Water Activity of Foods

Water activity refers to the availability of water in a food or beverage and represents the amount of water that is available to microorganisms. Pure water has an aw of 1.00.

Most microorganisms grow well at 0.91 to 0.99. Clostridium botulinum will grow and produce deadly botulism toxin if the pH is above 4.6 and the water activity is above 0.85.



Microorganisms grow at this aw and above

Food examples


Pseudomonas, Escherichia, Proteus, Shigella, Klebsiella, Bacillus, Clostridium perfringens, some yeasts

Highly perishable foods (fresh and canned fruits, vegetables, meat, fish), milk, cooked sausages, breads, foods with up to 4 oz sucrose or 7% NaCl


Salmonella, Vibrio parabaemolyticus, C. botulinum, Lactobacillus, some molds

Some cheese (Cheddar, Swiss, Provolone), cured meat, fruit juice concentrates with 55% sucrose or 12% NaCl


Many yeasts, Candida, Torulopsis, Hansenula micrococcus

Fermented sausage, sponge cakes, dry cheese, margarine, foods with 65% sucrose or 15% NaCl


Most molds, most Saccharomyces spp., Debaryomyces, Staphylococcus aureus

Most fruit juice concentrates, condensed milk, syrup, flour, high-sugar cakes, pulses containing 15-17% moisture


Most halophilic bacteria, Mycotoxigenic aspergilli

Jam, marmalade, glace fruits, marzipan, marshmallows


Xerophilic molds, Saccharomyces bisporus

Rolled oats with 10% moisture, jelly, molasses, nuts


Osmophilic yeasts, few molds

Dried fruits with 15-20% moisture, caramel, toffee, honey


No microbial proliferation

Noodles with 12% moisture, spices with 10% moisture


Whole egg powder with 5% moisture


Cookies, crackers, bread crusts with 3-5% moisture


Whole milk powder with 2-3% moisture, dehydrated soups


Source:Water Activity of Some Foods and Susceptibility to Spoilage by Microorganisms Adapted from Beuchat (1981).