Freezing proceseses used in the food industry

22

23 24 25 26 27

Rubio,F.M., Itak, J.A., Scutellaro,A.M, Selisker, M.Y.and Herzog, D.P. (1991) Food Agric. Immunol.3, 113-126 Winter,G. and Milstein,C. (1991) Nature 349, 293-299 Milstein,C. (1990) Proc. R. Soc. Lond. B 239, 1-16 Clackson,T., Hoogenboon, H.R., Griffiths, A.D. and Winter, G. (1991) Nature 352, 624-628 Marks,I.D., Hoogenboon, H.R., Bonnert,T.P., McCafferty,I., Griffiths, A.D. and Winter, G. (1991)/. Mol. BioL 222, 581-597 Marks,I.D., Griffiths,A.D., Malmquist, M., Clackson, T.P., Bye,I.M. and Winter, G. (1992) Biotedmology 10, 779-783

28

29

Lee, H.A., Alcocer, M.I.C., Lacarra, TI ,~,,J,, r ~A;n. E.N.C., ~,t.,t c~. nr,I.s, C., Wang, B. and Morgan, M.R.A. (1992) in American Chemical Society Division of Agrochemicals Symposium on EmergingPesticide Residue Analysis Techniqueson Immunoanalysis, San Francisco, p. 7, American Chemical Society Ward, V., Hammock, B.D., Maeda, S. and Choudray, P.V. (1992) in American Chemical Society Division of Agrochemicals Symposium on EmergingPesticide ResidueAnalysis Techniques on Immunoanalysis, San Francisco, p. 8, American Chemical Society I¥1111~ l

Review

Freezing processesused in the food industry

however, dynamic. Chilled food markets have experienced enormous recent growth I, and the improvements in the nutritional and sensory qualities of novel heatprocessed foods, which have comparable shelf lives to their frozen counterparts, have resulted in these preservation processes taking a considerable market share 2. Recet~t new product activity, by technology sector, is illustrated in Fig. 1. An emerging new player is the 'Superchill system', which maintains product temperaThe freezing of foods slows down, but does not stop, the tures between O°C and the temperature at which ice physical and biochemical reactions that govern the deterio- crystals start to form in the product (this varies slightly from one product to another; for example, beef freezes ration of foods. When properly handled and processed, frozen at -!.7°C, fish at -2°C). The superchill temperature foods are often perceived to have superior sensory and nu- range is cold enough to suppress bacterial growth and tritional qualities to foods preserved by other methods. These enzymatic reactions but does not cause cellular damage. qualities depend upon the control of the freezing process and The storage and distribution of sous-vide foods is also a upon careful pre-freezing preparation and post-freezing stor- growing application, particularly in Japan, where an estimated 50% of retail refrigerated displays are superage of the product. New insights into the physicochemical chilled .~. aspects of each stage of frozen food production has diagIt is very evident to the frozen food industries that the nosed sources of quality loss and identified appropriate strat- quality aspects of freezing are critical. Preserving the egies for improving the quality of frozen foods. Comparison of integrity of the product throughout the cold chain is a the freezing operations currently used indicates a trend commercial, as well as a legislative imperative. In order to obtain high-quality frozen foods, hightowards more rapid freezing, although at present the trade-off quality raw materials are necessary, and processing, between freezing rate and freezing economy may best be distribution and storage must be carefully controlled. Quality cannot be gained but it certainly can be lost. balanced by means of combination freezing processes. Many deteriorative processes are temperature dependent, and the control of temperatures following processing and during distribution, storage and retail display is necessary for compliance with European Directives 4.-~ Freezing has long been established as an excellent and to maintain the quality attributes of frozen foods. method for preserving high quality in food products. An emerging technology for monitoring the timeGenerally, freezing preserves the taste, texture and nu- temperature regime experienced by frozen foods during tritional value of tbods better than any other preservation distribution and storage are temperature and timemethod; as a result, ever-increasing quantities of food temperature indicators. These devices may be either are being frozen throughout the world. The picture is, threshold indicators, which show that a product has been held above a specified abuse temperature, or R.M. George is at the Campden Food and Drink Research Association, integrating devices that can reflect time- and temperatureChipping Campden, UK GL55 6LD. dependent safety ~nd quality changes. However, despite

R.M. George

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©1~,~3,ElsevieSci r encePublishersLid.(UK) 0924-2244/93/$06.00

Trends in Food Science& TechnologyMay 1993 !Vol. 4]

1400 I the proliferation of devices available 6, food industry specifications defining the technical standards required and procedures for the evaluation of such devices have only recently been established 7. In terms of the actual freezing operation, the primary objective has to be to maintain as much of the original and characteristic product quality as possible. This can normally be achieved by freezing rapidly. However, there is a trade-off between the speed of freezing and the economy of the freezing operation, particularly for large production volumes, where the economics often dictate the choice of freezing operation and hence the quality of the final product.

The freezing process It is useful to briefly review the thermodynamics of freezing in order to understand the physicochemical changes that occur during the process and how they affect the food product. The major thermal events that occur during the slow freezing of both water and a simple aqueous solution are shown in Fig. 2. In order to induce the crystallization of liquid water to solid ice (a process known as nucleation), an effect known as undercooling will often occur, analogous to an activation energy for the phase change 8. Nucleation can be defined as the formation within the metastable phase of a foreign, stable phase capable of growing spontaneou~!y. Two forms of nucleation can potentially occur: homogeneous and heterogeneous. Homogeneous nucleation occurs only in highly purified systems, with ice crystal sites formed at random accumulations of water molecules. Heterogeneous nucleation is found in the freezing of real food systems, and occurs when small particles in the solution (e.g. solutes) act as nucleation sites; mechanical impact and local variations of solute concentrations also contribute to the heterogeneous formation of ice. The mechanisms involved in nucleation are determined by the rate of tYeezing, which consequently affects crystal structure within the food and, hence, product quality 9. Following nucleation, the growth of the ice crystal occurs at a rate that is governed by the rate at which water molecules react at the crystal surface, the diffusion rate of water to the crystal surface, and the rate of heat removal. The transition of water to ice results in an increase in the concentration of the remaining unfrozen solution and, consequently, depression of the freezing point (see Fig. 2) until a eutectic point is reached. In the multi-solute situation of actual foods, several eutectic points may be reached, which may then not be evident due to their number. Crystal size varies inversely with the number of nuclei, which can be controlled by the rate of heat removal "). The freezing process is highly dependent upon product-related properties. Thermal properties govern the rates of heat transfer and removal 1I.,2, but of particular significance is the role of water and its change of state during freezing. Control of the factors promoting optimum conditions for the change of state of water is paramount for control of the freezing operation. Trends in Food Science ~. Technology May 1993 lVol. 4]

~

1200

O 1OOO Q"

800

73 o o

600

u,_

400

20O Z

o

1990

1991

1992

Year Frozen

~

Chilled

I....

I Ambient

Fig. 1 Recent new product activity in the UK by technology sector, highlighting the number of new food products in 1990-1992 (Food Products Intelligence Centre, Campden Food and Drink ResearchAssociation, unpublished).

Freezing rates and product quality The interaction between nucleation and crystal growth

has an effect on the size of the resulting ice crystals and consequently the quality of the frozen food. At rapid freezing rates, the propagation of the initial ice crystals is insufficient to keep pace with the rate of heat removal, resulting in undercooling and an increased frequency of nucleation. The result is that more nucleation sites become active and there is an increase in the number of ice crystals - with a corresponding decrease in crystal size. During slow freezing the propagation of ice can better keep pace with heat removal, resulting in fewer nuclei becoming active and the formation of larger ice crystals m. Commercial food freezers aim for rapid freezing. A major cause of product degradation is the amount of unfrozen water present in the frozen matrix. The unfrozen water is known to be reactive, particularly during frozen storage, rendering the product susceptible to

OOC-

Point

Eutect~ti~cPoint ~tion

Time

m.-

Fig. 2 A comparison of slow freezing curves for pure water and an aqueous solution containing one solute.

135

deteriorative and enzymatic reactions and limiting its frozen shelf life t3. Unfrozen water has a very low freezing point as a result of the freeze concentration process. However, there is now great potential for quality optimization during both the freezing process and frozen storage using the concepts of 'glassy state dynamics', which relate the temperature stability of frozen foods to the physical properties of the unfrozen matrix existing between the ice crystals 14'ts. As the product temperature is reduced by the freezing process, the concentration of solutes in the unfrozen medium increases, in turn increasing matrix viscosity. The viscosity eventually becomes so high that molecular motions and thus reaction rates are greatly inhibited, and water cannot be supplied to support further growth of ice crystals within practical timescales: the matrix has achieved a glassy or vitreous state. The glassy state concept may enable the application of optimum freezing rates to minimize the amount of unfrozen water. In addition, frozen shelf life can be optimized by storing frozen foods below the glassy state transition temperature (Tg) or by the reformulation of frozen foods (e.g. by the addition of certain compatible polymers) to increase the Tg to practical freezer temperatures. Databases of information on the relationships between frozen food composition and stability are currently being established tS. Food reformulation is also being used as a means of controlling and optimizing the freezing process. Antifreeze proteins, found in polar fish and cold-tolerant insects and plants, can affect freezing in several different ways. They not only lower the freezing temperature but also retard recrystallization on frozen storage. Possible applications of antifreeze proteins include improving the control of freezing damage to agricultural crops and the control of crystal growth during frozen storage. Other groups of proteins can promote ice nucleation, causing supercooled solutions to freeze more rapidly. Both types of proteins can now be synthesized chemically or using genetic engineering techniques, and may become commercially important in the future'.

