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.
Scott’s work is relevant to every product from fresh tree nuts and wheat berries to processed cheese and pharmaceuticals. The microbial growth limits he and his colleagues established apply to dried meats, cheesecake, powdered drink mix, and dog food, but also to non-food products like tree bark, hand lotion, and insulation.
Control water activity, prevent microbial growth
Like all organisms, microorganisms rely on water 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. The chart below shows water activity growth limits for many common microorganisms.
|0.97||Clostridium botulinum E |
|fresh meat, fruits, |
vegetables, canned fruit, canned vegetables
|low-salt bacon, cooked sausages,|
nasal spray, eye drops
|0.94||Clostridium botulinum A, B|
|0.93||Bacillus cereus||Rhizopus nigricans||some cheeses, cured meat (ham)|
evaporated milk, ral liquid
suspensions, topical lotions
|0.85||Aspergillus clavatus||sweetened condensed milk, aged cheeses (cheddar), fermented sausage (salami), dried meats (jerky), bacon, most fruit juice concentrates, chocolate syrup, fruit cake, fondants, cough syrup, oral analgesic suspensions|
|0.81||Penicillium Penicillium cyclopium|
|0.78||Aspergillus flavus||jam, marmalade, marzipan, glace fruits, molasses, dried figs, heavily salted fish|
|0.62||Saccharomyces rouxii||dried fruits, corn syrup, licorice, marshmallows, chewing gums, dried pet foods|
|0.60||No microbial proliferation|
|0.50||No microbial proliferation||caramels, toffees, honey, noodles, topical ointments|
|0.40||No microbial proliferation||whole egg powder, cocoa, liquid center cough drop|
|0.30||No microbial proliferation||crackers, starch-based snack foods, cake mixes, vitamin tablets, suppositories|
|0.20||No microbial proliferation||boiled sweets, milk powder, infant formula|
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 water activities 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.