influence of temperature, air velocity and pretreatments on drying thompson seedless grapes

drying is one of the most important steps in raisin processing. during this step, initial moisture content decreases up to 15-17% (db) for suitable storage. achieving optimum drying conditions can affect the processing time and improve raisin quality. temperature, air velocity and pretreatment are important factors in the quality of the grape drying process. in this research, the effect of temperature at 50ºc, 60ºc, 70ºc, and 80ºc; air velocity at 1, 2 and 3 m/s, and four pretreatments (hot water, 5% potassium carbonate, 0.4% olive oil, 0.5% sodium hydracids) and no pretreatment were measured. diffusivity and activation energy of all treatments were determined. the results of anova showed that temperature, air velocity and pretreatment have significant effects on drying time and average drying rate. pretreatment has a significant effect on the drying process and decreases the drying time up to 69% at some temperature levels. also, increasing the temperature decreases drying time up to 66% for some pretreatments. increasing the hot air velocity decreases it about 8.6%.