Which statement about water activity aw is correct?

• A. Distilled water has an aw of 2
• B. E. coli requires a minimum aw of 8.6 to grow in food
• C. At an aw < 0.6 all bacterial, yeast and mold growth is inhibited
• D. Fungi, in general, require higher aw than bacteria to grow

Answer: (C)

Water activity shows how much water is available for microbes to use, not just the amount of water present. It runs from 0 to 1, with distilled water having an aw of 1.0. The idea that dist water has an aw of 2 is incorrect, and a minimum aw of 8.6 for E. coli makes no sense because aw is never above 1 and E. coli generally requires a relatively high aw to grow.

The statement that at an aw below 0.6 all bacterial, yeast, and mold growth is inhibited captures a practical, widely used takeaway: when water is very unavailable, most microorganisms cannot grow. While some xerophilic molds can tolerate slightly lower aw and may initiate growth around that boundary, for typical food safety purposes aw < 0.6 effectively inhibits growth of the common microbes you’re concerned about. In contrast, fungi generally do not require higher aw than bacteria; they can often grow at lower aw than many bacteria, so the other option about fungi needing higher aw is not accurate.