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Water Activity

Water activity (aw) is the amount of water that is available to microorganisms. Many microorganisms prefer an aw of 0.99 and most need an aw higher than 0.91 to grow. Relative humidity and aw is related. Water activity refers to the availability of water in a food or beverage. Relative humidity refers to the availability of water in the atmosphere around the food or beverage.

Relative Humidity vs Water Activity
       RH = aw x 100

 

Raoult's Law
aw =   p  
 po

aw = water activity
RH = relative humidity
p = vapor pressure of a solution
po = vapor pressure of pure water

Water Activity of
Saturated Solutions at 25°C

aw Solutions
0.11 Lithium chloride
0.22 Potassium acetate
0.43 Potassium carbonate
0.53 Magnesium nitrate
0.74 Sodium nitrate
0.75 Sodium chloride
0.84 Potassium chloride
0.94 Potassium nitrate
0.97 Potassium sulfate

 

Water Activity Levels for Growth of Microorganisms
aw Microorganisms grow at this aw and above Food examples
0.95 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
0.91 Salmonella, Vibrio parabaemolyticus, C. botulinum, Lactobacillus, some molds Some cheese (Cheddar, Swiss, Provolone), cured meat, fruit juice concentrates with 55% sucrose or 12% NaCl
0.87 Many yeasts, Candida, Torulopsis, Hansenula micrococcus Fermented sausage, sponge cakes, dry cheese, margarine, foods with 65% sucrose or 15% NaCl
0.80 Most molds, most Saccharomyces spp., Debaryomyces, Staphylococcus aureus Most fruit juice concentrates, condensed milk, syrup, flour, high-sugar cakes, pulses containing 15-17% moisture
0.75 Most halophilic bacteria, Mycotoxigenic aspergilli Jam, marmalade, glace fruits, marzipan, marshmallows
0.65 Xerophilic molds, Saccharomyces bisporus Rolled oats with 10% moisture, jelly, molasses, nuts
0.60 Osmophilic yeasts, few molds Dried fruits with 15-20% moisture, caramel, toffee, honey
0.50 No microbial proliferation Noodles with 12% moisture, spices with 10% moisture
0.40 Whole egg powder with 5% moisture
0.30 Cookies, crackers, bread crusts with 3-5% moisture
0.03 Whole milk powder with 2-3% moisture, dehydrated soups

 

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