Final optical density and growth rate; effects of temperature and NaCl differ from acidity

Most predictive models used in food microbiology accurately describe microbial growth rate responses to conditions in the environment, but do not improve understanding of mechanisms. The effects of temperature, water activity and acid constraints on the growth of Escherichia coli are investigated using substrate-limited batch culture experiments. Final optical densities of substrate-limited batch cultures indicate the efficiency of substrate conversion to biomass and, therefore, the relative energetic burdens that different environmental conditions pose for microbial growth. Typical growth rate responses are observed. At suboptimal temperatures, the square root of growth rate declines linearly with temperature. With increasingly stringent water activity conditions, the growth rate declines linearly. It also declines linearly (but only slightly) with increasing hydrogen-ion concentration. Similar ΔOD (the change in optical density from the initial value to the value where the final population density is reached) responses are observed for temperature and water activity (adjusted using sodium chloride). Over most of the growth permissive ranges, the ΔOD remains high for both factors. Close to the growth boundaries, however, at the low water activity extreme and at the low and high temperature extremes, cell production declines to zero suddenly. The influence of water activity on growth rate is partly relieved by the compatible solute betaine. However, the main influence of betaine on ΔOD is to extend (to a lower water activity value) the water activity growth boundary and, therefore, the water activity value where cell production declines suddenly. In contrast to the temperature and water activity responses, the ΔOD declines steadily with increasing hydrogen-ion concentration. This indicates that temperature and water activity constraints, despite their marked influence on growth rate, may not impose large energetic burdens. However, when acid stress is applied, the efficiency of substrate conversion to biomass appears to be reduced.