduction of the fat and cholesterol contents in meat products is one of the challenges of the food industries to attend to consumer needs for a healthy diet and important for those consumers who desire to lose weight or those with a heart problem. Supercritical fluid extraction of cholesterol and fat from chicken (33), pork (34), fish (35), and beef (80), among others, has been studied. Results have revealed the process to be efficient, in some cases resulting in the removal reducing of more than two-thirds of the original cholesterol and fat content. Wehling (95) extracted fat and cholesterol in dehydrated beef using supercritical CO2 at pressures and temperatures ranging from 23.4 to 38.6 MPa and 45 to 55°C, respectively. At a solvent density of 0.9 g/cm3, about 87% of the total cholesterol and fat content from dehydrated beef powder was removed, and high temperatures favored the extraction of lipids, which were in the solid state at 45°C but were completely melted at 55°C. The authors also reported on the effects of particle size on the extraction efficiency and on the color alteration of the dehydrated beef powder due to removal of pigments, which could be very desirable as it allows the product to be a source of protein in various prepared foods. Lin et al. (34) observed this same loss in pigments during the removal of cholesterol from fried shredded pork using continuous supercritical carbon dioxide extraction. Operating at pressures and temperatures ranging from 7.3 to 34.4 MPa and 50 to 150°C, respectively, about 50–70% of the cholesterol in the meat sample was removed. The increase in temperature from 50°C to 150°C resulted in higher pigment removal, which in this case is not desirable as whiteness could be perceived to indicate a poor-quality product. Sensory analysis indicated that products obtained with these extraction conditions could not be differentiated from those bought in a local supermarket, with 34 MPa and 150°C being the optimal extraction conditions. Froning et al. (33) also demonstrated the efficiency of the supercritical fluid process in the extraction of cholesterol

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and fat from dehydrated chicken meat powder and chunks, along with recovery of important flavor components from the residues (extracted lipids). A summary of the main products containing cholesterol and their extraction with supercritical fluids is presented in Table 2. These results indicate the great potential of supercritical fluid extraction in the recovery of meat products with acceptable cholesterol and fat contents.

V. FUTURE PROSPECTS During the past 20 years, the use of supercritical fluid technology has advanced quite rapidly. Supercritical fluids have been applied in diverse areas, including the food and pharmaceutical industries, biotechnology, new materials technologies, petroleum, among others. Removal of cholesterol contents from different food products, including fish oil capsules, foodstuffs sausages, mayonnaise, noodles, and cheese (71, 96), have been recently reported, and the determination of cholesterol contents in food products and blood serum using capillary supercritical fluid chromatography is believed to be a potential and fast analysis technique for programmed control of the optimal consumption diets. The analytical determination of cholesterol in food, as described by Jime´nez-Carmona (57) using a reverse micelle formation to accelerate the extraction of cholesterol and in which a surfactant is added to the sample from which cholesterol will be extracted, is also a potential new application.

Table 2 Supercritical Fluid Extraction of Cholesterol with CO2 from Products of Animal Origin Cholesterol (mg/g) Product Dried egg yolk Dried egg yolk Dehydrated beef Beef patties (cooked) Pork (cooked) Dried chicken meat Milk fat Milk fata a

Ref.

P (Mpa)

T (°C)

Before

After

Yield (%)

39 68 86 70 34 33 20 22

16.5–37.8 24.1–37.8 23.4–38.6 17.2–55.1 7.3–34.4 30.6–37.6 10.1–36.4 8.0–24.0

40–55 45–55 45–55 40–50 50–150 45–55 40–70 40–70

18.52 18.94 1.56 1.94 0.80 4.96 2.50 2.50

6.34 0.38 0.19 0.12 0.22 0.54 0.21 0.20

65.8 98.0 87.8 93.8 70.1 90.0 91.5 93.4

Using supercritical ethane as solvent.

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In general, alternative processes using supercritical fluids have been claimed to be highly versatile techniques, which could be applicable in a number of situations where conventional processes could not be applied or have serious limitations. Some new applications of supercritical fluid processes for the reduction of cholesterol content include the enzymatic conversion of cholesterol to sterols that are unabsorbed by humans. In this application, a supercritical fluid as a reaction medium in enzymatic catalytic processes is considered as a means to increase the solubility of hydrophobic components and revealed to be highly successful as shown in the recent literature (97, 98). King et al. (99) reported the use of supercritical carbon dioxide as a reaction medium with lipase as a catalyst in a natural process, applicable to produce additives that can be incorporated directly in food formulations. Some sterol esters can be added to foods acting as cholesterol-lowering agents, with important implications for the food and nutraceutical industries. Enzyme-catalyzed esterification of cholesterol using vinyl acetate as a cosolvent in supercritical ethane and pressurized hexane was studied by Sarkari et al. (97), who concluded that the enhancement in the esterification rate is a strong function of the pressure and cosolvent concentration in the system when using supercritical ethane as a reaction medium,. When using hexane, the effect of pressure was less pronounced. These facts are much important as they point to the possibility of controlling the reaction rate by choosing the appropriate solvent and pressure system.

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25. J Chrastil. Solubility of solids and liquids in supercritical gases. J Phys Chem 86: 3016–3021, 1982. 26. MDA Saldan˜a, EM Homen, RS Mohamed. Uso de etano como co-solvente na extrac¸a˜o do colesterol com CO2 supercrı´tico. In: Anales del II Congresso Iberoamericano de Ingenierı´a de Alimentos. Universidad Nacional del Sur, Bahia BlancaArgentina, 1998, pp. 1–6. 27. JM Wong, KP Johnston. Solubilization of biomolecules in carbon dioxide based supercritical fluids. Biotechnol Prog 2:29–39, 1986. 28. RS Mohamed, GM Neves, TG Kieckbusch. Reduction in cholesterol and fractionation butter oil using supercritical carbon dioxide with adsorption on alumina. Int J Food Sci Technol 33(5):245–253, 1998. 29. SLJ Yun, KK Liong, GS Guardial, NR Foster. Solubility of cholesterol in supercritical carbon dioxide. Ind Eng Chem Res 30:2476–2482, 1991. 30. H Singh, SLJ Yun, SJ Macnaoughton, DL Tomasko, NR Foster. Solubility of cholesterol in supercritical ethane and binary gas mixtures containing ethane. Ind Eng Chem Res 22:2841–2848, 1993. 31. E Kosal, CH Lee, GD Holder. Solubility of progesterone, testerone, cholesterol in supercritical fluids. J Super Fl 5:169–179, 1992. 32. GW Froning, SL Cuppett, L Niemann. Extraction of cholesterol and other lipids from dehydrated beef using supercritical carbon dioxide. J Agric Food Chem 40: 1204–1207, 1992. 33. GW Froning, F Fieman, RL Wehiling, S Cuppett, L Nielmann. Supercritical carbon dioxide extraction of lipids and cholesterol from dehydrated chicken meat. Poultry Sci 73:571–575, 1994. 34. TY Lin, YJ Wang, PY Lai, FJ Lee, JTS Cheng. Cholesterol content of fried-shredded pork extracted by supercritical carbon dioxide. Food Chem 67:89–92, 1999. 35. I Hardardottir, JE Kinsella. Extraction of lipid and cholesterol from fish muscle with supercritical fluids. J Food Sci 53:1656–1658,1661, 1988. 36. GW Froning. Supercritical fluid extraction of cholesterol from dried eggs. Chap. 22. In: C Haberstroh, CE Morris, eds. Fat and Cholesterol Reduced Foods: Technologies and Strategies. Advances in Applied Biotechnology Series, Vol. 12. Woodlands, TX: Gulf Pub, 1991, pp. 277–288. 37. RA Novak, WJ Reightler, G Pasin, AJ King, G Zeidler. Supercritical fluid extraction of cholesterol from liquid egg. Chap. 23. In: C Haberstroh, CE Morris, eds. Fat and Cholesterol Reduced Foods: Technologies and Strategies. Advances in Applied Biotechnology Series, Vol 12. Woodlands, TX: Gulf Pub, 1991, pp. 289–298. 38. GW Froning, RL Wehiling, S Cuppett, L Nielmann. Moisture content and particle size of dehydrated egg yolk affect lipid and cholesterol extraction using supercritical carbon dioxide. Poultry Sci 77:1718–1722, 1998. 39. GW Froning, RL Wehiling, S Cuppett, MM Pierce, L Nielmann, DK Siekan. Extraction of cholesterol and other lipids from dried egg yolk using supercritical carbon dioxide. J Food Sci 55:95–98, 1990. 40. VW Kaufmann, G Biernoth, E Frede, W Merk, D Precht, H Timmen. Fraktionierig von buttergett durch extraktion mit u¨berkritischim CO2. Milchwisserscharft 37(2): 92–96, 1982.

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41. H Chen, SJ Schwartz, GA Spanos. Fractionation of butter oil by supercritical carbon dioxide. J Dairy Sci 75:2659–2669, 1992. 42. A Shishikura, K Fujimoto, T Kaneda. Modification of butter oil by extraction with supercritical carbon dioxide. Agric Biol Chem 50(5):1209–1215, 1986. 43. W Hubert, A Molero, C Pereyra, EM de la Ossa. Dynamic supercritical CO2 extraction for removal of cholesterol from anhydrous milk fat. Int J Food Sci Technol 31:141–151, 1996. 44. RR Chao, SJ Mulvaney, ME Bailey, LN Fernando. Supercritical CO2 conditions affecting extraction of lipid and cholesterol from ground beef. J Food Sci 56:183– 187, 1991. 45. K Bloch. The structure of cholesterol and bile acids. Trends Biochem Sci 7:334– 336, 1982. 46. E J Blanchette-Mackie. Intracellular cholesterol trafficking: role of the NPC1 protein. Biochim Biophys Acta 1486:171–183, 2000. 47. DE Vance, H Van den Bosch. Cholesterol in the year 2000. Biochim Biophys Acta 1529:1–8, 2000. 48. K Lu, MH Lee, SB Patel. Dietary cholesterol absorption; more than just bile. Trends Endocrinol Metab 12(7):314–320, 2001. 49. M Hernandez, J Montenegro, M Steiner, D Kim, C Sparrow, PA Detmers. Intestinal absorption of cholesterol is mediated by a saturable, inhibitable transporter. Biochim Biophys Acta 1486:232–242, 2000. 50. S Yokoyama. Release of cellular cholesterol: molecular mechanism for cholesterol homeostasis in cells and in the body. Biochim Biophys Acta 1529:231–244, 2000. 51. TA Miettinem. Cholesterol absorption efficiency and sterol metabolism in obesity. Atherosclerosis 153:241–248, 2000. 52. N Kumar, OP Singhal. Cholesterol oxides and atherosclerosis: a review. J Sci Food Agric 55:497–510, 1991. 53. RM Feeley, PE Criner, BK Watt. Cholesterol content of foods. J Am Diet Assoc 61:134–139, 1972. 54. DE LaCroix, WA Mattingl, NP Wong, JA Alford. Cholesterol, fat and protein in dairy products. J Am Diet Assoc 62:275–279, 1973. 55. N Bragagnolo. Determinac¸a˜o dos nı´veis de colesterol em carnes, ovos e macarra˜o com ovos. FEA, UNICAMP, 1993. In: Revista da Carne 222: 82, 1995. 56. JP Sweeney, JL Weihrauch. Summary of available data for cholesterol in foods and methods for its determination. CRC Crit Rev Food Sci Nutr 8:131–159, 1976. 57. MM Jime´nez-Carmona, MD Luque de Castro. Reverse miscelle formation for acceleration of the supercritical fluid extraction of cholesterol from food samples. Anal Chem 70:2100–2103,1998. 58. AOAC. Residue (unsaponifiable) of oils and fats-ether extraction method. In: Official Methods of Analysis. Arlington, VA: AOAC, 1990, p. 971. 59. WJ Hurst, MD Aleo, RA Martin. High-performance liquid-chromatographic analysis of cholesterol. J Dairy Sci 66:2192–2194, 1983. 60. DR Newkirk, AJ Sheppard. High-pressure liquid-chromatographic determination of cholesterol in foods. JAOAC 64:54–57, 1981. 61. K Sugino, J Terao, H Murakami, S Matsushita. High-performance liquid-chromato-

