Not being able to store foods at liquid nitrogen temperature (-196°C), or even the temperature of dry ice (-80°C), imposes certains constraints upon the process. Most mechanical freezers that are to be found in homes and grocery stores keep the temperature between -20°C and -30°C. 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.
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 fish muscle is more problematic than mammalian muscle as the fish meat is more susceptible to protein denaturation. Extensive cross- linking occurs at -20°C, 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.