Evaluation of Microwave and Convective Finish Drying Parameters and Drying Effects on Color of Dried Grapes

In this study, the possibility of microwave finish drying for two types of dried grapes (Natural and Sultana) was investigated. The samples were washed in water bath at given temperature ranges (25 and 45 C) for 30 to 60 minutes. After dewatering the samples, they were dried using microwave (at two powers: 270 and 450 W) and hot air convective dryer (at 50 C). Water gain (%), effective moisture diffusivity and Hunter values (L as lightness and a/b ratio) were determined. To determine a suitable model for describing finish drying behavior, ten commonly used mathematical models were compared according to the three statistical parameters: coefficient of determination (R2), chi-square (χ2), and root mean square error (RMSE). Results showed that water gain for sultana samples was 3-4.5 times more than that for natural raisins. Based on experimental drying rate data, drying took place in falling rate period. Among the models, two term model was most adequate showing highest R2 and lowest χ2 and RMSE. It was found that the drying method affect color indices of product and also the application of microwave finish drying technique depends on used microwave power and type of dried grape.