The effects of ohmic heating frequency on hot-air drying rate and juice yield

Ohmic pretreatment of fruit and vegetable samples has been shown to increas e hot-air drying rate, shift desorption isotherms, and increase juice yield s over raw samples or those pretreated with conventional or microwave heati ng. The frequency of alternating current has been found to alter heat and m ass transfer properties. In this study, the hot-air drying rate of yam and the juice yields of apples were compared using a 60 Hz sine wave and a 4 Hz sawtooth wave to determine if lowering the frequency would result in addit ional improvements to these processes. The 4 Hz sawtooth wave resulted in a faster hot-air drying rate of yam cylinders than the 60 Hz sine wave. The drying rates of the 4 Hz pretreated samples were significantly greater duri ng most of the drying process, with the most pronounced differences occurri ng during intermediate stages of drying. The electric field strength affect ed the drying curves in the range tested at 4 and 60 Hz. Apple juice yield was improved by ohmic pretreatment, with 4 Hz sawtooth samples yielding sig nificantly greater quantities than the 60 Hz sinusoidal pretreatment. Due t o increased electrical conductivity at 4 Hz, pretreatments at this frequenc y require considerably less time than pretreatments at 60 Hz The efficiency of mass transfer processes appears to be significantly dependent on wavefo rm and frequency of alternating current. These phenomena could have useful applications for food processing. (C) 1999 Elsevier Science Ltd. All rights reserved.