Effect of water activities of heating and recovery media on apparent heat resistance of Bacillus cereus spores

Spores of Bacillus cereus were heated and recovered in order to investigate the effect of water activity of media on the estimated heat resistance (i. e., the D value) of Spores. The eater activity (ranging from 0.9 to 1) of t he heating medium was first successively controlled with three solutes (gly cerol, glucose, and sucrose), while the,vater activity of the recovery medi um was kept near 1. Reciprocally, the water activity of the heating medium was then kept at 1, while the water activity of the recovery medium was con trolled from 0.9 to with the same depressors. Lastly, in a third set of exp eriments, the heating medium and the recovery medium were adjusted to the s ame activity. As expected, added depressors caused an increase of the heat resistance of spores with a greater efficiency of sucrose with respect to g lycerol and glucose. In contrast, when solutes were added to the recovery m edium, under an optimal water activity close to 0.98, a decrease of water a ctivity caused a decrease in the estimated D values. This effect was more p ronounced when sucrose was used as a depressor instead of glycerol or gluco se. When the heating and the recovery media were adjusted to the same water activity, a balancing effect was observed between the protective influence of the solutes during heat treatment and their negative effect during the recovery of injured cells, so that the overall effect of water activity was reduced, with an optimal value near 0.96. The difference between the effic iency of depressors was also less pronounced. It may then be concluded that the overall protective effect of a decrease in water activity is generally overestimated.