Relationship between the apparent heat resistance of Bacillus cereus spores and the pH and NaCl concentration of the recovery medium.

Conventional heat resistance data, D values, were previously established by other workers at optimal condition for spores outgrowth. However, in canned food conditions of outgrowth are generally suboptimal in term of pH, salt concentration, water activity. The combined effects of pH and NaCl level of the recovery medium for the D value and z(pH) value were studied. Spores of Bacillus cereus were heated at 95 degrees C in phosphate-citrate buffer media at pH 7. Cells were recovered at 25 degrees C in Nutrient Agar with pH ranging from 5 to 7 and 1% to 4% (w/w) NaCl concentration. For each condition D' values (decimal reduction time associated with the recovery media characteristics) were determined. The results show a major influence of the recovery pH on the D' values. This effect is characterised by the z'(pH) values, distance of recovery medium pH from optimum recovery pH* medium (6.7) which leads to a tenfold reduction time of D value. The increase of the salt concentration leads to a slight decrease of D' value. However z'(pH) values are not significantly affected by the salt concentration. A simple three parameter model describing the effects of pH and NaCl concentration of the recovery medium upon the heat resistance of spores is proposed. The interaction between pH and salt concentration is sufficiently low to be neglected by the model.