Water activity

Water activity (aw) is a measure of the energy status of the water in a system. It is defined as the vapor pressure of water above a sample divided by that of pure water at the same temperature; therefore, pure distilled water has a water activity of exactly one. As the water activity decreases, the amount of "free" water, water that can participate in chemical or physical reactions, decreases. Although it is not a direct measure of water content, it is used in many industries as a critical control point for product quality and safety.

Definition

Water activity is a thermodynamic quantity that measures the energy status of the water in a system. It is defined as the vapor pressure of water above a sample divided by that of pure water at the same temperature. This means that water activity is a ratio of the water vapor pressure in the air space surrounding the sample to the vapor pressure of pure water at the same temperature. Therefore, pure distilled water has a water activity of exactly one.

Measurement

Water activity can be measured using a variety of techniques, including gravimetric methods, dew point methods, and electronic methods. The most common method is the dew point method, which involves measuring the relative humidity of the air above a sample in a sealed container. The relative humidity is then converted to water activity using a psychrometric chart.

Applications

Water activity is used in many industries as a critical control point for product quality and safety. It is used in the food industry to predict the growth of microorganisms, in the pharmaceutical industry to predict the stability of drugs, and in the cosmetic industry to predict the shelf life of products. It is also used in the building industry to predict the likelihood of mold growth.

See also

• Relative humidity
• Vapor pressure
• Psychrometric chart
• Food safety
• Pharmaceutical industry
• Cosmetic industry
• Building industry

References

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