Thompson seedless grapes were dried under forced convection and also combined convection and microwaves in a modified microwave (MW) oven. Effect of chemical pretreatment on drying rate was compared with fruits with no pretreatment. Different air temperatures and power levels were investigated. Dipping of fruits in hot alkaline solution of ethyl oleate considerably reduced drying time resulting in better quality raisins. MW drying reduced drying time resulting in raisins of comparable product quality even without chemical pretreatment. An air temperature of 50 C was found to be optimum for MW drying. Pages equation fitted the MW drying data adequately well and the estimated parameters of the equation are presented.
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Tulasidas et al. (1993) studied this question.