Handbook of frozen foods

Handbook of frozen foods

author(s) nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage, or liability directly or indirectly caused or alleged to be caused by this book The material contained herein is not intended to provide specific advice or recommendations for any specific situation.

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ISBN: 0-8247-4712-7

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For centuries, frozen foods have been available to consumers in countries that experiencecold winters In some areas with severe winters such as Alaska, Russia, and others, foodsare routinely frozen by leaving them outside Since 1875, with the development ofmechanical 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 freshfoods as compared with other preserved or processed foods Frozen foods are popular andaccessible in most developed countries, where refrigerators and freezers are standard homeappliances Nowadays, frozen foods have become essential items in the retail foodindustries, grocery stores, convenience food stores, fast food chains, food services, andvending machines This growth is accompanied by the frequent release of new referencebooks for the frozen food industry

Several updated books on freezing preservation of foods or frozen foods have beenavailable in the past decade, and most of them are excellent books The science andtechnology of food freezing can be viewed from several perspectives:

Food engineering principles These principles explain such phenomena as heat andmass transfer, freezing time, convective and conductive processes, and otherprocesses and principles relevant to understanding the dynamics of freezing.Food science and technology principles These principles explain the chemistry andbiology of food components, their interactions during processing, and otherprinciples relevant to understanding how foods behave before, during, andafter the frozen stage

Food manufacturing principles.These principles explain how we can start with a rawingredient and end with a finished frozen product

Food commodities, properties and applications This approach takes an individualcommodity of food (e.g., fruits, vegetables, dairy, muscle foods) and explainsthe whole spectrum of factors that involve cooling, refrigeration, freezing, andthawing unique to that category of food and its properties Although theunderlying principles are the same, freezing carrots is definitely different fromfreezing 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 thetopics above When it comes to books on frozen foods, it is an endless venture The reason issimple: Every month and every year, food scientists, food technologists, and food engineerswitness rapid development in the science and technology of frozen foods We continually seenew knowledge, new equipment, and new commercial applications emerging

Based on the above premises of principles and applications, the Handbook of FrozenFoods uses the following approaches to covering the data:

Principles Chapters 1 through 8 cover principles applicable to the processing offrozen foods, such as science, technology, and engineering Topics include thephysical processes of freezing and frozen storage, texture, color, sensoryattributes, and packaging

Meat and poultry Seven chapters (Chapters 9–15) discuss freezing beef and poultrymeat, 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 CriticalControl Points), and product descriptions

Vegetables.Five chapters (Chapters 22–26) discuss frozen vegetables, covering productdescriptions, quality, tomatoes, French fries, and U.S grades and standards.Fruits Chapters 27 through 29 discuss frozen fruits and fruit products, coveringproduct descriptions, tropical fruits, and citrus fruits

Special product categories.Chapters 30, 31, and 32 provide details on some popularproducts: frozen desserts, frozen dough, and microwavable frozen foods.Safety.Chapters 33 through 36 discuss the safety of processing frozen foods coveringbasic considerations, sanitation of a frozen food plant, risk analysis inprocessing 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 internationaleditorial 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 frozenfood 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 thefruits of their labor

We know how hard it is to develop the content of a book However, we believe thatthe production of a professional book of this nature is even more difficult We thank theproduction team at Marcel Dekker, Inc., and express our appreciation to Ms TheresaStockton, coordinator of the entire project

You are the best judge of the quality of this book

Y H HuiPaul CornillonIsabel Guerrero Legarreta

Miang H Lim

K D MurrellWai-Kit Nip

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

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

Patti C Coggins and Roberto S Chamul

8 Texture in Frozen FoodsWilliam L Kerr

9 Frozen Muscle Foods: Principles, Quality, and Shelf LifeNatalia 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

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

Peggy Stanfield

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

PART VI FROZEN FRUITS AND FRUIT PRODUCTS

27 Frozen Fruits and Fruit Juices: Product Description

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

Y H Hui

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

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 StateUniversity, 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, NewZealand

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

Philip Cheng-Ming Chang National Taiwan Ocean University, Keelung, Taiwan

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 StateUniversity, 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, NewJersey, 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, NewZealand

Miang H Lim Department of Food Science, University of Otago, Dunedin, NewZealand

Ana Guadalupe Luque-Alcara´z Centro de Investigacio´n en Alimentacio´n y Desarrollo,A.C., Hermosillo, Sonora, Mexico

Domingo Martı´nez-Romero Miguel Hernandez University, Orihuela, Spain

J Mateo-Oyagu¨e Universidad de Leo´n, Leo´n, Spain

Janet E McFetridge Department of Food Science, University of Otago, Dunedin, NewZealand

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, MexicoEdith 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

Hua Zhao Rutgers University, New Brunswick, New Jersey, U.S.A

is probably the most popular freezing process, but other concepts such as contact freezingare also used in a wide range of applications The thermal contact resistance existingbetween the refrigerated surface and the product is often neglected; a focus is proposed onthis 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 basicallynonlinear 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 toobtain 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 yeastactivity 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 beoperated in a production line The refrigeration system used allows classifying freezers intwo other subfamilies: freezers using cryogenic fluids such as carbon dioxide or liquidnitrogen, and freezers using a mechanical refrigeration unit and a secondary refrigerationfluid (air, brine, etc.) Mechanical refrigeration units are used for a large majority ofindustrial 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 Heattransfer 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 initialtemperature of the product is equal to the phase change temperature, (b) phase changeoccurs at constant temperature, and (c) all thermophysical properties and heat transfercoefficients are constants Consequently, the initial cooling and final cooling after freezingare not taken into account The freezing time given by the Plank formula is proposed in

Based on this first approach, several authors attempted to improve the accuracy ofthe freezing (or thawing) time calculation Ramaswamy et al (1984) proposed a review ofthese equations Nagoaka et al (1955) proposed Eq (2), which takes into account theamount 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 productand the final temperature at the end of freezing (J? kg1).

