Water activity is the measurement of the amount of water available for chemical reactions and microbial growth within a product, quantifying how “bound” or “free” water is within a material.
It is expressed on a scale from 0 to 1, with 0 representing completely dry or waterless conditions and 1 representing pure water.
Water activity (aw) is a crucial property of any personal care product formulation, since it directly affects stability, shelf life, and microbial safety, as follows:
- Microbial Growth: High water activity levels can promote microbial growth, leading to spoilage, odor, and potentially harmful bacteria or mold contamination. This can be a significant concern in personal care products, as they are applied to the skin and can pose health risks if contaminated.
- Stability: Water can interact with other ingredients, potentially causing phase separation, texture changes, or ingredient degradation. Maintaining an appropriate water activity level helps preserve the product’s stability and consistency over time.
- Sensory Performance: Water activity can influence the texture, feel, and performance of personal care products. Products with higher water activity are often absorbed more quickly by the skin, providing a sensation of immediate hydration. On the other hand, products with lower water activity may feel heavier and may provide longer-lasting moisturization because they create a barrier on the skin’s surface that helps to lock in moisture.
- Shelf Life: Products with higher water activity tend to have shorter shelf lives because they are more susceptible to microbial contamination and ingredient degradation. Manufacturers often use preservatives to extend the shelf life of such products.
USP <1112> covers what levels of water activity are required to support growth of certain bacterial and fungal species. For example, shampoos typically have water activity of 0.99, and Staphylococcus aureus requires a minimum aw of 0.86 to proliferate. Products with an aw measurement lower than 0.60 do not support microbial growth.
If you are interested in adding water activity testing to your portfolio, please reach out to our laboratory at [email protected].