Explain the Mechanism of Drying?
Drying as a mechanism, you will realize, involves the removal of free moisture from the surface and also moisture from the interior of the material. How does this mechanism work? When hot air is blown over a wet food, heat is transferred to the surface, and the latent heat of vaporisation causes water to evaporate. Water vapour diffuses via a boundary film of air and is carried away by the moving air. This makes a region of lower water vapour pressure at the surface of the food, and a water vapour pressure gradient is well-known from the moist interior of the food to the dry air. This gradient offers the driving force for the elimination of water from the food.
Water moves to the surface by the subsequent mechanisms:
1) Liquid movement by capillary forces.
2) Diffusion of liquids, due to differences in concentration of solutes in different regions of food.
3) Diffusion of liquids that are adsorbed in layers at surfaces of solid components of the food.
4) Water vapour diffusion in air spaces within the food caused by vapour pressure gradients.