Table 1 Coefficient of the Plank Formula

(Eq 3) The International Institute of Refrigeration (1972) proposed Eq (3), which is onceagain very similar to the model of Nagoaka This time it is the enthalpy difference that istaken 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).

F0¼ PBiSteþ R

Ste¼ DH between Tf and Ta

P* ¼ 0:3751 þ 0:0999Pk þ Steð0:4008Pk þ0:0710

And for a sphere,

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 beconsidered as a good basis The accuracy can be greatly improved by using numericalmodels 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 majordifficulties

III CONVECTIVE PROCESSES: AIR FREEZING, BRINE FREEZING,

CRYOGENIC FREEZING

In the case of convective freezing, air, a cryogenic fluid (mainly liquid nitrogen), or a brinecan be used as refrigerant In the case of air, it can be admitted that the air velocity is in therange of 1 to 5 m? s1 for most industrial application, leading to the effective heat transfercoefficient 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 widelyused in the industry Individual quick freezing (IQF) consists of freezing small productsindividually with a high air speed (i.e., 1–5 m? s1) Freezing of larger products can berealized with blast air but will yield a low freezing rate and thus a low quality in terms ofice crystal size; plate freezers are preferred Some specificity in terms of air flow patternhave been developed in order to reduce water loss by dehydration (i.e., counter flow orpartial counter flow with air inlet a mid position between entrance and exit) Higher airvelocity can also be imposed in a local section such as the entrance of the freezer This hasbeen developed for small products (few centimeters thick) It improves heat transfer andpermits a superficial freezing which will minimize mass losses by reducing partial vaporpressure at the surface of the product This partial superficial freezing is also called crustfreezing 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 beforetraveling either into a conventional belt freezer at follow or in the vicinity of the cryogenicbath 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 Theproduct 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 unwrappedproducts (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 preventedfreezing 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 excessivewater 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 ofthe immersion technique is that the brine concentration is changing during the process,requiring a specific adjustment of it during the treatment Undesirable side effects mayoccur such as spoilage of the product by the brine (requiring filtering and cleaning of thebrine) 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 betweencontact freezing (contact of the solid flakes of CO2) and convective freezing (convectiveheat 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 highfreezing rate IQF and cryogenic refrigeration will yield yet higher freezing rates It iswidely used in the industry to produce slabs of frozen foods such as fish filets and mashedvegetables As for any freezing process, the geometry of the product will rule the freezingrate 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 frozenfrom 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 arefrigerated surface and by scraping it off after freezing Two concepts have beendeveloped, 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 refrigeratedsurface, but this will create a problem in removing the frozen product at the end of theprocess Rotating drum freezers (Fig 2) have been developed in the industry to freeze

Figure 1 Contact freezer: batch system.

liquid or viscous products or even solid products such as fish filets In this case, theproduct 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) Theproduct 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 processimproved the storage stability of cod relative to those frozen in a plate freezer Recentliterature on drum freezer technology mainly concerns patents Some of these patents arerelated to heat transfer improvement between the refrigerant and the drum, such asReynolds (1993) in the case of boiling refrigerant (R22 type) Specific patents are related tofood preparation and processing such as a patent (Hoogstad, 1988) for preparing tea orcoffee extracts destined to freeze drying, a patent (Dalmau, 1987) for citrus fruit freezing, apatent (Roth, 1982) for freezing and forming meat patties, and a patent (W.a.A., 1969) forshrimp processing Several refrigeration techniques are used: boiling refrigerant (i.e.,Reynolds (1993)), cryogenic refrigerant (Anonymous, 1980) or brine Cryogenicrefrigerant such as liquid nitrogen is an expansive solution, as gas is emitted to theambience Nevertheless, its very low phase change temperature permits it to achieve highheat flux and thus fast freezing Wentworth et al (1968) presented a development forincreasing 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 thelower 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 orsemiliquids 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 refrigeratedsurface In the former, the product is applied onto the film and is frozen during itstranslation on the refrigerated surface After freezing, the product is removed from thefilm, which is discarded Additional refrigerating effect is usually added by allowingFigure 2 Contact freezer: rotating drum freezer for liquid and viscous food.

Figure 3 Contact freezer: linear system with a sliding plastic film.

