Modelling the combined effects of pH, temperature and sodium chloride stresses on the thermal inactivation of Bacillus subtilis spores in a buffer system

Aims: The inactivation of Bacillus subtilis 168 spores subjected to the combined stress of pH, temperature and sodium chloride in a buffer system was modelled. Methods and Results: Bacillus subtilis 168 spore suspension in 50 mmol l(-1) potassium phosphate buffer was heated in an open system using a block heater. A second order polynomial equation was used to describe the relationship between pH, temperature, sodium chloride concentration and the logarithm of the decimal reduction time (D-value) of the spores. Response surface graphs were constructed to predict the inactivation within the experimental domain. The data obtained were also compared with those reported for B. subtilis in different media and foods included in a large reference-based database of thermal inactivation (ThermoKill Database, TKDB R9100), which was constructed in the laboratory. Conclusions: All the variables studied seemed to have a significant effect on the inactivation of B. subtilis 168 spores in potassium phosphate buffer. The coefficient of determination, r(2), and an analysis of the residuals from the model indicated the adequacy of the model to predict the inactivation of B. subtilis spores within the range of the experimental variables studied. Significance and Impact of the Study: The findings of this study will enable a better understanding of the inactivation of B. subtilis spores under the influence of the studied environmental variables. The model can be used by food industries to assess and monitor the shelf life of food products in the event of a chance contamination by B. subtilis spores.