Water activity (aw)

Definition

Water activity (aw) is a measure of the availability of water for microbial growth, represented on a scale from 0 to 1. It indicates how much water in a substance is free for microorganisms to use.

5 Must Know Facts For Your Next Test

1. Water activity (aw) directly affects microbial growth; most bacteria require an aw of at least 0.91.
2. Fungi can grow at lower water activities compared to bacteria, with some molds growing at aw as low as 0.70.
3. Reducing water activity in food preservation inhibits microbial growth and extends shelf life.
4. Water activity is different from moisture content; aw measures available water, whereas moisture content measures total water present.
5. Common methods to reduce water activity include drying, adding solutes like salt or sugar, and freezing.

Review Questions

• What is the minimum water activity required for most bacteria to grow?
• How does reducing water activity help in food preservation?
• What is the difference between water activity and moisture content?

Related terms

Osmotic Pressure:

The pressure exerted by solutes in solution which affects the movement of water across cell membranes.

Desiccation:

The process of drying out or removing moisture from a substance.

Halophile: Microorganisms that thrive in high-salt environments with low water activity.