Mode of Action:
The effect of low temperatures on microorganisms depends on the particular microbe and the intensity of application. For example, at temperatures of ordinary refrigerators (0 ˚ C), the metabolic rate of some microbes is so reduced that they cannot reproduce or synthesize toxins. In other words, ordinary refrigeration has a bacteriostatic effect, but does not kill many microbes. Heat is much more effective than cold at killing microorganism.
Disadvantage:
Yet psychrotrophs do grow slowly at refrigerator temperatures and will alter the appearance and taste of foods after a time. For example, a single microbe reproducing only three times a day would reach a population of more than 2 million within a week.
Advantage by Medical Point of View:
Pathogenic bacteria generally will not grow at refrigerator temperature.
Uses of Cold temperature:
Refrigeration is used to prevent food spoilage. Freezing, drying, and freeze-drying are used to preserve both foods and microorganism, but these methods do not achieve sterilization.
Optimum Conditions:
Surprisingly, some bacteria can grow at temperatures several degrees below freezing. Most foods remain unfrozen until -2oC or lower. Rapidly attained subfreezing temperatures tend to render microbes dormant but do not necessarily kill them. Slow freezing is more harmful to bacteria; the ice crystals that form and grow disrupt the cellular and molecular structure of the bacteria. Thawing, being inherently slower is actually the more damaging part of a freeze-thaw cycle. Once frozen, one third of the population of some vegetative bacteria might survive a year, whereas other species might have very few survivors after this time.
Results of Low Temperature Treatment:
Many eukaryotic parasites, such as the roundworms that cause trichinosis, are killed by several days of freezing temperatures.
Conditions:
Many fresh foods can be prevented from spoiling by keeping them at 5 ° C (ordinary refrigerator temperature).
Limitations:
However, storage should be limited to a few days because some bacteria and molds continue to grow at this temperature. To convince yourself of this, recall some of the strange things you have found growing on left over of the back of your refrigerator. In rare instances strains of Clostridium botulinum have been found growing and producing lethal toxins in a refrigerator when the organism were deep within a container of food, where anaerobic conditions exist.
Freezing:
Uses of Freezing:
Freezing at -20 ° C is used to preserve foods in homes and in the food industry. Although freezing does not sterilize foods, it does significantly slow the rate of chemical reactions so that microorganism does not cause food to spoil. Frozen foods should not be thawed and refrozen. Repeated freezing and thawing of foods causes large ice crystals to form in the foods during slow freezing. Cell membranes in the foods are ruptured, and nutrients leak out. The texture of foods is thus altered, and they become less palatable. It also allows bacteria to multiply while food is thawed, making the food more susceptible to bacterial degradation.
Freezing can be used to preserve microorganisms, but this requires a much lower temperature than that used for food preservation. Microorganism are usually suspended in glycerol or protein to prevent the formation of large ice crystal (which could puncture cells), cooled with solid carbon dioxide (dry ice) to a temperature of -78 ° C, and then held there. Alternatively, they can be placed in liquid nitrogen and cooled to – 180 ° C.
