Development of a laboratory scale ohmic heating system for pasteurization of grape juice

Abstract

The aims of this study were to develop a laboratory scale ohmic heating system for pasteurization of grape juice and to evaluate the effects of ohmic heating on the microbiological quality of the juice. The system was mainly composed of a cylindrical heating chamber (borosil glass), two electrodes (SS 304), and a variable auto transformer. It was equipped with an analog voltmeter and ammeter for measurement of voltage and current, respectively. A mercury thermometer was used to measure the temperature of the juice. The capacity of the system was 150 mL. The juice was pasteurized at different combinations of voltage gradient (10-40 V cm-1), temperature (55-85 °C) and holding time (1-5 min). Total plate count, yeast and mould count and system performance for grape juice were determined. The microbial counts of grape juice significantly decreased (p< 0.05) when the voltage gradient, temperature and holding time were increased. The higher electric field strength results in a greater reduction of pathogens. The microbial counts of the juice treated with the combination of 30 V cm-1:85 °C:5 min were below the recommended limit till 21 days of storage. As the voltage gradient increased, the system performance decreased. The system performance coefficients for grape juice samples were in the range of 0.57-0.99. Bubbling was observed above 75 °C especially at high voltage gradients. These results suggest that ohmic heating might be effectively and efficiently used to pasteurize fruit juices.