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98. MA Kane, GA Baker, S Pandey, FV Brigh. Performance of cholesterol oxidase sequestered within reverse miscelles formed in supercritical carbon dioxide. Langmuir 16(11):4901–4905, 2000. 99. JW King, JM Snyder, H Frykman, A Neese. Sterol ester production using lipasecatalyzed reactions in supercritical carbon dioxide. Eur Food Res Technol 212(5): 566–569, 2001.

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13 Solvent Extraction: Safety, Health, and Environmental Issues Phillip J. Wakelyn National Cotton Council Washington, D.C., U.S.A.

Peter J. Wan U.S. Department of Agriculture New Orleans, Louisiana, U.S.A.

I. INTRODUCTION Safety, health, and environmental issues for edible oil extraction facilities vary, depending on the extraction method used. For example, aqueous extraction, supercritical fluid extraction, and mechanical extraction facilities have different requirements than organic solvent extraction facilities. In the United States, edible oil extraction using organic solvents, is a mature industry that has been under increasing regulatory pressure in recent years. In this chapter, regulatory concerns and toxicity of extraction solvents, commercial hexane, and other potential alternative solvents are discussed along with the current methods of edible oil extraction (i.e., mechanical, prepress solvent, and solvent extraction). Fats, which are solid at ambient temperature, and liquid oils are recovered from diverse biological sources by mechanical separation, solvent extraction, or a combination of the two methods (1, 2). These materials include animal tissues (e.g., beef, chicken, and pork); crops specifically produced for oil or protein (e.g., soy, sunflower, safflower, rape/canola, palm, and olive); byproducts of crops grown for fiber (e.g., cottonseed and flax); crops for food and their coproducts (e.g., corn germ, wheat germ, rice bran, coconut, peanuts, sesame, walnuts, and almonds); nonedible oils and fats (castor, tung, jojoba); and other oil sources (oils and fats from microbial products, algae, and seaweed). There are

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many physical and chemical differences among these diverse biological materials. However, the similarities are that oils (edible and industrial) and other useful materials (e.g., vitamins, nutraceuticals, fatty acids, phytosterols) can be extracted from these materials by mechanical pressing, solvent extracting, or a combination of pressing and solvent extraction. The preparation of the various materials to be extracted varies. Some need extensive cleaning, drying (optional), fiber removal (cottonseed), dehulling, flaking, extruding, and so forth, all of which affect the solvent–substrate interaction and therefore the yield, composition, and quality of the oils and other materials obtained. Historically, the advancement of processing technology for recovering oils and other useful materials has been primarily driven by economics. Each extraction process was optimized through trial and error with the available technology to produce maximal yield of high-quality products at the lowest cost. For thousands of years, stone mills, and for several centuries, simple hydraulic or lever presses were used as batch systems. The continuous mechanical presses only became a reality during the early 1900s. It was not until the 1930s that extraction solvents were used more widely, which greatly enhanced the recovery of oil from oilseeds or other oil-bearing materials. In recent years, safety, health, and environmental regulations for the solvents used for extracting oil from oilseeds have prompted research efforts to find solvents to replace commercial hexane. These solvents, including ethanol, isopropanol, water, supercritical carbon dioxide, and others, are technically feasible as oil extraction solvents but at present are economically unacceptable (1). Recent research to use commercial isohexane in two separate cottonseed oil mills (3, 4) has demonstrated energy savings and throughput increases. In the near term, it appears that commercial isohexane can be used as an alternative to commercial hexane.

II. OIL EXTRACTION PROCESS Four types of processing systems are used to extract oil from oil-bearing materials: hydraulic press, expeller or screw press, prepress solvent extraction, and direct solvent extraction. Oil-bearing materials have to be prepared for extraction to separate the crude oil from the meal (1, 2) (Fig. 1). Careful control of moisture and temperature during processing must be exercised to maintain the quality of the protein in the meal, to minimize the damage to the oil, and to maximize oil extraction. Crude oils are refined by conditioning with phosphoric acid to promote the removal of phospholipids and treating with sodium hydroxide (alkali refining) (Fig. 2). Refined oil is bleached with activated clay to remove color pigments and residual soap. Bleached oils are then deodorized by steam distillation. When the soybean oil is of good quality and the free fatty acid (FFA) content is low, the oil can be physically refined. Physical refining involves water washing to recover lecithin followed by phosphoric acid pretreat-

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Figure 1 Preparation for extraction to separate the crude oil from the meal in oilbearing materials.

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Figure 2 Refining of crude oil.

ment and bleaching to remove the nonhydratable gum. After bleaching, the oil is steam deodorized to remove the FFAs. The refined, bleached, and deodorized oil (RBD oil) is used to produce finished products, e.g., salad and cooking oils, shortenings, and margarine. Some of the finished products also require the oil to be hydrogenated, which changes the consistency and solid content of the oil, and increases stability to oxidation, which extends the shelf life of the finished products. Also, some of the oils (e.g., cottonseed and sunflower) are winterized to remove the higher melting constituents or wax. The solid fraction removed from winterization can be used in confectionery products; the winterized oil is less likely to become cloudy in refrigerated storage.

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A. Preparation for Extraction 1. Storage For optimal extraction and quality of oil, the oil-bearing material should be stored dry and at a relatively low temperature. If the material is wet, it should be processed as soon as possible after harvest. Oils in the presence of water can deteriorate rapidly, forming FFAs and causing greater refining loss. 2. Seed Cleaning The first step in the commercial processing of oilseeds is “cleaning” to remove foreign materials, such as sticks, stems, leaves, other seeds, sand, and dirt using dry screeners and a combination of screens and aspiration. Permanent electromagnets are also used for the removal of trash iron objects. Final cleaning of the seed usually is done at the extraction plant just prior to processing. 3. Dehulling The dehulling process may include the removal of excess moisture by drying, cracking the seed, and removing the outer seed coat (hull) of the seed. The hull contains little or no oil, so that its inclusion makes the extraction less efficient and dilutes the protein content of the meal. Also, the hull will reduce the total yield of oil by absorbing and retaining oil in the press cake. An acceptable level of hull removal must be determined, depending on the desired protein level of the final meal. Hulls are removed by aspiration, and un-dehulled seeds are removed from the kernels by screening and returned to the huller. Some meats still adhere to the hulls, which are beaten, then screened again to obtain the meat. In the case of high oil content seed for direct solvent extraction, such as cottonseed, sunflower seed, etc., certain quantity of hull material is added to the kernels to provide the structural strength and matrix needed for the solvent extraction process. 4. Flaking After dehulling, the meats are reduced in size and “flaked” to increase surface area and to facilitate oil removal. Proper moisture content of the seeds is essential for flaking, and if the moisture level is too low, the seeds are “conditioned,” with water or steam, to raise the moisture to about 11%. In the case of soybean, heat will be applied to soften the meats prior to flaking. For solvent extraction, flakes are commonly not less than 0.203–0.254 mm (0.008–0.010 in), which can be solvent extracted efficiently with less than 1% residual oil. Thinner flakes tend to disintegrate during the solvent extraction process and reduce the miscella percolation rate.

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5. Cooking Prior to extraction, the flakes are heated. The purpose of cooking the flakes is to (a) break down cell walls to allow the oil to escape; (b) reduce oil viscosity; (c) control moisture content; (d) coagulate protein; (e) inactivate enzymes and kill microorganisms; and (f) fix certain phosphatides in the cake, which helps to minimize subsequent refining losses. Flakes are cooked in stack cookers to more than 87.8°C (190°F) in the upper kettle. Flakes with high phosphatide content may benefit from being cooked at slightly lower temperatures to avoid elevating refining losses. The temperature of the flakes is raised to 110–132.2°C (230–270°F) in the lower kettles. The seeds are cooked for up to 120 min. Overcooking lowers the nutritional quality of the meal and can darken both the oil and the meal. Poor-quality seeds with high levels of FFAs cannot be cooked for as long as high-quality seeds because of darkening. Darker oil requires additional refining to achieve a certain bleachable color. For soybeans, this heat treatment is often done prior to flaking. 6. Preparation of Collets with Expanders Sometimes low-shear extruders, called expanders, are used. This equipment has the capability to process both low and high oil content materials. The meats are fed to an extruder after dehulling, flaking, and cooking and are heated as they are conveyed by a screw press through the extruder barrel. The meats are under considerable shear, pressure, and temperature when they reach the exit of the extruder. The change in pressure as the material leaves the extruder causes it to expand whereupon most of the oil cells are ruptured, releasing the oil, which is rapidly reabsorbed to the porous “collets” or pellets. The expanded collets produced are then cooled and extracted with solvent. B. Oil Extraction 1. Mechanical Extraction Olive oil is still routinely obtained from olives by using a low-temperature hydraulic press process, referred to as “cold pressing.” This is done to minimize the heatrelated degradation of olive oil. Palm fruit and some cottonseed are extracted with an expeller or screw press, which is a continuous process. To achieve a higher yield of oil, often a higher heat treatment of the cottonseed flakes is carried out prior to expelling. This process can extract up to 90% of the available oil from the cottonseed kernels and leaves about 3–5% residual oil in the pressed cake. 2. Prepress Solvent Extraction In prepress solvent extraction, the oil-bearing material is first mildly pressed mechanically by means of a continuous screw press operation to reduce the oil

