Water activity measures how available water is within a product rather than simply how much water the product contains.

This distinction is important because ingredients such as sugars, salts and polyols can bind water and reduce its availability.

Two products may contain similar amounts of water while having significantly different water activity values.

Microbiological Importance

Microorganisms require available water to grow. Reducing water activity can therefore contribute to microbial control.

However, low water activity does not automatically mean that a product requires no preservation system. The microbial risk depends on the complete formulation and packaging.

Measurement

Water activity can be measured using a dedicated water-activity meter.

The result can be useful in combination with preservative efficacy testing, microbiological testing and formulation analysis.

References

ISO 18787. Foodstuffs — Determination of water activity.

ISO 17516. Cosmetics — Microbiology — Microbiological limits.

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