The importance of water activity for the safety and
Quality of cosmetic products

This powerful but under-used quality parameter is gaining interest in the Cosmetic Industry with the release of ISO 29621

The cosmetic industry is valued at $511 billion with hundreds of brands and product types. The product range is immense covering everything from soap and shampoo to eye liner and eye shadow. Each of these products is unique in its purpose, ingredients, and physical characteristics. All these products are applied topically to the skin in some way or another, and it is essential that they are safe and deliver the advertised functionality. Since cosmetic products can have a wide range of water activities (Table 1), measuring the water activity of a cosmetic product is critical to determining how susceptible the product is to microbial contamination and to delivering consistent product quality.

Introduction

In 2017, ISO 29621 Cosmetics — Microbiology — Guidelines for the risk assessment and identification of microbiologically low-risk products was published identifying water activity as one of the most important factors in determining the potential for microbial growth in cosmetics. Water activity, not moisture content, determines whether microorganisms in cosmetics can access water for growth. Each microorganism requires a unique water activity level to grow, while all organisms stop growing at water activities less than 0.600 aw. Water activity also has a stronger correlation with the chemical and physical stability of cosmetics. In general, as water activity is reduced, the rate of chemical reactions such as oxidation and hydrolysis are reduced, extending the shelf life of the cosmetic. However, lowering water activity also typically results in changes in the physical properties of cosmetic products, which may be undesirable. The key is to determine the ideal water activity range for each cosmetic product and to implement it as a required release specification. In addition, it would be necessary to implement water activity testing as a routine QA test to ensure that the product is being made to this ideal water activity.

Table 1. Water activity of common cosmetic products (adapted from Fontana and Schmidt, 2020)

What is water activity?

Water activity is defined as the energy status of water in a system and is rooted in the fundamental laws of thermodynamics through the Gibbs free energy equation. It represents the relative chemical potential energy of water as dictated by the surface, colligative, and capillary interactions in a matrix. Practically, it is measured as the partial vapor pressure of water (P) in a headspace that is at equilibrium with the sample, divided by the saturated vapor pressure (P0) of water at the same temperature (T). Water activity is equal to the Equilibrium Relative Humidity (ERH) divided by 100:

This water activity index covers a range from 0 for bone-dry conditions up to 1.00 for pure water, when the partial pressure and the saturated pressure are equal. Water activity is often referred to as “free water,” which is useful when referring to higher energy; however, it is misleading because “free” is not scientifically defined and is interpreted differently depending on the context. As a result, the concept of free water can cause confusion between the physical binding of water, a quantitative measurement, and the chemical binding of water to lower energy, a qualitative measurement. Rather than a water activity of 0.50, indicating 50% free water, it more correctly indicates that the water in the product has 50% of the energy that pure water would have in the same situation. The lower the water activity, the less the water in the system behaves like pure water.

Water activity is measured by equilibrating the liquid-phase water in the sample with the vapor-phase water in the headspace of a closed chamber and measuring the ERH in the headspace using a sensor. The relative humidity can be determined using a resistive electrolytic sensor, a chilled mirror sensor, or a capacitive hygroscopic polymer sensor. Instruments from Novasina, like the LabMaster NEO, utilize an electrolytic sensor to determine the ERH inside a sealed chamber containing the sample. Changes in ERH are tracked by changes in the electrical resistance of the electrolyte sensor. The advantage of this approach is that it is very stable and resistant to inaccurate readings due to contamination, a particular weakness of the chilled mirror sensor. The resistive electrolytic sensor can achieve the highest level of accuracy and precision with no maintenance and infrequent calibration, making it ideal for cosmetic testing. Water activity is an intensive property that describes the energy of the water in a system, whereas moisture content is an extensive property that determines the amount of moisture in a product. Although related, water activity and moisture content are not the same: moisture content is typically determined through loss-on-drying or chemical titration; though useful as a measurement of purity and a standard of identity, moisture content does not correlate as well as water activity with microbial growth, chemical stability, or physical stability. Water activity and moisture content are related through the moisture sorption isotherm.

Regulatory information

Federal Food, Drug and, Cosmetics Act (FD&C)

The FD&C, which outlines what is considered a cosmetic, is also the only available regulation that applies to cosmetics. It prohibits the marketing of adulterated or misbranded cosmetics but does not outline specific requirements for the safety of cosmetic products. Instead, that burden lies with the manufacturer, and they are legally required to take whatever steps are necessary to assure the safety of their product. The FDA has advised manufacturers to substantiate the safety through “performance of any addition toxicological and other tests that are appropriate in light of existing data and information” (Federal Register, March 3, 1975, page 8916). So, although water activity is a valid test to determine the susceptibility of cosmetics to microbial contamination, cosmetic manufacturers may not know to use water activity or may need additional validation that water activity is an acceptable parameter to use for safety.

ISO 29621 Cosmetics — Microbiology — Guidelines for the Risk Assessment and Identification of Microbiologically Low-Risk Products

ISO 29621 provides cosmetic manufacturers the validation they need based on the “existing data and information” precedent set by the FD&C. The document states that its purpose is to define those finished cosmetic products that present a low risk for microbial contamination and therefore, do not require challenge testing to validate their microbial safety. Section 4.2.2 outlines that products with a water activity below the limit for microbial growth do not need to be subjected to preservative challenge testing as their low water activity provides sufficient preservation.