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by one-half to two-thirds of its original level, followed by solvent extraction to remove the remaining oil in the prepressed cake. Pressing followed by solvent extraction is more commonly used when high oil content materials (e.g., canola/ rapeseed, flaxseed, corn germ, and cottonseed) are processed. This process reduces the amount of oil to be extracted by solvent and, therefore, requires a smaller extractor and less solvent than a direct solvent extraction facility of the same throughput. 3. Direct Solvent Extraction Direct solvent extraction involves the use of a nonpolar solvent, usually commercial hexane, to dissolve the oil from oilseed flakes or collets without removing proteins and other non-oil-soluble compounds. Solvent extraction yields about 11.5% more oil than does the screw press method, and less oil remains in the meal. The cooked flakes or collets are mixed with solvent in a batch or continuous countercurrent extraction operation. The vapor pressure of hexane limits the practical operating temperature of the extraction and its contents to about 50–55°C. The resulting miscella (or full miscella) (oil-solvent mixture), usually about 18–24% of oil by weight, and the marc (solvent-laden collets or flakes) are heated to evaporate the solvent, which is collected and reused. Occasionally, overheating of the oil-solvent miscella can cause irreversible color changes in the oil. The oil is freed from the full miscella, by using a series of stills (solvent evaporators), stripping columns, and associated condensers. The hexane-free oil (i.e., crude oil) is filtered and cooled before leaving the solvent extraction plant for storage or further treatment. This is the crude oil normally traded in the commodity market. To minimize the settling problem during storage and shipping, crude soybean oil is often degummed before it is traded. Due to economic factors and product quality concerns, most of the cottonseed mills in the United States further integrate oil refining as part of the routine operation. The majority of cottonseed mills conduct miscella refining with sodium hydroxide to produce a once refined or prime bleachable summer yellow (PBSY) cottonseed oil. The primary benefit of this additional refining operation is to achieve a more consistent oil quality in terms of its color and reduced refining loss. Several U.S. cottonseed oil mills further process the cottonseed oil to finished RBD cottonseed oil (Fig. 2). In some cases, the oil laiden bleaching clay (which is a source of calcium to livestock) is added back to the meal during the deodorization/toasting step. Some operation, besides refining (R), bleaching (B) and deodorization (D), may also do winterization (W) to reduce the amount of the saturated portion of the vegetable oil to produce a finished oil referred to as RBWD or RBDW oil. For these facilities, additional regulatory requirements may be necessary. The extraction solvent used in the extractor is normally recovered from the

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miscella and solvent-laden flakes or collets and reused. The small amount of solvent loss that occurs through vents, crude oil, and desolventized flakes is unavoidable (i.e., fugitive loss). Estimated solvent loss from each of these emission points is given in Fig. 3. Management and control of emission loss during solvent extraction is an important issue for worker safety and protection of our environment.

III. TOXICITY OF EXTRACTION SOLVENTS Many halogenated and aromatic solvents have been examined in the past and are effective in extracting edible oils (1). However, these solvents have various degrees of toxicity and therefore are not likely to be used as alternates/replacements for commercial hexane for edible oil extraction. Several hydrophilic solvents also have been studied as oil extraction solvents. They are not toxic but their overall performance as an oil extraction solvent is not acceptable. This is why hydrocarbon-based solvents will likely be used for oil extraction for the foreseeable future. A. Commercial Hexane Commercial hexane has been used for decades as the solvent to extract oils from biological sources. It is a mixture of six carbon saturated compounds, with n-hexane as the predominant component. Pure n-hexane causes peripheral nerve damage in rats and humans when inhalation exposures are maintained for several months at 500 ppm in rats or 125 ppm in humans (5, 6). However, commercial hexane, which contains 52% n-hexane and a mixture of hexane isomers (see composition below), does not cause peripheral nerve damage in animals. The composition of commercial hexane tested (7) is as follows: 52% n-hexane 16% methylcyclopentane 16% 3-methylpentane 13% 2-methylpentane 3% cyclohexane This was shown by extensive animal inhalation studies, which were mandated by the U.S. Environmental Protection Agency (EPA) under Section 4 of the Toxic Substances Control Act (TSCA) (8). The test results, summarized by Galvin (7), showed that this commercial hexane blend was not a neurotoxin. In addition, the following tests also were negative: acute toxicology, subchronic neurotoxicity, mutagenicity (both in vitro and in vivo studies), oncogenicity, and development and reproductive studies (with the above-described commer-

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Figure 3

Overview of oilseed extraction operation and identification of hexane emission sources.

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cial hexane) at a vapor concentration as high as 9000 ppm for 6 h/d, 5 d/week up to 13 weeks. B. Commercial Isohexane and Hexane Isomers An alternative to commercial hexane, which would require minimal retrofit of existing extraction facilities, is commercial isohexane. This solvent, a blend of hexane isomers [2-methylpentane or isohexane, 3-methylpentane, 2,3-dimethylbutane, 2,2-dimethylbutane and n-hexane (<1%)], has not been tested as extensively as commercial hexane. However, the individual components have been tested in various toxicological assays. Based on the available information, commercial isohexane is not a neurotoxicant (7). C. Other Solvents Many other solvents have examined by various research teams as potential alternative solvents for commercial hexane for the extraction of edible oils. Some toxicity information of the most common ones is summarized in Table 1. More discussion on these solvents can be found in the summary by Wakelyn and Adair (9). Most of the solvents mentioned in this section, with the exception of acetone (9a), can undergo photochemical oxidation in the atmosphere in the presence of sunlight and nitrogen oxides (NOx) to form ozone in a greater rate than ethane (C2H6) and, therefore, are classified as volatile organic compounds (VOCs). The main component of the hydrocarbon solvents used for edible oil extraction, n-hexane, is a neurotoxin and considered a hazardous air pollutant (HAP) by EPA. Thus, n-hexane containing solvents are more stringently regulated than isohexane (see IV. Regulatory Issues).

IV. REGULATORY ISSUES Many workplace/occupational health and safety, environmental, food safety, and other regulations (see Tables 2 and 3 for summary information on U.S. laws and regulations; a list of terms/abbreviations is given in the Glossary) apply to oilseed processors (10). Most of the environmental and workplace legislation came about in the 1970s in response to increased general public awareness, concern, and desire for a cleaner environment, safer workplace, and safer food supply. This has led to an increasing number of regulations, and it is expected that there will continue to be more and stricter regulation in the future. How are regulations established? In the United States, first Congress passes a law, e.g., the 1990 amended Clean Air Act, giving the legislative authority to the regulatory agency. Next the federal regulatory agency that has the

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Table 1 Some Toxicity Information for Potential Alternate Solvents Solvent (CAS no.)

LD50 (g/kg)

Acetone (67-64-1)

5.8 in rats 20 in rabbits 2.74 in rats 13 in rabbits 12.7 in rats

2-Butanone (methylethylketone) (78-93-3) Cyclohexane (110-82-7) Cyclopentane (287-92-3) Ethyl acetate (141-78-6)

Other toxic concerns A central nervous system depressant in animals and humans Eye and skin irritation and cause narcosis Moderate irritation to eyes and mucous membrane Narcotic Irritation to the eyes, mucous membranes, respiratory tract Irritation to the eyes, mucous membranes Central nervous system depressant Irritation to the eyes, mucous membranes Irritation to the eyes, nose, throat

Ethyl alcohol (ethanol) (64-17-5)

5.6 in rats 3.0 in cats 7.06 in rats

Heptane (142-82-5) Isopropyl acetate (108-27-4)

2.22 in mice 3.0 in rats

Isopropyl alcohol (isopropanol) (67-63-0) Methyl alcohol (methanol) (6756-1) Methylcyclohexane (108-87-2) Methylene chloride (dichloromethane) (75-09-2)

5.05 in rats 12.8 in rabbits 5.628 in rats Headaches and visual impairment 15.8 in rabbits — Similar to that of heptane — Decreased visual and auditory function; headaches, dizziness, nausea, and memory loss; a B2 probable human carcinogen 9.37 in rats Irritation to the eyes, respiratory system

n-Propyl acetate (109-60-4)

legislative authority establishes standards/regulations through notice and comment rule making. As long as there is legislative authority, regulations can be established or revised when necessary. A proposed standard is published in the Federal Register announcing that the agency is undertaking a rule making (notice) and asking for comments; a hearing can be held also. After comments are received and the comment period closes, the agency prepares and publishes a final standard in the Federal Register. The effective dates of various requirements of the new standard are usually phased in under a predetermined timetable (e.g., see the regulation for vegetable oil production under Hazardous Air Pollutants). Each facility is expected to establish programs to ensure proper compliance with the various regulations and that proper corrective actions are taken where necessary. Programs can include written programs, auditing, record keeping, reporting, warning labels, worker training programs, and so forth, according

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Table 2 U.S. Environmental Laws and Regulations Law or regulation Environmental Protection Agency (EPA) (established 1970) Clean Air Act (CAA) (42 U.S. Code 7401 et seq.)

Federal Water Pollution Control Act (known as the Clean Water Act) (CWA) (33 U.S. Code 1251 et seq.)

Toxic Substances Control Act (TSCA) (15 U.S. Code 2601 et seq.)

Resource Conservation and Recovery Act (RCRA) (42 U.S. Code 6901 et seq.) Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) (42 U.S. Code 9601 et seq.)

Emergency Planning and Community Right-to-Know Act (EPCRA; also “SARA Title III”) (42 U.S. Code 1101 et seq.)

Purpose To protect human health and welfare and the environment. To protect the public health and welfare. Provides EPA with the authority to set NAAQS, to control emission from new stationary sources, and to control hazardous air pollutants. The major law protecting the “chemical, physical and biological integrity of the nation’s waters.” Allows the EPA to establish federal limits on the amounts of specific pollutants that can be released by municipal and industrial facilities. Provides a system for identifying and evaluating the environmental and health effects of new chemicals and chemicals already in comerce. A system for handling and disposal of nonhazardous and hazardous waste. Known as “Superfund,” gives the EPA power to recover costs for containment, other response actions, and cleanup of hazardous waste disposal sites and other hazardous substance releases. (Part of Superfund) Provides authority for communities to devise plans for preventing and responding to chemical spills and release into the environment; requires public notification of the types of hazardous substances handled or release by facilities; requires state and local emergency plans.

to the particular regulations that pertain to that industry. Regulations are enforced by the appropriate federal or state agency. The regulations discussed here are required in the United States. Many other countries have similar requirements, but if they do not, it would be prudent for oilseed solvent extraction operations to consider meeting these regulations and for these industries to have environmental, health and safety, and quality management programs (11, 12).

