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Methods Of Food Preservation and Sterilisation (Part II) By Mumtaz Khalid Ismail
More Links Dehydration Refrigeration Use of salt and sugar Use of acid Use of oil and spices
Preservation of foods by drying is one of the earliest and simplest techniques used for centuries. Dehydration is the process by which surplus water is removed with out drastically reducing the taste and nutritive value of the foods. Foods such as meat, fish, figs, apples, raisins, apricots, gooseberries, herbs, and vegetables are generally preserved by this method. They keep well because of the combination of the physical changes. In fruits as the moisture is removed the concentration of sugar goes up and this increased sugar concentration adds to the unfavourable conditions for bacterial growth.
The principle of dehydration is that bacteria are unable to survive in the absence of moisture.
Sun drying is the most common method of dehydration. Foods such as vegetables and fish are often salted before drying. Other than sunlight, other methods such as cabinet dryer, and oven are been used. Papad, macaroni dried milk powder, dehydrated soups, pre-cooked legumes and cereals are some of the commonly used dried preserved food items.
Dried foods can be transported easily due to its lightweight and volume. In this method vitamin C and beta-carotene are lost mainly but the sugar and fibre concentrates. This method also causes severe loss of taste, aroma, texture etc.
Refrigeration works on the principle that bacterial and enzymatic activities are minimal at low temperatures. Foods should be placed in the refrigerator as quickly as possible and it should be covered. This will also prevent foods from acquiring the flavour of the other foods.
Whole fruits and vegetables can be wrapped in brown paper or newspaper to keep fresh for a longer period. Hot foods must be allowed to cool to room temperature before they are placed in the refrigerator. Top shelves maintain a lower temperature than the bottom ones so easily perishable foods such as milk must be kept in the upper shelves where as curd, vegetables, fermented batters can be placed in the lower shelves. If cooked foods have been lying at room temperature for a long time they should be heated again and cooled at room temperature before they are placed in refrigerator. When vegetables are brought in bulk they take up a lot of space in the refrigerator for short-term preservation. The vegetables can be cleaned and cut and kept in closed containers. Tomatoes can be blanched, crushed boiled a little and packed in bottles and can be stored. Prepared food such as sambhar and curries can also be stored in airtight containers for a week. Nutrients in general are retained well. There will be small reduction in the water-soluble vitamin and mineral content of vegetables and fruits, which are blanched before refrigerated or frozen. Each food has a recommended maximum storage time beyond which it slowly begins to deteriorate. Cold prevents the growth and multiplication of bacteria
With the advancement of technology more efficient refrigeration techniques have been developed. This has led to freezing techniques such as air blast freezing, contact freezing etc. The latest and most advanced method of refrigeration is the cryogenic method of refrigeration or the refrigeration using liquid nitrogen at extremely low temperatures. Currently liquid nitrogen is extensively used in the food industry.
Salting is a method for preservation of fish, meat, vegetables and fruits like tamarind, raw mango, lemon, goose berries, bitter gourd, green chillies etc. The high concentration of salt prevents the water from being available for bacterial growth. This is because the concentration of salt in the water is higher than that in the bacterial cells. Thus the cellular membrane of the bacteria cannot absorb the water and actually it loses water.
Principle of preservation with sugar is also the same as that of salting. Salting is generally a cold process while in case of sugar the mixture is heated. Sugar acts as a preservative because the high concentration of sugar solution exerts a high osmotic pressure and with draws water from the micro-organisms leading to its death.
When the medium in which the food is preserved is strongly acidic, most organisms cannot survive. The use of acetic acid (vinegar) and citric acid (lime juice) is common preservatives used in household. These are used in pickles, sauces, ketchup, fruit squashes etc.
Spices such as chillies, turmeric, pepper, mustard and fenugreek seeds are used in the preparation of pickles. They not only improve the flavour of foods but are also believed to create an environment unfavourable for the growth of bacteria. A top layer of oil prevents the micro-organisms in the air coming into contact with the food as well as deprives oxygen to the organism present in the food. Dhal can be preserved longer if a thin coating of oil is given to it. Grains such as rice and wheat also can be coated with oil while stored or can be mixed with whole dried red chillies that will prevent the spoilage with insects.
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