Effect of water activity on the mechanical glass transition and dynamical transition of bacteria.

The purpose of this study was to clarify the glass-transition behavior of bacteria (Cronobacter sakazakii) as a function of water activity (a). From the water sorption isotherm (298 K) for C. sakazakii, monolayer water content and monolayer a were determined to be 0.0724 g/g-dry matter and 0.252, respectively. Mechanical relaxation was investigated at 298 K. In a higher a range of over 0.529, the degree of mechanical relaxation increased with an increase in a. From the effect of a on the degree of mechanical relaxation, the mechanical a (a at which mechanical glass transition occurs at 298 K) was determined to be 0.667. Mean-square displacement of atoms in the bacteria was investigated by incoherent elastic neutron scattering. The mean-square displacement increased gradually with an increase in temperature depending on the a of samples. From the linear fitting, two or three dynamical transition temperatures (low, middle, and high Ts) were determined at each a. The low-T values (142-158 K) were almost independent from a. There was a minor effect of a on the middle T (214-234 K) except for the anhydrous sample (261 K). The high T (252-322 K) largely increased with the decrease in a. From the a dependence of the high T, the dynamical a was determined to be 0.675, which was almost equivalent to the mechanical a. The high T was assumed to be the glass-transition temperature (T), and anhydrous T was estimated to be 409 K. In addition, molecular relaxation time (τ) of the bacteria was calculated as a function of a. From the result, it is suggested that the progress of metabolism in the bacterial system requires a lower τ than approximately 6 × 10 s.