Virginia Food Processor Technical Assistance Program

Water activity (aw) is defined as the amount of water available for microbial (bacteria, yeast and mold) growth. Water activity is based on a scale of 0 to 1.0 with pure water having a water activity of 1.00.  Usually products that contain lower percent moisture have lower water activities.  For example, those products such as dried fruits, crackers and dried pastas have very low water activities.  Those foods with lower water activities are quite shelf-stable in that they contain very little water available for microorganisms to use for growth.  When microorganisms grow, they degrade the food causing spoilage as well as health dangers for the consumer.
Although there can be a correlation between total moisture content of a food and the water activity, the correlation does not occur at all times.  Food products can exist with high moisture content but have very little water activity.  Many natural ingredients can be added to a product to “bind” the water making it unavailable for the growth microorganisms.  Several common ingredients the bind water are sugar, salt (has six times the capacity to bind water when compared to sugar), pectin and glycerol.  A good example of a food product that contains a good deal of water with a fairly low water activity is jelly.  Although composed of 50-60% water, jams usually have water activities around 0.75.  The sugar and pectin in jams binds the water making it unavailable for microbial growth.
Meat products that utilize water activity for a means to increase shelf-stability include cured meats such as ham, fermented meats such as sausage and pepperoni and dried meats such as jerky.  Most meat products with lower water activity levels utilize salt to bind water as well as drying techniques to lower the total moisture content of the product which in turn lowers the water activity. 

aw

Microorganisms grow at this aw and above

Food examples

0.95

E. coli, Clostridium perfringens,  many spoilage organisms, 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, C. botulinum, Lactobacillus, some molds

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

0.87

Many types of yeast.

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

0.80

Most molds, most Saccharomyces sp.,  Staphylococcus aureus

Most fruit juice concentrates, condensed milk, syrup, flour, high-sugar cakes, some meat jerky products

0.75

Most halophilic (salt tolerant) bacteria, Mycotoxigenic aspergilli

Jam, marmalade, glace fruits, marzipan, marshmallows, some meat jerky products

0.65

Some molds.

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

0.60

Some yeasts, few molds.

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

0.50

No microbial growth.

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