When the availability of food is more than the present use it is preserved for future consumption. Preservation helps the food to be available in off-season and in any place. Delay in the use of fresh food alters its freshness, it palatability and its nutritive value hence such food is preserved and use for long time. Many foods cannot be preserved as such and need some type of treatment.
Spoilage of food refers to ulteration in food or undergoing some physical and chemical changes, which render the food inedible or hazardous to eat. The chief causes of food spoilage are:
All food preservation methods are based upon the general principle of preventing or retarding the causes of spoilage-microbial decomposition, enzymatic and non-enzymatic chemical reactions and damage from mechanical causes insects and rodents etc.
There are two types of preservation methods used:
Temporary preservation: In this method growth of microorganisms is only retarded or inhibited for short time.
Permanent preservation: In this method the growth of spoilage microorganisms are completely destroyed by different means.
Preservation by Low Temperature
Freezing may preserve foods for long periods of time provided the quality of the food is good to begin with and the temperature of storage is far enough below the actual freezing temperature of food for long preservation. In vegetables, enzyme action may still produce undesirable effects on flavour and texture during freezing. The enzymes therefore must be destroyed by heating before the vegetables are frozen.
It is also known as sharp freezing. In this method, the foods are placed in refrigerated rooms at temperatures ranging from 40C to 290C. Freezing may require from 3 to 72 hours under such conditions. Home freezing is done by sharp method.
The lower temperatures used 320 C to 400 C freeze foods so rapidly that fines crystals are formed and the time of freezing is greatly reduced over that required in sharp freezing. In quick freezing, large amount of food can be frozen in a short period of time.
Dehydro freezing consists of drying the food to about 50% of its original weight and volume and then freezing the food to preserve it. Generally fruits, vegetables, meat, pork and poultry products are preserved by this method.
Preservation by High Temperature
The temperature and time used in heat processing a food will depend upon what effect heat has on the food and what other preservative methods are to be employed.
Pasteurisation is a heat treatment that kills part but not all the microorganisms present and usually involves the application of temperatures below 1000 C. The heating may be by means of steam, hot water, dry heat or electric currents and the products are cooled promptly after the heat treatment.
Preservative methods used to supplement Pasteurisation include:
Pasteurisation temperature and time for various products:
Canning involves the application of temperatures (to food) that are high enough to destroy essentially all microrganisms present plus air tight sealing in sterilised containers to prevent re-contamination and to preserve the food as nearly as possible in the condition in which it is served when freshly cooked. The degree of heat and the length of time of heating vary with the type of food and the kinds of microorganisms that are likely to occur in it. Most canning is in tin cans which are made of tin coated steel or in glass containers but increasing use is being made of containers that are partially or wholly of aluminum, of plastics as pouches or solid containers.
Preservation by Preservatives
Preservatives are defined as "chemical agents, which serve to retard, hinder or mask undesirable change in food". These changes may be caused by microorganisms, by the enzymes of food, or by purely chemical reactions. Food preservatives are often used in conjunction with other methods of food preservation.
The preservatives generally used in fruit and vegetable products may be broadly classified as organic and inorganic preservatives. The organic preservatives are benzoic acid, esters of p-hydroxy-benzoic acid, or o-chlorobenzoic acid or salicylic acid. The only permitted inorganic preservative is sulphur dioxide, which is generally used in the form of sulphites. The preservatives permitted in fruit and vegetable products in India are sodium benzoate, sulphites and sorbic acid.
The sodium salt of benzoic acid has been extensively used as an antimicrobial agent in foods. Sorbic acid and its salts are useful additives against yeasts and moulds but are less effective against bacteria.
Preservation by High Osmotic Pressure
By the principle of osmosis jams jellies and pickles are preserved. Water is withdrawn from microbial cells when they are placed in solutions containing large amounts of dissolved substances such as sugar or salt. As a result of this water loss, microbial metabolism is halted. Like dehydration, high osmotic pressure may inhibit microbial growth but it cannot be relied upon to kill microorganisms.
Preservation by Dehydration
The word dehydration usually complies the use of controlled conditions of heating, with the forced circulation of air or artificial drying as compared with the use of sun drying. Drying as a means of preservation can be observed in cereal grains, legumes and nuts, which dry on the plants. Freeze drying is a method of drying involving freezing and then the sublimates of the ice under vaccum.
Dried foods are preserved because the available moisture level is so low that microorganisms cannot grow and enzyme activity is controlled.
Methods of drying
Removal of water from a product while it is frozen by sublimation is called freeze drying. After the preliminary preparation of food is carried out as for other methods of drying and then the prepared food is frozen, placed in a vacuum chamber and a small amount of heat is applied. Fresh flavours and textures are better preserved by freeze-drying than by sun-drying or other procedures of artificial drying without vacuum. This method is costly. Freeze-dried fruits, coffee, meats, poultry have been reported to have better texture tenderness.
It is a old and traditional method of drying. It is limited to climates with a hot sun and a dry atmosphere and to certain fruits such as raisins, prunes figs, apricots, pears and peaches. It is a slow process. Vegetables like french beans, curd chilli are preserved by this method.
In this case, the moisture is drawn out from all cell tissues. The water is then bound with the solute, making it an unavailable to the microorganisms. In osmotic dehydration of fruits, the method involves the partial dehydration of fruits by osmosis in a concentrated sugar solution or syrup.
Most methods of artificial drying involve the passage of heated air with controlled relative humidity over the food to be dried or the passage of the food through such air. The simplest drier is the evaporator, where the natural draft from the rising of heated air brings about the drying of the food. The optimal temperature for drying are between 520 C and 600 C. lower temperature for a longer time yield better quality and better retention of vitamin content. It takes 6-15 hours for vegetables and 6-24 hours for fruits. Vegetables are dried until they are brittle but fruits feel leathery when dried. Liquid foods, such as milk, juices and soups may be evaporated by the use of comparatively low temperatures and in a vacuum pan or similar devise, drum dried by passage over a heated drum with or without vacuum, or spray-dried by spraying the liquid into a current or dry heated air.
Milk and egg are dried to a powder in spray driers in which the liquid is atomized and sprayed into a hot air stream for almost instant drying. The rapid dissolving in water of non fat dry milk crystals is due to a second drying step that gives the particles a sponge like structure. Skim milk, whole milk whey and fat enriched milk are spray dried in large quantities.
Foam mat drying may be used commercially with orange and tomato juice. In this process a small amount of edible foam stabilizer such as monoglycerides or a modified soyabean protein with methyl cellulose is added to liquid and a stiff foam is produced by whipping. The foam is spread in a thin layer and dried in a stream of hot air. The product separates easily into small particles on cooling.
In smoking drying is the main preservative factor, especially drying at the surface of the food.
Factors in the control of drying
It is a new method of preservation. In this method, food is irradiated by cathode rays produced by high voltage generators, gamma rays produced by naturally occuring fission products or by such radio active materials as cobalt 60. Low levels of radiation result in radio pasteurization rather than sterilization. Such treatment extends the period during which pre packed meats can be stored under refrigeration and destroys trichinae organisms in pork. It increases the shelf life of cereals and dried fruit by destroying insects, of tubers by inhibiting sprouting and of citrus fruits by inhibiting microbial growth. Canned and packaged foods can be sterilised by an appropriate radiation dosage.