Influence of water activity and acidity on Bacillus cereus spore inactivation during combined high pressure-thermal treatment

Understanding the inactivation kinetics of bacterial spores during combined pressure-thermal processing is essential for process design. The knowledge on the influence of pH and water activity (a(w)) during combined pressure-thermal treatment is limited. Experiments were conducted using Bacillus cereus spores suspended in citrate-phosphate buffers, adjusted to different levels of pH (3.8-7.3) and a(w) (0.80-0.99). Spores were then treated with a pressure-heat combination (600 MPa, 70 degrees C) using a laboratory-scale high-pressure kinetic tester. Linear regression and Weibull models were used to fit the experimental data. The kinetic parameters of the spores inactivation estimated from the models were plotted as a function of pH and a(w). Results showed that the inactivation kinetic curves were characterized by tailing, particularly at low a(w) levels. The log reduction of B. cereus spores decreased with the decrease of a(w). During pressure-thermal treatment, the changes in acidity values for a given water activity have limited influence on decimal reduction values of the spores. Study findings will be helpful for designing pressure-thermal processes to produce microbially safe products with varying water activity and acidity levels.