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 plasticfilm represents a first thermal resistance Moreover, the uncontrollable deformation of theproduct will result in the apparition of an air film between the cold surface and the plasticfilm Nevertheless, this process remains very handy in achieving a superficial freezing ofthe product preventing dehydration during conventional freezing

Single-side contact freezing is of course less efficient than double-side contactfreezing Nevertheless, a continuous process is highly desirable in the industry in order tominimize contamination by handling and also for productivity Maltini (1984) studied theapplication of solid foods to contact freezing processes and did a comparison with airfreezing He suggested that the food should be regular in shape to ensure contact of greaterthan 20 mm2? g1

Donati (1983) did a similar study and compared the drum freezingtechnique with several other freezing processes

The scraped heat exchanger has been adapted to the case of freezing ice cream It typicallyconsists of a rotating drum equipped with one or two blades The rotating drum isinstalled in a refrigerated vessel The blades allow the scraping of the ice crystals formedonto the inner surface of the refrigerated drum They also permits whipping of the airinside 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, acertain 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, theback pressure, and the rotating speed will interact with the degree of fat destabilizationand foam structure

Straight blades are usually used Helical blades aiming to propel the ice cream mixtoward the exit of the system has been evaluated by Myerly (1998) More recently, a newdesign of continuous freezer has been developed by Windhab et al (1998) This system hasbeen developed from a twin screw extruder that has been adapted for the freezing of icecreams The enhanced local shear stresses acting in the extrusion channel resulted inimproved microstructure in comparison with conventional scraped heat exchanger Thisprocess, 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 freezercannot be used to fill forms or mold but does not need any further conventional hardening

The thermal performance of a given freezing process is related to the overall energyconsumption required to cool down a given product from an initial temperature down to afinal one An accurate evaluation of this parameter is difficult because it has to take intoaccount the type of refrigerating system (mechanical compression, cryogenic) beingconsidered, the geometry of the product, the freezing rate, the final temperature, and thebalance that will be considered between the refrigeration in the freezing process per se andthe refrigeration load that will be held by the storage system A first approach can berealized by comparing the heat transfer coefficient between the refrigerated medium

(convective freezing) and the surface (contact freezing) The order of magnitude of theeffective heat transfer coefficient as defined by Eq (17) for convective process or by Eq.(18) for contact freezing are summarized in Table 2.

ðTF TSÞ with TS andTF ¼ T @ surface and fluid ð17Þ

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 ofcontact freezing is not acceptable If the product is applied directly onto the refrigeratedsurface, 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 freezemashed vegetables (broccoli in the present case) was studied A parameter study showedthat the thermal contact resistance was higher for lower surface temperature Thissupposes that the mechanical stress in the product during freezing (stretching owing to iceformation) interacts with the quality of the mechanical and therefore the thermal contactbetween 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 Aplastic film seems to reduce slightly the heat transfer coefficient, whereas the presence ofcardboard 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)

Convective Liquid nitrogen, smooth

cylinders, warming regime

sample ¼ 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

1 layer polyethylene film 278 Corrugated fiberboard þ polyethylene 20

freezing rate is usually desired except for some specific products (i.e., bread dough) Thepackaging 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, forexample, chicken breast, is much used in the industry Recent studies have pointed toimmersion freezing in brine, even though some problems will necessarily occur for thetreatment of the brine Contact freezing is also widely used and is well adapted for massproduction 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 theefficiency 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 celldehydration and large ice crystals that might damage the texture of a food A fast freezingrate prevents the migration of water into the extracellular spacing and yields fine andnumerous ice crystals Side effects such as the increasing of the concentration of theremaining 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 freezingconditions 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 phasechange 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 initialfreezing temperature Based on this definition, several researchers calculated the freezingrate 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 becalled 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 thefreezing 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

A high freezing rate might result in cracks in the product Shi et al (1999) reportedthat stress as high as 2 MPa (20 atm) can be reached during the freezing of biologicaltissue 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 mmdiameter) A frozen mantle first appears at the surface of the product Meanwhile, theformation of ice at the core will yield an increase of the pressure Radial andcircumferential stresses develop during freezing Rapid temperature drop (i.e., cryogenicfreezing with surface temperature down to196 C for liquid nitrogen) will induce higherstress, which can be as high as 1.5 MPa, whereas freezing in a medium at 40 C yieldsstress in the range of 1 to 0.5 MPa (Shi et al., 1999) Even though the temperature of thefrozen external mantle is far from the glass transition, the tensile failure strength, whichwas around 0.5 MPa for potato, might be passed, leading to cracks On the other hand, adepression of the initial freezing point due to a pressure increase will result in a partialthawing (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 ofthe freezing time A focus proposed on the surface heat transfer coefficient includescontact freezing It shows that an infinite heat transfer coefficient can’t be assumed in thisprocess, which is widely used in the industry

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AC Cleland, (1990) Food refrigeration processes Analysis, design and simulation E Sciences,

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 0256.567.A2.

EP-A LeBail, et al (1996) EP-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 845349.

US-J Nagaoka, et al (1955) Experiments on the freezing of fish by the air-blast freezer US-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.

