Water Diffusion from a Bacterial Cell in Low‐Moisture Foods

We used a Fick's unsteady state diffusion equation to estimate the time required for a single spherical shaped bacterium (assuming Enterococcus faecium as the target microorganism) in low‐moisture foods to equilibrate with the environment. We generated water sorption isotherms of freeze‐dried E. faecium. The water activity of bacterial cells at given water content increased considerably as temperature increased from 20 to 80 °C, as observed in the sorption isotherms of bacterial cells. When the water vapor diffusion coefficient was assumed as between 10^?12 and 10^?10 m²/s for bacterial cells, the predicted equilibration times (t_eq) ranged from 8.24×10^?4 to 8.24×10^?2 s. Considering a cell membrane barrier with a lower water diffusion coefficient (10^?15 m²/s) around the bacterial cell with a water diffusion coefficient of 10^?12 m²/s, the t_eq predicted using COMSOL Multiphysics program was 3.8×10^?1 s. This result suggests that a single bacterium equilibrates rapidly (within seconds) with change in environmental humidity and temperature.