Effective vs. Ineffective Preservation Using Water Activity*

Important to understanding preservation in cosmetics and personal care is the discussion of water activity, as was discussed last week in Tony O'Lenick's Comparatively Speaking column, titled "Water Content vs. Water Activity." The present column takes this concept a step further with expert David Steinberg's explanation of how to use water activity for effective preservation.

*Editor's note: This column was adapted with permission from chapter 8 of Preservatives for Cosmetics, Steinberg's book published by Allured Books.

The concept of water activity to prevent the growth of microorganisms is as old as time and as new as the 1960s. All microorganisms need sufficient water and nutrients to grow. The oldest use of water activity to prevent spoilage goes back to honey.

Honey would appear to be the ideal growth medium for microorganisms. It contains water and sugar. Although honey is produced and used everywhere in the world, it does not need to be preserved. The reason lies in the sugar, which makes the water unavailable for microorganisms to utilize for growth. Therefore, there is an important difference between water content and the available water. This is known as water activity.

Water activity (aw) is defined as the ratio of a product's water vapor pressure compared to pure water at the same temperature, as shown in the following formula:

 aw = P/Po = (n2/(n1 + n2))

where P is the vapor pressure of the product, Po is the vapor pressure of pure water, n1 is the number of moles of solute and n2 is the number of moles of water.

Pure water has an activity of 1.00 while something considered “bone dry” is 0.00, which is numerically equivalent to 1/100 of the relative humidity (RH) generated by the product in a closed system. RH can be calculated from the direct measurements of partial vapor pressure or dew point or indirect measurements by sensors that are altered by being exposed to RH.

Therefore, the relationship between water activity and the equilibrium relative humidity (ERH) is expressed by the following formula.

 ERH (100%) = aw X 100

However, it is practically impossible to calculate water activity, therefore it is far better to measure it by instrumentation. The foods industry has used water activity for many years to determine the need for preservatives.

There are several ways to measure water activity including: vapor pressure manometry, electric hygrometry, hair hygrometry and dew point. The Association of Official Analytical Chemists (AOAC) has officially recognized the dew-point/chilled mirror method.

The approximate aw needed for growth of various microorganisms was published around 1960. While bacteria requires 0.94-0.99 aw for growth, yeast needs >0.7 aw and mold needs >0.6 aw for growth.

The specific aw needed for growth of the five common test organisms are: 0.77 aw for Aspergillus niger, 0.86 for Staphyloccous aures, 0.95 aw for Escherichia coli, 0.97 for Pseudomonas aeruginosa and 0.87 for Candida albicans.

It should be noted that some molds can grow with water activities as low as 0.7, so a good rule is to keep the water activity below 0.7 in order for the product to not need the addition of chemical preservatives.

Friedel published one of the earliest papers on the use of water activity in determining the requirements of preservation in topical products.1 This work showed how a formulator could measure water activity using a dew-point/chilled-mirror instrumentb and get reproducible and useful results. Among the examples given were creams, gels, anhydrous lip balms and ointments.

Water activity has become a valuable microbiologist tool for two purposes. First, it helps the microbiologist determine the potential for growth in raw materials. Secondly, as microbiological specifications are required for all ingredients in the EU, determining the aw and finding this to be below 0.7 (the threshold for growth) allows for an intelligent method to assign the need for microbial testing of ingredients.

Testing for the water activity of the finished formulation before the addition of a preservative can benefit formulators since, if they find that only mold or yeast can grow, they do not need to add preservatives that are only active against bacteria.

There are certain ingredients that lower aw;however, there appears to be no linear relationship between levels of these ingredients and aw. There have been models published to predict the effects of addition of water binding ingredients to lower the water activity, but due to the complex nature of cosmetic formulations, these really do not work well. One must measure by instrumentation to determine water activity.

Ingredients that lower water activity include salts, which can lower the water activity below growth such as in the Dead Sea if they are high enough; and glycols and polyols such as glycerin, propylene glycol, polyethylene glycols, etc. There are commercially available mixtures on the market that claim to reduce the aw  15-20%, making the cosmetic product “self-preserving.” There also have been attempts made to use high amounts of glycols to lower the water activity so no additional chemical preservative needs to be added; however, the aesthetics of the resulting products where unacceptable.

