Microorganisms need moisture to grow. When exposed to sunlight or subjected to dehydration, the moisture in the food is removed and the concentration of water is brought below a certain level. This prevents the growth of microorganisms and thereby spoilage of food.
Food preservation by drying is one of the oldest methods practised from ancient times. This method consists of exposing food to sunlight and air until the product is dry.
Treatment of foods before drying
1. Selection and sorting for size, maturity and wholesomeness.
2. Washing, especially of fruits and vegetables.
3. Peeling of fruits and vegetables by hand, machine or abrasion.
4. Subdivision into halves, slices, shreds or cubes.
5. Blanching or scalding of vegetables and some fruits like apricots and peaches.
6. Sulphuring of light coloured fruits and vegetables. Fruits are sulphured by exposure to sulphur di oxide gas produced by the burning of sulphur to a level of 1,000 to 3,000 ppm.
Methods Of Drying
It is limited to regions with hot climates and a dry atmosphere and to certain fruits such as raisins, prunes, figs, apricots, pears and peaches. It is a slow process.
Many Indian foods are preserved by sun drying. Papads and vathals are made using this principle. Vegetables like cluster beans and curd chilli and fruits like jack fruit are preserved by this method. Fish and meat are also sun dried.
Drying by mechanical driers
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.
Fruits, vegetables, nuts, fish and meat can be successfully preserved by this method. In the dehydration process, artificial drying methods (eg. spray drier) are used for drying foods. Although it is expensive when compared to natural sun-drying procedures, it is very advantageous because the temperature and relative humidity can be manipulated.
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.
Foam mat drying
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 the 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.
Drying by osmosis
Drying also results when fish is heavily salted. In this case, the moisture is drawn out from all cell tissues. The water is then bound with the solute, making it 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.
Removal of water from a product while it is frozen by sublimations is called freeze drying.
Factors to be considered in drying foods
1. The temperature employed, which will vary with the food and the method of drying.
2. The relative humidity of the air. It usually is higher at the start of drying than later.
3. The velocity of the air.
4. The duration of drying.