Effects of thermal and electrothermal pretreatments on hot air drying rateof vegetable tissue

Cylindrical samples of carrot, potato and yam were dried in a hot-air dehyd rator after preheating to 50C or 80C by three different heating methods (co nventional, microwave and ohmic). The results showed that enhancement of dr ying rate increased with pretreatment temperature. Ohmic pretreatment incre ased the drying rate more than conventional and microwave heating. Desorpti on isotherms showed that in the low a(w) range, desorption data of preheate d and raw materials were similar. However, the isotherms of preheated sampl es shifted when a(w) was high, which indicated that thermal pretreatments a ltered the structure, and apparently, the water distribution within these m aterials. For all samples, ohmic pretreatment showed stronger influences on isotherms than microwave heating, while the pretreatment effect of convent ional heating was only observed for potato tissue.