Microbiology of Canned Foods

Microbiology of Canned Foods

	Microbiology of Canned Foods - One method of preserving canned food is by the use of heat which destroys most, if not all the microorganisms. The type of heat treatment will however, depend on the nature of the food material and varies from pasteurization for milk, juices and other liquids to sterilization by Steam under pressure for canned vegetables or soups.
At milder heat treatment: a number of heat resistant organisms survive and these may subsequently grow and cause spoilage if conditions are favourable. Following heating, the canned or bottled food is then stored at a low temperature. Some foods which are processed at high temperatures can be stored at room temperature. Heated canned foods may undergo spoilage either due to chemical or biological reasons. The most important chemical spoilage of canned foods is the "hydrogen swell" produced as a result of the action of the food acid with the metal can. Such spoilage occurs mostly due to imperfect tinning and lacquring of the interior of the can used for canning acidic foods.
Biological spoilage of canned' foods by microorganisms may result either from the survival of organisms after the heat treatment or leakage of the container permitting entrance of microorganisms. Surviving organisms may be vegetative cells or spore formers depending on the heat treatment. Acid foods are processed at temperatures around 100°C which results in the killing of all vegetative cells of bacteria, yeasts and molds.
Only bacterial spores may survive but these do not grow in acid foods. On the other hand, meat, vegetables and milk are processed at lower temperatures. This may eliminate vegetative cells but not the spores, which germinate later and cause sP9Hage. Microorganisms that enter through leaks during cooling need not necessarily be heat resistant. Normally, the two ends of the food can should be flat indicating partial vaccum. If pressure develops inside, the ends bulge and the extent of bulging can lead to a 'flipper', a 'springer', soft swell or a hard swell depending on the pressure inside the can. Canned foods can be grouped as follows on the basis of acidity:
	Group I: Low acid foods, pH 5.3, vegetables such as peas, corn, beans etc. and meat, fish and poultry. Group II: Medium acid foods, pH 5.3-5.5, Beets, pumpkin, spinach etc. Group III: Acid foods, pH 4.5-5.7, tomatoes, pears etc. Group IV: High acid foods, pH 3.7 berries, etc. The type of microbial spoilage of canned foods is divided into those caused by thermophilic bacteria and those caused by mesophilic organisms and the kind of spoilage is classified as putrefaction, acid production, gas formation, blackening etc.

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