Effect of water activity on the mechanical glass transition and dynamical transition of bacteria-solute systems

This study examined the mechanical glass transition and dynamical transition of bacteria (Cronobacter sakazakii)-solute (glycerol and glucose) systems. From the effect of water activity (a_w) on the mechanical relaxation, the a_w at which mechanical glass transition occurs at 25 °C (a typical ambient temperature) was determined as 0.225 for the glycerol-added sample and 0.540 for the glucose-added sample. These values were lower than that (0.667) reported previously for non-added sample. The effect of temperature on the mean square displacement of atoms within the samples was investigated by incoherent elastic neutron scattering. The a_w at which dynamical transition occurs at 25 °C was determined as 0.156 for the glycerol-added sample and 0.475 for the glucose-added sample. These values were lower than that (0.675) reported previously for non-added sample. According to the Couchman-Karasz model, the T_g for the bacteria plasticized by the additive and that for the bulk additive were obtained.