When it comes to food safety and quality, there are two critical measurements that all food manufacturers must be concerned with: moisture content and water activity.
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 (aw) 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.
aw= P/P0 where aw 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 aw at which most food spoilage bacteria will grow is about 0.90.
Staphylococcus aureus under anaerobic conditions is inhibited at an aw of 0.91, but aerobically the aw level is 0.86. The aw for mold and yeast growth is about 0.61 with the lower limit for the growth of mycotoxigenic molds at 0.78 aw.
Measuring aw 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 aw 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.
Now that we understand the role of water activity in terms of food safety, it is important for food manufacturers to correctly manage and measure it to produce better quality food. At DKSH, we provide industry-proven solutions to fit your business needs for water activity measurement tools. Reach out to us to learn about these solutions.
Potchara Sungtong is the Director, Food and Beverage for DKSH Thailand overseeing the Asia Pacific region. With a background in food science and microbiology, he has over 20 years of experience in research and development, sales and marketing, channel management, and business development in food and beverages. Potchara has extensive knowledge in food safety, customer requirements, laboratory workflows, and lab efficiencies.