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 increase hot-air drying rate, shift desorption isotherms, and increase juice yields over raw samples or those pretreated with conventional or microwave heating. The frequency of alternating current has been found to alter heat and mass 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 additional 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 during most of the drying process, with the most pronounced differences occurring during intermediate stages of drying. The electric field strength affected 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 significantly greater quantities than the 60 Hz sinusoidal pretreatment. Due to increased electrical conductivity at 4 Hz, pretreatments at this frequency require considerably less time than pretreatments at 60 Hz The efficiency of mass transfer processes appears to be significantly dependent on waveform and frequency of alternating current. These phenomena could have useful applications for food processing. (C) 1999 Elsevier Science Ltd. All rights reserved.