Numerous problems have arisen in the application of freezing methods to the various types of food products. One problem is concerned with the determination of the direct effects of low temperatures upon the food itself and another problem is to determine the effects of low temperatures upon other factors which may in turn affect the quality of the food. We are especially interested in knowing the exact effects of freezing and other low temperatures upon the micro‐organisms associated with foods. Bacteria constitute the most significant group of micro‐organisms affecting the sanitation and keeping qualities of foods. Those bringing about the decomposition of food products, while they are many and vary greatly, depending upon the nature of the food, are chiefly organisms from the air, water and soil. The types of bacteria found in foods vary greatly in their action on the food and also in reaction or response to varying temperature conditions. The action of micro‐organisms on foods of high carbohydrate content results in fermentations, while the action of the micro‐organisms on foods of high protein content will result, chiefly, in putrefactive changes. The former type of change usually occurs at a more rapid rate, when conditions are favourable, but the latter change usually results in a more undesirable condition of the food. While certain types of bacteria grow best at temperatures well above human body temperatures and others even as low as the freezing point of water, a large majority of those found in foods and the ones normally responsible for the detrimental changes in foods, are active only between 50° and 100° F. It is this latter group which is most implicated in food spoilage and it is significant that this group will be most effectively suppressed by low temperatures. Bacteria are much less affected by low than by high temperatures. Cold alone does not kill most types of bacteria, but slows down their activities to such an extent that they multiply very slowly, if at all. Many bacteria will die off, however, when held at a temperature below that which permits growth and reproduction. Bacteria, generally speaking, will be more easily killed when frozen in pure water than when frozen in foods containing albuminous matter and fats. There are a few bacteria of the cold‐loving type, which may actually multiply and cause slow decomposition at temperatures of 0° C. or less, if substances in solution are present to depress the crystallising point of water. Cold not only retards the growth of bacteria by the direct physiological effect of slowing down the rate of metabolism, but also depresses bacterial activity through its effect on their water and food supplies. Bacteria cannot grow and multiply in a completely frozen or crystallised medium, since they are by nature aquatic and are unable to carry on their normal activities except in a liquid medium. There is no evidence that bacteria maintain a body temperature which would make water available from a completely frozen medium. Bacteria may only utilise food when it is in soluble form, and thus capable of diffusion through their semipermeable cell membranes. When the temperature is sufficiently low to cause the crystallisation of most of the water, the remaining constituents become relatively more concentrated and this will further suppress the activity of the bacterial cells by affecting their osmotic pressures. These effects are very similar to those of partial desiccation or drying. In the course of experimentation some very striking examples of bacterial resistance to low temperatures have been reported. Lactobacillus and aerobacter have been reported to survive in peas stored at −10° C. for two years; whilst bacteria of the genus Pseudomonas were reported to increase in numbers when stored at −4° C. In general it may be said that practically all pathogenic bacteria likely to be found in foods will die off rather rapidly at low temperatures. However, this should not be interpreted to mean that infected foods can be made safe by low temperatures alone. Among the disease producing bacteria transmitted through foods, those of special significance include the organisms and toxins of botulism, typhoid fever, the several organisms of food poisoning called ptomaine poisoning, belonging to the Salmonella group (Salmonella enteritidis, etc.), and various organisms causing infections of the general nature of dysenteries or summer complaints of infants and adults. Frozen foods present no greater threat of botulism than foods preserved by other methods, yet it has been shown that Clostridium botulinum spores may survive freezing at −16° C. for as long as 14 months. The vegetables when thawed become toxic in from three to six days. Experiments have shown that Clostridium botulinum in foods preserved by “quick freezing” and subsequent storage at temperatures below 10° C., show no toxin production for at least 30 days. The lower the temperature of storage the greater the protection against botulism. All foods in which Clostridium botulinum might be present, and which have not been thoroughly heated, should be refrigerated at or near the freezing point. All foods which may harbour the botulism organisms or toxins should be selected with special care, before they are frozen, and care should be taken to see that they are kept frozen until used by the customer. Frozen vegetables should be used immediately after thawing. Thawing and refreezing is always objectionable since such a practice leads to loss of quality, and since bacterial growth and activity may occur during the period of thawing. While the typhoid organisms (Eberthella typhosa) shows considerable variation in resistance to low temperatures, it has been shown that about 99 per cent. will be killed immediately by freezing. Temperatures below freezing apparently have little more effect than the freezing point temperature. Small numbers of the Salmonella and similar organisms of the food poisoning groups may survive in frozen foods for periods of several weeks. It has been shown, however, that no significant growth of activity of these organisms will occur if the foods are refrigerated at 5° C. (41° F.) or less. Moulds and yeasts are of relatively little importance in frozen foods, both from the standpoint of sanitation and food spoilage. While low temperatures will materially retard the rate of enzymatic changes within food products, there is evidence that such changes continue to take place in frozen foods, even considerably below the freezing point. These changes probably account, in part, for the fact that frozen foods once thawed, will decompose more rapidly than foods which have not been frozen.
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