Food freezing operations - available options Optimal product quality requires control, not only of

the pre-freezing and post-freezing regimes, but also of the freezing process itself. In modem freezing systems, freezing rates can be monitored and carefully controlled. Developments in computer control systems offer the prospect of modifying freezing conditions to match the characteristics of the food product '7. A major area of activity is in the prediction of freezing rates and times using mathematical formulae based upon the rate of heat transfer and simultaneous phase change t~. Such formulae allow the development of optimum freezing conditions and determination of tile effects of product- and process-related factors. Freezing systems can be characterized according to the rate of movement of the ice front from the product surface, where the initial freezing takes place, to the thermal centre of the food. This rate is related to the properties of the food product and to the efficiency 136

Table 1. Typical surface heat transfer coefficients (/!) for various freezing processesa Type of freezer

Conditions

h (W/m2K)

Cold store

Still air

5

Air blast freezer

Air velocity: 2.5 m/s Air velocity: 5 m/s

17 26

Plate freezer

Contact to cold surface

56

Fluidized-bed freezer

'Fluidizing air'

85

Cryogenic freezer

Gas zone (pre-cooling) Spray zone (freezing)

40-60 100-140

a Datatakenfrom Ref.9

of heat transfer from the freezing medium to the food product, as characterized by the heat transfer coefficient (Table l ). Slow freezers (e.g. still-air freezers and cold stores) are generally used for the storage of frozen foods. The low heat transfer coefficient of naturally circulating air necessitates long holding times to achieve product freezing, resulting in significant quality loss. A major criterion for their use as storage facilities is their temperature stability. Constant temperatures help to minimize recrystallization, a major cause of quality loss during the storage of frozen foods ''~. Quick freezers (e.g. air-blast, tunnel or plate freezers) are common within the food industry and offer several advantages to food producers, including flexibility, ease of operation, and economy of operation for large production volumes :°. Modern air-blast freezers use spaceefficient and energy-efficient vertical air flow via product lines on an upward spiralling helical belt, and are typically used for frozen meat patties, fish fillets, chicken pieces and packaged products. Rapid freezers (e.g. fluidized bed, immersion and scraped-surface freezers) have been largely utilized for individual quick-frozen (IQF) products, where the combination of efficient heat transfer and small size of the product (e.g. peas, cut beans, berries, diced vegetables and seafoods) contributes to rapid ice formation throughout the product and, consequently, greater retention of essential product qualities. The characteristics of IQF products also aid portioning, ease of handling and stock control 2'. Immersion freezers can provide extremely rapid freezing of individual food portions. The food sample can be immersed into either a cryogen (e.g. liquid nitrogen or carbon dioxide) or a fluid refrigerant that is capable of being cooled to low temperatures. In order to ensure that the food does not come into contact with liquid refrigerants, flexible membranes can be used to enclose the food completely while allowing rapid heat transfer. The advantages of immersion freezers include rapid freezing, uniform temperature distribution and a 'dry' process. Trends in Food Science & Technology May 1993 [Vol. 41

Ultra-rapid freezers (e.g. cryogenic freezers), which utilize expendable, liquefied gases such as nitrogen or carbon dioxide as the refrigerant, have shown enormous growth2L The potential benefits to food producers include rapid freezing rates, high product throughput rates, low floor-space requirements, flexibility (compatibility with various types of food products) and low capital entry. In addition, cryogenic freezing can greatly reduce the negative economic and quality effects of product dehydration by reducing the surface temperature very rapidly to minimize the evaporation rate. The rapid freezing obtained with cryogenic systems is also reported to aid food safety 22. As the food product is cooled very rapidly, particularly at the surface of the food, where post-cooking contamination can occur, it spends minimal time in the optimum temperature range for the growth of pathogenic microorganisms. The advantages listed above have driven the growth of this technique to such an extent that -10% of all foods frozen in the UK during 1990 were frozen by cryogenic freezing techniques 2~. Cryogenic freezing techniques are particularly suited to food products that have a high surface area to volume ratio and in which the thermal diffusivity of the food does not restrict the transfer of heat from the product to the freezing medium. Typical examples are fish fillets, shellfish, pastries, burgers, meat slices, sausages, pizzas and extruded products. Cryogenic gases can also be advantageously applied to produce a hard, frozen crust on 'soft' products (e.g. ice cream, bacon sides, cake products and whole fish products) to allow for handling, packaging or further processing :3. Box l summarizes some of the major advantages of the various freezing options available to food processors.

