Influence of several environmental factors on the initiation of germination and inactivation of Bacillus cereus by high hydrostatic pressure

The influence of pH, aw, l-alanine, and fat concentration of milk on the initiation of germination and inactivation by high hydrostatic pressure (HHP) (250 mPa at 25°C for 15 min and 690 mPa at 40°C for 2 min) of Bacillus cereus sporulated at 20, 30 and 37°C was investigated. B. cereus sporulated at the lowest temperature was found to be the most resistant to the initiation of germination and inactivation by HHP. At ambient pressure, the rate and extension of germination induced by l-alanine were also lower in B. cereus sporulated at 20 than 30 or 37°C. The optimum pH for the germination and inactivation of B. cereus depended on the sporulation temperature. At 250 mPa the extent of germination for the three suspensions increased with higher pH. At 690 mPa, the pH barely affected the germination of B. cereus sporulated at 20°C (3 log cycles), but the inactivation increased as the pH of the medium was lowered. After the same treatment, pressure optimally initiated the germination of B. cereus sporulated at 30 and 37°C (6–7 log cycles) around neutral pH. Higher inactivation was obtained at pH 6. High concentrations of sucrose protected the spores from the germinating and inactivating effect of HHP. At aw 0.92, no germination was detected when the spores were pressurized at 250 mPa, and only 1 log cycle of B. cereus sporulated at 20 and 30°C and 2 log cycles of B. cereus sporulated at 37°C were germinated at 690 mPa. In addition, no inactivation was observed at aw 0.92 even at 690 mPa. The presence of l-alanine in the medium of pressurization increased the germination initiated by HHP at 250 mPa, but not at 690 mPa. A combination of 250 mPa at 25°C with l-alanine (100 mM) was found to give an additive response. The initiation of germination and inactivation by HHP were not affected by the fat concentration of the milk.