Food products vary dramatically in terms of how much water is accessible to the organisms. Fresh meats and milk, for example, have ample water and will support growth of many microorganisms. Bread, nuts, and dried foods, on the other hand, provide a relatively arid environment. Some sugar-rich foods, such as jams and jellies, are seemingly moist, but most of that water is chemically interacting with the sugar, making it unavailable for use by microbes. Highly salted foods, for similar reasons, have little available moisture.
The term water activity (aw) is used to designate the amount of water available in foods. By definition, pure water has an aw of 1.0. Most fresh foods have an aw above 0.98, whereas ham has an aw of 0.91, jam has an aw of 0.85, and some cakes have an aw of 0.70.
Most bacteria require an aw above 0.90 for growth, which explains why fresh moist foods spoil more quickly than dried, sugary, or salted foods. Fungi can grow at an aw as low as 0.80, which explains why forgotten bread, cheese, jam, and dried foods often become moldy. Staphylococcus species, which are adapted to grow on the dry, salty surfaces of human skin, can grow at an aw of 0.86, which is lower than the minimum required by most common spoilage bacteria. Staphylococcus species normally do not compete well with other bacteria, but on salty products, such as ham and other cured meats, they can multiply with little competition. Ham, when improperly handled, is a common vehicle for S. aureus food poisoning. ■ Staphylococcus aureus foodborne illness, p. 812
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