Thin layer mathematical modeling of convective, vacuum and microwave drying of intact and brined onion slices

Abstract

Drying of onions by convective, vacuum and microwave methods in terms of drying kinetics were investigated at various conditions. Kinetics of intact and brined onions at 50, 60, and 70°C for convective and vacuum drying, and 80, 240, and 400 W for microwave drying were obtained. Onion slices of 3 and 7 mm thicknesses either dipped into 8% NaCl solution for 40 min or intact were dehydrated. Fitting of the experimental data to 13 thin layer drying models resulted in Sigmoid model as the most suitable model for all investigated drying techniques. Sigmoid model was followed by Cubic and Midilli models with respect to R2, RMSE, and χ2. Diffusion coefficients varied between 1.962 × 10−9 and 1.372 × 10−8 m2/s for convective, 9.757 × 10−9 and 1.723 × 10−8 m2/s for vacuum and 3.193 × 10−8 and 9.139 × 10−7 m2/s for microwave drying. Activation energy values were in the range of 3.28–34.13 kJ/mol for convective and vacuum drying and 2.25–6.08 W/kg for microwave drying. Practical applications Onion (Allium cepa L.) is famous for authentic sensorial properties and used in a lot of dishes. Fresh or dried form of this vegetable contains numerous healthful compounds such as organosulfur compounds, flavonoids, fructans, vitamins, and minerals. Convective, vacuum and microwave drying of food materials are common methods applied in dehydration of foodstuffs for preservation and new product development. The aim of this study is to reveal the effectiveness and behaviors of indicated drying techniques. The comparison and selection of most appropriate dehydration procedure for onion might be of interest in food and market suppliers.