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Table 3 U.S. Worker Health and Safety Laws and Regulations Laws: Occupational Safety and Health Act of 1970 (OSH Act) (PL 91-596 as amended by PL 101 552; 29 U.S. Code 651 et. seq.) OSHA Health Standards: Air Contaminants Rule, 29 CFR 1910.1000 Hazard Communication Standard, 29 CFR 1910.1200 Occupatioanl Exposure to Hazardous Chemicals in Laboratories, 29 CFR 1910.1450 Bloodborne Pathogens, 29 CFR 1910.1030 OSHA Safety Standards: Process Safety Management, 29 CFR 1910.119 Emergency Action Plan, 29 CFR 1910.38(a)(1) Fire Prevention Plan, 29 CFR 1910.38(b)(1) Fire Brigades, 29 CFR 1910.156 Permit-Required Confined Space, 29 CFR 1910.146 Lockout-Tagout, 29 CFR 1910.147 Powered Industrial Truck Operator Training, 29 CFR 1910.178 Occupational Noise Exposure, 29 CFR 1910.95 and Hearing Conservation Program, 29 CFR 1910.95(c) Personal Protection Equipment: General Requirements, 29 CFR 1910.132 Eye and Face Protection, 29 CFR 1910.133 Respiratory Protection, 29 CFR 1910.134 Head Protection, 29 CFR 1910.135 Foot Protection, 29, CFR 1910.136

A. Environmental Protection The purpose of the EPA, which administers all regulations affecting the environment and chemicals in commerce, is to protect human health and welfare and the environment. The individual states and state environmental regulatory control boards implement and enforce most of the regulations. The legislation that serves as the basis for the regulations can be divided into: 1. Statutes that are media specific [Clean Air Act (CAA) and Clean Water Act (CWA)]; 2. Statutes that manage solid and hazardous waste [Resources Conservation and Recovery Act (RCRA) and Comprehensive Environmental Response, Compensation and Liability Act (CERCLA)]; and, 3. Statutes that directly limit the production rather than the release of

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chemical substance [Toxic Substances Control Act (TSCA) and Federal Insecticide, Fungicide and Rodenicide Act (FIFRA)]. The reader is referred to Table 2 for a summary of the information on environmental laws and regulations, Table 4 for an overview of environmental requirements for air and water, Table 5 for a summary of air threshold emissions, and Table 6 for an overview of environmental requirements for waste. 1. Clean Air Act (CAA; 42 U.S. Code 7401 et seq.): The purpose of the CAA is to protect the public health and welfare. To satisfy the CAA requirements, states and state air control boards are required to implement regulations and develop state implementation plans (SIPs) (13, 14). Criteria pollutants [e.g., ozone (O3), particulate matter (PM), nitrogen oxides (NOx), sulfur oxides (SOx), carbon monoxide (CO), and lead (Pb)] are regulated with

Table 4 U.S. Environmental Regulations, Air and Water Chemical name (CAS no) n-Hexane (110-54-3) Commercial hexane (none) n-Heptane (148-82-5) Cyclohexane (110-82-7) Cyclopentane (287-92-3) Hexane isomers (none) Commercial isohexane (none) 2-Methyl pentane (isohexane) (107-83-5) (a hexane isomer) 3-Methyl pentane (96-14-0) (a hexane isomer) Methyl cyclopentane (96-37-7) (a hexane isomer) 2,2-Dimethyl butane (neohexane) (75-83-2) (a hexane isomer) 2,3-dimethyl butane (79-29-8) (a hexane isomer) Methyl cyclohexane (108-87-2) Isopropyl alcohol (2-propanol) (67-17-5) Ethyl alcohol (ethanol) (64-17-5) Acetone misc. (67-64-1) a

VOC

HAP

CWAa

Yes Yes Yes Yes Yes Yes (Same as hexane isomers) Yes

Yes

Yes

No No No No No

Yes Yes Yes Yes Yes

No

Yes

Yes Yes Yes

No No No

Yes Yes Yes

Yes Yes Yes

No No No No No

Yes Yes Yes Yes Yes

Nob

Under the Clean Water Act there could be storm water and NPDES permit requirements; none of the solvents are listed as priority toxic pollutants in 40 CFR 401.15. b Acetone is considered by the U.S. EPA not to be a VOC (60 FR 31643; June 16, 1995). CAS no., Chemical Abstracts Service Registry number; VOC, volatile organic chemical; HAP, hazardous air pollutant; CWA, Clean Water Act.

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Table 5 Summary of Threshold Emission Levels Regulation

Threshold emission level

Major source (40 CFR 70) 100 t criteria pollutant (or lessb ) or 10/25 t HAP (per year) Hazardous Air Pollutants 10 t of one HAP/25 t total (HAP) HAP (per year) 10–100 t (depending on deNAAQS: 03 (VOC)c gree of severity of nonattainment) PM 70 t nonattainment; 100 t attainment 250 t NSR: PSDd

Nonattainment NSR

100 t/a

Toxic Release Inventory (TRI) 112(r)e

10,000–25,000 lb/a

PSMf

Requirementa Title V permit (federal operating permit) NESHAP/MACT standard RACT/BACT standard

RACM/BACM standard Preconstruction/building permit, BACT; much paperwork Preconstruction permit, LAER, emission offsets Annual reporting (Form R)

Hexane/hexane isomers not covered >10,000 lb in one tank (all Written plan, controls, trainflammable solvents including, etc. ing hexane/hexane isomers are covered)

a

For NESHAP, MACT, RACM, BACM, RACT, BACT, and LAER, see Glossary. For example, the threshold can range from 10 to 100 t depending on degree of severity of ozone (O3) nonattainment. c Ground level ozone (O3) is not emitted directly into the air but is formed when sunlight acts on emissions of nitrogen oxides (NOx) and volatile organic compounds (VOC) like hexane. d PSD = prevention of significant deterioration, a requirement of New Source Review (NSR) in attainment areas. e section 112(r) of the Clean Air Act (40 CFR 68) is for prevention of chemical accidents. f PSM = process safety management (29 CFR 1910.119); OSHA standard to prevent or minimize the consequences of catastrophic releases. b

National Ambient Air Quality Standards (NAAQS) and hazardous air pollutants (HAPs), such as n-hexane, with National Emissions Standards for Hazardous Air Pollutants (NESHAP). The 1990 CAA expanded the list of HAPs to 188, including n-hexane, and more strictly regulates nonattainment areas for criteria pollutants such as O3, PM, CO and NOx.

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Table 6 U.S. Environmental Regulations, Solid Waste (EPCRA/SARA Title III)a (RCRA) Chemical name (CAS no.)

RCRA Codeb

n-hexane (110-54-3) Commercial hexane (none) n-heptane (142-82-5) Cyclohexane (110-87-7) U056 Cyclopentane (287-92-3) Hexane isomers (none) Commerical isohexane (none) 2-Methyl pentane (isohexane) (107-83-5) 3-Methyl pentane (96-14-0) Methyl cyclopentane (96-37-7) 2,2-Dimethyl butane (neohexane) (75-83-2) 2,3-Dimethyl butane (79-29-8) Methyl cyclohexane (108-87-2) Isopropyl alcohol (2-propanol) (67-63-0) Ethyl alcohol (64-17-5) Acetone (67-64-1) U002 a

Sec. 304 CERCLA Sec.313 (RQ) Sec.311/312 (TRI) 5000c

1000

5000

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

Yes Yesd No Yes No Noe Noe Noe Noe Noe Noe Noe No Nof No No

From Title III Lists of Lists, U.S. EPA, EPA 740-R-95-001 (April 1995); 40 CFR 52-99; (59 FR 4478; January 31, 1994) hexane added to TRI list; (60 FR 31633; June 16, 1995) acetone removed from TRI list. b 40 CFR 261.33, listed hazardous waste—EPA RCRA Hazardous Waste Number. All the solvents that are on the RCRA list are listed because of Section 3001 of RCRA (part for identification and listing of hazardous waste) except hexane which is on because of CAA Section 112 (HAP). c RQ for hexane finalized June 12, 1995 (60 FR 30939) d Only the amount of commercial hexane that is n-hexane has to be reported (e.g., if the commercial hexane is 62% n-hexane, only 62% of the emissions have to be reported for TRI). e The EPA clarified that the listing for hexane was only for n-hexane, other isomers of hexane are not included. (59 FR 61457; Nov. 30, 1994). f The EPA has indicated (62 FR 22318; April 25, 1997) that IPA itself does not meet the criteria for listing on the TRI list. The EPA will remove IPA from the TRI list. RQ, reportable quantity in pounds.

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Hazardous Air Pollutants (HAPs) or Air Toxics (40 CFR 61). If a facility is a major emitter (i.e. a major source or significant area source) of any of the chemicals on the CAA list of HAPs (presently 188), EPA requires sources to meet national emissions standards (13, 15, 16). n-Hexane is on the HAP list but isohexane, acetone, and other solvents listed in Table 4 are not. The air toxic requirements of the CAA for establishing control measures for source categories are technology-based emission standards (not health based) established for major sources (10 t/a of one HAP or 25 t/a of total HAPs per facility) that require the maximal degree of reduction emissions, taking costs, other health and environmental impacts, and energy requirements into account. Standards are set based on known or anticipated effects of pollutants on the public health and the environment, the quantity emitted, and the location of emissions. Compliance with a NESHAP involves the installation of maximum achievable control technology (MACT)—MACT essentially is maximal achievable emission reduction. For new sources, MACT standards must be no less stringent than the emission control achieved in practice by the best-controlled similar source. On April 12, 2001 (66 FR 19006) EPA published the NESHAP for solvent extraction (40 CFR 63) for vegetable oil production. On April 5, 2002 (66 FR 16317) U.S. EPA amended this NESHAP. The amendments clarify startup, shutdown, and malfunction (SSM) requirements for owners or operators subect to the NESHAP rules applicable to vegetable oil production facilities and also clarifies the applicability of NESHAP General Provisions (40 CFR63). EPA considers solvent extraction for vegetable oil production processes as major sources of the HAP n-hexane. Figure 3 shows a general flow diagram of a typical vegetable oil production facility, identifying the most common emission sources of hexane (17). Hexane emissions occur from ten general sources: (a) the main vent; (b) meal dryer vent; (c) meal cooler vent; (d) crude meal; (e) crude oil; (f) equipment leaks; (g) solvent storage tanks; (h) process wastewater collection; (i) facility startup/shutdowns; and (j) operational upsets. Facilities covered are those that produce crude vegetable oil and meal products by removing crude oil from listed oilseeds (corn germ, cottonseed, flax, peanuts, rapeseed, safflower, soybeans, and sunflower) through direct contact with solvent. The rule requires all existing and new solvent extraction processes that are major sources (potential to emit 10 t/a or more of n-hexane) to meet these HAP emission standards as a 12-month rolling average based on a 64% n-hexane content. HAP emission standards (solvent lose factors) vary for each oilseed (Table 7) and reflect the application of MACT. Industry will have 3 years to achieve compliance (i.e., Apr 12, 2004). Since the emission loss factor values are 12-month rolling averages, the first compliance report would be due 48 months after the standard is promulgated (i.e., Apr 12, 2005). The requirements cover normal operations and SSM, which was further clarified by EPA (4/5/2002; 67 FR

