Effect of ohmic pretreatment on the drying rate of grapes and adsorption isotherm of raisins

Effects of ohmic pretreatment on the drying rates of seedless red grapes and adsorption isotherms of the raisins produced were investigated. The treated grape samples were heated in a medium containing 2% citric acid to a Final medium temperature of 60 degrees C using an alternating current at 14 V/cm field strength and 30 Hz, 60 Hz, and 7.5 kHz frequencies. Results of this study reveal that the drying rate of the grapes was significantly increased by the ohmic pretreatment, especially at low electrical frequencies. The effect of the ohmic pretreatment on equilibrium moisture content of the raisins produced was evident at 0.75 or higher water activities but there was no or limited effect at low to moderate water activities.