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 concentratedliquid, while ice is removed by sublimation in freeze-drying, yielding a dried material Theremoval 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 thetheoretical principles behind these processes is necessary for minimization of detrimentalchanges, operating strategies, and optimization purposes Thus the fundamental aspects offreeze-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 aqueoussolution with an initial relatively low concentration by the partial freezing of water andsubsequent separation of the resulting ice [1,2] Freeze-concentration is based on thefreezing temperature-concentration diagram (Fig 1) [3]

It is necessary briefly to review the physicochemical changes that occur during afreezing 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), andconditions of maximal freeze-concentration The freezing curve corresponds to solution–ice crystals equilibrium Along this curve, as water is removed as ice, the concentration ofsolute increases during the freeze-concentration process The solubility curve representsequilibrium between the solution and supersaturated solution in a rubbery state Thefreezing 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 equilibriumwith 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; T0

g)lower than point B (C0g; T0

m) represents a characteristic transition in the state diagram Theglass transition curve (DFG) represents the glass–rubber transition of the solute–watermixture, and the type and concentration of the solute and the temperature define it Abovethe 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 theequilibrium ice melting temperature of ice (T0m) [4,5] The liquid solute–water mixture isthe maximum freeze-concentrated and has become glassy The glass transitiontemperature 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 andtemperature 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) arefrigeration 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 crystallizerwhere part of the water crystallizes Cooling causes ice crystal growth and an increase insolute concentration The resulting mixture of ice crystals and concentrated solution ispumped through a separator where crystals are separated and the concentrated solution isdrained off Ice crystals are removed and melted by hot refrigerant gas The final productsare cold water and concentrated solution, which flow separately [1,8]

Figure 1 Typical solid–liquid state diagram for a food system.

1 Crystallizers

The heat of crystallization can be taken out directly or indirectly In direct-contactcrystallizers, 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 Adisadvantage 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 liquidfoods, owing to the aroma losses and deterioration of the product by the refrigerant Incrystallizers with indirect heat removal, the refrigerant (R22 or ammonia) is separatedfrom 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 usedcommercially 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 ofpresses, 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 thecake, which is the limiting factor of this method Since the presses are completely closed,aroma loss is negligible [1,9]

Figure 2 Schematic diagram for the freeze-concentration process of foods.

Ice and concentrated solutions may be separated by centrifugation at about 1000 G.Centrifugation must be conducted under inert atmospheres to reduce oxidation and aromaloss Solute losses may occur if concentrated solution remains adhered to the crystalsurface, but washing of the cake with water will minimize such losses This washing stagerenders the centrifugation operation more efficient than pressing [1,2,9].

In washing columns, the ice–solution mixture is introduced at the bottom of thetower, and the solution is drained off The crystals move toward the top of the column incountercurrent to the wash liquid, which is obtained by melting part (5–3%) of the washedcrystals leaving the column In this process the loss of dissolved solids with the ice is lessthan 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 obtainlarge and symmetrical crystals Large crystals can be more easily separated from theconcentrated solution Large crystals also reduce the loss of solutes due to occlusion andadherence to the small crystals [1,8] During crystallization, two kinetic processes takeplace: the formation of nuclei and the growth of crystals Nucleation is the association ofmolecules (at some degree of subcooling) into a small particle that serves as a site forcrystal growth Once a nucleus is formed, crystal growth is simply the enlargement of thatnucleus Nucleation and growth of crystals are dependent on solute concentration, bulksupercooling, residence time of the crystals in the crystallizer, freezing rate, moleculardiffusion coefficient of water, and heat transfer conditions These factors should becarefully controlled to regulate crystal formation [2,10]

2 Bulk Supercooling

Supercooling is the driving force responsible for the creation of crystal nuclei and theirgrowth The nucleation rate is proportional to the square of the bulk supercooling At highbulk supercooling values, the nucleation rate decreases, owing to the inhibition ofmolecular mobility Crystal growth exhibits a first-order dependence of the bulksupercooling [1,9,10]

3 Residence Time of the Crystals in the Crystallizer

At constant bulk supercooling and solute concentration, the crystal size is proportional tothe crystal residence time At short residence times the crystals produced are very small[1,10]

4 Freezing Rate

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 rateresults 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 (ashigh viscosity) cause the growth rate to decline Very large uniform crystals require largeexchange surface at relatively high temperatures [1,2,9]

7 Viscosity of the Liquid

Viscosity increases markedly as concentration increases, ice crystals grow very slowly athigh viscosity, and large crystals become difficult to separate The maximum concentrationobtainable 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 pointformation 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 theconcentrate and directly proportional to the square of the mean diameter of the crystals asexpressed by the equation

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 representthe solid, liquid, and gaseous states of water in a closed system The points along theseparating lines represent the combinations of temperature and pressure at which twostates 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 ofwater are simultaneously in equilibrium, and it is called the triple point [4,12]

Freeze-drying can also be conducted at moderated pressures and even atatmospheric pressure The principle of this process is to produce a vapor pressuredifference as large as possible by blowing dry air over the frozen material In practice, theprocess is very long because of the low mass and energy transfer rates, but problemsrelated to the application of vacuum do not exist, resulting in an important reduction ofoperation costs [13,14]

Freeze-drying is used to obtain dry products of higher quality than those obtainedwith conventional drying methods Freeze-dry products have high structural rigidity, highrehydration capacity, and low density, and they retain the initial raw material propertiessuch as appearance, shape, taste, and flavor This process is generally used for thedehydration 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, drying is an expensive process, since it takes large operation times and consumes largeamounts of energy Energy is required to freeze the product, heat the frozen product tosublimate ice, condense water vapor, and maintain the vacuum pressure in the system[15,16]

freeze-B Basic Components of a Freeze-Dryer

The typical freeze dryer consists of a drying chamber, a condenser, a vacuum pump, and aheat 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 havesufficient condensing surface and cooling capacity to collect water vapor released by theproduct As vapors contact the condensing surface, they give up their heat energy and turninto ice crystals that will be removed from the system A condenser temperature of65 CFigure 3 Pressure–temperature phase diagram of pure water.

