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Preface
For centuries, frozen foods have been available to consumers in countries that experience cold winters. In some areas with severe winters such as Alaska, Russia, and others, foods are routinely frozen by leaving them outside. Since 1875, with the development of mechanical ammonia freezing systems, the frozen food industry has grown steadily, especially in the past two decades.
Frozen foods have the advantages of being very close in taste and quality to fresh foods as compared with other preserved or processed foods. Frozen foods are popular and accessible in most developed countries, where refrigerators and freezers are standard home appliances. Nowadays, frozen foods have become essential items in the retail food industries, grocery stores, convenience food stores, fast food chains, food services, and vending machines. This growth is accompanied by the frequent release of new reference books for the frozen food industry.
Several updated books on freezing preservation of foods or frozen foods have been available in the past decade, and most of them are excellent books. The science and technology of food freezing can be viewed from several perspectives:
Food engineering principles. These principles explain such phenomena as heat and mass transfer, freezing time, convective and conductive processes, and other processes and principles relevant to understanding the dynamics of freezing. Food science and technology principles. These principles explain the chemistry and
biology of food components, their interactions during processing, and other principles relevant to understanding how foods behave before, during, and after the frozen stage.
Food manufacturing principles.These principles explain how we can start with a raw ingredient and end with a finished frozen product.
Food commodities, properties and applications. This approach takes an individual commodity of food (e.g., fruits, vegetables, dairy, muscle foods) and explains the whole spectrum of factors that involve cooling, refrigeration, freezing, and thawing unique to that category of food and its properties. Although the underlying principles are the same, freezing carrots is definitely different from freezing salmon. These data are a combination of the three principles above
and are the basis of our ability to enjoy winter vegetables during summer and 100 flavors of ice cream all year round.
Over the past two decades, books have been published that cover some or all of the topics above. When it comes to books on frozen foods, it is an endless venture. The reason is simple: Every month and every year, food scientists, food technologists, and food engineers witness rapid development in the science and technology of frozen foods. We continually see new knowledge, new equipment, and new commercial applications emerging.
Based on the above premises of principles and applications, the Handbook of Frozen Foods uses the following approaches to covering the data:
Principles. Chapters 1 through 8 cover principles applicable to the processing of frozen foods, such as science, technology, and engineering. Topics include the physical processes of freezing and frozen storage, texture, color, sensory attributes, and packaging.
Meat and poultry. Seven chapters (Chapters 9–15) discuss freezing beef and poultry meat, covering operations, processing, equipment, packaging, and safety. Seafoods’ Chapters 16 through 21 discuss frozen seafoods, covering principles,
finfish, shellfish, secondary products, HACCP (Hazards Analysis and Critical Control Points), and product descriptions.
Vegetables.Five chapters (Chapters 22–26) discuss frozen vegetables, covering product descriptions, quality, tomatoes, French fries, and U.S. grades and standards. Fruits. Chapters 27 through 29 discuss frozen fruits and fruit products, covering
product descriptions, tropical fruits, and citrus fruits.
Special product categories.Chapters 30, 31, and 32 provide details on some popular products: frozen desserts, frozen dough, and microwavable frozen foods. Safety.Chapters 33 through 36 discuss the safety of processing frozen foods covering
basic considerations, sanitation of a frozen food plant, risk analysis in processing frozen desserts, and U.S. enforcement tools for frozen foods. This volume is the result of the combined effort of more than 50 contributors from 10 countries with expertise in various aspects of frozen foods, led by an international editorial team. The book contains eight parts and 36 chapters organized into eight parts. In sum, the approach for this book is unique and makes it an essential reference on frozen food for professionals in government, industry, and academia.
We thank all the contributors for sharing their experience in their fields of expertise. They are the people who made this book possible. We hope you enjoy and benefit from the fruits of their labor.
We know how hard it is to develop the content of a book. However, we believe that the production of a professional book of this nature is even more difficult. We thank the production team at Marcel Dekker, Inc., and express our appreciation to Ms. Theresa Stockton, coordinator of the entire project.
You are the best judge of the quality of this book.
Y. H. Hui Paul Cornillon Isabel Guerrero Legarreta Miang H. Lim K. D. Murrell Wai-Kit Nip
Contents
Preface Contributors
PART I. FREEZING PRINCIPLES 1. Freezing Processes: Physical Aspects
Alain Le Bail
2. Principles of Freeze-Concentration and Freeze-Drying
J. Welti-Chanes, D. Bermu´dez, A. Valdez-Fragoso, H. Mu´jica-Paz, and S. M. Alzamora
3. Principles of Frozen Storage
Genevie`ve Blond and Martine Le Meste 4. Frozen Food Packaging
Kit L. Yam, Hua Zhao, and Christopher C. Lai
PART II. FROZEN FOOD CHARACTERISTICS
5. Frozen Food Components and Chemical Reactions Miang H. Lim, Janet E. McFetridge, and Jens Liesebach 6. Flavor of Frozen Foods
Edith Ponce-Alquicira 7. Food Sensory Attributes
8. Texture in Frozen Foods William L. Kerr
PART III. FROZEN MEAT AND POULTRY
9. Frozen Muscle Foods: Principles, Quality, and Shelf Life Natalia F. Gonza´lez-Me´ndez, Jose´ Felipe Alema´n-Escobedo, Libertad Zamorano-Garcı´a, and Juan Pedro Camou-Arriola 10. Operational Processes for Frozen Red Meat
M. R. Rosmini, J. A. Pe´rez-Alvarez, and J. Ferna´ndez-Lo´pez 11. Frozen Meat: Processing Equipment
Juan Pedro Camou-Arriola, Libertad Zamorano-Garcı´a,
Ana Guadalupe Luque-Alcara´z, and Natalia F. Gonza´lez-Me´ndez 12. Frozen Meat: Quality and Shelf Life
M. L. Pe´rez-Chabela and J. Mateo-Oyagu¨e
13. Chemical and Physical Aspects of Color in Frozen Muscle-Based Foods J. A. Pe´rez-Alvarez, J. Ferna´ndez-Lo´pez, and M. R. Rosmini
14. Frozen Meat: Packaging and Quality Control Alfonso Totosaus
15. Frozen Poultry: Process Flow, Equipment, Quality, and Packaging Alma D. Alarcon-Rojo
PART IV. FROZEN SEAFOODS
16. Freezing Seafood and Seafood Products Principles and Applications Shann-Tzong Jiang and Tung-Ching Lee
17. Freezing Finfish B. Jamilah
18. Freezing Shellfish
Athapol Noomhorm and Punchira Vongsawasdi 19. Freezing Secondary Seafood Products
Bonnie Sun Pan and Chau Jen Chow 20. Frozen Seafood Safety and HACCP
Hsing-Chen Chen and Philip Cheng-Ming Chang 21. Frozen Seafood: Product Descriptions
PART V. FROZEN VEGETABLES
22. Frozen Vegetables: Product Descriptions Peggy Stanfield
23. Quality Control in Frozen Vegetables
Domingo Martı´nez-Romero, Salvador Castillo, and Daniel Valero 24. Production, Freezing, and Storage of Tomato Sauces and Slices
