Design of hot air drying for better foods

Drying of food consists of three steps, namely pre-drying processing, dehydration and post-drying handling of the material. The pre-drying processes depend on the physical state of the material subjected to drying. Liquids are vacuum concentrated, treated with enzymes or foamed, while solids are sulfited, soaked in solutions of different compounds, dewatered by osmosis, blanched, frozen or treated by high pressure. Drying of the pre-treated material can be done under vacuum or at atmospheric pressure. Material undergoing drying can be heated by surface heating or by volumetric heating, and it can be stationary or set in motion. Storage stability of a dry material depends on the state at which the drying process is finished. The effect of all these steps on quality of the final product are discussed. It is shown that to design a process of hot air drying a thorough understanding of all the operations affecting quality is needed.