Heating and Refrigeration Methods of Food Preservation

c. Heat Processing. This is method uses heat to kill microorganisms or suppress their growth and to inactivate enzymes.

1. Canning. In canning the food is placed in containers, heated, and then sealed, usually under vacuum. Canning is used for products such as fruit juices, syrups, and sauces. The advantage of canning process compared to other preservation is that in canning the stable vitamins are retained and color and flavor of food item is retained better. The temperature to which the food is heated generally exceeds 212°F and the food is heated long enough to inactivate the most heat-resistant pathogens (disease-causing organisms) and spoilage organisms. Factors such as the kind and number of microorganisms present, acidity of foods, presence of preservatives (salt, sugar) affect the length of time the food must be heated.
2. Pasteurization. Pasteurization is a pressurized heating for a short time, it is consider a mild form of heat treatment. The temperature used during pasteurization is below 212°F (100°C). Milk is pasteurized to destroy pathogens, but other, more heat-resistant organisms are only reduced in number, so that pasteurized milk must generally be stored under refrigeration to keep bacterial growth in check. In addition to destroying some microorganisms, pasteurization also inactivates some enzymes. Pasteurization does not change the color and flavor to any significant level.
3. Blanching. Blanching is a mild heat treatment applied to fruits and vegetables primarily to inactivate enzymes. This process is usually done when such foods are to be dried or frozen.

d.Refrigeration. This procedure uses low temperatures to suppress microbial growth and hinder biochemical changes. The nutritional value of frozen foods is often very close to its fresh equivalents, even though frozen foods cannot be kept for as long as canned foods, it retains color and flavor value better than canned foods. The two forms of refrigeration are:

1. Cooling. When food is kept at temperatures below room temperature but about freezing, the growth of spoilage microorganisms is slowed, and the rate of biochemical changes is reduced. Meat, poultry, fish and many fruits and vegetables are quite perishable, however, when kept at 32°F (0°C) retain their freshness for less than two weeks.
2. Freezing. When foods are kept at temperatures below freezing, most chemical changes take place at such a reduced rate that only minor changes are noticeable even after long periods of storage. But microorganisms are generally more resistant to cold than to heat, and although some are killed by freezing, most bacterial spores and a large number of organisms survive and are revitalized when the food is thawed.

e. Irradiation. In this process gamma rays or high speed electrons are used to destroy micro-organisms. Experiments have shown that the doses required for eliminating microorganisms are so high that they also affect the food's molecular structure, particularly that of the proteins and fats, causing off-flavors and the breakdown of some ingredients. Irradiated foods are not radioactive but they are not for sale in the market and they are not approved by the food regulating authorities. Even though it is not proven that irradiated food intake is associated with any complication, it appears food irradiation is best be avoided whenever possible...

Back to first page -->> Methods of Food Preservation (dehyration and chemical additives