The spoilage of foods is caused chiefly by microorganisms. Since growth of microorganisms depends upon the temperature, lowering the temperature is of great importance for food preservation. In order to describe these effects, it is therefore necessary to understand the influence of low temperatures on the growth of bacteria and other microorganisms. External agents of chemical changes, microorganisms, also alter the color, flavor, texture, aroma, etc. Bacteria and mold on fruits and vegetables break down the supporting pectic material and make the tissues soft and mushy through loss of structure. Certain bacteria also convert the carbohydrates in vegetables to lactic acid, which is the material in curdled milk having a sour smell and taste. Foods subjected to the action of such microorganisms become unpalatable in the earlier stages of decomposition. It was pointed out that the killing effect on bacteria is even greater in the range -4°C to -10°C than in the range -15°C to -30°C (Koelet, 1992). For this reason, if the freezing process is slow the effect of the cold shock is lost, meaning that the bacteria will have time to adapt themselves to the new conditions.
In relation to temperature level, there are four main bacteria according to type of growth: thermophilic, mesophilic, psychrophilic, and psychrotrophic (Hui, 1992). Psychrophils, particularly, spoil the protein-rich foods (e.g. meat, fish, poultry) and grow well above 0°C. Psychrotrophs are able to grow at temperatures close to the freezing point. It is useful to mention that every product has a different freezing point, which will be discussed later. Lowering the temperature slows the growth rate of all bacteria, and at temperatures used in commercial storage of frozen foods all microbiological growth ceases completely. The time it takes to decrease the temperature to below the freezing point is critical. A product temperature of -10°C is normally considered safe with regard to microbiological growth.