is typical for most commercial freeze-dryers The vacuum pump removes noncondensablegases to achieve high vacuum levels (below 4 mm Hg) in the chamber and condenser Theheating 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, andsecondary drying The objective of the freezing stage is to freeze the mobile water of theproduct The product must be cooled to a temperature below its eutectic point, which isthe temperature and composition combination that produces the lowest point at which aproduct will freeze Freezing has an important influence on the shape, size, anddistribution 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 removefrozen water by sublimation, while the frozen product is held below the eutectictemperature During the primary drying, approximately 90% of the total water in theproduct, 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 fromthe 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 reducingthe partial pressure of water vapor in the dryer [12,20]

The secondary drying stage requires 30 to 50% of the time needed for primary dryingbecause of the lower pressure of the remaining bound water than free water at the sametemperature, yielding a slow process Freeze-drying is complete when all the free andbound water has been removed, resulting in a residual moisture level that assures desiredstructural 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 occurswithin the product: energy is transported to the sublimation zone and water vapor isgenerated In contrast with mass transfer, which always flows through the dry layer, heattransfer can take place by conduction through the dry layer (Fig 5a)or through the frozenFigure 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 penetratedeeply 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 adried porous layer, undergoing one-dimensional freeze-drying [12,23] The interfacebetween 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

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 isthe thickness of the slab (m), and x is the relative height of the ice front

If heat is transferred through the frozen layer,

Figure 6 Schematic representation of freeze-drying of a slab.

sublimation (NW, kg/s m2) is given by

NW ¼ eDK

MwRT

where riceis the density of ice (kg/m3)

Assuming that all supplied heat is used for sublimation of ice, the enthalpy balancegives

where DHsis the latent heat of sublimation (J/kg)

For temperature differences not too large, the Clausius–Clapeyron equation can belinearized, and the above equations can be solved analytically The following expressionscan be derived for the total drying time:

where kice is the thermal conductivity of ice (W/m K)

Several mathematical equations describing mass and energy transfer have beendeveloped for modeling the freeze-drying process Such models account for the removal offrozen water only (sublimation model) or for the removal of frozen and bound water

(sorption sublimation model) These models also examine the methods of supplying heatand the diffusion mechanisms, describe steady or non-steady state processes, or analyzeboth transfers under various processing conditions Some models have been found todescribe accurately experimental drying rates and freezing times However, a majorproblem in the application of some models is the requirement of reliable data on thermaland mass transport properties of food materials such as diffusivity within the porousmedium, Knudsen diffusion, water vapor concentration in the dry layer, porosity, effectivethermal conductivity, permeability, etc [15,19,21,24].

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 productand will change the physical characteristics of the dried material Excessive heat may causethe dry cake to char or shrink The heating rate can be optimized during operationmodifying conveniently product temperatures in the dried zone and at the sublimationfront [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 thetransport parameters that influence the kinetics of vapor removing At a giventemperature, 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 ratemay be limited by the transport of water vapor through the product, if the transport ofwater vapor falls in the free molecular flow regime [21,25,26]

Chamber pressure affects the transport properties, thermal conductivity, and watervapor diffusivity Thermal conductivity of the dry layer is higher at higher chamberpressures, within the range of freeze-drying operation, resulting in high heat transfer ratesfrom 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 islow (low sublimation temperature), freeze-drying is often a heat-controlled process, but atrelatively high pressures freeze-drying becomes a mass-controlled process In mostsituations, 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 operationpressures, since this is a fundamental factor for the sublimation temperature Normally, avacuum 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 ishigher than product’s temperature, water vapor will tend to move toward the product, anddrying will stop [26,28]

When freeze-drying temperature is high enough, the product cake suffers a drasticloss of its structure and is said to have undergone collapse Collapse affects aromaretention, caking and stickiness, rehydration capacity, and final moisture of the product Acollapse temperature (Tc) is related to the glass transition temperature (Tg), which in turndepends 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 canflow, and structural collapse can occur A critical viscosity in the range of 105–108Pas hasbeen reported to observe collapse [20,29]

III CONCLUSIONS AND RECOMMENDATIONS

Despite the reduced use at the industrial level of freeze-concentration and freeze-dryingprocesses within the food area, both are important to obtain high quality products Deepknowledge of the fundamentals of phase changes of water in foods and of the effect of thevariables on the processes’ effectiveness and cost can open new opportunities for theapplication of both processes to obtain high-quality preserved foods

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.

9 HG Schwartzberg Food freeze concentration In: HG Schwartzberg, MA Rao, eds Biotechnology and Food Process Engineering New York: Marcel Dekker, 1990, pp 127–202.

10 JG Muller Freeze concentration of food liquids: theory, practice, and economics Food Technol 21:49–61, 1967.

11 WW Rothmayr Basic knowledge of freeze-drying Heat and mass transfer In: SA Goldblith, L Rey, WW Rothmayr, eds Freeze Drying and Advanced Food Technology New York: Academic Press, 1975, pp 203–222.

12 M Karel Heat and mass transfer in freeze-drying In: SA Goldblith, L Rey, WW Rothmayr, eds Freeze Drying and Advanced Food Technology New York: Academic Press, 1975,

16 MJ Millman, AI Liapis, JM Marchello Note on the economics of batch freeze dryers J Food Technol 20:541–551, 1985.

17 SE Charm The Fundamentals of Food Engineering 2nd ed Westport, Connecticut: AVI,

23 CJ King Freeze-drying of foodstuffs Critical Reviews in Food Technology 9:379–451, 1970.

24 NK Sharma, CP Arora Prediction of transient temperature during freeze drying of yoghurt Drying Technol 11(7):1863–1883, 1993.