Sheryl A. Barringer
25. Frozen French Fried Potatoes and Quality Assurance Y. H. Hui
26. Frozen Peas: Standard and Grade Peggy Stanfield
PART VI. FROZEN FRUITS AND FRUIT PRODUCTS
27. Frozen Fruits and Fruit Juices: Product Description Peggy Stanfield
28. Frozen Guava and Papaya Products Harvey T. Chan, Jr.
29. Frozen Citrus Juices Louise Wicker
PART VII. FROZEN DESSERTS, FROZEN DOUGH, AND
MICROWAVABLE FROZEN FOODS 30. Ice Cream and Frozen Desserts
H. Douglas Goff and Richard W. Hartel 31. Effect of Freezing on Dough Ingredients
Marı´a Cristina An˜o´n, Alain Le Bail, and Alberto Edel Leon 32. Microwavable Frozen Food or Meals
Kit L. Yam and Christopher C. Lai
PART VIII. FROZEN FOODS SAFETY CONSIDERATIONS
33. Safety of Frozen Foods
Phil J. Bremer and Stephen C. Ridley 34. Frozen Food Plants: Safety and Inspection
35. Frozen Dessert Processing: Quality, Safety, and Risk Analysis Y. H. Hui
36. Frozen Foods and Enforcement Activities Peggy Stanfield
Appendix A: FDA Standard for Frozen Vegetables: 21 CFR 158. Definitions: 21 CFR 158.3; FDA Standard for Frozen Vegetables: 21 CFR 158. Frozen Peas: 21 CFR 158.170
Appendix B: Frozen Dessert Processing: Quality, Safety, and Risk Analysis. Special Operations
Contributors
Alma D. Alarcon-Rojo Universidad Auto´noma de Chihuahua, Chihuahua, Mexico Jose´ Felipe Alema´n-Escobedo Centro de Investigacio´n en Alimentacio´n y Desarrollo, A. C., Hermosillo, Sonora, Mexico
S. M. Alzamora Universidad de Buenos Aires, Buenos Aires, Argentina Marı´a Cristina An˜o´n Universidad Nacional de La Plata, La Plata, Argentina
Sheryl A. Barringer Department of Food Science and Technology, The Ohio State University, Columbus, Ohio, U.S.A.
D. Bermu´dez Universidad de las Ame´ricas—Puebla, Puebla, Mexico Genevie`ve Blond ENSBANA–Universite´ de Bourgogne, Dijon, France
Phil J. Bremer Department of Food Science, University of Otago, Dunedin, New Zealand
Juan Pedro Camou-Arriola Centro de Investigacio´n en Alimentacio´n y Desarrollo, A.C., Hermosillo, Sonora, Mexico
Salvador Castillo Miguel Hernandez University, Orihuela, Spain
Roberto S. Chamul California State University, Los Angeles, Los Angeles, California, U.S.A.
Harvey T. Chan, Jr. HI Food Technology, Hilo, Hawaii, U.S.A.
Hsing-Chen Chen National Taiwan Ocean University, Keelung, Taiwan
Chau Jen Chow National Kaohsiung Institute of Marine Technology, Kaohsiung, Taiwan
Patti C. Coggins Department of Food Science and Technology, Mississippi State University, Mississippi State, Mississippi, U.S.A.
J. Ferna´ndez-Lo´pez Miguel Hernandez University, Orihuela, Spain
H. Douglas Goff Department of Food Science, University of Guelph, Guelph, Ontario, Canada
Natalia F. Gonza´lez-Me´ndez Centro de Investigacio´n en Alimentacio´n y Desarrollo, A.C., Hermosillo, Sonora, Mexico
Richard W. Hartel Department of Food Science, University of Wisconsin–Madison, Madison, Wisconsin, U.S.A.
Y. H. Hui Science Technology System, West Sacramento, California, U.S.A. B. Jamilah University Putra Malaysia, Selangor, Malaysia
Shann-Tzong Jiang National Taiwan Ocean University, Keelung, Taiwan
William L. Kerr Department of Food Science and Technology, University of Georgia, Athens, Georgia, U.S.A.
Christopher C. Lai Pacteco Inc., Kalamazoo, Michigan, U.S.A. Alain Le Bail ENITIAA–UMR GEPEA, Nantes, France
Tung-Ching Lee Department of Food Science, Rutgers University, New Brunswick, New Jersey, U.S.A.
Martine Le Meste ENSBANA–Universite´ de Bourgogne, Dijon, France Alberto Edel Leon Universidad Nacional de Co´rdoba, Co´rdoba, Argentina
Jens Liesebach Department of Food Science, University of Otago, Dunedin, New Zealand
Miang H. Lim Department of Food Science, University of Otago, Dunedin, New Zealand
Ana Guadalupe Luque-Alcara´z Centro de Investigacio´n en Alimentacio´n y Desarrollo, A.C., Hermosillo, Sonora, Mexico
J. Mateo-Oyagu¨e Universidad de Leo´n, Leo´n, Spain
Janet E. McFetridge Department of Food Science, University of Otago, Dunedin, New Zealand
H . Mu´jica-Paz Universidad Auto´noma de Chihuahua, Chihuahua, Mexico Athapol Noomhorm Asian Institute of Technology, Pathumthani, Thailand Bonnie Sun Pan National Taiwan Ocean University, Keelung, Taiwan J. A. Pe´rez-Alvarez Miguel Hernandez University, Orihuela, Spain
M. L. Pe´rez-Chabela Universidad Auto´noma Metropolitana, Mexico City, Mexico Edith Ponce-Alquicira Universidad Auto´noma Metropolitana, Mexico City, Mexico Stephen C. Ridley College of Agriculture, Food, and Environmental Science, University of Wisconsin–River Falls, River Falls, Wisconsin, U.S.A.
M. R. Rosmini Universidad Nacional del Litoral, Santa Fe, Argentina Peggy Stanfield Dietetic Resources, Twin Falls, Idaho, U.S.A.
Alfonso Totosaus Universidad Auto´noma del Estado de Hidalgo, Hidalgo, Mexico A. Valdez-Fragoso Universidad Auto´noma de Chihuahua, Chihuahua, Mexico Daniel Valero Miguel Hernandez University, Orihuela, Spain
Punchira Vongsawasdi King Mongkut’s University of Technology Thonburi, Bangkok, Thailand
J. Welti-Chanes Universidad de las Ame´ricas—Puebla, Puebla, Mexico
Louise Wicker Department of Food Science and Technology, University of Georgia, Athens, Georgia, U.S.A.
Kit L. Yam Rutgers University, New Brunswick, New Jersey, U.S.A.
Libertad Zamorano-Garcı´a Centro de Investigacio´n en Alimentacio´n y Desarrollo, A. C., Hermosillo, Sonora, Mexico
1
Freezing Processes: Physical Aspects
Alain Le Bail
ENITIAA–UMR GEPEA, Nantes, France
I. INTRODUCTION
This chapter presents the freezing process. Selected models permitting estimates of the freezing time are proposed and discussed. These models are based on the classical model established by Plank; improvements of this model are presented and discussed. The different types of freezing processes used in the industry are then presented. Blast freezing is probably the most popular freezing process, but other concepts such as contact freezing are also used in a wide range of applications. The thermal contact resistance existing between the refrigerated surface and the product is often neglected; a focus is proposed on this aspect. The discussion ends with an evaluation of the freezing rate.
II. FREEZING PROCESS
A. Heat Transfer DuringFreezing
The heat transfer phenomenon involved in freezing of biological material is basically nonlinear heat transfer. The latent heat of water represents a large amount of heat that has to be removed from the foodstuff. Generally, a high freezing rate is desired in order to obtain numerous small ice crystals. Nevertheless, this is not always the case. For example, consider frozen dough, for which a slower freezing gives a better preservation of yeast activity. Freezers can be classified in two families; batch freezers, for which a given amount of product will be frozen in the same batch, and continuous freezers, which can be operated in a production line. The refrigeration system used allows classifying freezers in two other subfamilies: freezers using cryogenic fluids such as carbon dioxide or liquid nitrogen, and freezers using a mechanical refrigeration unit and a secondary refrigeration fluid (air, brine, etc.). Mechanical refrigeration units are used for a large majority of industrial freezers. Cryogenic fluid will be used for special applications requiring (a) minimal investment, (b) specific use (i.e., meat grinding), or (c) high freezing rate. Heat transfer conditions and thus the freezing rate are closely related to the type of freezer.
