Water Activity – Cosmetic Uses in Preservation and Product Stability

What Is Water Activity?

Water activity (aw) describes the amount of water in a cosmetic formulation that is available for chemical reactions and, particularly, for the growth of microorganisms. It is different from the total amount of water present in a product.

Water can be present in a formulation but may be strongly bound to ingredients such as salts, sugars, polyols or polymers. This bound water is less available to microorganisms. Therefore, two products containing similar amounts of water can have different water activity values and different microbiological risks.

Water activity is expressed on a scale from 0 to 1, with pure water having an aw of approximately 1.

Water Activity in Cosmetic Formulation

Water activity is particularly relevant when developing products containing water, humectants, salts and other ingredients that influence the availability of water.

Factors that can affect water activity include:

• Total water content
• Concentration of dissolved ingredients
• Humectants such as glycerin and sorbitol
• Salts and other electrolytes
• Polymers and other water-binding materials
• Product composition and physical structure

Reducing water activity can make conditions less favourable for microbial growth, but it does not automatically make a cosmetic product microbiologically safe.

Water Activity and Preservation

Water activity is an important consideration when designing a preservation system. Microorganisms require available water for growth, and different groups of microorganisms have different minimum water-activity requirements.

For this reason, measuring water activity can provide useful information when assessing the microbiological risk of a formulation and selecting an appropriate preservation strategy.

Water activity should, however, be considered together with other factors such as pH, preservative concentration, packaging and manufacturing hygiene. A low water activity value does not by itself replace appropriate microbiological testing or preservation efficacy testing.

Water Activity and Cosmetic Stability

Water activity can also influence chemical and physical stability. Changes in the availability of water may affect ingredient interactions, crystallisation, viscosity and other properties of the finished product.

Measurement of water activity can therefore be included in the physicochemical evaluation of cosmetic products, particularly when developing formulations in which the available water content is an important part of the product's stability and preservation strategy.

References

ISO 18787. Foodstuffs — Determination of water activity.

ISO 17516. Cosmetics — Microbiology — Microbiological limits.