The concept of using water activity (a_w) as a means of controlling foodborne illness in the retail food industry is a relatively recent addition to the applied science of food safety. As part of the regulatory community, I can safety say that it really wasn’t even a blip on the radar screen until some 20 years ago. I guess we sanitarians have always used it but in a rather circuitous way. For example, as part of my state-issued inspection “tool kit,” I was given a salinity refractometer. When I encountered manufactured brined foods not requiring refrigeration, I would take a measurement with the instrument; record it in a notebook and use those data, albeit speculative, as some sort of a standard when I found a similar food prepared in a restaurant I was inspecting. This was limited to only to those foods that lent themselves to this type of analysis. However, it seemed to work, even in the absence of defining a particular set of circumstances, including such factors as pH.

We also used a Brix refractometer to measure the specific gravity of foods containing sugar; these crude subjective data were sometimes the basis for suggesting alternative food safety measures to the operator.

In the mid-1990s, the first truly portable water activity measuring device significantly broadened the range of foods we could assess in the field for potential risk of foodborne illness. It rapidly became a valuable must-have tool, particularly for those of us who worked with correctional kitchens. The burgeoning incarcerated population put a strain on all foodservice equipment, specifically refrigeration. By measuring the a_w of the various foods kept “on ice,” we could assess many foods that heretofore were deemed on the cusp of “potentially hazardous” and were treated as worst-case scenario. It eliminated collecting samples, carrying them to the laboratory and waiting for results. In real time, we could now maximize valuable refrigeration space for those foods most sensitive to temperature controls. The science and accompanying technology proved invaluable.

Regulatory Impact of a_w
It has only been the last decade that water activity had a real impact on regulatory criteria. And, it has been only a few short years that water activity has had any impact in the way we go about our retail regulatory business. From all that I’ve read, using water activity as a microbiological control finds its grounding in parts and paragraphs of Good Manufacturing Practice (GMP) regulations from Title 21 of the Code of Federal Regulations. Expanding on this, the use of water activity in relation to control measures and food safety comes in the form of in-process detection through Hazard Analysis and Critical Control Points (HACCP). Here, water activity is specifically targeted as a microbial control measure during the production process. This was followed in short order by both the U.S. Department of Agriculture (USDA) and its Food Safety and Inspection Service (FSIS) including water activity as a means of microbial control in their Generic HACCP Model 10, Heat-treated, Shelf-stable Meat and Poultry Products.

Earlier versions of the FDA Food Code stipulated what science already knew: that below a certain water activity level (a_w 0.85) most foods would not support pathogenic bacterial growth. The concept of water activity and food safety were merged as a standard when in November 2000, NSF International issued ANSI/NSF 75-2000, Non-potentially hazardous foods. The purpose of this standard is “… to serve as a communication tool between manufacturers of product, retailers, and public health officials.” It provides test methods and evaluation criteria to allow for the determination that a food product meets FDA Food Code criteria for a “non-potentially hazardous food” and does not require refrigeration for safety.

Finally, about four years ago, the Conference for Food Protection made the giant leap through its recommendation to include a_w and pH in the latest iteration of the Food Code. The 2005 Food Code considers the interaction of a_w and pH under certain conditions of heat treatment and packaging. This synergism controls or eliminates pathogens that would otherwise be ineffective when used alone. Additionally, heat treatment that destroys vegetative cells and the effect of packaging, which prevents recontamination, is now widely considered in the evaluation of foods offered to the public.

The NSF standard and Food Code together ushered in a new era in which the definition of potentially hazardous food was dramatically expanded. It gave the sanitarian broader latitude in assessing risk of foodborne illness beyond the more traditional time/temperature controls. There are some new interpretation issues proffered by the FDA on water activity and enforcement that I heard about at a recent meeting. The FDA has yet to formally detail and disseminate them to industry and the regulatory community. They did, however, sound promising.

