Influence of drying conditions on diffusivity, energy and color of seedless grape after dipping process

Abstract

Thin layer drying was carried out for Sultana seedless grape after dipping in olive oil and potassium carbonate solution pretreatment. Simulation of raisin production process was performed using experimental data and mathematical models. A laboratory scale convective dryer was used for drying experiments. Second law of Fick was used as a major model to compute the moisture diffusivity with some simplifications. Drying conditions of fixed bed dryer with air temperature levels of 50, 60 and 70 °C and air velocities of 0.25, 0.5, 0.75 and 1 m s -1 were applied in this study. Computed values of moisture diffusivity for dried grape varied from a minimum of 7.77E-11 to a maximum of 2.62E-10 m 2 s -1 under drying conditions. Moisture diffusivity values were increased as air temperature was increased. Activation energy values varied from a minimum of 44.806 kJ mol -1 to a maximum of 46.302 kJ mol -1 in this study. Maximum value of activation energy was obtained at air velocity of 1 m s -1 . Lightness value for dried grapes was highest at air velocity of 1 m s -1 and 70°C. Color analysis of dried grape proved that the best condition for raisin production process was achieved at air temperature between 60-70°C and air velocity of 1 m s -1 . Specific energy consumption values for grape thin layer drying were found to be in the range of 547 and 1904 MJ kg -1 . Increase in drying temperature for each air velocity level caused a decrease in specific energy consumption. These grape properties are necessary to determine the best point of drying process and designing the apparatus.