Water Diffusion from a Bacterial Cell in Low-Moisture Foods

摘要

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 ℃, as observed in the sorption isotherms of bacterial cells. When the water vapor diffusion coefficient was assumed as between 10~(-12) and 10~(10) m~2/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) m~2/s) around the bacterial cell with a water diffusion coefficient of 10~12 m~2/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.