Food Processing: Hot air drying

Drying may be defined as the vaporization and removal of water or other liquids from a solution, suspension, or other solid-liquid mixture to form a dry solid. It is a complicated process that involves simultaneous heat and mass transfer, accompanied by physicochemical transformations.

During the drying process, two types of resistance control water transport: internal resistance to the water movement inside the material and external resistance between the solid surface and the air. Internal resistance is a characteristic of the material, while external resistance depends on the thickness of the diffusion boundary layer.

Among drying medium, hot air is the commonest, in spite of superheated steamhas shown higher efficiency and higher product quality in some special cases. For removing moisture and preserving the food, drying technique is one of the earliest techniques. By means of this way, the shelf-life of a product can be prolonged significantly. Drying technique also influences other characteristics, such as palatability of food, flavor, aroma, viscosity, hardness, microbial spoilage and enzymatic activity.

For drying fruits and vegetables, hot air drying is the most common one, which is both the simplest and most economical technology. In this technique, the need for elevated temperatures or lengthy drying durations may provoke severe harm to flavor, color, and nutrients of the product and it may also reduce rehydration capacity and bulk density of the dried fruit.
Food Processing: Hot air drying