Future trends - combination techniques and new technologies The use of CFC (chlorofluorocarbon) and HCFC (hydrochlorofluorocarbon) compounds as refrigerants is being phased out in accordance with international agreements and national legislation; this is having a great impact on the freezing industry. The agreements have been tightened several times, resulting in an increasingly hectic search fbr alternatives. Candidate refrigerant liquids need to exhibit favourable physical characteristics such as nontoxicity and nonflammability along with appropriate thermodynamic efficiency and zero environ-

mental interaction. Although many of the larger frozen food processors such as frozen food warehouses and meat, dairy and food-processing plants utilize 'environmentally friendly' ammonia in the refrigeration cycle, many smaller units such as distribution systems, food service installations and household equipment rely almost exclusively on CFC or HCFC refrigerants. The immense economic burden associated with the necessary transition will be paid for by society and by the users of both chilled and frozen refrigeration systems24. The redesign of freezing systems will parallel the development of new refrigerants. However, it is evident that the freezing process itself can also be redesigned: freezing techniques can and should benefit from an interdisciplinary approach by physicists and engineers as well as chemists. These considerations have perhaps further fuelled the drive towards the cryogenic freezing techniques discussed earlier. At present, however, economic considerations are taking precedence in the choice of freezing operation, and have limited the use of cryogenic techniques as the sole mechanism of freezing. Mechanical techniques (air blast or contact freezing) are therefore more widespread. A cost-effective alternative to a 100% cryogenic IQF freezer is the use of combination freezers, which use a combination of cryogenic and mechanical techniques. The principle of operation of combination systems is to use a cryogenic freezing unit for initial freezing of the outer surface of the product, followed immediately by mechanical freezing to reduce the temperature of the bulk of the product to the desired frozen temperature. The initial 'crust-freezing' stage ensures that product quality is maintained, particularly in the case of delicate products such as seafoods, berries and certain fruits, while the second mechanical stage ensures efficiency and economy of the overall freezing operation. Advantages of this two-stage process include active separation and crust freezing of sticky or delicate products and better control of freezing time, resulting in a more uniform, higher-quality product. Commercial applications of the combination freezing technique have been widely publicized. The 'CRUSTOFREEZE' system, developed by Frigoscandia and its parent company AGA AB (Frigoscandia Food Process Systems AIS, Helsingborg, Sweden), consists

Box 1. Some advantages of the freezing options available Mechanical (air blast)

Mechanical (contact)

Economical to construct and operate

Low capital investment

Cryogenic Low product dehydration

Allows continuous, in-line processing

Low operating costs

High product quality

Controlled heat transfer

Efficient heat transfer

Efficient freezing of small products

Flexible product line

Improved bulk product handling

No refrigerationplant required

Low maintenance

Trends in Food Science& Technology May 1993 lVol. 4]