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Table 7 U.S. Oilseed Solvent Loss Factors for Allowable HAP Loss (12-Mo. rolling ave.) Oilseed solvent loss factor (gal/t) Type of oilseed process Corn germ, wet milling

Corn germ, dry milling

Cottonseed, large

Cottonseed, small

Flax Peanuts Rapeseed Safflower Soybean, conventional Soybean, specialty

Soybean, small combination plant

Sunflower

A source that . . . processes corn germ that has been separated from other corn components using a wet process of centrifuging a slurry steeped in a dilute sulfurous acid solution. processes corn germ that has been separated from other corn components using a dry process of mechanical chafing and air sifting. processes 120,000 t or more of a combination of cottonseed and other listed oilseeds during all normal operating periods in a 12-month operating period. processes less than 120,000 t of a combination of cottonseed and other listed oilseeds during all normal operating periods in a 12-month operating period. processes flax. processes peanuts. processes rapeseed (e.g., canola). processes safflower. uses a conventional style desolventizer to produce crude soybean oil products and soybean animal feed products. uses a special style desolventizer to produce soybean meal products for human and animal consumption. processes soybeans in both specialty and conventional desolventizers and the quantity of soybeans processed in specialty desolventizers during normal operating periods is less than 3.3% of total soybeans processed during all normal operating periods in a 12-month operating period. The corresponding solvent loss factor is an overall value and applies to the total quantity of soybeans processed. processes sunflower.

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Existing sources

New sources

0.4

0.3

0.7

0.7

0.5

0.4

0.7

0.4

0.6 1.2 0.7 0.7 0.2

0.6 0.7 0.3 0.7 0.2

1.7

1.5

0.25

0.25

0.4

0.3

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16317). A plant processing an oil that is not a listed oil, i.e., not a regulated entity (40 CFR 63.2872; e.g., rice bran oil plant or meadfoam oil processing plant, etc.), and that is a major source, is not covered by the MACT standard but the state is likely to require it to meet a particular level as part of its Title V permit. There are also variable emission requirements depending on the oilseed for allowable emissions for vegetable oil processing in Europe (Table 8) (18). The EEC directive for Europe is to be fully implemented by all member states by Oct. 30, 2004 for new plants and Oct. 30, 2007 for old plants. However, there are some intermediate targets before full compliance—the target solvent consumption is 1.5 times the final level starting Oct. 30, 2001 for new plants and starting Oct. 30, 2005 for old plants. The EEC counts the total input of solvent into a plant per calendar year or any 12-month period and also requires a solvent management plan, which contains the following and is to be updated yearly: (a) verification of compliance to the EEC regulation through a mass balance; (b) identification of future reduction options; and (c) development of a waste minimization plan. In the United States a health-based standard would be for a boundary line level of a solvent (e.g., n-hexane) based on the inhalation reference concentration (RfC) (19). The current RfC for n-hexane is 200 µg/m3. Recent research suggests that the RfC for n-hexane should be at least 10 times higher (>2000 µg/m3).

Table 8 European Maximal Solvent Loss Factors (EEC Directive for Vegetable Oil Extraction) (18) Max VOC usage/t of seed/yr Oilseed

kg/t

gal/t

Olives Castor Rapeseed Sunflower Soybeans Soy flash Other seeds Oil refining and fractionation: Fractionation w/o degumming Degumming plant

2.5 3.0 1.0 1.0 0.8 1.2 3.0

1.0 1.2 0.4 0.4 0.3 0.5 1.2

1.5 4.0

0.6 1.6

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NAAQS (40 CFR 50). The NAAQS are set at levels sufficient to protect public health, including the health of sensitive populations (primary air quality standards) and public welfare (secondary air quality standards; “welfare effects” include protection against decreased visibility, damage to wildlife, crops, vegetation, and buildings, and effects on personal comfort and well-being) from any known or anticipated adverse effect of the pollutant with an adequate (appropriate) margin of safety. VOCs are essentially considered the same as the criteria pollutant ozone (14–16). Ground level ozone is not emitted directly into the air but is formed when sunlight acts on emissions of NOx and VOCs. VOCs are very broadly defined by the EPA (40 CFR 51.100): any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, that participates in atmospheric photochemical reactions. This includes any organic compound other than those specifically listed as having been determined to have negligible photochemical reactivity. Reactive VOCs are essentially all those judged to be clearly more reactive than ethane— the most reactive member of the “negligibly reactive” class. C4 —C6 paraffins are of relatively low kinetic reactivity but produce NO2 and potentially ozone (20). n-Hexane, hexane isomers, and the other solvents discussed, except acetone, would be considered VOCs (Table 4) that can undergo photochemical oxidation in the atmosphere to form ozone. In the United States, acetone was added to the list of compounds excluded from the definition of VOCs in 1995 because it was determined to have negligible photochemical reactivity (11). Most U.S. vegetable oil extracting facilities would be major sources of VOCs and would be covered by the requirements for ozone emissions and attainment, unless they used a solvent that was not classified as a VOC. The definition of “major source” changes as the severity of the ozone nonattainment area increases. Plants in marginal and moderate areas are major if they emit 100 t VOC/a; in serious areas, 50 t/a; in severe areas, 25 t/a; and in extreme areas, 10 t/a. All facilities in ozone nonattainment areas could be required to reduce emissions through implementing reasonable available control technology (RACT) or best available control technology (BACT) standards. Particulate matter (PM) is the solid or liquid matter suspended in the atmosphere. Most vegetable oil production facilities are major sources of PM. Depending on the oilseed processed, PM emissions can be 0.045–0.136 kg of total suspended particulate (TSP) per ton of seed processed. This PM is about 50% PM10 (particulate 10 µm and less) and less than 3% PM2.5 (particulate 2.5 µm or less), assuming that PM emissions from oilseed handling operations are similar in particle size distribution to other agricultural operations, such as cotton gins (21). PM controls (Table 5) would also have to be part of a facility’s federal and state permits. Vegetable oil production facilities probably also have to include NOx, SOx, and CO emissions in their federal and state permits.

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New Source Review. Any new or significantly modified facility would have to comply with the new source review (NSR) requirements. NSR is a preconstruction-permitting program. If new construction or making a major modification will increase emissions by an amount large enough to trigger NSR requirements (Table 5), then the source must obtain a permit before it can begin construction. Permits for sources in attainment areas are prevention of significant deterioration (PSD) permits and those in nonattainment areas are nonattainment NSR permits. For a PSD permit a source must apply BACT and for nonattainment NSR lowest achievable emission rate (LAER) is required. Odor. There are no specific federal regulations for odor. However, states can regulate odor if they choose to. For example, Colorado requires hog lagoons to be covered because of a state referendum vote. Also, odor can generate complaints that cause states to require more stringent emission controls. Federal Permits (40 CFR 70). All major sources of regulated solvents are required to have federally enforceable operating permits (FOPs) (13, 14) (also referred to as Title V permits). State Permits. Most states require state permits for facilities that emit listed air pollutants (13, 14). In some states federal permits and state permits are combined, whereas in other states facilities are required to have both a state and a county (air district) permit and a federal permit. As part of annual emission inventory reporting requirements, many states already require reporting of HAPs and VOCs because of their state implementation plan (SIP). 2. Clean Water Act (CWA; 33 U.S. Code 1251 et seq.) The CWA is the major law protecting the “chemical, physical and biological integrity of the nation’s waters.” Under it, the EPA establishes water quality criteria used to develop water quality standards, technology-based effluent limitation guidelines, and pretreatment standards and has established a national permit program [National Pollution Discharge Elimination System (NPDES) permits; 40 CFR 122] to regulate the discharge of pollutants. The states have responsibility to develop water quality management programs. Oilseed processing and oil refining are covered by: (a) basic discharge effluent limitations (40 CFR 122); (b) storm water regulations (40 CFR 122 and 123); and (c) oil spill prevention and response plans (40 CFR 112) (14). Basic Discharge and Stormwater. Vegetable oil extracting facilities and oil refining are covered by basic discharge effluent limitations [direct discharges to receiving waters or indirect discharges to publicly owned treatment works (POTWs)], and stormwater regulations (14). The amount of solvent in effluent discharges and in stormwater (for those covered) needs to be determined and

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possibly monitored as part of an NPDES permit and as part of the visual examination or testing of stormwater quality. None of the solvents normally used in oilseed extraction and refining are listed as priority toxic pollutants (40 CFR 401.15). Oil and Hazardous Substances Spills and Response Plans. Under Oil Pollution Prevention and Response (40 CFR 112) there are requirements for oilseed extraction and oil refining for storage and transportation of vegetable oil. In 2002 EPA amended this rule (67 FR 47042; July 17, 2002). This rule includes requirements for Spill Prevention Control and Countermeasure (SPCC) Plans (i.e., prevention plans) and for Facility Response Plans (FRPs). SPCC plans are required for on site storage and are intended to prevent spills of oil (of any kind; this includes animal fats and vegetable oils) by non-transportationrelated on-shore and off-shore facilities into the waters of the United States or adjoining shorelines. The requirements for preparation and implementation of SPCC plans state that if a facility discharges or could reasonably be expected to discharge oil in harmful quantities [a discharge of oil that can cause a sheen (indecent appearance) on the surface of water; 40 CFR 110.3] into navigable waters of the United States or adjoining shoreline, an SPCC plan is required. In the 2002 amended rule facilities with total above-ground storage capacity greater than 5000 L (1320 gal) are covered but underground storage tanks (UST) are exempt if regulated under the federal UST rule (40 CFR 280). Also, under the final rule as published, SPCC Plans must be in compliance by Feb. 17, 2003 and implementation of the amended SPCC Plan must be completed by Aug. 18, 2003. Some of the amendments are controversial and EPA is planning to suspend this deadline at least by one year. Spill reporting is required for any spill ≥3788 L (1000 gal) and for any two spills in any consecutive 12-month period of 159 L (42 gal) or greater. Pursuant to the Oil Pollution Act of 1990 (OPA-90), EPA amended the oil pollution prevention regulations by adding response plan requirements for nontransportation-related on-shore facilities that handle, store, or transport oil. Facilities that could cause substantial harm to the environment are required to prepare and submit response plans to EPA. The “Flowchart of Criteria for Substantial Harm” (14) was published in the Federal Register on July 1, 1994 (59 FR 34104); if the facility transfers oil over water to or from vessels and has a total oil storage capacity greater than or equal to 159,000 L (42,000 gal) or the facility has a total storage capacity greater than or equal to 3.8 × 106 L (1 million gal), a response plan is required. EPA further amended this rule (65 FR 40776; June 30, 2000) to provide guidance for handling, storing, or transporting of vegetable oils and animal fats. Under OPA-90, comprehensive oil spill response plans are also required for transportation of nonpetroleum oils (i.e., vegetable oil) by rail and road in

