Water Distribution in Bacterial Spores: A Key Factor in Heat Resistance

Abstract : The role of water, its distribution and its implication in the heat resistance of dried spores was investigated using DSC (Differential Scanning Calorimetry). Bacillus subtilis spores equilibrated at different water activity levels were heat treated under strictly controlled conditions. The temperature was increased linearly in pans with different resistances to pressure. Data from the heat-related transitions occurring in the spores were recorded and spore viability was assessed at different stages during DSC. The thermodynamic transitions observed were related to the water status in the spores and spore survival. The results demonstrated that water still remained in the spore core when water activity was as low as 0.13. The first transition occurred at around 150 °C and was assumed to be related to a mobile fraction of water from the outer layers of the spore. The second occurred at around 200 °C, which could correspond to a fraction of water embedded in the spore core. Moreover, the results showed that spore destruction during heating was favored by the amount of water remaining in the spore. The changes in their structure were also evaluated by FTIR (Fourier Transform Infrared Spectroscopy). This work offers new understanding about the distribution of water in spores and presents new elements on the heat resistance of spores in relation to their water content.