B. FreezingTime
A basic analytical model has been proposed by Plank (1941) assuming that (a) the initial temperature of the product is equal to the phase change temperature, (b) phase change occurs at constant temperature, and (c) all thermophysical properties and heat transfer coefficients are constants. Consequently, the initial cooling and final cooling after freezing are not taken into account. The freezing time given by the Plank formula is proposed in Eq. (1). t¼ DH ? r ? X ðTa TfÞ ? N ? 1 hþ X 4lF ðsÞ ð1Þ
where t is freezing (thawing) time (s);DH is enthalpy difference over the freezing plateau (J? kg1); r is density of the frozen food (kg ? m3); Ta is medium temperature (K or8C);
Tf is initial freezing temperature (K or8C); lFis thermal conductivity of the food in the
frozen state (W? m1? K1); H is heat transfer coefficient (W? m2? K1); X is characteristic dimension (m); and N is coefficient (see Table 1).
Based on this first approach, several authors attempted to improve the accuracy of the freezing (or thawing) time calculation. Ramaswamy et al. (1984) proposed a review of these equations. Nagoaka et al. (1955) proposed Eq. (2), which takes into account the amount of heat to be removed during the pre- and postfreezing periods. In this equation, DH represents the enthalpy difference between the initial temperature (Ti) of the product
and the final temperature at the end of freezing (J? kg1). t¼ ½1 þ 0:008*Ti DH ? r Tf Ta ð Þ ? PX h þ RX2 lF ðsÞ ð2Þ
with P and R geometric factors as defined in the table below.
Geometry X P R
Slab Thickness 0.5 0.125
Cylinder Diameter 0.25 0.0625
Sphere Diameter 0.167 0.0416
Levy (1958) proposed an expression extrapolated from the model of Nagoaka that differs mainly in that the temperature difference between initial and final conditions is explicitly taken into account, the enthalpy difference being considered between the initial temperature (Ti) of the product and the final temperature at the end of freezing (J? kg1)
Table 1 Coefficient of the Plank Formula
Geometry L N
Slab Thickness 2
Cylinder Diameter 4
(Eq. 3). The International Institute of Refrigeration (1972) proposed Eq. (3), which is once again very similar to the model of Nagoaka. This time it is the enthalpy difference that is taken into account between the initial freezing temperature and the final temperature Tc
(DH ¼ enthalpy between Tf and final temperature of the product Tcin J? kg1) (Eq. 4).
t¼ ½1 þ 0:008ðTi TfÞ DH ? rF Tf Ta ð Þ? PX h þ RX2 lF ðsÞ ð3Þ t¼ DH ? rF Tifp Ta ? PX h þ RX2 lF ðsÞ t ¼ DH0? r0 ðTf TaÞ? Pd h þ Rd2 lF ðsÞ ð4Þ
Cleland and Earle (Cleland et al., 1979) used an approach based on numerical models and experimental results. The Plank equation is proposed in nondimensional form using the Fourier number Fo based on the P and R factors and on the Biot number (Bi) and Stefan number (Ste). F0¼ P BiSteþ R Ste ð5Þ
Ste¼ DH between Tf and Ta
DH between Tf and Tfinal ð6Þ
Bi¼ hX
2lF ð7Þ
These authors introduced a dimensionless Plank number [Pk, Eq. (8)] and defined new coefficients P* and R* of the new Plank equation for a slab.
Pk ¼ DH between Ti and Tf
DH between Tf and Tfinal ð8Þ
P* ¼ 0:5072 þ 0:2018Pk þ Steð0:3224Pk þ0:0105
Bi þ 0:0681Þ ð9Þ
R* ¼ 0:1684 þ Steð0:2740Pk þ 0:0135Þ ð10Þ
t¼DH ðbetween Tf and TfinalÞ ? r
ðTf TaÞ ? R*X h þ R*X2 lF ðsÞ ð11Þ
This equation was acceptable for initial temperature Ti < 40 C and medium temperature
between15 C< Ta < 40 C, heat transfer coefficients between 10< h < 500 Wm2? k1 and maximum slab thickness of 12 cm. For an infinite cylinder, they found Eqs. (12) and (13).
P* ¼ 0:3751 þ 0:0999Pk þ Steð0:4008Pk þ0:0710
Bi 0:5865Þ ð12Þ
And for a sphere,
P* ¼ 0:1084 þ 0:0924Pk þ Steð0:2310Pk 0:3114
Bi þ 0:6739Þ ð14Þ
R* ¼ 0:0784 þ Steð0:0386Pk 0:1694Þ ð15Þ
These latter were applicable for 0:155 < Ste < 0:345; 0:5 < Bi < 4:5, and 0 < Pk < 0:55. The numerical parameters were obtained from experiments realized with thylose gels (77% w.c.). The accuracy of these relationships for freezing time prediction were within +3% for slabs, +5.2% for infinite cylinders and +3.8% for spheres Ramaswamy and Tung (1984) established a new model extrapolated from the previous one. They use a regression approach: t¼ ½0:3022CðTi TfÞ þ L þ 2:428C0ðTf TcÞ r 0 ðTf TaÞ? Pd h þ Rd2 lF ðsÞ ð16Þ
With C and C0 ¼ specific heat of the foodstuff respectively before and after freezing (J? kg1? K1) and L¼ enthalpy over the freezing plateau (J ? kg1). This formula was established in the following conditions:
1 C< Ti < 25 C; 18 C< Tc < 10 C; 178 C< Ta < 18 C;
13:9 < h < 68:4 W ? m2? k1:
Other expressions are available in the literature but the one proposed above can be considered as a good basis. The accuracy can be greatly improved by using numerical models for which extensive studies have been done (Cleland, 1990). These models allow us to take into account the time-dependent heat transfer coefficient and the temperature. Modern software is now available to realize this type of modeling without major difficulties.
III. CONVECTIVE PROCESSES: AIR FREEZING, BRINE FREEZING, CRYOGENIC FREEZING
In the case of convective freezing, air, a cryogenic fluid (mainly liquid nitrogen), or a brine can be used as refrigerant. In the case of air, it can be admitted that the air velocity is in the range of 1 to 5 m? s1 for most industrial application, leading to the effective heat transfer coefficient in the range of 10 to 50 W? m2k1 between the medium and the product. Large-scale spiral freezers have been developed by equipment companies and are widely used in the industry. Individual quick freezing (IQF) consists of freezing small products individually with a high air speed (i.e., 1–5 m? s1). Freezing of larger products can be realized with blast air but will yield a low freezing rate and thus a low quality in terms of ice crystal size; plate freezers are preferred. Some specificity in terms of air flow pattern have been developed in order to reduce water loss by dehydration (i.e., counter flow or partial counter flow with air inlet a mid position between entrance and exit). Higher air velocity can also be imposed in a local section such as the entrance of the freezer. This has been developed for small products (few centimeters thick). It improves heat transfer and permits a superficial freezing which will minimize mass losses by reducing partial vapor pressure at the surface of the product. This partial superficial freezing is also called crust freezing or cryomechanical freezing (Macchi, 1995; Agnelli et al., 2001); it can be realized
by using a liquid nitrogen bath in which the products are floating for a short period before traveling either into a conventional belt freezer at follow or in the vicinity of the cryogenic bath to gain the benefit of the vaporized gas for the final freezing (see Mermelstein, 1997). Freezing in a brine will yield a much higher heat transfer coefficient than in blast air. The product can be wrapped; in this case, a brine made of CaCl2, NaCl, propylene glycol,
ethanol or mixtures of them can be used (Venger et al., 1990). In the case of unwrapped products (Lucas et al., 1999) the freezing process can be combined with a soaking effect. Soaking resulted in a salt concentration at the surface of the product and prevented freezing in an external layer (a nonfrozen layer of ca. 1 mm has been observed [Lucas et al., 1999b]). Brine freezing is also used for small products such as shrimp to prevent excessive water loss by dehydration and eventually to enhance solute intake; in the case of shrimp, for example, a brine solute is generally a mixture of salt and sugar. One major drawback of the immersion technique is that the brine concentration is changing during the process, requiring a specific adjustment of it during the treatment. Undesirable side effects may occur such as spoilage of the product by the brine (requiring filtering and cleaning of the brine) and cross-contamination of pathogenic microorganisms, as shown by Berry et al. (1998).
The use of solid carbon dioyde as a refrigerant is an intermediate solution between contact freezing (contact of the solid flakes of CO2) and convective freezing (convective
heat transfer between the sublimated CO2and the product). An optimal design will result
in a temperature of the gaseous CO2 as high as possible.
