METHODS OF PRESERVATION OF FOODS

METHODS OF FOOD PRESERVATION

There are different methods of preserving foods. However the type of method used depends on the nature of the food, length of preservation required, the facilities available and the type of dish to be prepared. The different methods of food preservation are:-

DRYING: This is the process by which the water content of the food is reduced. Both food enzymes and micro- organisms require water for action and growth, therefore the removal of water from the food creates an unfavourable environment for their actions, hence they are unable to cause food spoilage.

Examples of food preserved by drying are legumes, cereals, meat and fish. There are different types of drying.

Solar drying: – This is drying by mere spreading the food stuff in the sun on a mat or concrete floors e. g. cassava, maize, melon etc.
Oven drying: – This is drying by putting the food in pre- heated oven or enclosed chamber e. g. melon, legume.
Freeze drying: – The water content of the food stuff is frozen and then sublimed under vacuum. Heat is supplied by conduction or radiation. Example of food dried by these methods is, fish, meat and vegetables.
Roller drying: – The liquid or slurry to be dried is uniformly applied on a hot rotating stainless steel drum or roller. As the drum rotates, the food dries and is scraped of by a knife. Example are potato flakes, cocoa- oats, yam flakes etc.
Vacuum drying: – Water is removed by creating vacuum. Heat is supplied by condition or radiation. Example, milk.
Tunnel drying: – Fried food placed in conveyor belt or perforated trays and passed through a warm air tunnel. Example, vegetables.
Spray drying: – Liquid is forced through a small nozzle in the form of a spray into a hot chamber, the spray mixes with the warm air and the water is evaporated while a fine powder is produced. Examples of foods preserved by this method are milk, egg white, bournvita, cocoa beverages.

FACTORS AFFECTING THE RATE OF DRYING

The efficiency of the drying equipment.
The method of arrangement of the foodstuff on the drying material. e. g. tray, mat, concrete floor in relation to the heat source or medium.
The physical and chemical properties of the foodstuff.
The amount of surface area of the foodstuff exposed to the dry air or heat.

LOW TEMPERATURE TREATMENT: This is keeping the food at a low temperature by freezing below O C in the freezer and chilling at a higher temperature in the refrigerator. E. g. fish, meat, milk fruits, vegetables.

HIGH TEMPERATURE TREATMENT: This is the application of heat to food stuff to destroy the food enzymes and micro- organisms. It could be by pasteurization- to destroy only the pathogenic micro- organism with minimum effect on the nutritional properties of the food. Usually at a temperature below 100oc e. g. milk.

USES OF CHEMICAL: This is the addition of chemical to food to inhibit the action of food enzymes, Create an unfavourable environment for the growth of micro- organisms and also prevent chemical reaction from taking place in the food.

There are three types of chemical preservation.

Natural chemical
Synthetic
Antibiotic

The natural chemical preservatives are the chemical that are present naturally in the food substance themselves or in other natural substance that interact with the food. Examples of natural preservatives are vinegar, salt, sugar, phenols and Aldehydes. Examples are synthetic chemical preservatives are benzoic acid; nitrites butylated hydroxy toluene (BHT) Examples of antibiotics are – theabendazole, Nisin and tetracyclines.

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CANNING AND BOTTLING: This is keeping food aseptically in cans or in bottles. The food is sterilized and scaled in the vacuum or in the presence of inert gases and them in cans or bottles. E. g. fruit juices, meat, fish, beverages, etc.

IRRADIATION: This is involves the use of radioactive element like cobalt, to destroy micro-organism and food enzymes e.g. potato, yam tuber and onion.

FERMENTATION: This very old method of preserving food is still being practiced. It involves the decomposition of some of the food component to produce acid. The acid so produced will reduce the pH of the food and hence make the food unfavourable to the mirco- organisms. Example of food preserved by fermentation are locust beans seed, (iru) melon paste (ogiri) and wines .Other product of fermentation in the foods are cheese, pitto, fermented fish, fermented plantain, garri and corn paste/ dough.

SMOKING: This is drying of food stuff over a fire or heated charcoal, the smoke produced contains some aldeheydes and phenols which have a strong bacteria effect e. g. fish, meat.

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