Water activity controls microbial growth

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.

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.

awBacteriaMoldYeastTypical Products
0.97Clostridium botulinum E
Pseudomonas fluorescens
fresh meat, fruits,
vegetables, canned fruit, canned vegetables
0.95Escherichia coli
Clostridium perfringens
Salmonella spp.
Vibrio cholerae
low-salt bacon, cooked sausages,
nasal spray, eye drops
0.94Clostridium botulinum A, B
Vibrio parahaemolyticus
Stachybotrys atra
0.93Bacillus cereusRhizopus nigricanssome cheeses, cured meat (ham)
bakery goods,
evaporated milk, ral liquid
suspensions, topical lotions
0.92Listeria monocytogenes
0.91Bacillus subtilis
0.90Staphylococcus aureus
Trichothecium roseumSaccharomyces
0.87Staphylococcus aureus
0.85Aspergillus clavatussweetened 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
0.84Byssochlamys nivea
0.83Penicillium expansum
Penicillium islandicum
Penicillium viridicatum
Deharymoces hansenii
0.82Aspergillus fumigatus
Aspergillus parasiticus
0.81Penicillium Penicillium cyclopium
Penicillium patulum
0.80Saccharomyces bailii
0.79Penicillium martensii
0.78Aspergillus flavusjam, marmalade, marzipan, glace fruits, molasses, dried figs, heavily salted fish
0.77Aspergillus niger
Aspergillus ochraceous
0.75Aspergillus restrictus
Aspergillus candidus
0.71Eurotium chevalieri
0.70Eurotium amstelodami
0.62Saccharomyces rouxiidried fruits, corn syrup, licorice, marshmallows, chewing gums, dried pet foods
0.61Monascus bisporus
0.60No microbial proliferation
0.50No microbial proliferationcaramels, toffees, honey, noodles, topical ointments
0.40No microbial proliferationwhole egg powder, cocoa, liquid center cough drop
0.30No microbial proliferationcrackers, starch-based snack foods, cake mixes, vitamin tablets, suppositories
0.20No microbial proliferationboiled sweets, milk powder, infant formula
Table 1. 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, 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.

Measure water activity in 5 minutes

Not a “quick mode” reading, but a direct measure of water activity to 0.003. No calibration, no approximation.

Combine water activity with other hurdles

Learn more about how to use hurdles to prevent microbial growth even at water activities higher than 0.91.