IV. CONDUCTIVE PROCESSES: CONTACT FREEZERS
Contact freezing can be considered as a mass production process that has a relatively high freezing rate. IQF and cryogenic refrigeration will yield yet higher freezing rates. It is widely used in the industry to produce slabs of frozen foods such as fish filets and mashed vegetables. As for any freezing process, the geometry of the product will rule the freezing rate as described by the Plank equation (Plank, 1941). Two classes of contact freezers can be defined, continuous and batch systems. In batch systems, the product is usually frozen from both sides, by plane heat exchangers applying a certain pressure against the product (Fig. 1). Continuous systems are usually operated by applying a thin product on a refrigerated surface and by scraping it off after freezing. Two concepts have been developed, namely rotating drum freezers and linear belt freezers (Marizy et al., 1998).
In batch plate freezers, the product is usually installed in a cardboard box containing a plastic film or pouch. It can eventually be installed directly against the refrigerated surface, but this will create a problem in removing the frozen product at the end of the process. Rotating drum freezers (Fig. 2) have been developed in the industry to freeze
liquid or viscous products or even solid products such as fish filets. In this case, the product is directly applied against a rotating metallic drum refrigerated from the inside by a brine, for example. Such a process has been modeled by Marizy et al. (1998). The product is applied on one side (thickness between 1 mm and a few cm) and is scraped after a rotation. Madsen (1983) studied drum freezing of codfish and showed that this process improved the storage stability of cod relative to those frozen in a plate freezer. Recent literature on drum freezer technology mainly concerns patents. Some of these patents are related to heat transfer improvement between the refrigerant and the drum, such as Reynolds (1993) in the case of boiling refrigerant (R22 type). Specific patents are related to food preparation and processing such as a patent (Hoogstad, 1988) for preparing tea or coffee extracts destined to freeze drying, a patent (Dalmau, 1987) for citrus fruit freezing, a patent (Roth, 1982) for freezing and forming meat patties, and a patent (W.a.A., 1969) for shrimp processing. Several refrigeration techniques are used: boiling refrigerant (i.e., Reynolds (1993)), cryogenic refrigerant (Anonymous, 1980) or brine. Cryogenic refrigerant such as liquid nitrogen is an expansive solution, as gas is emitted to the ambience. Nevertheless, its very low phase change temperature permits it to achieve high heat flux and thus fast freezing. Wentworth et al. (1968) presented a development for increasing the efficiency of the cryogenic fluid distribution thanks to a jacket. Anonymous (1980) describes a process in which the disadvantage of the stagnant cryogenic fluid at the lower part of the drum is used as an advantage to remove the product from the drum (owing to the thermal shock caused by the sudden cooling).
Machinery using the linear design (Fig. 3) has been recently developed and proposed on the market, while the rotating design has been used several years to freeze liquid or semiliquids foods. The linear continuous contact freezer consists of a refrigerated surface on which a plastic film is sliding, or on which food is frozen on a mobile refrigerated surface. In the former, the product is applied onto the film and is frozen during its translation on the refrigerated surface. After freezing, the product is removed from the film, which is discarded. Additional refrigerating effect is usually added by allowing Figure 2 Contact freezer: rotating drum freezer for liquid and viscous food.
refrigerated air on top of the product. This kind of process is not extremely efficient owing to thermal contact resistance between the product and the refrigerated surface. The plastic film represents a first thermal resistance. Moreover, the uncontrollable deformation of the product will result in the apparition of an air film between the cold surface and the plastic film. Nevertheless, this process remains very handy in achieving a superficial freezing of the product preventing dehydration during conventional freezing.
Single-side contact freezing is of course less efficient than double-side contact freezing. Nevertheless, a continuous process is highly desirable in the industry in order to minimize contamination by handling and also for productivity. Maltini (1984) studied the application of solid foods to contact freezing processes and did a comparison with air freezing. He suggested that the food should be regular in shape to ensure contact of greater than 20 mm2? g1. Donati (1983) did a similar study and compared the drum freezing technique with several other freezing processes.
V. SCRAPED FREEZERS
The scraped heat exchanger has been adapted to the case of freezing ice cream. It typically consists of a rotating drum equipped with one or two blades. The rotating drum is installed in a refrigerated vessel. The blades allow the scraping of the ice crystals formed onto the inner surface of the refrigerated drum. They also permits whipping of the air inside the mix. Indeed, a certain overrun (amount of air entrapped in the final ice cream) must be obtained to ensure an acceptable texture of the ice cream. For this purpose, a certain back pressure must be applied at the exit of the system (between ca. 100 and 500 kPa). Fat and air structures in ice creams have been investigated by several authors (Bolliger et al., 2000). The temperature of the refrigerated surface, the formulation, the back pressure, and the rotating speed will interact with the degree of fat destabilization and foam structure.
Straight blades are usually used. Helical blades aiming to propel the ice cream mix toward the exit of the system has been evaluated by Myerly (1998). More recently, a new design of continuous freezer has been developed by Windhab et al. (1998). This system has been developed from a twin screw extruder that has been adapted for the freezing of ice creams. The enhanced local shear stresses acting in the extrusion channel resulted in improved microstructure in comparison with conventional scraped heat exchanger. This process, known as cold extrusion, can yield an ice cream at a much lower temperature than a conventional scraped heat exchanger. Thus the ice cream obtained with such a freezer cannot be used to fill forms or mold but does not need any further conventional hardening.
VI. THERMAL PERFORMANCE OF FREEZING PROCESSES
The thermal performance of a given freezing process is related to the overall energy consumption required to cool down a given product from an initial temperature down to a final one. An accurate evaluation of this parameter is difficult because it has to take into account the type of refrigerating system (mechanical compression, cryogenic) being considered, the geometry of the product, the freezing rate, the final temperature, and the balance that will be considered between the refrigeration in the freezing process per se and the refrigeration load that will be held by the storage system. A first approach can be realized by comparing the heat transfer coefficient between the refrigerated medium
(convective freezing) and the surface (contact freezing). The order of magnitude of the effective heat transfer coefficient as defined by Eq. (17) for convective process or by Eq. (18) for contact freezing are summarized in Table 2.
U ¼ h ¼ F
ðTF TSÞ
with TS andTF ¼ T @ surface and fluid ð17Þ
U ¼ 1
TCR¼
F ðTRS TFSÞ
with TCR¼ thermal contact resistance ð18Þ
TRS and TFS¼ T @ refrigerated and food surface.
One can see from Table 2 that assuming a ‘‘perfect’’ thermal contact in the case of contact freezing is not acceptable. If the product is applied directly onto the refrigerated surface, heat transfer coefficients as high as 500 to 1000 can be expected, but are a function of process parameters as detailed by LeBail et al. (1998). A drum freezer used to freeze mashed vegetables (broccoli in the present case) was studied. A parameter study showed that the thermal contact resistance was higher for lower surface temperature. This supposes that the mechanical stress in the product during freezing (stretching owing to ice formation) interacts with the quality of the mechanical and therefore the thermal contact between the surface and the frozen product. The roughness of the metallic surface is also an important parameter. Specific study of LeBail (unpublished data) showed that a factor of 10 can be observed between a smooth and a rough surface of stainless steel (ca. 70 W? m2? K1 and 700 W? m2? K1 for the rough and the smooth surface, respectively). The presence of packaging drastically deteriorates the heat transfer. A plastic film seems to reduce slightly the heat transfer coefficient, whereas the presence of cardboard results in a heat transfer coefficient that can be as low as 20 W? m2? K1 (Creed et al., 1985), which is comparable to blast air freezing.
The thermal efficiency of the freezing process is thus highly related to the geometry of the product, its physical state (solid, liquid), and the process that is considered. A high
Table 2 Effective Heat Transfer Coefficients U for Convective and Conductive Freezing Processes as Defined by Eqs. (17) and (18)
Process Conditions U (W? m2? K1) Ref.