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amounts of 159,000 L (42,000 gal) or more [regulated by Research and Special Programs Administration (RSPA), Department of Transportation (DOT); Final Rule, 61 FR 30533; June 17, 1996] and for marine transportation–related facilities [regulated by the DOT, Coast Guard; Final Rule, 65 FR 40820; June 30, 2000]. 3. Resource Conservation and Recovery Act (RCRA; 42 U.S. Code 6901 et seq.) RCRA gives EPA authority to regulate the handling and disposal of hazardous and nonhazardous waste. RCRA subtitle C (40 CFR 261) is a federal “cradleto-grave” system to manage hazardous waste (including provisions for cleaning up releases and setting statutory and regulatory requirements). Subtitle D covers nonhazardous wastes. Materials or items are hazardous wastes if and when they are discarded or intended to be discarded. The act requires generators, transporters, and disposers to maintain written records of waste transfers, and requires EPA to establish standards, procedures, and permit requirements for disposal. The act also requires states to have solid waste management plans, prohibits open dumping, and requires EPA to establish criteria for sanitary landfills. EPA under RCRA also regulates underground storage tanks that store or have stored petroleum or hazardous substances. Hazardous wastes are either listed wastes (40 CFR 261.30-33) or characteristic wastes (40 CFR 261.21-24). The EPA defines four characteristics for hazardous waste: ignitability (40 CFR 260.21); corrosivity (40 CFR 260.22); reactivity (40 CFR 260.23); and toxicity (40 CFR 260.24). Any waste that exhibits one or more of these characteristics is classified as hazardous under RCRA. The ignitability definition includes a liquid that has a flash point of less than 60°C (140°F); the EPA included ignitability to identify wastes that could cause fires during transport, storage, or disposal (e.g., used solvents). Since all of the solvents in Table 6 have flash points less than 60°C, all could be an RCRA ignitability waste. Spent bleaching clay is not an RCRA hazardous waste (40 CFR 302). It is usually disposed of by taking it to a regular landfill. Sometimes a spontaneous combustion (oxidation of unsaturated fatty acids in the retained oil causing selfheating leading to combustion) may occur when it is taken to the landfill. The potential for spontaneous combustion in spent bleaching earth depends on the type and amount of oil retained and rises with increasing unsaturation of the fatty acids in the retained oil. U.S. DOT classifies materials liable to spontaneous combustion as Class 4.2 hazardous materials [49 CFR 173.124(b) and Appendix E3]. Spent bleaching clay can be finely ground and put in small quantities into the animal meal in operations that do oil extraction. Zschau (22) describes other ways to utilize spent bleaching clay, including environmentally friendly ways.

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4. Comprehensive Environmental Response, Compensation and Liability Act (CERCLA, “Superfund”; 42 U.S. Code 9601 et seq.) CERCLA “Superfund” gives EPA the authority to force those responsible for hazardous waste sites or other releases of hazardous substances, pollutants, and contaminants to conduct cleanup or other effective response actions. Section 103 of CERCLA requires the person in charge of a facility to immediately report any release of a hazardous substance in an amount equal to or greater than its reportable quantities (RQ) to the National Response Center (NRC) (see also discussion below on EPCRA Section 304). n-Hexane, acetone, and other chemicals in Table 6 as well as NOx (RQ = 4.5 kg/24 h [10 lb/24 h]), hydrogen sulfide, and ammonia (both 45 kg/24 h [100 lb/24 h]) are CERCLA hazardous substances and have a CERCLA RQ for releases. However, CERCLA and EPCRA do not require notification to the NRC, the State Emergency Response Commissions (SERCs), and the Local Emergency Planning Committees (LEPCs) of “federally permitted releases” of hazardous air releases/emissions as defined in CERCLA Section 101(10)(H): any emission into the air subject to a permit or control regulation under Section 111, Section 112, Title I Part C, Title I Part D, or state implementation plans in accordance with Section 110 of the CAA. In 2002 EPA published final guidance clarifying the CERCLA Section 101(10)(H) federally permitted release definition for “Certain Air Emissions” (67 FR 18899; April 17, 2002) and for “Clean Air Act ‘Grandfathered’ Sources” (67 FR 19750; April 23, 2002). Essentially if the CAA requirements are being met, even without a permit, it is a federally permitted release exempt from reporting. 5. Emergency Planning and Community Right-to-Know Act (EPCRA; 42 U.S. Code 11001 et seq.) Enacted as Title III of the 1986 Superfund Amendments and Reauthorization Act (“SARA”), the Act mandates EPA to monitor and protect communities regarding release of chemicals into the environment. It requires states to establish emergency planning districts with local committees to devise plans for preventing and responding to chemical spills and releases. Section 304 (40 CFR 355.40). Section 304 of EPCRA requires the owner or operator of a facility to immediately notify the State SERCs and LEPCs of any accidental releases (that are not “federal permitted”), in quantities equal to or greater than their RQ, of an EPCRA designated extremely hazardous substance (EHS) or a CERCLA hazardous substance (40 CFR 302, Table 302.4) and provide written follow-up notice as soon as practicable thereafter (also see discussion on CERCLA Section 103). n-Hexane, cyclohexane, acetone, and

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some of the other solvents discussed are CERCLA hazardous substances and have CERCLA RQ for air releases (Table 5). Section 311, 312 (40 CFR 370.20-.21). Businesses must make MSDSs, for chemicals that are required to have an MSDS, available to state and local officials. Since all of the solvents discussed require MSDSs under the OSHA HCS, all are covered by these requirements. Section 313 (40 CFR 372), Toxic Release Inventory (TRI). Businesses are required to file annual reports with federal and state authorities of releases to air, water, and land above a certain threshold for chemicals on the TRI/ Section 313 list (40 CFR 372.65) by July 1 each year for the previous year’s releases (23). TRI requirements are triggered if a facility is involved in manufacturing with 10 or more full-time employees, manufactures, processes, or otherwise uses one or more listed substance(s) in a quantity above the statutory reporting threshold of 11 t/a (25,000 lb/yr) (manufactured or processed) or 4.5 t/a (10,000 lb/yr) (otherwise used). Beginning with the 1991 reporting year, such facilities also must report pollution prevention and recycling data for such chemicals pursuant to Section 6607 of the Pollution Prevention Act (42 U.S. Code 13106). n-Hexane was added to the TRI list in 1994 with reporting for 1995 emissions (11, 23). The other solvents discussed are not on the TRI list. The EPA can add new chemicals to or delete chemicals from the TRI list as is deemed necessary and any person may petition the EPA to add chemicals or delete chemicals from the list. Acetone (11) in 1995 (60 FR 31643; June 16, 1995) and phosphoric acid in 2000 (65 FR 39552; June 27, 2000) were deleted and no longer require reporting under TRI. 6. Toxic Substances Control Act (TSCA; 15 U.S. Code 2600 et seq.) If a chemical’s manufacture, processing, distribution, use, or disposal would create unreasonable risks, the EPA, under the TSCA (40 CFR section 700, et seq.), can regulate it, ban it, or require additional testing. Section 4(a). Under Section 4(a) of TSCA, the EPA can require testing (referred to as a “Section 4 test rule”) of a chemical substance or mixture to develop data relevant for assessing the risks to health and the environment. Section 5(a)(1). Section 5(a)(1) of TSCA (40 CFR 720) mandates the EPA to monitor and control the use of toxic substances by requiring the Agency to review the health and environmental effects of new chemicals (referred to as “Premanufacturing Notice” or “PMN”) and chemicals already in commerce. The EPA also has Significant New Use Rules (SNURs) under Section 5(a)(2) of

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TSCA (40 CFR 721), which provides a way for EPA to restrict use of a chemical substance already in commerce that is proposed for new uses. n-Hexane, hexane isomers, acetone, and the other solvents discussed are already commercially available, so a PMN would not apply. However, some solvents other than n-hexane could be subject to an SNUR (40 CFR 721, subpart A), since they are not presently being used as extraction solvents in large quantities. Section 8. Reporting and Retention of Information: Section 8(d) of TSCA (Health and Safety Data Reporting; 40 CFR 716) requires that lists of health and safety studies conducted or initiated with respect to a substance or mixture be submitted to the EPA. Section 8(e) of TSCA (no 40 CFR Ref.) requires that all new toxicological data of the effects of a chemical not previously mentioned must be reported immediately, if the data reasonably support the conclusion that such substance or mixture presents a substantial risk of injury to health or the environment. Testing (Section 4 test rule) was required for several of the solvents earlier [e.g., commercial hexane for which new toxicological information was reported to the EPA since 1992 (24)], and any new toxicological information will have to be reported to the EPA under Section 8(e) and 8(d). Inventory Update Rule (IUR) (40 CFR 710). The IUR was established in 1986 to require manufacturers and importers of chemicals listed on the master TCSA inventory to report current data every four years on the production volume of chemicals imported or produced. Food and feed products produced from natural agricultural product, such as oilseeds, are not required to be reported, but all oil and meal products obtained by solvent extraction that is sold for other than food or feed use (e.g., oils as chemical raw materials and meal as fertilizer) are. This list from 1990 was used to determine the high production volume (HPV) chemicals (greater than 453 t/a [1 million lb/yr]) that are part of the HPV testing program. Vegetable oils are listed as a category 1 that does not require toxicity testing at this time. B. Workplace/Occupational Safety and Health In the United States, workplace regulations (Table 3) are promulgated and enforced by the Occupational Safety and Health Administration (OSHA), which is part of the U.S. Department of Labor. The purpose of OSHA is to ensure that the employers maintain a safe and healthful workplace. OSHA general industry standards (29 CFR 1910) apply to oilseed extraction and oil refining, and several of these workplace standards that particularly apply to oilseed extraction facilities are discussed. Many other health and safety standards (e.g., blood-borne pathogens, noise, operation of fork-lift trucks, and lockout/tagout) apply that cover all industries (25). Also even if there is not a specific standard, OSHA can cite a

