Influence of Water Activity on Foodborne Bacteria — A Review

Abstract

The influence of water activity on some characteristics of bacterial growth is presented. Bacteria are able to overcome the plasmolytic effect of reduced water activity by intracellularly accumulating compatible solutes such as glutamic acid or proline. In food systems, water activity is one of several preservative factors which interact to form a preservative system. Other preservative factors considered in this review are temperature, pH, O/R potential and chemical preservatives. Control of water activity in foods is receiving more attention as new food products and new technologies are developed. This review has been organized to summarize current knowledge about the influence of water activity (a w) on bacterial growth and its importance in food bacteriology. The review does not include the influence of a w on bacterial toxin production or on the heat resistance of bacterial cells and spores. INFLUENCE OF WATER ACTIVITY ON BACTERIAL GROWTH Influence on growth rate A reduction in water activity has a dramatic effect on bacterial growth. One can observe, for instance, an increase in the lag phase and decreases in the growth rate and cell yield as water activity is reduced (50). Figure 1 shows the general response of bacterial growth rate to decreasing water activity. As the a w is decreased from 1.0, the growth rate increases (39). Many bacteria have their maximum growth rates between a w 0.990 and 0.995. From the maximum point, the growth rate diminishes until the minimum a w for growth is reached. The growth rate of Staphylococcus aureus at a w 0.90 is only 10% of its maximum growth rate (50). At a w values lower than the minimum for growth, cells either remain dormant or die.