Enhanced destruction of spoilage microorganisms in apple juice during continuous flow microwave heating

Destruction kinetics of two selected spoilage microorganisms, Saccharomyces cerevisiae and Lactobacillus plantarum in apple juice were evaluated under continuous flow microwave heating conditions and compared with conventional batch heating in a water bath. Inoculated apple juice was heated in a microwave oven (700 W, 2450 MHz) under continuous-flow conditions to selected exit temperatures (52.5-65 degrees C). Aliquots of inoculated juice were also subjected to batch thermal treatments (50-80 degrees C) in a well-stirred water bath. Time-temperature profiles obtained both during heating and cooling of test samples were used to correct both come-up and come-down times. The time-corrected D-values under microwave heating were 4.8, 2.1 and 1.1 s at 52.5, 55 and 57.5 degrees C, respectively, for S. cerevisiae and 14, 3.8 and 0.79 s at 57.5, 60 and 62.5 degrees C, respectively, for L. plantarum with corresponding z-values of 7 and 4.5 degrees C. D-values under batch thermal heating were 58, 25, 10 and 1.9 s at 50, 55, 60 and 70 degrees C, respectively, for S. cerevisiae and 52, 22, 8.4 and 1.2 s at 55, 60, 70 and 80 degrees C, respectively, for L. plantarum with corresponding z-values of 13.4 and 15.9 degrees C. Microbial destruction thus occurred much faster under microwave heating than under thermal heating suggesting some contributory enhanced effects to be associated with microwave heating. (C) 1999 Published by Elsevier Science Ltd on behalf of the Canadian Institute of Food Science and Technology. All rights reserved.