Understanding the Importance of Water Activity Towards Food Safety Our expertise

While these measurements may sound similar, they are not the same; nor are they interchangeable. Moisture content and water activity are measured for two different purposes. Each test reveals its own insights about the yield, quality, and safety of your foods.

Difference Between Moisture Content and Water Activity

Moisture content is a measurement of the total amount of water contained in a food, usually expressed as a percentage of the total weight. It is used for determining the dry weight of your food and ingredients, as well as to calculate the total yield. It can also be used to confirm whether the drying process of food is completed.

Meanwhile, water activity (a_w) is a measurement of the availability of water for biological reactions. It determines the ability of microorganisms to grow. It is primarily used to determine the necessary food storage requirements and shelf life of your products.

Closer Look at Water Activity

The higher the water activity, the faster microorganisms like bacteria, yeast, and mold will be able to grow. This results in higher standards of food storage.

a_w= P/P₀ where a_w is expressed as the ratio of the vapor pressure of food (P) to the vapor pressure of pure water (P₀) under those same conditions. It predicts whether water is likely to move from the food product into the cells of micro-organisms that may be present.

The water activity is a value between 0 and 1. A value of 0 means there is absolutely no available water, which is very rare in food products. A value of 1 means all water in the product is available, which is pure water. Most foods hover within a range of 0.2 and 0.99 for water activity.

Importance in Food Safety

Microorganisms have a limiting water activity level below which they will not grow. Water activity, not water content, determines the lower limit of available water for microbial growth. The lowest a_w at which most food spoilage bacteria will grow is about 0.90.

Staphylococcus aureus under anaerobic conditions is inhibited at an a_w of 0.91, but aerobically the a_w level is 0.86. The a_w for mold and yeast growth is about 0.61 with the lower limit for the growth of mycotoxigenic molds at 0.78 a_w.

Measuring a_w in Food

Water activity can be measured using a water activity meter. The required amount of sample is placed in a water activity sample vessel and is brought to ambient temperature.

The sample is then placed within the airtight measuring cell of a calibrated water activity meter. Once the atmosphere within the cell has equilibrated, and the digital read-out has stabilized, the a_w value can be recorded.

Precise control of temperature is very important when taking measurements. The equipment should be calibrated at the same temperature that is used for analyzing the samples. The sample to be measured should be uniform.