Mathematical models to describe thin layer drying kinetics and determine the drying rate of grapes in raisin production processed by different convectional drying methods

Abstract

During the peak season of harvesting, grapes are flooded onto the market, causing price decreases and farmer losses. To reduce the losses, an alternative is to dry the product. In this context, the main objective of this article was to describe the thin layer drying of seedless grapes. To describe the convective drying processes (such as fast heating (107 º C) (T1), slow heating (55 º C) (T2), Open sun drying (T3) and drying under shade (T4)) some mathematical models are normally used. In this article, there are three empirical models were selected to simulate experiments of thin layer drying accomplished with grapes. In the selection, it was imposed that mathematical expressions must be obtained from each model to calculate the moisture ratio and also the processing time. The processing time ranged from 14 hours (107 º C) up to 35 days (drying under shade). The maximum drying rate occurs at the beginning of the process and varied as 13.73 g/100g/hour (107 º C), 0.724 g/100g/hour (55 º C), 0.299 g/100g/hour (open sun drying) and 0.18 g/100g/hour (drying under shade). The statistical indicators showed that the Page model was the best one to describe the drying kinetics of grapes. The best-fitted model was selected based on the R2 and constants values. While R2 values were 0.9923, 0.9750, 0.9721 and 0.9638 and k values were 3.1702, 5.4672, 4.9687 and 6.0645 for T1, T2, T3 and T4 respectively. As a result, the empirical equation for the page model allows for the creation of mathematical expressions for the moisture ratio and process duration, and it is best suited for all treatments.