I. Hot Air Drying Principle for Drying Fruits and Vegetables
● What is hot air drying method
Hot air drying is the conventional drying method, and it transfers heat from the drying medium to the material based on the heat conduction effect, then this results in two diffusion after the material absorbs heat: the external diffusion ( the moisture from the material surface to the drying medium ) and the internal diffusion ( the moisture from material interior to the material surface ), and the two diffusion goes on working until the moisture of material drops to a certain degree to achieve the drying purpose. Hot air drying happens under high temperature ( above 55℃ ) and aerobic conditions in the hot air drying oven, and there are many chemical changes happened in the drying process, such as the oxidation of phenolic compounds being catalyzed by oxidase, vitamins being damaged easily under high temperature, amino acid and sugar presenting Maillard reaction under high temperature, etc. Hot air drying temperature and time are the main factors affecting the changes of nutrients in the fruits and vegetables.
● Heat transfer and mass transfer happens in the hot air drying processes
In the process of hot air drying, heat transfer and mass transfer are happening at the same time. Heat is transferred to the surface of the fruits or vegetables by convection and transmitted to the interior by the surface of the material, then the internal moisture of fruits or vegetables diffuses to surface and spreads from surface to the gas phase after intensifying finally. The driving force of heat transfer is the temperature difference, the driving force of mass transfer is the concentration difference of water or the difference of vapor partial pressure, and the directions of heat transfer