Like all organisms, microorganisms rely on available water in food for growth. They take up water by moving it across the cell membrane. This water movement mechanism depends on a water activity gradient – on water moving from a high water activity environment outside the cell to a lower water activity environment within the cell.
When WATER ACTIVITY outside the cell becomes low enough, it causes osmotic stress: the cell cannot take up water and becomes dormant. The microorganisms are not eliminated, they just become unable to grow enough to cause infection.
Different organisms cope with osmotic stress in different ways. That’s why there are different growth limits for each organism. Some types of moulds and yeasts have adapted to withstand very low water activity levels. Table below shows water activity growth limits for many common microorganisms.
Water Activity and FDA, FSIS, FSMA
If you measure the water activity of any material, you will know which BACTERIA, MOULD or FUNGI can grow on and in it. By reducing water activity, you can rule out the growth of certain classes of microbes. At low water activity, you can preclude the growth of anything at all.
Water activity is not a kill step. It is a control step, and an integral part of many HACCP plans. These well-established microbial growth limits have been incorporated into FDA, FSIS and other regulations. Water activity is part of the 2013 Food Code’s definition of potentially hazardous foods, which is referenced by Food Safety Modernisation Act (FSMA).