Convective Blast air 10–50
Convective Brine 50–500
Convective Liquid nitrogen, smooth cylinders, warming regime U¼ 1860
ðTFTSÞþ 125 W ? m
2? K1
120–200 Macchi, 1995
Convective Liquid nitrogen, strawberry, meat balls (experimental)
170–230 Agnelli and
Mascheroni, 2001 Conductive Drum freezer with direct contact,
sample¼ mashed broccoli mean value ranging between (vs. process parameters) 214 1000–166 Marizy et al., 1998; LeBail et al., 1998 Conductive Plate freezer: sample: copper block;
310 kPa pressure
Creed et al., 1985
No wrapping 481
1 layer polyethylene film 278 Corrugated fiberboardþ polyethylene 20
freezing rate is usually desired except for some specific products (i.e., bread dough). The packaging plays a major role. It permits a reduction of water loss but has a negative effect on the heat transfer rate. At present, individual quick freezing of meat products, for example, chicken breast, is much used in the industry. Recent studies have pointed to immersion freezing in brine, even though some problems will necessarily occur for the treatment of the brine. Contact freezing is also widely used and is well adapted for mass production. The presence of packaging is required for obvious handling reasons (removal of the frozen product from the refrigerated surface) but has a very negative impact on the efficiency of the process. Direct freezing in cardboard should be avoided.
VII. FREEZING RATE
The freezing rate is central to the final quality of frozen foods. A slow rate results in cell dehydration and large ice crystals that might damage the texture of a food. A fast freezing rate prevents the migration of water into the extracellular spacing and yields fine and numerous ice crystals. Side effects such as the increasing of the concentration of the remaining aqueous solution might affect the integrity of cell membranes or of proteins. The freezing rate is a very general statement used most of the time to compare freezing conditions. The freezing rate is numerically presented in two ways in the literature: Plank (1941) proposed an expression of the freezing rate evaluated as the velocity of the phase change front (dimension/time). The International Institute of Refrigeration (IIR, 1972) defined the nominal freezing time as the duration between 0 C and 10 C above the initial freezing temperature. Based on this definition, several researchers calculated the freezing rate by a ratio of temperature difference and the respective duration (Eq. (19) in K/time). This approach, which can be called temperature formulation, yields a freezing rate unit in K? s1 or in K? min1 (practical unit). The approach proposed by Plank (1941) will be called the Plank formulation and yields freezing rate in m? s1 or cm? h1(practical unit). Plank calculated the velocity of the phase change front by deriving the expression of the freezing time. This yielded Eq. (19a–c), respectively, for slab, cylinder, and sphere with x¼ distance from center (slab), r ¼ radius, and ro¼ outer radius of the geometry.
wðxÞ ¼ ðTa TfÞ r ? DH ? 1 hþ x lF h i m? s1 ð19aÞ wðrÞ ¼ ðTa TfÞ r ? DH ? r ro?hþ r lFLn ro r h i ðm ? s1Þ ð19bÞ wðrÞ ¼ ðTa TfÞ r ? DH ? r2 r2 o?hþ rðr2=r orÞ lF h i ðm ? s1Þ ð19cÞ FTðrÞ ¼ T1 T2 t1 t2 ðK ? min 1Þ ð19dÞ
In the case of the temperature formulation, a beginning criterion and an ending criterion for freezing must be defined [subscripts 1 and 2 in Eq. (19)]. LeBail et al. (1996, 1998b) showed that the freezing rate value is dependent on these criteria. Thus the use of the freezing rate from Plank expression or the evaluation of the freezing rate from the ratio of the lower thickness by the corresponding freezing time [i.e., determined by the nominal freezing time (IIR, 1972)]. In this latter case, a mean freezing rate will be obtained.
A high freezing rate might result in cracks in the product. Shi et al. (1999) reported that stress as high as 2 MPa (20 atm) can be reached during the freezing of biological tissue. This result was obtained from a mathematical model (viscoelastic model coupled to a thermal model) developed to study the freezing of a sample of potato (17.8 mm diameter). A frozen mantle first appears at the surface of the product. Meanwhile, the formation of ice at the core will yield an increase of the pressure. Radial and circumferential stresses develop during freezing. Rapid temperature drop (i.e., cryogenic freezing with surface temperature down to196 C for liquid nitrogen) will induce higher stress, which can be as high as 1.5 MPa, whereas freezing in a medium at 40 C yields stress in the range of 1 to 0.5 MPa (Shi et al., 1999). Even though the temperature of the frozen external mantle is far from the glass transition, the tensile failure strength, which was around 0.5 MPa for potato, might be passed, leading to cracks. On the other hand, a depression of the initial freezing point due to a pressure increase will result in a partial thawing (Otero et al., 2000) leading to a release of the stress.
VIII. CONCLUSION
This chapter offers a general presentation of the freezing processes including evaluation of the freezing time. A focus proposed on the surface heat transfer coefficient includes contact freezing. It shows that an infinite heat transfer coefficient can’t be assumed in this process, which is widely used in the industry.
REFERENCES
IIR (1972). Recommendations for the processing and handling of frozen foods. International Institute of Refrigeration, 2nd Ed. Paris, 1972.
M, Agnelli, et al. (2001). Cryomechanical freezing. A model for the heat transfer process. Journal of Food Engineering 47:263–270, 2002.
Anonymous (1980). Method and Apparatus for Cooling and Freezing. Patent UK-2023.789A. ED Berry, et al. (1998). Bacterial cross-contamination of meat during liquid-nitrogen immersion
freezing. Journal of Food Protection 61(9):1103–1108.
S Bolliger, et al. (2000). Correlation between colloidal properties of ice cream mix and ice cream. International Dairy Journal 10(4):303–309.
A Cleland, et al. (1979b). A comparison of methods for predicting the freezing times of cylindrical and spherical foodstuffs. Journal of Food Science 44:958.
AC Cleland, (1990). Food refrigeration processes. Analysis, design and simulation. E. Sciences, p. 284.
D Cleland, et al. (1986). Prediction of thawing times for food of simple shape. International Journal of Refrigeration 10:32–39.
PG Creed, et al. (1985). Heat transfer during the freezing of liver in a plate freezer. Journal of Food Science 50:285–294.
G Dalmau, (1987). Method for freezing citrus fruit portions. Patent EP.0248.753.A2.
L Donati, (1983). Freezing of foods. Effects of freezing on thermophysical properties of foods. Technologie-Alimentari 6(6):21–31.
B Hoogstad, (1988). Method of preparing a freeze-dried food product. Unilever. Patent EP-0256.567.A2.
A LeBail, et al. (1996). Application of freezing rate expressions and gassing power to frozen bread dough. Proceedings of the International ASME Congress, Atlanta, GA, USA.
A LeBail, et al. (1998a). Continuous Contact Freezers for Freezing of Liquid or Semi-Liquid Foods. Influence of the Thermal Contact Resistance Between Food and Refrigerated Surface. Symposium of the International Institute of Refrigeration, Nantes, France.
A LeBail, et al. (1998b). Influence of the freezing rate and of storage duration on the gassing power of frozen bread dough. Symposium of the International Institute of Refrigeration, Nantes, France.
F Levy, (1958). Calculating freezing time of fish in airblast freezers. Journal of Refrigeration 1(55). T Lucas, et al. (1999a). Mass and thermal behaviour of the food surface during immersion freezing.
Journal of Food Engineering 41(1):23–32.
T Lucas, et al. (1999b). Factors influencing mass transfer during immersion cold storage of apples in NaCl/sucrose solutions. Lebensmittel Wissenschaft und Technologie 32(6):327–332.
HMacchi, (1995). Conge´lation alimentaire par froid mixte. Proce´de´ avec pre´traitement par immersion dans l’azote liquide. ENGREF, Paris.
A Madsen, (1983). Drum-freezing and extrusion of fish. Boletim de Pesquisa, EMBRAPA Centro de Technologia Agricola e Alimentar (Brazil) 1:204–205.
