By Mumtaz Khalid Ismail
These methods are used when a large quantity are to be preserved and for a prolonged period.
Canning is the method of food preservation in which food is sealed in containers after heating using steam under pressure. Heat processing preserves food and extends shelf life by arresting spoilage and by destroying harmful micro-organisms partly and rest is rendered inactive. Canning is used for fluid products such as fruit juices, syrups, and sauces. Selection of the food item, sterilisation and sealing of cans should be carefully monitored. The advantage of canning process compared to other preservation is that in canning the stable vitamins are retained and colour and flavour of food item is retained better. It also makes the starches and proteins in food easier to digest.
The principal of pasteurisation is pressurised heating for a short time. There are two methods, which are commonly used. In the conventional method food is heated at least 62 degree centigrade and kept at that temperature for at least 30 minutes. In the other method the high temperature is employed for a short time, that means food is heated to 71 degree centigrade and kept at that temperature for at least 15 seconds. The vitamin loss is similar to canning. Milk and milk products are generally pasteurised. Pasteurisation does not change the colour or flavour to any significant level.
The nutritional value of frozen foods is often very close to fresh equivalents. The main losses are the water soluble vitamins from vegetables and fruit during blanching prior to freezing. Even though frozen foods cannot be kept for as long as canned and tinned foods, it retains colour, flavour, and nutritive value better than canned or tinned food. Vegetables, fruits, fish, and meats are commonly preserved in cold storage Freezing does not sterilise the food although it in activates the enzymes and reduces bacterial activity. But bacterial cells have a unique resistance to low temperatures and they regain their survival power once they return to normal temperature. Foods to be frozen must be care fully selected for quality and maturity and before freezing the vegetables are usually blanched to inactivate the oxidative enzymes. All foods do not freeze well.
In dehydro freezing it is partial dehydration and freezing. The dehydro freezed vegetables and fruits are first heated to lose about half their water content and then frozen. By this method one can preserve food for longer period.
In freeze drying the frozen food is placed under vacuum to remove the water and it is then packaged in the presence of an inert gas such as nitrogen. The advantage of this method is that the product retains its original volume and shape and re-hydrates easily. Meat products, soups, food mixtures, and coffee are subjected to freeze drying. They have long shelf life light in weight and can be stored in refrigerator. They are considered inferior to frozen or canned foods.
Food preservation by irradiation is controversial method. In this process gamma rays or high speed electrons are used to destroy micro-organisms. This method can be used for sterilising spices, extending the shelf life of shellfish, strawberries, and other soft fruits. Preventing potatoes from sprouting and for destroying dangerous micro-organisms on poultry are the other uses. Irradiated foods are not radioactive but they are not for sale in the market and they are not approved by the food regulating authorities. Even though it is not proved that irradiated food intake is associated with any complication, it appears food irradiation is best be avoided whenever possible.
Fermentation method is used in many foods such as matured cheese, bread, yoghurt, soya sauce, and wines. Cultures or naturally occurring organisms may be encouraged to flourish. Cultures are ingested or sprayed on to foods to promote mould growth with distinctive tastes and textures. Nutrient loss is very less in fermentation but there is an increase in vitamins of B groups such as thiamine, riboflavin, and niacin to nearly double the original amounts. Vitamin C, methionine, and folic acid levels are also increased. Iron gets released from its bound form during fermentation. Due to enzyme action and microbial fermentation starches under go changes in their physical and chemical composition. Fermented foods is soft easy to digest and provides variety in texture.
| Cereals, pulses etc. |
In clean tightly covered container |
| Vegetables |
Cool dry place or in refrigerator |
| Onions and potato |
Dry open basket |
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