Water Activity Analysis

Scientists analyze water for bacterial activity.

Scientists analyze water activity for bacteriological growth. This analysis is useful for determining how likely it is that bacteria will grow in a particular water source, such as water-based food. Scientists use special tools to determine the rate of bacteriological growth in water sources. This information is helpful when food producers are deciding how best to preserve food items.

Definition

Water activity is a measurement used to determine which portion of the water is ideal for bacteriological growth and how quickly bacteria will grow on the water. Scientists measure water activity (Aw) using the formula Aw = vapor pressure of a solution @ T in Celsius / vapor pressure of pure water @ T in Celsius. T represents the value of the temperature. In other words, to measure water activity, scientists measure the vapor pressure of the water taken from the water source at a particular temperature and measure the vapor pressure of pure water at the same temperature, then divide the first reading by the second.

Instruments Used

To measure water activity, scientists use an electronic device to measure vapor pressure. The device measures the pressure of aqueous solutions and gives an approximate reading. Microbiol.org describes one such device, the AquaLab CX-2, which measures condensation on a mirror to find the optimal temperature for measuring water activity, then performs the calculation and gives a reading that is accurate within .003 Aw.

Physical Preservation and Water Activity

According to Microbiol.org, water activity is a reliable predictor of microbiological growth in water. Food producers can use this information to determine how to best preserve food items. They pack food in water that has low water activity; they also use this information to determine whether to add preservatives to food.

Preservatives and Water Activity

If water activity is not low enough to keep bacteria from growing on food, producers can add preservatives to lower the water activity. Preservatives that lower water activity include alcohol, sugar, calcium salt, glycerin, gum and protein.

Water Activity Range

Water activity levels can range from 0.0 to 1.0. The higher the activity rate, the more quickly bacteria grow. Microbiol.org says that mold grows in water with an activity rate of 0.61 to 0.94, while yeasts require a level of 0.62 to 0.92, and food-spoiling bacteria such as salmonella require a level of 0.91 to 1.00.

References

• Microbiol: Water Activity (Aw ) Analysis as a Predictor of Microbiological Growth Potential in Personal Care Products