Yet, with all this information whirling about us, we often act as if we’ve just entered the dawning of the Age of Aquarius, with complete innocence and oblivious to all the work that has being done and gone before. I’m still called upon to arbitrate a battle of wits stemming from a violation cited for not placing the croutons, raisins, sunflower seeds and artificial bacon bits on ice in a salad bar, or compelling a short order cook to keep butter and bacon at “safe” temperatures and to maintain the peanut butter, mayonnaise, jams and table condiments refrigerated at all times. So that we can avoid these minor storm clouds, greet the sunshine and ensure that we’re all singing from the same hymnal, here is the definition and some of the wonderful benefits attributed to the measurement of a[W] that we can take to the field with us.

Definition of Water Activity
My simple definition is that water activity is the relative humidity of food. Here’s the one that good science gave us: a_wis the relative availability of water in a substance. It is defined as the vapor pressure of water divided by that of pure water at the same temperature; therefore, pure water has a water activity of exactly one. As a rule, when the ambient temperature or the temperature of the food increases, a_w typically decreases, except in some salt or sugar solutions. Higher a_w substances tend to support more microorganisms than foods with a lower a_w. Consider in evaluating foods that bacteria usually require at least a water activity of 0.91, whereas fungi require a water activity of at least 0.7. Consider too that water tends to migrate from high a_w substances to low a_w substances.

Water Content Alone is Not a Reliable Predictor 
We often confuse “water activity” with “moisture content” and “water content.” Here’s the differentiation between these terms: Water in products or ingredients focus on moisture or water content, which is a quantitative or volumetric analysis that determines the total amount of water present. Whereas, moisture content determination is essential in meeting product nutritional labeling regulations, specifying recipes and monitoring food manufacturing processes. If using only the water content values, it’s impossible to know how “available” the water in the product is to support microbial growth or influence product quality. Available water is another name for water activity.

The water content of a safe product varies from product to product and from formulation to formulation. As an example cited in much of the literature on this topic: One safe, stable product might contain 15% water while another containing just 8% water is susceptible to microbial growth. Although the wetter product contains proportionally more water, its water is chemically bound by other components, making it unavailable to microbes.

Let’s look at it a little differently, water activity is sometimes described in terms of the amounts of “bound” and “free” water in a product. These terms fail to define all aspects of the concept of water activity. For instance, free water is not subjected to any force that reduces its energy. Therefore, all water in food is bound water. Water content includes water that is chemically bound and unavailable to microbes. Microbial and chemical processes are related to this bound energy status in a fundamental way. Because water is present in varying energy states, analytical methods that attempt to measure total moisture in samples don’t always agree or relate to safety and quality.

Taking it one step further, the issue is not whether water is bound, but how tightly it is bound. Water activity is a measure of how tightly water is bound in relation to the energy required to remove water from the system. There are several factors including osmotic, matrix and capillary action that act together and control water activity in a system. It is a combination of these factors in a product that reduces the energy of the water, which in effect, has water activity somewhere between chemically bound water (theoretically, 0.0) and free water (1.0).

Water Activity and Shelf Stability
Water activity, unlike water content, can determine a food’s shelf stability. It can predict which microorganisms will be potential sources of spoilage and infection (the difference between bacterial pathogens and fungal physiology, or, an a_w of 0.91 versus that of 0.70). The water activity of a food is instrumental in maintaining its chemical stability. Consider that water activity is partially responsible for minimizing non-enzymatic browning reactions and spontaneous autocatalytic lipid oxidization reactions; prolonging the activity of enzymes and vitamins; and optimizing the physical properties of products such as moisture migration, texture, flavor, odor and shelf life. Not bad for a little relative humidity measurement. Every retail food establishment needs to know what will happen to their products as they sit on the shelf, even under ideal conditions of temperature and humidity. Shelf stability means the product “won’t get moldy,” but it also affects the food’s texture, moisture migration and caking and clumping.

So, in the greater scheme of things, water activity and its measurement in the field does considerably more than provide the ability to judge a food with a thermocouple thermometer. Water activity opens the door to both food safety and food quality assurance.