The hot air drying technique is the most frequently used technique to produce dehydrated vegetables and fruits, in which heat is transferred from the hot air to the product by convection, and evaporated water is transported to the air also by convection.
Air that has a low moisture concentration is blown through the material bed so that it absorbs water molecules from the product away. In some dryers the air flow is also what is used to heat the material being dried.
Hot air driers are classified as parallel flow, counter flow, direct flow, or cross flow, pending on the direction in the product moves in direction to the direction of flow of the heated air.
Hot-air drying is efficient at removing free water as long as it’s present as continuous liquid phase (high water content) preventing thermal deterioration of the products; this removal is governed by Fick’s Law. The water elimination generates shrinkage decreasing the internal water diffusivity and increasing the thermal conductivity of the product and thus the internal heat transfer.
Hot air drying
