Universal drying rate constant of seedless grapes: A review

Drying kinetics of materials may be described completely using their transport properties together with those of the drying medium. In the case of food drying, the drying rate constant ‘k’ was used instead of transport properties. The drying rate constant combines all the transport properties and may be defined by the thin layer equation. A large number of investigators have worked on solar drying of grapes and the drying rate constant has been calculated through conditions of drying product temperature, equilibrium relative humidity, equilibrium moisture content and drying time. Several mathematical models have been proposed to describe the moisture movement in the drying product. Among the thin layer models, the exponential model is found to be simple and most suitable to describe drying characteristics of grapes. The exponential model considers only the surface resistance, implying that all the resistance is concentrated in a layer at the surface of the drying product. Drying characteristics obtained from experimental results of some investigators were taken into consideration to estimate the value of drying rate constant for grapes. The best fit for drying rate constant value was selected from among the various drying curves obtained experimentally by investigators till date.