Normal drying curve

Foods do not loose water at a constant rate down to complete dryness. At starting of drying, the rate is constant and then it changes. Zero water is never reached under normal drying conditions. During the process of drying, food will loose moisture from its surface and then gradually a thick dried layer develops (an insulative barrier to heat) confining moisture to centre. From the centre, a moisture gradient will develop. During drying, moìsture in the centre has a longer path to travel than moisture near the surface.

The drying curve varies according to the type of food and the kind of drier that is used, and also the way the food material also responds to temperature, humidity, air velocity and direction of air and thickness of food. Oil-in-water emulsion dries only quickly than water -in-oil emulsion. Case hardening is common with foods that contain dissolved sugars and other solutes in high temperature. As water leaves cells of the food, solutes are retained by cell membranes and air spaces in surface layers can act by capillary action whereby water caries solutes to the surface during drying and leaves them there. In some fruits, this can cause sticky sugary exudates on the surface, which shrinks and clogs pores leading to case hardening.

It is a good drying style to create a state of porosity on the food material. This can be achieved by whipping or foaming a food liquid or puree prior to drying or by a vacuum drier by rapid escape of water vapour into high vacuum. The major advantage of this state of dried food is that there is quick reconstitution and greater volume appearance. While the drawback of this state is shorter stòrage ability due to increase surface exposure to air, light etc and also increase bulk.

Some food material that have been dehydrated exhibits thermoplasticity (the property of softening upon heating). A cellular food (plant and animal tissue) has structure and some rigidity, fruit or vegetables juices do not and are high in sugars and such that soften and melt at drying temperature. For example, if sugar syrup is dried in a pan, when the water is removed, the solids will be in a thermoplastic tacky condition that stick to the pan and are difficult to remove and with cooling, harden into a crystalline glass form.