E Maltini, (1984). Contact freezing, Industrie-Alimentari. 23 218:573–580.
C Marizy, et al. (1998). Modelling of a drum freezer. Application to the freezing of mashed broccoli. Journal of Food Engineering 37(3):305–322.
NHMermelstein, (1997). Triple-pass immersion freezer eliminates need for separate mechanical freezer. Food Technology 51(7):133.
RMS Myerly, (1998). Stepped helical scraper blade for ice cream maker. United States Patent US-845349.
J Nagaoka, et al. (1955). Experiments on the freezing of fish by the air-blast freezer. Journal of Tokyo University of Fischery 42(1):65.
L Otero, et al. (2000). High pressure shift freezing. Part 1. Amount of ice instantaneously formed in the process. Biotechnol. Prog. 16:1030–1036.
R Plank, (1941). Beitrage zur berechnung und bewertung der gefriergeschwindigkeit von lebensmittel. Beiheft zur Zeitschrift fu¨r die gesamte Ka¨lte-industrie 3(10):1–16.
HS Ramaswamy, et al. (1984). A review on predicting freezing times of foods. Journal of Food Process Engineering 7:169–203.
M Reynolds, (1993). Drum Contact Freezer System and Method. US Patent US-5199.279. E Roth, (1982). Method of Freezing and Forming Meat Patties. US Patents US-4849.575.
X Shi, et al. (1999). Thermal fracture in a biomaterial during rapid freezing. Journal of Thermal Stresses 22:275–292.
KP Venger, et al. (1990). Freezing of fish by immersion in non-boiling liquid (in Russian). Kholodil’naya Tekhnika 5:30–32.
Wentworth and Associates Inc. (1969). Shrimp Processing. Patent UK-1.173.348. A Wentworth, et al. (1968). Quick Freezing Apparatus. US Patent US-3410.108.
EJ Windhab, et al. (1998). Low temperature ice-cream extrusion technology and related ice-cream properties. European Dairy Magazine 1:24–29.
2
Principles of Freeze-Concentration and
Freeze-Drying
J.Welti-Chanes and D.Bermu´dez
Universidad de las Ame´ricas–Puebla, Puebla, Mexico A.Valdez-Fragoso and H.Mu´jica-Paz
Universidad Auto´noma de Chihuahua, Chihuahua, Mexico S.M.Alzamora
Universidad de Buenos Aires, Buenos Aires, Argentina
In freeze-concentration and freeze-drying processes, water is first frozen in the material. Ice is removed by mechanical means during freeze-concentration, leaving a concentrated liquid, while ice is removed by sublimation in freeze-drying, yielding a dried material. The removal of water by these methods yields high-quality products, but in both processes it is a very expensive operation owing to the high consumption of energy. Knowledge of the theoretical principles behind these processes is necessary for minimization of detrimental changes, operating strategies, and optimization purposes. Thus the fundamental aspects of freeze-concentration and freeze-drying are presented in this chapter.
I. FREEZE-CONCENTRATION
A. Introduction
Freeze-concentration is the term used to describe the solute redistribution in an aqueous solution with an initial relatively low concentration by the partial freezing of water and subsequent separation of the resulting ice [1,2]. Freeze-concentration is based on the freezing temperature-concentration diagram (Fig. 1) [3].
It is necessary briefly to review the physicochemical changes that occur during a freezing process before relating them to the freezing of foods. The phase diagram (Fig. 1) allows identifying different phase boundaries in a mixture. It consists of the freezing curve (AB), solubility curve (CE), eutectic point (E), glass transition curve (DFG), and conditions of maximal freeze-concentration. The freezing curve corresponds to solution– ice crystals equilibrium. Along this curve, as water is removed as ice, the concentration of solute increases during the freeze-concentration process. The solubility curve represents equilibrium between the solution and supersaturated solution in a rubbery state. The freezing and solubility curves intersect at the eutectic point E (Ce, Te), which is defined as
the lowest temperature at which a saturated solution (liquid phase) can exist in equilibrium with ice crystals (solid phase). The water content at point E is the unfreezeable water. Below Te only ice crystals embedded in a solute–water glass exist. The point F (C0g; T0g)
lower than point B (C0g; T0m) represents a characteristic transition in the state diagram. The glass transition curve (DFG) represents the glass–rubber transition of the solute–water mixture, and the type and concentration of the solute and the temperature define it. Above the DFG curve, solutions are in an unstable rubbery or liquid state; below the DFG curve, solutions transform into the glassy state (amorphous solid). The maximum freeze-concentration (maximum ice formation) only occurs in the region above T0g, but below the equilibrium ice melting temperature of ice (T0m) [4,5]. The liquid solute–water mixture is the maximum freeze-concentrated and has become glassy. The glass transition temperature of this unfrozen glassy mixture is designated T0g, and C0g is the solid content of this glass [3–7].Figure 1also shows the aqueous solution with initial concentration and temperature Ci and Ti undergoing freeze-concentration.
B. Freeze-Concentration System
A typical freeze-concentration system (Fig. 2)consists of three fundamental components: (a) a crystallizer or freezer, (b) an ice–liquid separator, a melter–condenser, and (c) a refrigeration unit. In the freeze-concentration system, the solution is usually first chilled to a prefreezing temperature in a cooler (Fig. 2), and then the solution enters the crystallizer where part of the water crystallizes. Cooling causes ice crystal growth and an increase in solute concentration. The resulting mixture of ice crystals and concentrated solution is pumped through a separator where crystals are separated and the concentrated solution is drained off. Ice crystals are removed and melted by hot refrigerant gas. The final products are cold water and concentrated solution, which flow separately [1,8].
1. Crystallizers
The heat of crystallization can be taken out directly or indirectly. In direct-contact crystallizers, the original solution is allowed to get in contact with the refrigerant, and heat is withdrawn by vacuum evaporation of part of the water, usually at pressures below 3 mm Hg, and by evaporation of the refrigerant. The refrigerants (CO2, C1–C3 hydrocarbons)
form icelike gas hydrates, which sequester water at temperatures above 0 C. A disadvantage of this method is that part of the aromas will be lost during the evaporation. Direct heat removal is applied in seawater desalinization but is not suitable for liquid foods, owing to the aroma losses and deterioration of the product by the refrigerant. In crystallizers with indirect heat removal, the refrigerant (R22 or ammonia) is separated from diluted solution by a metal wall. So crystallization takes place on chilled surfaces, from which ice crystals are removed by a scraper. This kind of process has been used commercially for orange juice and coffee concentration [1,9].
2. Separators of Ice-Concentrated Solution
The separation of ice crystals from concentrated solutions can be performed by the use of presses, centrifuges, and wash columns, operating in either batch or continuous mode.
Hydraulic and screw presses are used for pressing ice-concentrated slurries to form an ice cake. Pressures around 100 kg/cm2 are needed to avoid occlusion of solids in the cake, which is the limiting factor of this method. Since the presses are completely closed, aroma loss is negligible [1,9].
Ice and concentrated solutions may be separated by centrifugation at about 1000 G. Centrifugation must be conducted under inert atmospheres to reduce oxidation and aroma loss. Solute losses may occur if concentrated solution remains adhered to the crystal surface, but washing of the cake with water will minimize such losses. This washing stage renders the centrifugation operation more efficient than pressing [1,2,9].
In washing columns, the ice–solution mixture is introduced at the bottom of the tower, and the solution is drained off. The crystals move toward the top of the column in countercurrent to the wash liquid, which is obtained by melting part (5–3%) of the washed crystals leaving the column. In this process the loss of dissolved solids with the ice is less than 0.01%, and aroma losses are negligible. Wash columns are preferred in freeze-concentration of low-viscosity liquids such as beer and wine [1,8,9].