25 JI Lombran˜a, MC Villaran The influence of pressure and temperature on freeze-drying in an adsorbent medium and establishment of drying strategies Food Research Int 30:213–222, 1997.

26 J Welti-Chanes, B Lafuente Liofilizacio´n de disgregados (‘‘comminuted’’) de naranja Efecto

de los para´metros de proceso sobre la velocidad de secado y la calidad del producto en polvo Rev Agroquim Tecnol Aliment 25(4):532–540, 1985.

27 RJ Litchfield, AI Liapis, FA Farhadpour Cycled pressure and near-optimal policies for a freeze dryer J Food Technol 16:637–646, 1981.

28 K Niranjan, JM Pardo, DDS Mottram The relation between sublimation rate and volatile retention during the freezing drying of coffee In: J Welti-Chanes, GV Barbosa-Ca´novas, JM Aguilera, eds Engineering and Food for the 21st Century Boca Rato´n, Florida: CRC Press,

2002, pp 253–268.

29 G Levi, M Karel Volumetric shrinkage (collapse) in freeze-dried carbohydrates above their glass transition temperature Food Research Int 28:145–151, 1995.

Principles of Frozen Storage

Genevie`ve Blond and Martine Le Meste

ENSBANA-Universite´ de Bourgogne, Dijon, France

The use of freezing for food preservation has rapidly developed; the fact that products,mainly meat and fish, could be stored for considerable periods and served, after thawing,

as fresh products are at the origin of its development as one of most common methods offood preservation

Freezing implies two linked processes: a lowering of temperature and a change ofphase of water from liquid to solid Both processes tend to reduce rates of physical andchemical changes and might be expected to enhance the shelf life of the products

At the same time, quality degradation during storage remains the most commonproblem for manufacturers and for bringing food freezing to the fullest possible extent So

it is vital to understand well the essentials of the technology Freezing seldom improves thequality of food products; the raw material quality is of primordial importance, and thisquality must be preserved during processing and storage There is no single universal rulegoverning frozen food preservation; just as with optimal freezing rates, which vary fromproduct to product, the storage time depends not only on the temperature but also on thetype of product and packaging Most physical and chemical reactions are slowed with thedecrease in temperature, but they are not stopped at common storage temperatures Theproduct deterioration during cold storage is a slow, continuous, cumulative, andirreversible process

The physical properties of food products change dramatically depending on wateravailability and temperature The major assumption relating to quality, and thus shelf life,

is that stability is maintained in the glassy state In a glass, the diffusion of solutes anddegradation reactions should be strongly limited, and long-time stability during storageexpected The shelf life of frozen food products should be largely controlled by thephysical state of the freeze-concentrated fraction produced by the ice separation It isdesirable to know better how this physical state changes with temperature

I PHYSICAL STATE AS A FUNCTION OF TEMPERATURE

A Physical Changes During the Freezing Process

According to the freezing process presented in the first chapter, it is obvious that thefreezing of food is more complex than the freezing of water The removal of water byseparation of ice produces supersaturation of dispersed substances For that to occur,

temperature of the product must be lowered sufficiently The chemical potential of water isreduced by increasing concentration in solutes The ice formation requires a decrease intemperature, which depends on the product composition.

For practical purposes, the freezing process is considered complete when most ofwater at the center of the food product has been converted into ice At15 C, more than

80% of total water is transformed into ice [1] The system is segregated into a crystallinephase of pure water and an amorphous domain, which contains solutes and residual water

As the temperature decreases, the viscosity of the interstitial fluid increases rapidly as aresult of both concentration increase and temperature decrease When the viscosityreaches 1011–1012Pa? s, a solidification (vitrification) occurs, and the concentrated phasesurrounding the ice crystals becomes a glass The temperature at which this transitionappears is called Tg0, the glass transition temperature of the maximally freeze-concentrated system [2] The freezing of water is stopped at this temperature; the waterstill unfrozen at Tg0 is often called ‘‘unfreezable’’ water

Since the water content of foods is often close to 80–90%, its behavior during thefreezing process can be considered as that of an aqueous solution The proposed model isoften a sucrose solution, which could be representative of the freezing behavior of a widerange of solutions when no solute crystallizes during cooling If cooling is slow enoughcompared to the kinetics of ice formation, the concentration in the liquid phase dependsonly on the temperature for a given product and is independent of its initial water content

As a consequence, Tg0 is also independent of the initial water content Fruits, fruit juices,vegetables like tomatoes, and ice creams present very similar thermal changes Theequilibrium temperatures and concentrations vary with the nature and composition of theproduct

B Determination of Tg0

Differential scanning calorimetry (DSC) is the technique most often used to measure theglass transition temperature; this transition appears as a typical heat capacity jump on theDSC traces and can be characterized by different temperatures (Fig 1) In the presence ofice, the glass transition, which is visible just before the ice-melting peak on the heatingscans, appears more complex, presenting a larger heat capacity change than predicted

Figure 1 DSC thermogram of a 80 % sucrose solution (cooling and heating rates: 108C/min).