137

It is evident that in order to preserve the high quality of a cryogenic pre-freezer utilizing liquid nitrogen; it is designed for installation and retro-fitting immediately of frozen foods, rapid freezing techniques such as ahead of a finishing freezer23. The crust-freezing process fluidized bed and cryogenics are becoming more wideis controlled by the rate of flow of liquid nitrogen spread. To date, the major obstacle to their application and the speed of the belt driving the product through has been the relatively low economy of such operations. the cryogenic zone, the objective being to optimize This factor is being addressed by the development of the degree of pre-freezing and the rate of consumption cryo-mechanical combination techniques, which will of the liquid nitrogen. The rapid freezing rate of form the next generation of freezing processes. the product crust is such that the retention time for Simultaneous developments in the innovative reformuthe product in contact with the liquid nitrogen is lation of food products for freezing will enhance the short: 3-5 seconds in a dip pan, followed by 5-6 quality attributes of frozen foods and provide additional seconds on the crust-freezing belt. The nearly impetus for further growth of this market sector. instantaneous freezing of the product crust allows reduced agitation of the freezing air, aiding the References economy of the operation by reducing both dehydration 1 ~tringer,M.F. (1990) FoodManuf. 65(4), 39-42 losses and mechanical losses (e.g. products falling 2 Bond,S. (1992) Food TechnoL Int. Eur. 1992, 109-111 3 Morris,C.E.(1991) Food Eng.63(7), 61-63 from belt). The dynamic pre-freezing stage also guaran4 CommissionDirective 92/1/EECof 13thJanuary, 1992,on the tees a fully IQF product. Monitoring of Temperaturesin the Meansof Transport, Warehousing A combination freezing system has also been develand Storageof Quick-frozen FoodstuffsIntendedfor Human oped for the freezing of liquid or semi-liquid foods such Consumption (1992) Off. J. Eur. Commun. V35 L34, 18 as fruit or vegetable purees, dairy products and baby 5 CommissionDirective 92/2/EECof 13thJanuary, 1992,LayingDown foods 23. The technique utilizes a cryogenic gas to create the SamplingProcedureand the CommunityMethodof Analysisfor the a frozen crust on the fluid product; the product may then Official Control of the Temperaturesof Quick-frozen FoodsIntended be conveyed to the conventional mechanical freezer (air for Human Consumption(1992) Off. J. Eur. Commun.V35 L34, 30 blast or contact-type freezer). The cryo-mechanical sys- 6 Selman,J.D. (1990) FoodManuf. 65(8), 30-34 tem can thus offer the advantages of both systems: the 7 George,R.M. and Shaw, R. (1992) TechnicalManual No. 35, Campden flexibility and other benefits of cryogenic systems toFood and Drink ResearchAssociation,Chipping Campden, UK 8 Got, H.D. (1992) FoodRes.Int. 25, 317-325 gether with the lower unit-cost factors of the mechanical 9 Heldman,D.R. and Singh, R.P.(1981) Food ProcessEngineering,AVI systems. The flexibility of such systems also allows Publishing rapid adjustment to the requirements of new markets, a 10 Reid, D.S. (1990) Food Technol. Int. Eur. 1990, 89-90 considerable advantage for food processors within the 11 Lewis,M.J. (1987) PhysicalPropertiesof Foodsand FoodProcessing modem food-freezing industry. Systems, Ellis Horwood A combination treatment involving freezing in con12 Rao,M.A. and Rizvi, S.S.H.,eds (1986) EngineeringPropertiesof junction with irradiation has recently been proposed as a Foods, Marcel Dekker means of retarding spoilage prior to, or following, pro- 13 Reid,D.S.(1983) Food Technol. 37(4), 110-115 cessing -'~.This combination treatment allows products to 14 Noel,T.R., Ring, S.G.and Whittam, M.A. (1990) TrendsFoodSci. be harvested in a more mature state and the distribution Technol. 1,62-67 of products to more distant markets, which are currently 15 Slade, L. and Levine, H. (1991) Crit. Rev. Food5ci. Nutr. 30, 115-360 poorly accessible due to problems with preserving the 16 Feeney, R.E.and Yeh, Y. (1993) Food Technol.47(1), 82-90 integrity of the frozen food chain. It has been reported 17 Reid, D.S. (1990) Food Technol.49(7), 78-84 that in 1991-1992, parts of Europe were irradiating 18 Pham, Q.T. and Willix, I. (1990)I. FoodSci. 55(5), 1429-1434 frozen seafoods emerging from Asia to eliminate mi- 19 Singh, R.P.and Wang, C.Y. (1977)J. FoodProcessEng. 1,97-127 crobial pathogens such as Salmonella 25. The potential of 20 Eek, L. (1991) in FoodFreezing:Todayand Tomorrow(Bald,W.B., ed.), pp. 143-155, Springer-Verlag this approach for the poultry market is also being recogMiller, I.P. (1991) in FoodFreezing: Todayand Tomorrow(Bald,W.B., nized, although it is considered that the major obstacle ed.), pp. 157-170, Springer-Verlag to wider acceptance will be consumer perception of 22 Leeson, R. !1990) Br. Foodl. 92(4), 42-46 irradiated foodstuffs, which at present is preventing 23 Londahl, G. and Karlsson,B. (1991) Food Technol. Int. Eur. 1991,90-91 irradiated tbods becoming a serious challenger to the 24 Anon. (1992) FoodEng. Int. 17(2), 29-30 frozen food market sector. 25 Salvage, B. (1992) FrozenFood Rep. 3, 28-33

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Trends in Food Science & Technology May 1993 IVol. 41