Cylindrical samples of carrot, potato and yam were dried in a hot‐air dehydrator after preheating to 50C or 80C by three different heating methods (conventional, microwave and ohmic). The results showed that enhancement of drying rate increased with pretreatment temperature. Ohmic pretreatment increased the drying rate more than conventional and microwave heating. Desorption isotherms showed that in the low a w range, desorption data of preheated and raw materials were similar. However, the isotherms of preheated samples shifted when a w was high, which indicated that thermal pretreatments altered the structure, and apparently, the water distribution within these materials. For all samples, ohmic pretreatment showed stronger influences on isotherms than microwave heating, while the pretreatment effect of conventional heating was only observed for potato tissue.
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Wang et al. (2000) studied this question.