C. Influence of Process Parameters
Crystallization is the main step in freeze-concentration, so it is very important to obtain large and symmetrical crystals. Large crystals can be more easily separated from the concentrated solution. Large crystals also reduce the loss of solutes due to occlusion and adherence to the small crystals [1,8]. During crystallization, two kinetic processes take place: the formation of nuclei and the growth of crystals. Nucleation is the association of molecules (at some degree of subcooling) into a small particle that serves as a site for crystal growth. Once a nucleus is formed, crystal growth is simply the enlargement of that nucleus. Nucleation and growth of crystals are dependent on solute concentration, bulk supercooling, residence time of the crystals in the crystallizer, freezing rate, molecular diffusion coefficient of water, and heat transfer conditions. These factors should be carefully controlled to regulate crystal formation [2,10].
1. Solute Concentration
In general, an increase in solute concentration produces an increase in nucleation and a decrease in the growth velocity of the ice crystals and in the mean diameter of the crystal. At critical concentration, solutes may solidify along with ice and are difficult to separate. Practical maximum concentrations for freeze-concentration are between 45–55% range [1,9,10].
2. Bulk Supercooling
Supercooling is the driving force responsible for the creation of crystal nuclei and their growth. The nucleation rate is proportional to the square of the bulk supercooling. At high bulk supercooling values, the nucleation rate decreases, owing to the inhibition of molecular mobility. Crystal growth exhibits a first-order dependence of the bulk supercooling [1,9,10].
3. Residence Time of the Crystals in the Crystallizer
At constant bulk supercooling and solute concentration, the crystal size is proportional to the crystal residence time. At short residence times the crystals produced are very small [1,10].
A high freezing rate results in a strong local supercooling near the heat-removing interface, thus leading to high nucleation rates and to small crystals. A decrease in freezing rate results in large, uniform crystals with small surface area [1,10].
5. Molecular Diffusion Coefficient of Water
A decrease in the value of the molecular diffusion coefficient of water results in a decrease in diameter of the crystals [1].
6. Heat Transfer Conditions
The growth rate of ice crystals increases greatly as the rate of heat removal is increased, until some very low sample temperature is reached, at which mass transfer difficulties (as high viscosity) cause the growth rate to decline. Very large uniform crystals require large exchange surface at relatively high temperatures [1,2,9].
7. Viscosity of the Liquid
Viscosity increases markedly as concentration increases, ice crystals grow very slowly at high viscosity, and large crystals become difficult to separate. The maximum concentration obtainable in freeze-concentration depends on the liquid viscosity. Generally, concentra-tion can be carried out to the point where the slurry becomes too viscous to be pumped. For essentially all liquids, this viscosity limit is encountered before eutectic point formation occurs (Fig. 1). The viscosity of cold concentrated liquid and ice is very high, and agitation, which is necessary for proper crystal growth, becomes more difficult [9,10]. In all the ice separators, capacity is inversely proportional to the viscosity of the concentrate and directly proportional to the square of the mean diameter of the crystals as expressed by the equation
Q¼DPgd 2 e 0:2 ml * e3 1 e ð Þ2 ð1Þ
where Q is the draining rate from the crystal bed (cm3/cm2s);DP is the pressure difference exerted over the bed by compression or by centrifugal or pressure drop of the filtrate (kg/ cm2); deis the diameter of the crystals (cm);m is the viscosity of the liquid (poise); l is the
thickness of the bed (cm); g is the gravity acceleration (cm/s2); ande is the volume fraction in the bed filled by the liquid phase [1].
II. FREEZE-DRYING A. Introduction
Freeze-drying, or lyophilization, is the process of removing water from a product by sublimation and desorption [11]. Sublimation is the transformation of ice directly into a gas without passing through a liquid phase. Sublimation occurs when the vapor pressure and the temperature of the ice surface are below those of the triple point (4.58 mm Hg, 0 C), as shown in the pressure–temperature phase diagram of pure water (Fig. 3) [12].
The phase diagram of Fig. 3 is separated by lines into three regions, which represent the solid, liquid, and gaseous states of water in a closed system. The points along the separating lines represent the combinations of temperature and pressure at which two states are in equilibrium: liquid–gas equilibrium (DB line), liquid–solid equilibrium (DA
line), and solid–gas equilibrium (DC line), which is of main concern in freeze-drying. Point D represents the only combination of temperature and pressure at which all three states of water are simultaneously in equilibrium, and it is called the triple point [4,12].
Freeze-drying can also be conducted at moderated pressures and even at atmospheric pressure. The principle of this process is to produce a vapor pressure difference as large as possible by blowing dry air over the frozen material. In practice, the process is very long because of the low mass and energy transfer rates, but problems related to the application of vacuum do not exist, resulting in an important reduction of operation costs [13,14].
Freeze-drying is used to obtain dry products of higher quality than those obtained with conventional drying methods. Freeze-dry products have high structural rigidity, high rehydration capacity, and low density, and they retain the initial raw material properties such as appearance, shape, taste, and flavor. This process is generally used for the dehydration of products of high added value and sensitivity to heat treatments, produced by the pharmaceutical, biotechnological, and food industries.
Compared to air drying processes, which remove water in a single stage, freeze-drying is an expensive process, since it takes large operation times and consumes large amounts of energy. Energy is required to freeze the product, heat the frozen product to sublimate ice, condense water vapor, and maintain the vacuum pressure in the system [15,16].
B. Basic Components of a Freeze-Dryer
The typical freeze dryer consists of a drying chamber, a condenser, a vacuum pump, and a heat source (Fig. 4).
The drying chamber, in which the sample is placed and heating/cooling take place, must be vacuum tight and with temperature-controlled shelves. The condenser must have sufficient condensing surface and cooling capacity to collect water vapor released by the product. As vapors contact the condensing surface, they give up their heat energy and turn into ice crystals that will be removed from the system. A condenser temperature of65 C Figure 3 Pressure–temperature phase diagram of pure water.
is typical for most commercial freeze-dryers. The vacuum pump removes noncondensable gases to achieve high vacuum levels (below 4 mm Hg) in the chamber and condenser. The heating source provides the latent heat of sublimation, and its temperature may vary from 30 to 150 C [13,17,18].
C. Freeze-DryingStages
Freeze-drying involves three essential stages: initial freezing, primary drying, and secondary drying. The objective of the freezing stage is to freeze the mobile water of the product. The product must be cooled to a temperature below its eutectic point, which is the temperature and composition combination that produces the lowest point at which a product will freeze. Freezing has an important influence on the shape, size, and distribution of the ice crystals and thus on the final structure of the freeze-dried product. In the primary drying, the frozen product is heated under vacuum conditions to remove frozen water by sublimation, while the frozen product is held below the eutectic temperature. During the primary drying, approximately 90% of the total water in the product, mainly all the free water and some of the bound water, is removed by sublimation [19,20]. In the secondary drying, bound water (unfrozen) is removed by desorption from the dried layer of the product, achieving a product that should contain less than 1–3% residual water. This final stage is performed by increasing the temperature and by reducing the partial pressure of water vapor in the dryer [12,20].
The secondary drying stage requires 30 to 50% of the time needed for primary drying because of the lower pressure of the remaining bound water than free water at the same temperature, yielding a slow process. Freeze-drying is complete when all the free and bound water has been removed, resulting in a residual moisture level that assures desired structural integrity and stability of the product [12,16].
D. Heat and Mass Transfer in Freeze-Drying
During the freeze-drying operation, a coupled heat and mass transfer process occurs within the product: energy is transported to the sublimation zone and water vapor is generated. In contrast with mass transfer, which always flows through the dry layer, heat transfer can take place by conduction through the dry layer (Fig. 5a)or through the frozen Figure 4 Simple schematic representation of a freeze-dryer system.
layer (Fig. 5b), and by heat generation within the frozen layer by microwaves (Fig. 5c) [12,20]. Microwaves are used as a heat source for drying because they are able to penetrate deeply into the product, giving a more effective and uniform heating [21,22].