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facility under the “general duty clause” [Sec. 5(a)(1) of the OSH Act], since the OSH Act requires the employer to maintain a safe and healthful workplace. 1. Air Contaminants Standard (29 CFR 1910.1000) Air contaminant standards are intended to reduce risk of occupational illness for workers by reducing permissible exposure limits (PELs) for chemicals. Table 9 lists the PELs for n-hexane, hexane isomers, and some other solvents and chemicals. PELs are 8-h time-weighted average exposures. To achieve compliance with PEL, administrative or engineering controls must first be determined and implemented, whenever feasible. When such controls are not feasible to achieve full compliance, personal protective equipment, work practices, or any other protective measures are to be used to keep employee exposure below the PEL. In the case of a mixture of contaminants, an employer has to compute the equivalent exposure when the components in the mixture pose a toxic effect on the same target organ to a worker’s health (26, 27). The mixture calculation is expressed as: Em = (C1/L 1) + (C2/L 2) + . . . + (Cn/L– n) where:

(1)

Em is equivalent exposure for the mixture (Em should be ≤1 for compliance), C is concentration of a particular substance, and L is PEL (the exposure limit for that substance specified in 29 CFR 1910). 2. Hazard Communication Standard (HCS) (29 CFR 1910.1200) The HCS requires all employers to provide information to their employees on the hazardous chemicals to which they are exposed through a written hazard communication program, labels and other forms of warning, material safety data sheet (MSDS), training programs, and record keeping. A substance is a “hazardous chemical” if it is a “physical hazard” or a “health hazard” [29 CFR 1910.1200(c)]. A flammable or explosive liquid is a “physical hazard.” A flammable liquid means “any liquid having a flashpoint below 37.8°C (110°F), except any mixture having components with flashpoints of 37.8°C (110°F) or higher, the total of which make up 99% or more of the total volume of the mixture.” “Health hazard” means “a chemical for which there is statistically significant evidence based on at least one valid study that acute or chronic health effects may occur in exposed employees.” Hexane requires an MSDS, since all flammable liquids (physical hazard), as defined by OSHA and/or possible health hazards that have an U.S. OSHA PEL, require an MSDS. n-Hexane isomers (e.g., isohexane) do not have an OSHA PEL but do have an American Conference of Governmental Industrial Hygienists (ACGIH)

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Table 9 U.S. Workplace Regulations,a Air Contaminants Chemical name (CAS no.) n-Hexane (110-54-3) Commercial hexanea (none) nHeptane (148-82-5)

Cyclohexane (110-82-7) Cyclopentane (287-92-3) Hexane isomers Commercial isohexanea (none) 2-Methyl pentane (2-MP) (isohexane) (107-83-5) 3-Methyl pentane (3-MP) (96-14-0) Methyl cyclopentane (MCP) (96-37-7) 2,2 Dimethyl butane (2,2-DMB) (neohexane) (75-83-2) 2,3 Dimethyl butane (2,3-DMB) (7929-8) Methyl cyclohexane (107-87-2) Isopropyl alcohol (IPA) (2-propanol) (67-17-5) Ethyl alcohol (ethanol) (64-17-5) Acetone (67-64-1) Particulate not otherwise regulated (PNOR): Total dust Respirable dust Phosphoric acid (7664-36-2) Sodium hydroxide (1310-73-2) Sulfuric acid (7664-93-9)

Permissible exposure limit (PEL) [Health risk: basis for the PEL] 500 ppm-1800 mg/m3; new PEL was 50 ppm–180 mg/m3 same as ACGIH (TLV); [neuropathy] (Same as n-hexane) 500 ppm–2050 mg/m3; new PEL was 400 ppm–1640 mg/m3, (500 ppm STEL) same as ACGIH (TLV); [narcosis] 300 ppm–1050 mg/m3; ACGH (TLV) 300 ppm–1030 mg/m3; [sensory irritation] None; new PEL was 600 ppm, same as ACGIH (TLV); [narcosis] None; new PEL was 500 ppm–1760 mg/m3 (1000 ppm STEL) same as ACGIH (TLV); [narcosis] (Same as hexane isomer) (Same as hexane isomer) (Same as hexane isomer) (Same as hexane isomer) (Same as hexane isomer) (Same as hexane isomer) 500 ppm; new PEL was 400 ppm/1610 mg/m3, same as ACGIH (TLV); [narcosis] 400 ppm–980 mg/m3; ACGIH (TLV) same plus 500 ppm–1230 mg/m3 STEL; [sensory irritation] 1000 ppm–1880 mg/m3; ACGIH (TLV) same; [narcosis, irritation] 1000 ppm/2400 mg/m3; ACGIH (TLV) 750 ppm (1000 ppm STEL); [sensory irritation]

15 mg/m3 5 mg/m3; [physical irritation] 1 mg/m3; [sensory irritation] 2 mg/m3; [sensory irritation] 1 mg/m3; [sensory irritation]

a

CAS No. is the Chemical Abstracts Service Registry Number; PEL is from 29 CFR 1910.1000, Table Z-1; American Conference of Governmental Industrial Hygienists (ACGIH), threshold limit value (TLV); under the HCS, a MSDS is required for all of the compounds (physical and/or chemcial hazard); all of the solvents are flammable liquids or gasses, under the OSHA definition, and are regulated under the PSM standard. b Commercial hexane as used in the U.S. is usually about 64% n-hexane, and the rest is hexane isomers [e.g., methyl cyclopentane (MCP), 2-methyl pentane (2-MP), and 3-methyl pentane (3MP)], and it contains less than 10 ppm benzene. c Mixture of 2-MP (45–50%), 3-MP, 2,2-DMB, and 2,3-DMB (3).

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threshold limit value (TLV) (28) of 500 ppm (Table 9), which many states and countries enforce as a mandatory standard. Chemical manufacturers and importers are required to review the available scientific evidence concerning the hazards of chemicals they produce or import, and to report the information to manufacturing employers who use their products [29 CFR 1910.1200(b)]. If a chemical mixture has not been tested as a whole to determine whether it is a hazardous chemical, the mixture is assumed to present the same hazards as the components that comprise 1% or more of the mixture or a carcinogenic hazard if it contains a component in concentration of 0.1% or more that is a carcinogen [29 CFR 1910.1200(a)(5)]. Commercial hexane, containing 52% n-hexane, has been tested and found not to be neurotoxic, unlike pure n-hexane (7, 29, 30). So mixtures with less than 52% n-hexane should not be considered neurotoxic, although n-hexane would have to be listed on the MSDS if present in greater quantity than 1% of the mixture. 3. Process Safety Management Standard (29 CFR 1910.119) Process safety management (PSM) is for the prevention or minimization of the consequences of catastrophic releases of toxic, reactive, flammable, or explosive chemicals. This regulation applies to all processes that involve one or more of 137 listed chemicals (29 CFR 1910.119, Appendix A) above their threshold quantities or have 4.5 t (10,000 lb) or more of a flammable liquid or gas, as defined by the OSHA HCS [29 CFR 1910.1200(c)]. This includes n-hexane, hexane isomers, and the other solvents listed in Table 9. The requirements for meeting this regulation are described in more detail by Lajeunesse (31) and Strube (32). In addition to the PSM standard, OSHA has been enforcing two other regulations for operations/processes with flammable liquids. First, under Personal Protective Equipment—General Requirements (29 CFR 1910.132), OSHA has cited or obtained voluntary agreement from organizations relative to flameresistant (FR) clothing. Operators and other employees working in the area of a flammable process are being required to wear flame-resistant work clothing. For facilities that use commercial hexane and other flammable solvents, it would be prudent to require FR clothing for all personnel working in areas where there is an exposure to a flammable liquid. Second, OSHA has cited organizations for failure to meet related safety regulations under Fire Brigades (29 CFR 1910.156), specifically for standards such as training, both initial and annual refresher training; protective equipment availability and testing; and fitness for duty including periodic physicals. If an on-site fire brigade is part of the site’s emergency response plan (29 CFR 1910.38), then these requirements must also be met. In addition, the requirement of the PSM standard for an emergency response plan triggers the requirements of emergency action plan [29 CFR 1910.38(a)].

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C. Food Safety Oilseed extraction solvents and food processing substances, to be legally used in the United States, must have been subject to an approval by the U.S. Food and Drug Administration (FDA), which regulates all aspects of food, including food ingredients and labeling, or the U.S. Department of Agriculture (USDA) during 1938–1958 for this use (“prior sanction”); be generally recognized as safe (GRAS) for this use; or be used in accordance with food additive regulations promulgated by the FDA. Many prior sanctions and GRAS determinations are not codified in the FDA regulations. However, extracting solvents used in food manufacturing, such as n-hexane, have been labeled as food additives, solvents, defoaming agents, component of a secondary food and color additives, minor constituent, or incidental additives (i.e., “additives that are present in a food at significantly low levels and do not have any technical or functional effect in that food”) depending on the application. Incidental additives can be “processing aids,” (i.e., “substances that are added to a food during processing but removed from the food before it is packaged”). Most food processing substances, including solvents, can be regarded as “incidental additives” and thus are exempt from label declaration in the finished food product. Even if exempt from label declaration, all extraction solvents must be used in accordance with the FDA good manufacturing practice (GMP; 21 CFR 100). Commercial hexane, containing about 50–85% n-hexane, has been in major use since the 1940s as an oilseed extraction solvent on the determination that it is GRAS and it may also be subject to a prior sanction. Like many other food processing substances, there is no FDA regulation specifically listing nhexane as GRAS or prior sanctioned. However, under FDA regulations hexane has been cleared as a solvent in the manufacture of food additives and has been cleared as a minor constituent (not more than 5 ppm) of a cocoa butter substitute that is a direct food additive that has been affirmed by FDA as GRAS for food use (52 FR 47918, December 17, 1987; 21 CFR 184.1259). In Europe the maximum residue limit (MRL) in vegetable oils has been established as 5 ppm nhexane [European Union (EU) Communitee Directive 88/344/EEC of 13 June 1988; Off. J. Eur. Commun. L 157, 24 June 1988, pp. 0028–0033]. The Flavor and Extract Manufacturers Association (FEMA) has conducted a program since 1958 using a panel of expert pharmacologists and toxicologists to determine substances that are GRAS. This safety assessment program (“FEMA GRAS”) is widely accepted and considered an industry/government partnership with the FDA (33). A number of papers published since 1961 (34–36) list the substances that the panel has determined to be GRAS and the average maximal levels in parts per million (ppm) at which each has been reported to be GRAS for different categories of food. The FDA has not incorporated these substances

