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Dielectric properties of grapes at 2.45 GHz were measured at moisture contents varying from 80 to 15% (wet basis) at temperatures ranging from 25-80 degrees C using the Open Ended Coaxial Transmission Line Technique. Both dielectric constant and loss factor decreased with decreasing moisture content. In low moisture samples these properties were found to be considerably influenced by higher temperatures. Dielectric properties of sugar solutions of varying concentrations (20-250% by weight in water) were also determined at different temperatures, and these values were compared to those of grapes of corresponding moisture concentration. Predictive models of the dieletric properties as functions of moisture content and temperature were generated using response surface methodology. The results are useful in estimating the volumetric heating of grapes by microwave energy, and these methods can be extended to sugar-based foods in general.

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The effects of different dipping pretreatments on microwave drying of grapes was studied in a multimode cavity. Grapes pretreated with 2% ethyl oleate in 0.5 % sodium hydroxide (NaOH) solution resulted in comparatively good quality raisins with lowest drying times. Pretreatment with 3% ethyl oleate in 0.5% NaOH solution led to similar product without any major advantage over the former. Grapes treated with 3% ethyl oleate in 2.5% potassium carbonate solution took longer time to dry. Grapes treated with only NaOH solution resulted in raisins of inferior quality in terms of color and appearance. The intelphase mass transfer coeficients also described the effect of these chemical pretreatments. Studies conducted on the effect of washing and time of holding afrer the pretreatment indicated that both of these factors had no influence on drying time. potential of dielectric heating with microwaves (MW) for drying of grapes has been reported (Tulasidas et al. 1993). Several pretreatments have been developed to reduce the drying time of grapes and to enhance the quality of raisins in terms of color and appearance. In most of the pretreatment methods, the fruits are first fumigated with sulphur dioxide to attain a concentration of