New Insight into the Thermal Properties and the Biological Behaviour of the Bacterial Spores

The physicochemical properties of spores were studied in relationship of their structure, which was modulated by chemical or genetic methods. The Bacillus subtilis spores were equilibrated at different water activities (from 0.113 to ~1) and investigated by differential scanning calorimetry (DSC). The isothermal sorptions at 25 °C of the native and the modified spores were also used to analyse the DSC results. As already reported in literature, an endothermic peak in DSC was found at about 70 °C, but a previously unreported baseline shift, a ∆Cp step, was also observed at −69 °C. The endothermic peak found at 70 °C was assigned to a material relaxation which corresponded to a structure change from a less mobile state to a more mobile state. The spore cortex material seems to be mainly implicated in this event. The ∆Cp step observed at −69 °C was identified as a glass transition of the water in the spore protoplast. These results showed that at room temperature, the physical state of the components within B. subtilis spores equilibrated at water activity levels below 0.3 was different: The cortex material is in a low mobility state whereas confined structure of protoplast and its internal hydration level allow a certain mobility of water molecules.