Figure 6 illustrates a frozen food sample in the form of a slab, with a frozen and a dried porous layer, undergoing one-dimensional freeze-drying [12,23]. The interface between the dried and the frozen layers is referred to as the sublimation or ice front, and it is assumed to move at a uniform rate. The vapor flows through the pores and channels. In case heat is supplied through the dry layer, the heat flux to the ice front is given by q¼ kd
Te Tf
ð Þ
1 x
ð ÞL ð2Þ
where q is the heat flux (J/m2s), kdis the thermal conductivity of the dry layer (W/m K), Te
is the temperature at slab surface (8C), Tfis the temperature of sublimation front (8C), L is
the thickness of the slab (m), and x is the relative height of the ice front. If heat is transferred through the frozen layer,
q¼ kf
Tp Tf
xL ð3Þ
where kf is the thermal conductivity of the frozen layer (W/m K) and Tp is the heating
plate temperature (8C).
If vapor flows in the pores mainly by Knudsen diffusion, the collisions with the dry walls are numerous compared with collisions between water molecules. Thus the rate of ice Figure 5 Basic types of freeze-drying.
sublimation (NW, kg/s m2) is given by NW ¼ eDK Mw RT pf pe ð1 xÞL ð4Þ
where e is the volume fraction of ice, DK is the Knudsen diffusion coefficient of water
vapor (m2/s), Mwis the molecular mass of water (kg/mol), R is the gas constant (J/mol K),
T is the absolute temperature (K), pf is the vapor pressure at the ice front, and pe is the
vapor pressure at the slab surface (Pa).
The Knudsen diffusion coefficient is related to pore diameter (dp) and temperature
by Dk ¼ 2 3dp 2RT pMw 0:5 ð5Þ
At the ice front, Tf represents the sublimation temperature, and by assuming it to be in
equilibrium, it is related to vapor pressure (pw) by the Clausius–Clapeyron equation,
ln pð Þ ¼ 28:9 w
6138
Tf ð6Þ
From a mass balance for water vapor around the drying slab, NW ¼ erice
d xLð Þ
dt ð7Þ
where riceis the density of ice (kg/m3).
Assuming that all supplied heat is used for sublimation of ice, the enthalpy balance gives
q¼ DHsNW ð8Þ
where DHsis the latent heat of sublimation (J/kg).
For temperature differences not too large, the Clausius–Clapeyron equation can be linearized, and the above equations can be solved analytically. The following expressions can be derived for the total drying time:
t¼aL 2 DK 1þ bDK ð Þ ð9Þ in which a ¼1 2rice RTf Mw 1 pf pe ð Þ ð10Þ
If heat transfer takes place through the dry layer, thenb ¼ 0; and if heat transfer occurs through the frozen layer,
b ¼ eMw2DHs2pf
R2T3 fkice
ð11Þ
where kice is the thermal conductivity of ice (W/m K).
Several mathematical equations describing mass and energy transfer have been developed for modeling the freeze-drying process. Such models account for the removal of frozen water only (sublimation model) or for the removal of frozen and bound water
(sorption sublimation model). These models also examine the methods of supplying heat and the diffusion mechanisms, describe steady or non-steady state processes, or analyze both transfers under various processing conditions. Some models have been found to describe accurately experimental drying rates and freezing times. However, a major problem in the application of some models is the requirement of reliable data on thermal and mass transport properties of food materials such as diffusivity within the porous medium, Knudsen diffusion, water vapor concentration in the dry layer, porosity, effective thermal conductivity, permeability, etc. [15,19,21,24].
E. Influence of Parameters
A number of operation variables influence the performance of the freeze-drying process and the characteristics of the final product.
1. Freezing
The freezing rate has an important influence on the ice configuration and thus on the final structure of the freeze-dried product. Slow freezing rates allow the growth of large ice crystals, leading to larger pores, to higher mass flow, and thus to shorter freeze-drying times [12,23].
2. Heat Flux
Heat flux that reaches the product is an important factor to reduce the drying rate. However, if the drying proceeds too rapidly (high heat flux), the product may melt, collapse, or be blown out of the container [18]. This may cause degradation of the product and will change the physical characteristics of the dried material. Excessive heat may cause the dry cake to char or shrink. The heating rate can be optimized during operation modifying conveniently product temperatures in the dried zone and at the sublimation front [15,25].
3. Chamber Pressure
The most important operation variable in the freeze-drying process is chamber pressure. The pressure both controls the mean of the sublimation temperature and modifies the transport parameters that influence the kinetics of vapor removing. At a given temperature, a decrease of the pressure in the drying chamber reduces the vapor pressure at the product’s external surface (pe), thus the driving force (pf pe) for drying is enlarged,
and the total drying time is reduced. Nevertheless, at low pressures, the sublimation rate may be limited by the transport of water vapor through the product, if the transport of water vapor falls in the free molecular flow regime [21,25,26].
Chamber pressure affects the transport properties, thermal conductivity, and water vapor diffusivity. Thermal conductivity of the dry layer is higher at higher chamber pressures, within the range of freeze-drying operation, resulting in high heat transfer rates from the surface to the ice front. Water vapor diffusivity through the dry layer is, however, less at higher chamber pressures, producing low mass transfer rates. So, when pressure is low (low sublimation temperature), freeze-drying is often a heat-controlled process, but at relatively high pressures freeze-drying becomes a mass-controlled process. In most situations, the drying rate is limited by the rate of heat transfer through the dry layer [25–27].
4. Temperature
Aroma diffusivities are very similar to that of water when the water content is still high; therefore maintaining low temperatures during primary drying will reduce aroma losses. The melting point of products has a significant effect on the selection of operation pressures, since this is a fundamental factor for the sublimation temperature. Normally, a vacuum must be kept so high that no melting occurs in the product during the process, and a true freeze-drying or sublimation takes place. If the temperature of ice in the condenser is higher than product’s temperature, water vapor will tend to move toward the product, and drying will stop [26,28].
When freeze-drying temperature is high enough, the product cake suffers a drastic loss of its structure and is said to have undergone collapse. Collapse affects aroma retention, caking and stickiness, rehydration capacity, and final moisture of the product. A collapse temperature (Tc) is related to the glass transition temperature (Tg), which in turn
depends on temperature and moisture content (Fig. 1). At temperatures higher than Tg,
the viscosity of the amorphous matrix decreases drastically, this decrease being a function of (T Tg). As the viscosity decreases to a level that facilitates deformation, the matrix can
flow, and structural collapse can occur. A critical viscosity in the range of 105–108Pas has been reported to observe collapse [20,29].
III. CONCLUSIONS AND RECOMMENDATIONS
Despite the reduced use at the industrial level of freeze-concentration and freeze-drying processes within the food area, both are important to obtain high quality products. Deep knowledge of the fundamentals of phase changes of water in foods and of the effect of the variables on the processes’ effectiveness and cost can open new opportunities for the application of both processes to obtain high-quality preserved foods.
REFERENCES
1. HAC Thijssen. Freeze concentration. In: A Spicer, ed. Advances in Preconcentration and Dehydration of Foods. London: Applied Science Publishers, 1974, pp. 115–149.
2. M Karel. Concentration of foods. In: M Karel, OR Fennema, DB Lund, eds. Physical Principles of Food Preservation. New York: Marcel Dekker, 1975, pp. 287–294.
3. TR Noel, SG Ring, MA Whittam. Glass transition in low-moisture foods. Trends in Food Sci and Technol 62–67, 1990.
4. HD Goff. Low-temperature stability and the glassy state in frozen foods. Food Research International 25:317–325, 1992.
5. Y Bai, MS Rahman, CO Perera, B Smith, LD Melton. State diagram of apple slices: glass transition and freezing curves. Food Research International 34:89–95, 2001.
6. J Welti-Chanes, JA Guerrero, ME Ba´rcenas, JM Aguilera, F Vergara, GV Barbosa-Canovas. Glass transition temperature (Tg) and water activity (aw) of dehydrated apple products. J Food
Process Eng 22:91–101, 1999.
7. JMV Blanshard. The glass transition, its nature and significance in food processing. In ST Beckett, ed. Physico-Chemical Aspects of Food Processing. London: Blackie Academic and Professional, 1995, pp. 15–48.
8. SS Deshpande, HR Bolin, DK Salunke. Freeze concentration of fruit juices. Food Technol 68– 82, 1982.
