Moisture is necessary for the growth of spoilage organisms, which is why bacteria, molds and yeasts are not active in sufficiently dried foods.
However, water activity is not the same as moisture content. According to an article from the University of California Davis, “Although moist foods are likely to have greater water activity than are dry foods, this is not always so. ... Some foods with exactly the same moisture content have quite different water activity.”
Water activity is the amount of free water that is available in food. It is a measurement of the water that is not bound to components in the food such as sugar, salt or protein, and therefore is available for microbial growth. Water activity is measured in values from 0.0 (bone dry) to 1.0 (pure distilled water). The lower the water activity value, the more “dry” a food item is considered. The higher the water activity, the faster that microorganisms like bacteria, yeast and mold will be able to grow.
Most pathogenic bacteria need a water activity of 0.90, while spoilage molds need at least 0.70 or higher for growth. Shelf-stable foods are below 0.85. For example, bread has a water activity of about 0.95. Therefore, there is enough water available in bread to support mold growth, unless preservatives are added. Crackers, on the other hand, have a water activity about 0.50, which is too low, or dry, for mold growth.
Consider some ways that water activity applies to food preservation. It can be controlled by adding salt or sugar to foods. Both salt and sugar work by binding with free water in the food product so that water is no longer available to microorganisms for growth. Salt is more effective at binding water than sugar. Think of the salt in pickles or in cured meat.
Water activity may also be decreased by removing water from the food product. Decreasing water can be done by “cooking down” a recipe, which allows water to evaporate. Examples of this are long-cooking jams and marmalades. Water can also be removed by dehydration as in drying fruits, vegetables and meats (jerky).
Strawberries, Jellies and Jams
Let’s use strawberries to illustrate each of the above points. Fresh strawberries have a high level of available water, about 0.98. Traditional strawberry jam has a reduced water activity, about 0.75 to 0.80 — because of added sugar and long cooking that evaporates liquid. Dried strawberries have a very low water activity, about 0.60, because most of the available water is removed by dehydration. From the example of strawberries, you can see that water activity in foods can be controlled by the addition of sugar and/or salt and by the use of time and temperature to remove free water.
Water activity is more suitable for predicting food safety and quality than the water content of a food. Jams contain about 30% water, but that water is bound to sugar and is not free to support the growth of bacteria. Before refrigeration, it was common to store the opened jelly jar on the kitchen table from one meal until the next. This was possible because the water activity of traditionally made jelly is low. Opened jars of low- and no-sugar added jellies should not be stored at room temperature, because they contain more free water and can support the growth of microorganisms.
According to the U.S. Food and Drug Administration, many foods have a water activity above 0.95, which will support the growth of bacteria, yeast and molds. Research-based protocols for canning, freezing and drying foods take water activity into account to control these spoilage organisms.
This article is based on work by Nancy Wiker, a retired Penn State Extension educator.
If you have a food preservation question, contact your local Penn State Extension office and they will forward it to the proper educator.