How water activity controls microbial growth

In 1953, William James Scott showed that microbial growth in food is governed not by water content, as most people thought, but by water activity. Four years later, he established the concept of a minimum water activity for microbial growth. Water activity is now routinely used by food manufacturers to determine whether or not a product is susceptible to microbial proliferation.

Control water activity, prevent microbial growth

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 molds and yeasts have adapted to withstand very low water activity levels.  Table 1 shows water activity growth limits for many common microorganisms.

Frequently asked questions

What is the FDA water activity threshold for shelf-stable food?

The FDA classifies foods at or below aw 0.85 as generally not requiring refrigeration for safety under 21 CFR and FSMA Preventive Controls — this threshold is based on the minimum growth requirements of most bacterial pathogens. Products between aw 0.85 and 0.91 require careful evaluation because Staphylococcus aureus can still grow in that range even when other pathogens cannot.

How is water activity used as a critical control point in HACCP plans?

Water activity is used as a critical control point (CCP) in HACCP plans by establishing a specific aw limit — typically at or below 0.85 — as a measurable, verifiable preventive control against pathogen growth. Because water activity is directly tied to defined microbial growth limits and can be confirmed at the point of manufacture, it is one of the most reliable parameters for documenting food safety without requiring direct pathogen testing on every batch.

Does lowering water activity kill bacteria and mold already present in a food?

No, reducing water activity below the growth minimum forces microorganisms into dormancy but does not kill them, and spores can remain viable at very low water activity for years before resuming growth if conditions change. This is why hurdle technology combines water activity with other controls like pH or temperature rather than relying on water activity alone.

What is the minimum aw for Staphylococcus aureus and why does it matter?

Staphylococcus aureus has one of the lowest minimum water activity thresholds of any bacterial pathogen at aw 0.86, which is why products between aw 0.86 and 0.91 can carry Staph risk even though most other pathogens cannot grow in that range. This makes the 0.85 FDA threshold a conservative but critical control point for intermediate-moisture foods.

Can high salt or sugar content reduce water activity enough to prevent microbial growth?

Yes, dissolved solutes like salt and sugar bind water molecules and lower their energy state, which is the science behind traditional preservation methods like curing, brining, and jam-making. A sufficiently concentrated brine or high-sugar product can achieve water activity levels low enough to stop spoilage, though direct measurement is required since complex food matrices do not follow simple formulas.

Water activity and FDA, FSIS, FSMA

If you measure the water activity of any material, you will know which bacteria, molds, 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 activities, you can preclude the growth of anything at all. Water activity is not a kill step. It's 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 the Food Safety Modernization Act (FSMA).

While temperature, pH, and several other factors can influence whether an organism will grow in a food product and the rate at which it will grow, water activity may be the most important factor. Most bacteria, for example, do not grow at a water activity range below 0.91, and most molds cease to grow at water activities below 0.70. Water activity in combination with other hurdles, such as pH, temperature, or modified atmosphere packaging, will limit microbial growth even at water activities higher than 0.91.

Learn more—watch Water Activity 102

Everyone knows water activity is related to microbial growth. But how can you use that knowledge to your advantage in formulation, specification, production, and packaging? In this 30-minute webinar, learn:

• what you need to know about how water activity predicts microbial growth
• how to use specific organism aw limits relevant to your industry in setting your specs
• how to use different formulation techniques (including humectants, films, coatings) to hit the water activity you need
• why you should consider hurdle technology to address certain challenges