Microwave drying of grapes in a single mode cavity at 2450 MHz - I: Drying kinetics

Grapes were dried into raisins using combined microwave and convective drying in a single mode cavity specially designed for the studies. Effect of processing parameters, viz., microwave power density (P) air temperature (T) and velocity (u) on drying kinetics was studied in an effort to optimize the operating conditions. The individual and interactive effects of these variables were studied using three levels of P, four levels of T and two levels of u. Higher drying rates observed with the higher P and T were attributed to the increased product temperature and higher partial vapour pressure difference between the surface of the fruit and the air stream. Increase of air velocity increased the drying times. Low drying rate at higher air velocity was attributed to decreased fruit temperature due to rapid convective heat transfer at the surface.