Drying and Dehydration of Fruits and Vegetables

Fruits and vegetables are highly perishable, and the need for preserving them from post-harvest spoilage using economical and efficient technology is incredible. Drying or dehydration is considered an evergreen technology as it is been practised since traditional times, as well as until recently, using modern, sophisticated equipment. Both drying and dehydration involve the process of removing moisture from horticultural commodities like fruits and vegetables. The process facilitated under controlled conditions is referred to as dehydration, and one that occurs naturally is deemed drying. The process leads to a reduction of water activity of the commodity, thereby delaying the growth of microorganisms. These drying and dehydration operations involve a complex phenomenon of heat and mass transfer, which are dependent on the heterogeneous composition and thermo-physical properties of the particular commodity. There are two stages in which moisture is removed from the biological material, where in the first stage, unbound moisture evaporates from the surface of the commodity due to the surrounding heat application, and in stage II, bound moisture is removed from the interior of the material to its surroundings. The surface of the commodity serves as the boundary for heat transfer within the commodity to the surroundings. In the case of horticultural commodities, higher dry bulb temperature, lower relative humidity, and high air velocity are ideal for better drying. Understanding the drying kinetics of each commodity is crucial for optimising the correct drying time, temperature, and airflow rate so that the product quality is maintained to the maximum. When the commodity approaches a normal equilibrium with the surrounding relative humidity and temperature, drying ceases. At this point, the microbial as well as the enzymatic activity becomes very minimal, leading to the extended shelf life of the commodity. Research advancements have credited various dryers intended for fruit and vegetable applications. However, the selection of dryers depends on the end application and the nature of the commodity chosen for drying.