Determination of parameters and pretreatment solution for grape drying

The drying of Iranian seedless white grapes (sultana) was investigated in a batch operation in a laboratory dryer. The effect of pretreatment and temperature on the drying rate of grapes at constant air velocity were studied. Pretreatment solutions contained different alkaline materials in different concentrations and temperatures. Dipping grapes in an alkaline solution increased the drying rate substantially. Increasing the air temperature in the dryer from 60 to 70 degreesC accelerated the drying rate (for a decrease in drying time) of the pretreated grape. Grapes dried in 450-900 min depending on pretreatment and air temperature. The shorter drying time and best quality dried product were obtained with grapes dipped in a solution of potassium carbonate of 5% at 42 degreesC. The mean apparent diffusivity of moisture for K2CO3 pretreatment was calculated at 50, 60, and 70 degreesC, and the values were fitted to an Arrhenius type model. Moisture sorption isotherms of currants were determined at 20, 30, and 40 degreesC using static gravimetric methods. A modified Halsay equation was used to predict experimental data for water activity in a range of 0.1-0.85.