Water activity in food preservation: why some foods are likely to spoil | Cooking and country recipes

Moisture is necessary for the growth of pests, which is why bacteria, molds and yeasts are not active in sufficiently dried foods.

However, the activity of water is not the same as the moisture content. According to an article from the University of California Davis, “Although moist foods are likely to have greater water activity than dry foods, this is not always the case. … Some foods with exactly the same water content have very different water activity.

Water activity is the amount of free water available in food. It is a measure of water that is not bound to components of the food such as sugar, salt or protein, and is therefore available for microbial growth. Water activity is measured by values ​​from 0.0 (sec) to 1.0 (pure distilled water). The lower the value of the water activity, the more “dry” a food is. The higher the activity of the water, the faster microorganisms such as bacteria, yeasts and molds will grow.

Most pathogenic bacteria need a water activity of 0.90, while spoilage molds need at least 0.70 or more to grow. Long-life foods are less than 0.85. For example, bread has a water activity of about 0.95. Therefore, there is enough water available in bread to promote mold growth, unless preservatives are added. Crackers, on the other hand, have a water activity of about 0.50, which is too low, or dry, for mold growth.

Consider some ways in which water activity applies to food preservation. It can be controlled by adding salt or sugar to food. Both salt and sugar work by binding to the free water in the food product so that the water is no longer available for the growth of microorganisms. Salt is more effective at binding water than sugar. Think about salt in pickles or cold cuts.

Water activity can also be decreased by removing water from the food product. Decreasing water can be done by “cooking” a recipe, which allows the water to evaporate. Long cooking jams and marmalades are examples. Water can also be removed by dehydration such 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, around 0.98. Traditional strawberry jam has reduced water activity, about 0.75 to 0.80 – due to the added sugar and the long cooking that evaporates the liquid. Dried strawberries have a very low water activity, around 0.60, because most of the available water is removed by dehydration. From the strawberries example, you can see that the water activity in food can be controlled by adding sugar and / or salt and using time and temperature to remove. free water.

Water activity is more appropriate for predicting the safety and quality of food than the moisture content of a food. Jams contain about 30% water, but this water is related to sugar and is not free to promote the growth of bacteria. Before refrigeration, it was common to keep the jar of jelly open on the kitchen table from one meal to the next. This was possible because the water activity of the traditional jelly is low. Open jars of jellies with little or no added sugar should not be stored at room temperature, as they contain more free water and may promote the growth of microorganisms.

According to the United States Food and Drug Administration, many foods have water activity greater than 0.95, which will promote the growth of bacteria, yeasts, and molds. Research-based protocols for canning, freezing and drying foods take into account the activity of water to control these spoilage organisms.

This article is based on the work of Nancy Wiker, a retired Penn State Extension educator.

If you have a question about food preservation, contact your local Penn State Extension office and they will forward it to the appropriate educator.

The Well Preserved news column is prepared by Penn State Extension.