Drying or dehydration can be defined as the removal of water from the food by controlled processes, which can be achieved by either of 3 ways –

1. Evaporation due to heating of the food product
2. Osmotic dehydration
3. Sublimation, or freeze drying

Drying can be explained as a 2 stage process in which in the first stage, the surface water removal takes place which totally depends on the state of air surrounding that food particle such as its temperature, relative humidity, and speed. On the other hand in the second stage of the drying, the moisture which is present within the food particle is allowed to move onto the surface which can be then again removed by the process of the first stage.
The rate of drying basically depends on the time that moisture takes to come onto the surface of the food from its inside. Heating of any particle accelerated its time of drying. The shortest time period has been noted in starchy foods.
There are 5 basic methods of drying –

1. TUNNEL DRYING –
   Many years ago, for the purpose of preservation of agricultural products by the means of drying, a solar tunnel dryer was developed. It is an amazing device which can be used for the hygienic preservation of food and also acts as a good alternative for the seasonal agricultural surplus. Also the foods which are dried using this process are more hygienic and superior rather than other methods.
2. SOLAR DRYING –
   I t is also known as sun drying and is a method of drying any substance using natural sunlight. This drying technique offers faster drying rates and greater protection of the product. There are several problems or limitations which are associated which this type of drying technique, such as the drying areas must be free of dust and whenever there are chances of a dust storm or rain, the drying process cannot be continued.
   There is also a process known as shade drying which can be carried out for the product which can easily lose their color and turn brown if kept in sunlight i.e. they are non-photosynthetic.
3. SPRAY DRYING –
   This is a unique technique which ultimately transforms the substrate from a liquid to dried form by spraying the feed into a hot drying medium. It is a continuous process and takes time. The dried products which are ultimately obtained can be in the form of powders, granules or agglomerates. It is a multi-step process.
4. VACUUM DRYING –
   Since evaporation of water takes place more easily and effectively at lower pressures, so vacuum drying was developed which is faster than the process of evaporation. This is an expensive method and is used for some specific products.
5. FREEZE DRYING –
   This method uses the process of sublimation, which is a process which directly converts solid particles to their gaseous state, without first melting them. It is a technique which is very used in preserving food at low temperatures because at less temperature, chemical reactions occur very rarely and food is rarely attacked by pathogens. It is also a multi-step process which includes freezing, vacuuming, heating and finally condensation of the product.