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Food Preservation Methods |
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Food Preservation Methods
Food preservation aims at preventing the microbial spoilage of food products and the growth of the food borne pathogens. Thus, the two principal goals of food preservation methods are, (i) increasing the shelf life of the food and (ii) ensuring the safety for human consumption.
There are a variety of food preservation methods:
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1. Heat:: Heat kills microorganisms by changing the physical and chemical properties of their proteins. The most common use of heat is in the process of canning. The food product is washed, sorted, and graded and then subjected to steam heat. for three to five minutes. This last process called blanching, destroys many enzymes in the food product and prevents further cellular metabolism.
The food is then peeled and cored, and diseased portions are removed. For canning, containers are evacuated and placed in a pressurised steam steriliser, similar to an autoclave at 121°C. This removes especially Bacillus -and Clostridiurn spores.
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If canning is defective, foods may become contaminated by anaerobic; bacteria which produce gas. These are species of Clostridium, and coliform bacteria (a group of Gram-negative nonspore-forming rods which ferment lactose to acid and gas at 32°C in 48 hours).
2.Low temperature:Exposure of microorganisms to low temperatures reduces their rates of growth and reproduction. This principle is used in refrigeration and freezing. Microbes are not killed. In refrigerators at 5°C, foods remain unspoiled. In a freezer at -5°C the crystals formed tear and shred microorganisms. It may kill many of the microbes.
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However, some are able to survive. Salmonella spp. and streptococci survive freezing. For these types rapid thawing and cooking is necessary. Deep freezing at -60°C forms smaller crystals. It reduces biochemical activities of microbes.
Blanching of fruits and vegetables, by scalding with hot, water or steam prior to deep freezing, inactivate plant enzymes that may produce touhness, change in colour etc. A brief scalding prior to freezeing also reduces the number of microorganisms on the food surface by up to 99 per cent, enhances the colour of green vegetables.
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3. Drying or Desiccation: Water from foods is removed in different ways. It may be done by spray dryer which expels a fine mist of liquid such as coffee into a barrel cylinder containing hot air. There may be used a heated drum onto which liquids like soup may be poured. Another machine is a belt heater that exposes liquids as milk to a steam of hot air that evaporates water and produces dried milk solids.
A common process of freeze drying or lyophilization is used these days. The food is deep frozen, after which the water is drawn off by a vacuum pump in a machine. They dry product is then sealed in foil and is reconstituted with water. This method is very useful for storing, transporting and preserving bacterial cultures.
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4. Osmotic pressur: The principle of osmosis is applied. Foods are preserved by adding salts and sugars to them. These chemicals remove the water out of microbial cells causing them to shrink. Thus stopping their metabolism. Jams, jellies, fruit syrups, honey etc. are preserved by high sugar concentration. Fish, meat beef and vegetable products are preserved with salt.
5. Chemical preservatives: The most commonly used are the acids, such as sorbic acid, benzoic acid and propionic acid. These check mainly the growth of yeasts and molds. Sorbic acid is used for preservation of syrups, salads jellies and some cakes. Benzoic acid is used for beverages, margarine, apple cider etc.
Propionic acid is an ingredient of bread and bakery products. Sulphur dioxide, as gas or liquid is also used for dried fruits, molasses and juice concentrates. Ethylene oxide is used for spices, nuts and dried fruits.
6. Radiation:. UV is used in meat storage facilities which reduce surface contamination, on meat products. Gamma rays are also used for some meat products.
7. Anaerobiosis: Packaging of food products under anaerobic conditions - anaerobiosis is effective in preventing aerobic spoilage process. Vacuum packing in an airtight container is used to eliminate air.
8. Controlled atmospheres: Such atmospheres containing 10% CO2 are used to preserve stored food products as apples and pears. This checks fungal growth. Ozone can also be added.
9. Other methods: These are asepsis i.e. washing utensils that come in contact with food; and filtration and centrifugation, used, to remove microbes. Filtration is used for fruit juices, other drinks etc. Bacteriological filters are used in industries .
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