Measuring aw is especially useful for formulations that are “atypical” such as w/o emulsions, low water content products, anhydrous products like lipsticks, powders, etc., and for water extracts. When using water extract, the addition of high levels of glycols (they can even be the extracting solvent) can be used to lower the water activity below 0.7 and avoid the need to add additional preservatives.

The typical water activity of some cosmetics is: 0.97 aw for shampoos, 0.96 aw for conditioners, 0.91 aw for liquid soap, 0.86 aw for hand cream, and 0.98 aw for hand lotion.2

It is critical to understand that aw is not cidal (it does not kill, it only prevents growth), and it can only be applied to products that are free of contamination. Hence cGMP’s and hazard analysis critical control point (HACCP) are critical to apply water activity preservation to raw materials and finished formulations. Remember that aw can only be determined by measurement.

HACCP was originally developed for the space industry in conjunction with Pillsbury Company to assist in the production of food stuffs for astronaut consumption. The purpose was to have a high degree of quality and microbiologically safe foods. Hazard analysis identifies critical control points (CCP), as it relates to contamination. HAACP is now mandatory in the food industry and the US Food and Drug Administration (FDA) is working on this for drugs. It is useful for the production of cosmetics.

CCP can exist in every process and design in the manufacturing of cosmetics. It is used to identify locations where contamination could possibly occur and then establish procedures to monitor and control these specific areas. If one chooses to not incorporate traditional active preservatives or rely on water activity or other “non-preservative” methods, HACCP is a valuable tool to ensure quality.

It has been a belief of the cosmetic industry that the purpose of preservatives was to make a “clean” cosmetic under normal consumer use. In reality, the formulator is asking the preservative or preservative system to also prevent possible contaminations that may occur during production. A formulator never knows if the production of a clean cosmetic was achieved by strict adherence to cGMP’s or that the preservative system took care of any incidental contamination.

Although many people will disagree with these comments, even the FDA recognizes this thought. When a batch is produced and is contaminated, it is referred to as a “preservative failure.” Also, the FDA does not permit the post manufacturing treatment by external means, like radiation, heat sterilization, pressure, etc. to reduce contamination to acceptable levels.

Challenge tests of products to determine the adequacy of preservation usually challenge with “pure” cultures, and it is strongly recommend to include “house” isolates to be sure the preservative system is strong enough. How would a consumer possibly contaminate a cosmetic with “house” bugs? How would a consumer possible contaminate a product with a pure stain of bacteria? This illustrates the need need to concentrate on controlling and preventing microbial contamination during manufacturing.

HACCP is one of the best tools available to achieve this (see Seven Basic Principles of HACCP). It has been clearly shown that a well-designed and implemented HACCP studies reduce reliance on routine microbiological testing and provide a high degree of confidence in cosmetic product safety. HACCP's role is to determine points where microbial hazards exist. These points serve as CCP, which can be monitored and controlled. Critical limits can than be established for these points. 

More information on HACCP can be found through a few papers published by Preservatch and the International Life Sciences Institute.3-5 In addition, training software is available for cosmetic company employeesa.

1. RR Friedel and AM Cundell, The Application of Water Activity Measurement to the Microbiological Attributes Testing of Nonsterile Over-the-Counter Drug Products, Pharmacopeial Forum 24(2) 6087-6090 (1998)
2. JJ Kabara and DS Orth, Preservative-free and Self-Preserving Cosmetics and Drugs, New York, NY: Marcel Dekker Inc. (1997)
3. G Borovian, HACCP Application in the Manufacturing of Drugs and Personal Care Products: Microbiological Testing Centered on Prevention, Preservatech USA (1999)
4. M Easter, Implementing HACCP in the Manufacture of Personal Care Products, Preservatech (1996)
5. International Life Sciences Institute, Simple Guide to Understanding and Applying the Hazard Analysis Critical Control Point (HACCP) Concept

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