Modelling the effects of temperature and osmotic shifts on the growth kinetics of Bacillus weihenstephanensis in broth and food products.

Bacillus weihenstephanensis is a psychrotolerant bacterium belonging to the Bacillus cereus group. Some strains may be cytotoxic although they have not been described as food-poisoning agents so far. The objective of this work is to model the effects of temperature and a(w) downshifts on the lag time of B. weihenstephanensis and the dependence of μ(max) on the growth conditions (temperature and a(w)). Effects of temperature downshifts were studied on 30 experimental conditions (shifts magnitude ranging from 2 to 20 °C, temperature after shift from 10 to 20 °C and a(w) ranging from 0.977 to 0.997). Osmotic shifts were studied for 13 conditions (shift magnitude ranging from 0.008 to 0.020 units of a(w), temperature from 10 to 30 °C, a(w) after shift from 0.977 to 0.997). Experimental results show that temperature downshifts were able to induce considerable lag times (up to 20 days) when occurring near the growth limits. At lower temperatures, osmotic shifts had also a significant effect. Validation experiments in food subjected to changing conditions of temperature showed that the model provided valid predictions in diluted creamed pasta but overestimated bacterial growth in carrot soup (fail safe predictions).