In considering the effect of freezing on those microorganisms that are unable to grow at freezing temperatures, it is well known that freezing is one means of preserving microbial cultures, with freeze drying being perhaps the best method known. However, freezing temperatures have been shown to effect the killing of certain microorganisms of importance in foods.
Ingram32 summarized the salient facts of what happens to certain microorganisms upon freezing: 1. There is a sudden mortality immediately on freezing, varying with species. 2. The proportion of cells surviving immediately after freezing die gradually when stored in the frozen state. 3. This decline in numbers is relatively rapid at temperatures just below the freezing point, especially about −2◦C, but less so at lower temperatures, and it is usually slow below −20◦C.
Bacteria differ in their capacity to survive during freezing, with cocci being generally more resistant than Gram-negative rods. Of the food-poisoning bacteria, salmonellae are less resistant than Staphylococcus aureus or vegetative cells of clostridia, whereas endospores and food-poisoning toxins are apparently unaffected by low temperatures.21 The effect of freezing several species of Salmonella to −25.5◦C and holding up to 270 days is presented in Table 16–5. Although a significant reduction in viable numbers occurred over the 270-day storage period with most species, in no instance did all the cells die off.
From the strict standpoint of food preservation, freezing should not be regarded as a means of destroying foodborne microorganisms. The type of organisms that lose their viability in this state differ from strain to strain and depend on the type of freezing employed, the nature and composition of the food in question, the length of time of freezer storage, and other factors, such as temperature of freezing.
Low freezing temperatures of about −20◦C are less harmful to microorganisms than the median range of temperatures, such as −10◦C. For example, more microorganisms are destroyed at −4◦C than at −15◦C or below. Temperatures below −24◦C seem to have no additional effect. Food constituents such as egg white, sucrose, corn syrup, fish, glycerol, and undenatured meat extracts have all been found to increase freezing viability, especially of food-poisoning bacteria, whereas acid conditions have been found to decrease cell viability.21 Consider some of the events that are known to occur when cells freeze: