The influence of Two-Temperature and pretreatments on Drying of Thompson seedless Grape

Drying is one of the most important step in raisin processing. During this step, initialmoisture content is decreased up to 15 – 17 percent (d.b). In this research, the effect of two-tempreture and pretreatments on time and drying rate of Thompson seedless grape was studied. Drying was in two steps: First step; temperature at four levels of 50,60,70, and 80 ºC and secound step which samples were placed at two temperatures. In this step, samples were placed at 80 ºC and while moisture content was reduced to 50%, temperature was reduced to 70,60 and 50 ºC. Four pretreatments including hot water, %5 potassium carbonate, %0.4 olive oil, %0.5 sodium hydracids and no pretreatment was performed. According to the results of the ANOVA, factors such as temperature, two-temperature and pretreatment have significant effects on drying time and average drying rate. Drying time of grape with two-temperature in comparison with drying time of constant temperature was decreased, up to 61% for some pretreatment. Maximum and minimum activation energy were determind 893.9, 4538.8 kj/kg.