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.
Commercial wheat germ lipase inactivation was analysed at various temperatures with reference to conventional and microwave heating. A temperature controlled water-bath was used for conventional heating. A custom designed microwave temperature control system was used to maintain test samples at selected temperatures either by full exposure to microwaves (microwave heating) or partial exposure by immersion in water (in a beaker) maintained at the desired temperature using the microwave oven (mixed mode). Lipase inactivation was analysed using first-order kinetics resulting in activation energies (Ea) of 20.9, 25.4 and 18.7 kcalmole-' for the conventional, microwave and mixed mode heating, respectively. The activation energies of conventional and mixed mode heating were somewhat more comparable, while the microwave mode resulted in a slightly higher activation energy. The higher enzyme inactivation rate constants observed under microwave heating conditions were attributed to possible non-thermal effects associated with microwave energy.
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