The different preservation methods commonly employed are as discussed below:
Food is a living system and in its natural form it has its own protective mechanisms. When the food is removed from the field or protective skin or peel, it begins to deteriorate. Asepsis is a process of keeping microorganisms out of food. An aseptic environment can be created by
Removal of microorganisms
- Proper packaging of the product, which separates the internal environment from the surroundings.
- Maintenance of general cleanliness and sanitary conditions while processing and handling the product from raw material to finished stage can help in preventing the entry of microorganisms into the product.
It is well known fact that microbes are everywhere. The dust and dirt adhering to the raw material contain microorganisms and by applying various pre-treatment/ cooking methods, number of microorganisms can be reduced considerably. Such steps include:
Maintenance of anaerobic conditions/ packaging
- Trimming ingredients
- Discarding dirt
Packaging food in a vacuum environment, usually in an air-tight bag or bottle results in anaerobic environment. As bacteria need oxygen for survival, the vacuum environment in the package slows down the spoilage by them.
Drying Drying is one of the oldest and the simplest method of preserving food. It refers to removal of water from the food. Dried foods are preserved because the available moisture level is so low that the microorganisms cannot grow and the enzyme activity is also controlled. Drying can be accomplished by a number of methods viz. sun drying, mechanical/ artificial drying and freeze drying etc.
Dried foods are compact and lightweight; do not require refrigeration and last much longer than the fresh foods. Dried foods should be stored in airtight containers to prevent moisture from rehydrating them and allowing microbial growth.
- Sun drying
Sun-drying takes heat from sun rays but it is a slow process involving risk of contamination and spoilage. The limitation for sun drying is availability of climate with a hot sun and a dry atmosphere.
- Mechanical/ artificial drying
Dehydration process usually implies the use of controlled conditions of heating, with the forced circulation of air or artificial drying (mechanical drier) in contrast to sun drying. Using mechanical driers, fruits, fruit leathers, banana chips, tea, coffee, milk, soups, fish, meat, eggs and vegetables can all be dried year-round.
- Freeze drying
Freeze-drying is a form of dehydration in which the product is first frozen and then water is removed under vacuum as vapour by sublimation. The principle behind freeze drying is that under certain conditions of low vapour pressure, water in the form ice evaporates as water vapour directly without turning into liquid phase. The advantage is that the food structure and nutritional properties are better conserved but the equipment and its maintenance is costly.
Smoking has been used as a method of food preservation from time immemorial. In this method, foods are exposed to smoke by burning some special kinds of wood, which has two main purposes, adding desired flavouring and preserving. Smoke contains chemicals like formaldehyde, which is bactericidal. And also the dehydration occurring due to smoking is responsible for its preservative action. The smoke is obtained by burning wood like oak, maple, walnut and mahogany under low breeze/ wind. Most meat is smoked after curing to aid their preservation. Examples of smoke preserved foods are smoked beef, ham, bacon, fish and meat.
Relatively few liquid foods are preserved by concentration, involving preservative action of reduction in water activity (aw) and development of osmotic pressure, which retard the microbial growth and enzymatic reactions. Concentration of food is usually done for many reasons: reduction in volume and weight; reduction in packaging, storage and transport costs; better microbial stability; and convenience. Examples of food preserved by concentration are tomato paste, fruit juice concentrate, soup and condensed milk. The rate of heating should be controlled to prevent localized burning of the product, particularly when it has become thickened towards the end of boiling.
Use of high sugar or salt content
Use of organic acids Organic acids are used in food preservation because acid conditions inhibit growth of many spoilage microorganisms. Bacteria are generally pH sensitive. Organic acids penetrate the bacteria cell wall and disrupt its normal physiology and thus preserve the food. Acetic acid, lactic acid, citric acid and malic acid are widely used for preservation in food products.
A strong sugar solution (about 68 per cent or more) draws water from the microbial cells and thus, inhibits the growth of microbes. Examples of food preserved by high sugar concentrations are fruits in heavy sugar syrup (preserve or murraba), jams, jellies, marmalades, candies and sweetened condensed milk.
Pickles are the relishing accompaniments in the Indian meals. Microorganisms do not grow well in acidic solutions. And this is the basis of preserving fruits and vegetables by pickling. Pickling uses the salt combined with the acid, such as acetic acid (vinegar). Some of the fruits and vegetables, which are generally pickled, are raw mangoes, limes, Indian gooseberry (aonla), ginger, turmeric and green chillies.
- Salting or curing
Salting is being done in case of meat and fish preservation since ancient times. Curing preserves the food by drawing moisture from the meat through osmosis and makes it unavailable for microbial growth and enzyme action. Meat is generally cured with salt or sugar, or a combination of the two. Nitrates and nitrites are also often used to cure meat, which contribute the characteristic pink colour to meat, as well as inhibition of Clostridium botulinum. Dry salting is used in India for the preparation of preserved tamarind, raw mango, Indian gooseberry (aonla), fish and meat etc.