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in their regulations but recognizes the findings of the Expert Panel of FEMA as GRAS substances. Since vegetable oil and other human food grade oils undergo deodorization (steam distillation) and other purification processes (i.e., refining and bleaching) as part of the manufacturing process prior to being used as a food product, they should not contain any of the extraction solvent, if proper manufacturing practices are followed. Refining removes nonglyceride materials (e.g., phospholipids, color, and trace metals) and free fatty acids; bleaching with acidactivated bleaching earth or clay (e.g., bentonite), removes color-producing substances; and deodorization, the last major processing step in edible oils refining, removes volatile compounds (undesirable ingredients occurring in natural oils and those that may be imparted by prior unit processes or even storage, many of which are associated with undesirable flavors and odors) (37, 38). Most commercial deodorizers operate at a temperature of 245–275°C (475–525°F) under a negative pressure of 2–10 mm Hg (37, 38). It has been reported that no nhexane residue remains in the finished oil after processing due to its high volatility (39). In addition, animal feeding studies with expeller and solvent-extracted meals have not indicated any adverse health effects related to the extraction solvent (40). In summary, GRAS status may be determined by a company (“GRAS self-determination”), an industry, an independent scientific organization (e.g., FEMA GRAS), or the FDA. The Federal Food, Drug and Cosmetic Act (FFDCA; 21 U.S. Code 321 et seq.) does not provide for the FDA to approve all ingredients used in food, and the FDA explicitly recognizes that its published GRAS list is not meant to be a complete listing of all substances that are in fact GRAS food substances. Although there is no requirement to inform the FDA of a GRAS selfdetermination or to request FDA review or approval on the matter, the FDA has established a voluntary GRAS affirmation program under which such advice will be provided by the agency. If a facility is considering changing its extracting solvent for the extraction of the various edible biological materials, solvents that do not have prior sanction, a GRAS determination, or a tolerance set probably should be evaluated for compliance under food safety requirements.

V. SUMMARY In summary, there most likely will be new demands for highly specialized extraction solvents as newly domesticated species that make useful novel oils (41) and other products and new or altered biological products with enhanced nutritional and industrial properties will be developed through conventional breeding and genetic engineering for use as “functional foods” (42) (e.g., phytosterols to achieve cholesterol lowering); as oils with altered lipid profiles (43) (e.g., for

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lower saturated fat) or with more vitamin E; new drugs/nutraceuticals, industrial chemicals (e.g., fatty acids for lubricants, as cosmetics, coatings, detergents, surfactants, flavors, polymers, etc.); as sources for specialty chemicals; as valueadded products; and so forth (42–49). There will be demands for solvent systems for simultaneous removal of undesirable meal components (e.g., mycotoxins, gossypol, flavors, and odors) that offer the potential for upgrading meal for use as higher value animal feeds and human foods. Solvents that offer energy savings and that pose lower health, environmental, and fire hazards will also be sought. However, it will be more necessary than ever to be aware of regulatory requirements for and the toxicity of the solvents used.

GLOSSARY ACGIH

BACM BACT CAA CERCLA (Superfund) CFR

CWA EPA EPCRA

FR

GRAS

American Conference of Governmental Industrial Hygienists, an independent standards setting organization. best available control measures. best available control technology. Clean Air Act, 42 U.S. Code 1251 et seq. Comprehensive Environmental Response, Compensation, and Liability Act, 42 U.S. Code 9601 et seq. Code of Federal Regulations. This is where the U.S. federal regulations after promulgation are codified. The preceding number is the Title, the succeeding number (after CFR) is the Part of Section (e.g., 29 CFR 1910 is Title 29 Code of Federal Regulations at Part 1910). Clean Water Act (Federal Water Pollution Control Act), 33 U.S. Code 1251 et seq. Environmental Protection Agency, 42 U.S. Code 4321 et seq. Emergency Planning and Community Right-toKnow Act, part of CERLA/Superfund, Title III of SARA, the 1986 amended Superfund. Federal Register. This is where regulatory announcements and new rules and their justification are published. The preceding number is the volume; the succeeding number (after FR) is the page, usually followed by the date when it appeared (e.g., 51 FR 27956 is Volume 51 Federal Register, page 27956). generally recognized as safe.

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Solvent Extraction

HAP HCS IUR LAER MACT MSDS NAAQS NESHAP Nonattainment NPDES

NSR OPA-90 OSHA

Ozone (O3)

NOx PEL PM

POTW PSD PSM RACM RACT RCRA

423

hazardous air pollutant, 40 CFR 61. Hazard Communication Standard, 29 CFR 1910.1200. Inventory Update Rule, 40 CFR 710. lowest achievable emission rate. maximum achievable control technology. material safety data sheet, required under OSHA HCS. National Ambient Air Quality Standard, 40 CFR 50. National Emission Standard for Hazardous Air Pollutants under the CAA. Areas that do not meet NAAQS, 40 CFR 51.100 et seq. National Pollution Discharge Elimination System. The national permit program under the CWA, 40 CFR 122. new source review. Oil Pollution Act of 1990. Occupational Safety and Health Administration (part of the U.S. Dept. of Labor), 29 U.S. Code 651 et seq. One of the compounds on the NAAQS list that is formed through chemical reaction in the atmosphere involving VOC, NOx, and sunlight; also a primary constituent of smog. nitrogen oxides. permissible exposure limit for an air contaminant under OSHA standards. particulate matter. One of the NAAQS; denotes the amount of solid or liquid matter suspended in the atmosphere. The EPA regulates PM as PM10 (“coarse” particulate 10 µm and less) and PM2.5 (“fine” particulate 2.5 µm or less). publicly owned treatment works, for indirect wastewater discharge. prevention of significant deterioration, a requirement of NSR. process safety management standard. reasonably achievable control measures. reasonably available control technology. Resource Conservation and Recovery Act, 42 U.S. Code 6901 et seq.

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RCRA-Characteristic Wastes RCRA-Listed Wastes SARA TCLP Title V TLV TRI TSCA TWA U.S. Code

UST

VOC

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hazardous wastes that are ignitable, corrosive, reactive, or toxic, 40 CFR 260.64. Specially listed hazardous wastes in 40 CFR 261.3033. Superfund Amendments and Reauthorization Act. toxic characteristic leaching potential under RCRA, 40 CFR 261.24. The part of the Clean Air Act that deals with federal permits, 40 CFR 70. threshold limit value for an air contaminant under ACGIH regulations. toxic release inventory, under section 313 of EPCRA. Toxic Substances Control Act. time weighted average. The United States Code where legislation, including health, safety, and environmental legislation, is codified once passed by Congress (e.g., 42 U.S. Code 7401 is Title 42 U.S. Code at paragraph 7401). Underground storage tank (any tank completely covered with earth); Technical Standards and Corrective Action Requirements for Owners and Operators of UST (40 CFR 280). volatile organic compounds. A group of chemicals that react in the atmosphere with nitrogen oxides (NOx) in the presence of heat and sunlight to form ozone; does not include compounds determined by EPA to have negligible photochemical reactivity.

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6. Berichte Deutsche Gesellschaft fur Mineralolwissenschaft und Kohlechemie. Effects of n-Hexane in Man and Animals. Research Report 174-2. Hamburg: BDGNK e.V., 1982. [Also see U.S. EPA Integrated Risk Information System (IRIS), n-hexane http://www.epa.gov.iris.subst/0486.htm] 7. JB Galvin. Toxicity data for commercial hexane and hexane isomers. In: PJ Wan, PJ Wakelyn, eds. Technology and Solvents for Extracting Oilseeds and Nonpetroleum Oils. Champaign, IL: AOCS Press, 1997, pp. 75–85. 8. U.S. Environmental Protection Agency. Commercial Hexane Final Test Rule. (53 Federal Register 3382; February 5, 1988.) 9. PJ Wakelyn, PK Adair. Toxicity data for extraction solvents other than hexane and isohexane/hexane isomers. In: PJ Wan, PJ Wakelyn, eds. Technology and Solvents for Extracting Oilseeds and Nonpetroleum Oils. Champaign, IL: AOCS Press, 1997, pp. 86–100. 9a. PJ Wakelyn, RJ Hron, FJ Flider, PJ Wan. Acetoneian environmental preferable choice? INFORM 12(9):887–893, 2001. 10. PJ Wakelyn. Regulatory considerations for extraction solvents for oilseeds and other nonpetroleum oils. In: PJ Wan, PJ Wakelyn, eds. Technology and Solvents for Extracting Oilseeds and Nonpetroleum Oils. Champaign, IL: AOCS Press, 1997, pp. 48–74. 11. PJ Wakelyn, PK Adair. Assessment of risk and environmental management. In: SS Koseoglu, KC Rhee, RF Wilson, eds. Emerging Technologies, Current Practices, Quality Control, Technology Transfer, and Environmental Issues, Vol. 1. Proceedings of the World Conference on Oilseed and Edible Oil Processing. Champaign, IL: AOCS Press, 1998, pp. 305–312. 12. PJ Wakelyn, PK Adair, SR Gregory. Management of quality and quality control in cottonseed extraction mills. Oil Mill Gaz 103(6):23–30, 1997. 13. PJ Wakelyn. Overview of the Clean Air Act of 1990: Cottonseed oil mills. Cotton Gin and Oil Mills Press 92(17):12–15, 18, 1991. 14. PJ Wakelyn, LA Forster. Regulatory issues regarding air, water, solid waste and salmonella. Oil Mill Gaz 99(12):21–30, 1994. 15. PJ Wan, PJ Wakelyn. Regulatory considerations of VOC, HAP. Inform 9:1155– 1160, 1998. 16. PJ Wan, PJ Wakelyn. Regulatory considerations of VOC and HAP from oilseed extraction plants. Oil Mill Gaz 104(6):15–24, 1998. 17. C Zukor, BL Riddle. Vegetable oil process description. Memorandum to EPA Vegetable Oil Project File, May 29, 1998. 18. L Venne. Volatile organic compounds. Paper presented at the 1997 AOCS Annual Meeting, Seattle, WA, 1997. 19. U.S. Environmental Protection Agency. n-Hexane. U.S. EPA Integrated Risk Information System (IRIS) Substance File, 1999 (www.epa.gov/ngispgm3/IRIS/subst/ 0486.htm). 20. U.S. Environmental Protection Agency. Study of Volatile Organic Compound Emissions from Consumer and Commercial Products. U.S. Environmental Protection Agency, Office of Quality Planning and Standards, 1995 (EPA–453/R-94-066-A). 21. SE Hughs, PJ Wakelyn. Physical characteristics of cyclone particulate emissions. Appl Eng Agric 13(4):531–535, 1997.

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