The eects of ohmic heating frequency on hot-air drying rate and juice yield 1 Marybeth Lima a, * , Sudhir K. Sastry b a Department of Biological and Agricultural Engineering, Louisiana State University Agricultural Center, 149 E.B. Doran Building, Baton Rouge, LA 70803-4505, USA b Department of Food, Agricultural and Biological Engineering, Ohio State University, 590 Woody Hayes Drive, Columbus, OH 43210, USA Received 19 July 1998; accepted 26 April 1999 Abstract 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 signi®cantly greater during most of the drying process, with the most pronounced dierences occurring during intermediate stages of drying. The electric ®eld strength aected 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 signi®cantly 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 eciency of mass transfer processes appears to be signi®cantly dependent on waveform and frequency of alternating current. These phenomena could have useful applications for food processing. Ó 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Ohmic heating has been shown to in¯uence mass transfer properties. Halden, de Alwis and Fryer (1990) observed an increase in beet dye diusion during ohmic heating. Schreier, Reid and Fryer (1993) demonstrated that the diusion enhancement with ohmic heating is a linear function of applied voltage and is directly pro- portional to the surface area of the particle. Lima (1996) showed that diusion from a food sample depends on the electrical conductivity (r) of the food, and that at steady-state temperature, dierences in sample r be- tween conventional and ohmic conditions account for the dierence in the extent of diusion. Several other studies have demonstrated the eect of frequency and wave shape of alternating current on the eect of ohmic heating of foods. The ohmic heating of turnip as a function of frequency and wave shape of Journal of Food Engineering 41 (1999) 115±119 www.elsevier.com/locate/jfoodeng Notation m mass of sample (kg) n number of repetitions M moisture content (% dB) M(t) moisture content at time t (% dB) M e equilibrium moisture content (% dB) M 0 initial moisture content (% dB) MR dimensionless moisture ra- tio Mt M e =M 0  M e  P probability t time (s) T temperature (°C) V volume of extracted juice (ml) Y juice yield (ml/kg apple) r electrical conductivity (S/m) Subscripts e equilibrium 0 initial * Corresponding author. 1 Salaries and Research support provided in part by the Ohio Agricultural Research and Development Center, The Ohio State University, the Louisiana State University Agricultural Center; and in part by a USDA National Needs Fellowship in Food Engineering from USDA Grant No. 91-38420-6088. This publication has been approved by the Director of the Louisiana Agricultural Experiment Station as manuscript number 99-07-0097. Reference to commercial products or trade names does not imply endorsement or discrimination by The Ohio State University or Louisiana State University. 0260-8774/99/$ - see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 9 9 ) 0 0 0 8 0 - 1