[Microbial Contamination] Water Activity vs. Water Content

You probably know that you need to measure and control water activity in your product, but if you truly understand what water activity is, how it differs from moisture content and why it is important to food quality and safety—pat yourself on the back. You are among the relative few who do.

In simple terms, moisture content reflects quantity—the amount of water in a product. Water activity reflects qualities of that water—that is, the degree to which the water is "bound" within a product vs. that which is "free" to be used, i.e., contaminated, by microorganisms.

While understanding of the concept is not widespread even within university teachings today, it has been used since the early days of man to preserve food. One of the most common scenes for Americans to envision is the preservative salting of hunted game by Westward-bound pio-neers. Adding of the humectant did not dramatically reduce the water content of the meat, rather it bound that water so it was not available for microbial growth.

However, it wasn’t until the 1950s that microbiologist W.J. Scott described the availability of water in a food as water activity. As explained in Food Preservatives by Nicholas Russell and Grahame Gould (2003), "Although the term had been used previously, it was Scott who demonstrated the correlation between microbial growth in food and the availability of water in the food medium."

"Scott realized it doesn’t have to do with how much water is there, but with its energy," said Brady Carter, Research Scientist for Decagon Devices.

As evidenced—if not understood—by the pioneers, water activity is controlled through dehydration or adding ingredients, such as sugar or salt, that will interact with and bind the free water. Implementing such controls to lower water activity increases food safety because, said Anthony Fontana, Technical Director of Chemistry for Silliker, "water activity needs to be higher than 0.86 for pathogens to grow and multiply." In fact, control of water activity is, today, a regulated food safety measure and is also known to impact food quality.

In the 2005 Food Code revision, FDA updated the definition of potentially haz-ardous foods to implement criteria based on the interaction of water activity (aw) and pH. "If you want to make a product shelf stable, you can adjust down the water activity or the pH or both," Fon-tana explained. It is such adjustment that enables beef jerky to remain safe at room temperature, while luncheon meats need refrigeration.

Aw Quality Impacts. While food safety benefits from increasingly lowered water activity, there is a point at which a low level begins to impact quality—one effect of which can be chemical reactivity, such as rancidity or browning of a product. Fontana gave the example of a manufacturer’s single-serve vanilla pudding cup. By lowering the water activity, the manufacturer was able to create a product that was shelf stable at room temperature, however it also caused the prod-uct to darken in color. By changing the humectant from high fructose corn syrup to sucrose, the manufacturer was able to maintain a low water activity for stability without having the product discolor.

Another example of quality degradation is that of moisture migration, as detailed by Carter. It is water activity, not water content, that causes moisture migration of foods packaged together, such as cheese-and-cracker sandwiches or fruit-and-flake cereals. The two items can have different moisture contents if the aw is equal. But if one item has a higher water activity, the free water will migrate to the product with the lower aw to effect an equilibrium. In these examples, the result will be dry cheese/soggy crackers or dry fruit/soggy flakes.

Although one would intuitively think that this is caused by varying moisture contents, Carter said, "It is not about amounts, it’s about energies."

Measuring for water activity is today a standard step for safety and quality controls. However, Carter cautioned, "water activity lowering is not a kill step." Lowering of the water activity will stop growth of microorganisms, but it will not eliminate that which was already in the product. For example, he said, Salmonella can survive in very low water activity; then if the water activity is increased for any reason, it would restart its growth.

The author is Managing Editor of QA magazine. She can be reached at llupo@giemedia.com.