Development of a drying model for combined microwave-fan-assisted convection drying of spinach

The drying of spinach leaves was investigated in a combined microwave-fan-assisted convection oven. Spinach leaves were dried by using a microwave, a microwave-fan combination and fan drying. The effects of microwave drying (180, 360, 540, 720 and 900 W); fan-assisted convection (100, 180, 230 degrees C); combined fan-assisted convection (100, 180, 230 degrees C) and microwave (180 and 540W) on drying time, drying ratio and colour change of spinach leaves have been investigated. The drying data were applied to 11 different mathematical models, namely, Newton, Page, Modified Page, Henderson and Pabis, Logarithmic, Wang and Singh, Diffusion Approach, Verma, Two Term Exponential, Simplified Fick's Diffusion and Midilli-Kucuk Equation Models. The performances of these models were compared according to the coefficient of determination (R-2), standard error of estimate (SEE) and residual sum of square (RSS), between the observed and predicted moisture ratios. It was found that the Midilli-Kucuk model described the drying ratio satisfactorily in all drying methods. In order to determine the colour change of spinach leaves, a Minolta Chroma. CR-100 colour meter (Minolta Co., Osaka, Japan) was used. In every drying method, it was found that L* and a* values were not significantly different from the values of the fresh leaves (P > 0.05). On the other hand, b* values, Chroma, C* and Hue angle alpha of dried spinach leaves differed significantly (P < 0.01) from fresh spinach leaves. (c) 2008 IAgrE. Published by Elsevier Ltd. All rights reserved.