from the solid content in macromolecular solutions [3] For frozen solutions of smallsolutes (sugars, polyols), the thermograms show a two-step endotherm (Fig 2) Thesefeatures appear to be very similar to those observed with synthetic polymers and areinterpreted as a glass transition with associated enthalpy relaxation [4,5] There is still noagreement on how the DSC data should be interpreted The reported Tg0 values, according

to some authors, is the temperature of the midpoint or the onset of the first transition; forothers, it is the midpoint of the second step They are called, respectively, Tgonset, Tg1, and

Tg2 on Fig 2 This explains the variations in published Tg0 values [6]

Because of the ambiguity of the Tg0 determination from simple DSC traces, it wassuggested to use simplified state diagrams of solute–water blends, which represent theirdifferent states in the low-temperature range The two curves, corresponding, respectively,

to the equilibrium liquids or ice melting curve (Tm curve), and the kinetically controlledglass transition curve (Tg curve), are drawn The diagram representing the various states

of the sucrose–water system as a function of concentration and temperature is appropriate

to illustrate the demonstration (Fig 3).The curve Tm represents the temperature at whichice begins to separate as a function of initial concentration, or the concentration of thefreeze-concentrated phase as a function of temperature For all points on this curve, thefreeze-concentrated phase is in equilibrium with ice; its partial water vapor pressure isequal to that of ice at the same temperature For soluble solutes, the temperature of theglass transition depends on the water content; it decreases when the water contentincreases The intersection of the two curves could provide a better estimation of Tg0 [7].The freezing of water is stopped at this temperature; the concentration of the maximallyfreeze-concentrated phase is called Cg0 and its water content considered as the

‘‘unfreezable’’ water

There are obvious problems with the accuracy of the Tm and Tg curves to build aprecise diagram The experimental Tm values as obtained by DSC for the mostFigure 2 DSC thermogram of a frozen sucrose solution (50 % w/w) (cooling and heating rates:

10 8C/min).

concentrated solutions are considered as more or less reliable; an extrapolation of the Tmcurve can be subjective; therefore the curve represented in Fig 3 is derived from excessproperties and solid–liquid equilibrium using a UNIQUAC model [8] For the Tg curve,the experimental data taken into account can be the temperature of the beginning Tgonset,

of the middle of the transition Tgmid-point(value most often used) or even of the end of thetransition (SeeFig 1 for sucrose solution.) The cooling/heating rates contribute to somechanges in temperature due to the time dependence of the glass transition feature TheDSC values of Tgonset and Tg1 of frozen sucrose solutions (in Fig 2) were found to beclose to the intersection of the Tm curve with the Tgonset and Tgmp curves in Fig 3 The

Tg0–Cg0 coordinates vary from 45 C, 82.2% to 41 C, 81.2% [8] depending on the Tgvalue taken into account Using the method of optimal annealing temperature, Abblet andcoauthors have obtained very similar data 40 C, 82% for Tg0–Cg0 [9].

The liquid–glass conversion must be considered to occur in a temperature range that

is characteristic of the fraction forming the glassy matrix The coordinates Tg0–Cg0can befar away from the eutectic point predicted by equilibrium thermodynamics, for example,with sucrose–water binary Te, Ce 14 C, 64%

Thermomechanical spectroscopy (DMTA) studies of frozen sucrose solutions showthat the change in mechanical properties becomes perceptible at Tgonset as seen by DSCand that a maximum in the loss modulus (E00 or G00) occurs at a temperature between Tg1and Tg2 [10] Apart from its scientific interest, the sucrose–water phase diagram (Fig 3)could be considered as representative for a wide range of products containing lowmolecular weight solutes; very similar diagrams are reported for some fruits [11]

C Storage Conditions and Shelf Life of Frozen Foods

The choice of the storage temperature is of considerable practical importance in the frozenfood industries It is the major variable, which can affect stability; a product retaining agood quality for months at20 C can lose it in a few days at10 C Moreover, it is wellFigure 3 State diagram of sucrose–water binary (Adapted from Ref 8.)

demonstrated that frozen foods stored at fluctuating temperatures have not the same shelflife as products stored at constant temperatures [12,13] The weakest links of the frozenfood chain are handling time, temporary storage, and transport temperatures The lastlink of the chain, the retail shops, is often critical Frozen products delivered to the retailoutlets are not always placed in refrigeration immediately; the problem includestemperature increases during defrosting In Europe, where food stores are not open 24 hper day as in the United States, using night covers and programming the defrosting duringthe night may help to minimize the temperature fluctuations.

In order to ensure product quality, temperature control is necessary throughout thecold chain, and the required temperature must be maintained from production toconsumption European and international regulations concerning the storage temperature

of frozen foods set 18 C as the highest temperature during storage and distribution,although many studies have shown that slow degradation of the product quality will occureven at this temperature This temperature must be stable and maintained at all points inthe product (therefore at the surface); possible brief upward fluctuations of 3 C maximumduring transport are admitted, as well as a tolerance of 3 C during local distribution and

in retail display cabinets [14] To maintain an optimal quality, the products are generallyheld at temperatures colder than this in primary cold stores ( 20, 25 C).

