Inactivation of Bacillus cereus spores in a tsuyu sauce using continuous ohmic heating with five sequential elbow-type electrodes

Abstract

Aims: The effect of ohmic heating (OH) in a pilot plant system which had a zig-zag shaped (elbow-type) ohmic heater with five sequential voltage electrodes was investigated on Bacillus cereus spores in a commercial tsuyu sauce. Methods and Results: The electrical field was fixed at 26.7 V cm(-1) with an alternating current frequency of 25 kHz. Raw tsuyu sauce (50 l) inoculated with B. cereus spores was submitted in a 4 9 3 factorial design to the OH system and heated at 95, 105, 115, and 125 degrees C each for 30, 60, and 90 s. Survival of B. cereus spores and colour change in the commercial tsuyu sauce were both measured before and after treatment. As the treatment temperature and time increased, the number of surviving B. cereus spores decreased. The OH treatment in a bath-type process at 105 degrees C for more than 30 s resulted in the total inactivation of the inoculated B. cereus spores (average 5.4 log reductions to undetectable levels after treatment). The OH protocol of heating at 105 degrees C for 60 s which ensure complete eradication of the inoculated spores without compromising product quality was chosen and investigated for its suitability for commercial application on bulk quantities of samples (500 l). Reliable and reproducible reductions in B. cereus spore counts of 4.7 5.5 log CFU ml(-1) (mean +/- standard deviation = 5.1 +/- 0.3 CFU ml(-1)) were achieved by the selected protocol of the continuous OH treatment (105 degrees C for 60 s). Conclusion: This study suggests that OH treatment with five sequential elbow-type electrodes has great potential as an industrial sterilizing method for liquid food contaminated with B. cereus spores. Significance and Impact of the Study: This procedure will enhance the microbiological quality of liquid foods while minimizing quality deterioration.