Preservation by High Temperature

Preservation by High Temperature

The temperature and time used in heat processing a food depends upon the effects of heat on food and the other pre-servative methods employed.

Pasteurization

Pasteurization is a heat treatment that kills part but not all the micro organisms pres-ent and involves the application of tem-peratures below 100°C. The heating, may be by means of steam, hot water, dry heat or electric currents and the products are cooled immediately after the heat treat-ment. Milk is usually pasteurized.

Pasteurized products are not ster-ile. They contain vegetative organisms and spores which are still capable of growth. Hence many pasteurized foods must be stored under refrigeration. Pasteurized milk can be stored for over a week under refrigeration while pasteurized milk stored at room temperature will spoil within a day.

Blanching

Blanching is a heat treatment like pasteur-ization. It is done by dipping the products in boiling water for two to three minutes at 180°F to 190°F. Blanching focuses on deaerating the product and inactivating degradative enzymes before further pro-cessing. Blanching is an important step in freezing food, as frozen foods can develop off flavour, vitamin losses and colour changes while in storage.

Blanching

·        Prevents bacterial growth.

·        Fixes the natural colour of vegetables – holds the colour.

·        Shrinks the product, better for filling the container.

Canning

Canning involves the application of tem-peratures to food that is high enough to destroy essentially all micro organisms present. It also involves airtight sealing in sterilized containers to prevent recontam-ination. The degree of heat and the length of time of heating vary with the type of food and the kinds of micro organisms. Large quantities of food are canned for preservation. In developed countries, canned foods form a major part of the diet of the people. Items often canned are meats and meat products, fruits and vege-tables, fish products, soups, etc.

The process of canning involves the following steps:

·        Receiving, cleaning, grading and inspecting of raw commodity.

·        Blanching to inactivate enzymes.

·        Placing in the container with added brine or syrup and deaeration of the product.

·        The next process is exhausting. Exhaust-ing is done to expel the air and gas from the can so that its internal pressure, after heating and cooling, is the same as the atmospheric pressure.

After exhausting, the filled cans are per manently sealed mechanically.

·        The sealed containers are subjected to high temperatures, to destroy the most heat resistant organisms.

·        After this, the cans are cooled by water in a cooling canal to about 38°C, before storage.

The final step is casing and storing the cans.