There is a clear effect of temperature on storage life, with lower temperaturesresulting in extended shelf life; but 18 C corresponds to an optimum between thefinancial costs and the shelf life of frozen foods As it is the same temperature for all frozenproducts, the marketing shelf life varies from 6 months to 2 years (Table 1) [1] T1

At this temperature frozen foods are not fully frozen, nor inert, the Tg0 of mostproducts being below 18 C. Table 2 shows the Tg0 of some food products or moreexactly the temperatures corresponding either to the second step of the transition (called

Tg2 on Fig 2) or to the midpoint for a single transition The Tg data for solutions ofnumerous simple ingredients (sugars, polysaccharides, proteins) can be found in an article

by Levine and Slade [23] It must be emphasized that these temperatures, and consequentlythe glassy state, will be readily attained by a very limited number of frozen food products

at temperatures commonly employed in the distribution chains That means that thefreeze-concentrated phase is more or less fluid, and solute mobility may be significant.Frozen storage usually continues for long periods, so products undergo deterioration,which can be as important as those due to the freezing and thawing processes Theunderlying reason for necessary extensive studies about change of quality during storage is

Table 1 Practical Storage Life (PSL) in Months at Several Temperatures

Source: Adapted from IIR-IIF [1].

that we do not know enough about the causes of deterioration to be able to proposeefficient predictive models.

II CHANGES IN FROZEN FOODS DURING STORAGE

The main factors affecting frozen food quality during storage can be divided into twocategories: processing and compositional factors The quality factors associated withprocessing parameters are mostly related to the ice phase Defects associated withcompositional factors are related to chemical reactions and affect flavor, texture,appearance, color, and nutritional properties

A Physical Changes

1 Water (Moisture) Migration

The major physical change that occurs during storage of frozen foods results from watermigration Solid/liquid and especially liquid/crystal transformations can occur withtemperature fluctuations, but important water motions could take place withouttemperature changes There is always, inside a package or a product, some difference ofwater vapor pressure due to a temperature gradient or a surface energy difference Aswater molecules are not completely immobilized by low temperatures [26], a significantredistribution can be observed in frozen products during the storage time because of theduration usually involved The water migration causes either water content changes or

Table 2 Tg 2 Values for Frozen Food Products.a

Commercial ice creams 27.5 to 40 a

Levine (23) Commercial ice creams 34 a

Blond (24) Model ice creams 25 to 43 a

Hagiwara (25)

changes in ice crystal size called recrystallization Water migration is highly temperaturedependent, and it occurs at any storage temperature, although good conditions (e.g., lowtemperature, no temperature fluctuations) minimize its importance.

a Migration with Change in Water Content Water migration can correspondeither to a water loss through surface dehydration or to only a water location change forproducts with heterogeneous water contents

The surface dehydration adds to dehydration suffered during the freezing process,which can vary from almost zero for packaged products and cryogenic freezing to 3–4%for naked products and poorly designed freezers [27] These moisture losses have the sameorigin, i.e., a temperature difference between the product and the surrounding atmosphere,resulting in a difference of water pressure that produces a water molecular flux from thesurface of the frozen product to a colder area Ice sublimation progresses during storage,and it is more pronounced when the storage temperature is higher A common example isobserved with frozen fruits and vegetables that are not protected by transparent films inclose contact with them The films cool faster than the surface of the product, setting up awater pressure gradient; thus a small amount of vapor water migrates from the product tothe inside surface of the package When the freezer temperature increases, the process isreversed and the water vapor condenses on the product surface The dehydration must beconsidered irreversible, reabsorption of water in the product being not possible; thisremoved water crystallizes and remains inside the package as frost As this cycle isrepeated, the buildup of frost in the package becomes noticeable, this effect beingamplified by the frequency and magnitude of temperature fluctuations; fluctuations ofmore than 2 C promote an important sublimation of ice The consequences are more orless important depending on the products, as the water losses can reduce the weight of theproduct, thereby reducing the market value for large pieces of meat sold beforetransformation; overall, the dehydrated surface of a product reduces its appeal to theconsumer Meat appears darker when frozen, since oxygen cannot produce the bright redcolor of oxymyoglobin (its ‘‘natural’’ color returns on thawing) A surface dehydration onthis product leads to the important defect known as freezer burn, particularly for meatcarcasses, cuts, and poultry, stored without an adequate packaging Dehydrated surfacescome in grayish patches, because the disappearance of ice crystals by sublimation formssmall cavities on the surface that appear grayish because of the light scattering.Dehydration of unprotected fish can lead to irreversible quality loss when the fish surfacebecomes dry with a ‘‘woody’’ texture This dehydration also contributes to increase therate of rancidity and discoloration The dehydrated surfaces of vegetables may also appearclearer before thawing, but color changes for these products are predominantly caused byoxidative reactions

The dehydration effects almost completely disappear on thawing and cooking,except if the product has been stored in bad conditions and dehydration was too severe.But the appearance in the frozen state needs to be taken more into consideration, because

it is decisive for the consumer considering whether to buy the product or not Moreover,the melting of frost during thawing causes an unpleasant appearance when a wet layerappears on the product surface

Today these defects are more rarely observed, because efficient packaging preventsproducts from dehydration Each product to be stored in a freezer over an extended periodshould be wrapped Moisture-impermeable films offer considerable protection againstmoisture loss, but products must be packed with a minimum head space to reduce frost;vacuum packing, which ensures maximum contact of film to product, suppresses all frostand also limits the thermal gradient Its use is limited because it is more expensive and