Introduction
This document discusses the basic principles of water activity and the importance it has in the manufacture of pharmaceuticals. It also provides direction on when and where testing for water activity can be most beneficial.
Water activity (aw) is defined as the ratio of product vapour pressure to pure water vapour pressure. It is a measure of the water available for chemical or microbiological activity within a product. It is not a measure of the total water content of an item, as water can be chemically bound and not be available for use. Its values are typically expressed as a decimal value and can range from 0.0 (completely dry) up to 1.0 (pure water).
Though the principles of water activity have been used for centuries (e.g. salting, drying, mummification), its use by the FDA occurred in the 1980’s when water activity testing was added to existing strategies for microbiological control in food products. Water activity is significant to the pharmaceutical industry in that it affects the quality of ingredients and finished product through their chemical stability, a reduced need for chemical preservatives, and a potential reduction in the need for microbial limits testing.
Microbial growth requires water. Water dissolves solutes within a viable cell and is
required for metabolic function. When an (aw) value is associated with a micro-organism it serves as an indication of potential metabolic activity. While organism proliferation ceases below certain water activity levels, some species have the ability to adapt slightly and continue to grow at levels below their optimum range.
Organisms grown in an environment outside of their optimal aw range will most likely be more resistant to thermal means of destruction.
There is a comprehensive understanding of microorganisms and their associated aw
values. The optimal aw range for microorganism growth lies between 0.995 and 0.980.
Most bacteria cannot grow below levels of 0.90. Yeasts typically survive down to levels of 0.87. Molds can tolerate aw levels down to about 0.80. Microbiological growth at any level ceases at aw values of about 0.60. Thus, by controlling the water activity of products, the growth of microorganisms, when present, can be controlled as well.
The aw also has an effect on the chemical processes within a product or formulation. Lowering the aw may increase chemical stability. The activity of proteins and enzymes, which may lead to chemical changes, requires maintenance of specific aw levels that can be controlled or minimized. Browning, or Maillard reactions, can be controlled by altering aw as well.
In addition to the benefits of increased microbiological and chemical stability, reduced need for chemical preservatives may be another important benefit. Pressure from regulatory bodies provides a constant challenge to the pharmaceutical industry to limit, and eliminate where possible, the use of chemical preservatives. Controlling aw is a means of natural preservation that can be used alone, in combination with lower amounts of chemical preservatives, or with other restrictive attributes such as pH. Using multiple approaches can further reduce proliferation of microorganisms.
Freezing, freeze drying, salting, syruping, and drying are preservation methods that take advantage of lowering aw. These methods either bind or eliminate the available water in a product. While some resistance/sensitivity variability within a microorganism population will always exist, micro-organisms have not been shown to develop a tolerance to low aw levels as they can to chemical means of preservation.
Recommendations & Rationale
Ideally, water activity should be evaluated early in the product development process. Low aw can be achieved by limiting the amount of water added to a product during formulation, driving off water, or binding remaining water.
Water is often used to dissolve product constituents. When limiting water in the formulation is not possible, it may be driven out of the formulation or rendered inactive by binding it. Driving off water occurs on a routine basis in the pharmaceutical industry through heating, drying, and freeze-drying processes.
Binding of available water occurs through the use of solutes (e.g. sugars, salts, fatty acids, glycols, etc.). Different solutes provide different aw results. Salts are typically more effective at lowering aw than sugars.
Though aw is a key component, it can not be solely relied upon for ensuring the microbial quality of products. Other factors that affect microbial quality of the final product include quality of ingredients, method of manufacture, quality of the manufacturing environment, equipment sanitization, final packaging, and storage. Low aw does not necessarily kill microorganisms, but limits or halts proliferation. Any microorganisms present have the potential for growth should the proper environment become present.
Once ingredients are qualified, aw testing can assist in verification of the potential for microbial growth. Raw materials of natural origin (e.g., starch, sugars, proteins, etc.) can possess higher microbial populations than synthetic materials. 5 Manufacturing holding times of aqueous solutions, wet processes, and cleaning of equipment play important roles in controlling any microbial growth present in starting materials. Manufacturing does not always include processing steps that can eliminate indigenous bioburden.
Packaging also plays an important role in protecting product from ambient moisture. If a low aw is achieved, the packaging should be capable of maintaining it. Some materials and/or processes create condensation or allow migration of moisture into the final container. Likewise, if product has to be transported prior to final packaging, controlling ambient humidity during storage and transport may be necessary.
Aw Measurement
Aw can be measured using various methodologies such as hygrometers, vapour pressure manometry, and dew point. 1 The Association of Official Analytical Chemists (AOAC) publishes a guide of accepted and tested analytical procedures that are recognized globally. The method preferred by the AOAC is the chilled-mirror dewpoint methodology. This testing equipment is relatively inexpensive, easy to operate, and provides results in a timely (results can be obtained in approximately five minutes per sample) and consistent manner. In simple terms, this equipment operates by placing a sample of the item to be tested in a sealed chamber where the relative equilibrium humidity is measured. This measurement is directly related to water activity by dividing the relative humidity by 100 to obtain the decimal figure aw.
The aw of a product may also be examined over time to support product stability. This is of particular importance for multi-use products that may be subject to moisture uptake once the primary container has been opened.
Conclusion
If water activity levels can be achieved and maintained below predetermined levels known to support growth, microbiological and chemical stability of products can be maximized and the potential exists for a reduction in routine microbiological testing.
A sufficient database linking aw results and acceptable microbial stability can support reduced microbiological testing for raw materials and finished products alike. The determination of aw is a rapid and relatively inexpensive tool that can be used to predict the microbiological stability of pharmaceutical products and assist in determining the amount of full scale microbiological testing required.