Preservation of Food by Freezing

Freeze-Preservation of FoodStuffs

Substantial differences exist between the cryopreservation of mammalian tissues to preserve viability and the preservation of food. The scale, of course, is vastly different, with tons upon tons of foods requiring preservation on a daily basis. The competance and attentiveness of the recipients of the preserved goods must also be considered; trusting everyone to be familiar with the handling precautions associated with liquid nitrogen is simply not practical, nor can one insist upon a precise, multi-step thawing protocol before ingestion of the food. Indeed, the food preservationist must come to grips with the Darwinian maxim: "If you make a process idiot-proof, nature will provide a better idiot".

Not being able to store foods at liquid nitrogen temperature (-196C), or even the temperature of dry ice (-80C), imposes certains constraints upon the process. Most mechanical freezers that are to be found in homes and grocery stores keep the temperature between -20C and -30C. At these temperatures, chemistry will still occur, so the time of storage is an important parameter. The amount of ice formed will be less than that seen in cryopreservation and recrystallization or ice lensing may dramatically alter the texture of the food upon thawing.

The size of foodstuffs also imposes certain limitations on the feasibility of preservation. The rate of cooling and warming, critical parameters for cryopreservation of cells and tissues, is severely restricted due to the large size of most foods. The addition of molar quantities of cryoprotectants into food, where preservation is used to maintain taste, smell, and texture, can only be counterproductive. Although assays for successful food preservation are subjective, everyone seems to be able to agree when freeze-preservation has altered the quality of food. Perhaps these assessments work so well because the frozen sample is always compared to a fresh control.

Though the aims of the cryopreservationist and the food preservationist are quite different, there are more similarities than differences and each discipline has much to teach the other. Besides, the economic scale of food preservation is so much greater than medical cryopreservation that the pragmatic cryobiologist may wish to aquaint himself of the current practice of food preservation so that any insights that might apply to that arena are not simply discarded. After all, whoever figures out how to freeze a tomato so that it tastes just like a fresh tomato will not want for many earthly goods.

Freezing of Plants

One of the principle factors that affects plant food quality is their texture, the mechanical properties that the food exhibits when a bite is taken and it is chewed. Fresh fruits and vegetables that are eaten without cooking are given texture through cellular turgor pressure (hydrostatic pressure developed osmotically within a cell against its cell wall). The osmotic pressure is only present when their is an intact plasma membrane to provide a semi-permeable membrane, so any damage to the plasma membrane leads to a loss of turgor and hence, the degradation of food texture. Freezing and thawing target the plasma membrane as the primary site of injury, thus the texture of fresh fruits and vegetables is inevitably impaired by freezing a thawing. Common examples are the freezing of strawberries or lettuce, where the stiff and sometimes crunchy food turns to mush after freezing and thawing.

Sugar can be added to a fruit preparation before freezing to partially dehydrate the cells, thereby decreasing the damage to the plasma membrane during freezing, but the size and lack of a perfusion network (e.g. the vasculature in animals) makes this of limited applicability. Another strategy for preserving texture, especially common with strawberries, is to eat them while they are only partially thawed; the ice providing the extra firmness to offset the loss of turgor pressure.

Freezing of vegetables that are to be cooked do not suffer from the same problem of loss of turgor, since cooking usually destroys turgor as well. There are other problems associated with freezing that cause problems for vegetable preservation. The main problem is one of chemistry; cells normally keep enzymes and substrates compartmentalized in membrane bound vesicles, only bringing them together at appropriate rates and times. Freeze induced disruption of intracellular membranes allows these enzymes and substrates to come together and react, thereby changing the chemistry (hence the taste) of the food. To counteract these chemical changes, the food is usually blanched (partially cooked) to denature the enzymes prior to freezing. There are also chemical treatments that are used to inactivate enzymes, such as the addition of sulphur dioxide.

Freezing of Meats

Muscle tissue, or meat as the common folk choose to call it, differs from plant tissue primarily in its homogeneity (from the perspective of the food-preservationist). Following death of the organism, the muscle tissue continues to use ATP until it is exhausted and then enter rigor (a rigid state). Enzymes present in the tissue continue to digest the muscle tissue, softening it until it becomes meat (properly aged meat). The freezing of properly aged meat presents few problems but if there is any residual ATP present in the tissue during freezing, the muscle will contract upon thawing and be ruined. The only further problem encountered with meat in the frozen state is the oxidation of lipids which alters the taste with time in the frozen state (due to the high temperature of conventional freezers).

Freezing fish muscle is more problematic than mammalian muscle as the fish meat is more susceptible to protein denaturation. Extensive cross- linking occurs at -20C, producing foul tasting byproducts such as formaldehyde, ammonia and others. Although these reactions are primarily enzymatic, heat inactivation of the enzymes is problematic as the muscle structure is changed during heating. The simplest solution is to lower the temperature of storage (and thereby slow down the enzymatic reactions), however this precludes long-term storage in the home.

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Document last updated Mar. 5, 1999.
Copyright © 1999, Ken Muldrew.