Determining the water activity of a product or ingredient is necessary when conducting a hazard analysis for many products. Knowing the water activity of a food is important when preparing a Hazard Analysis Critical Control (HACCP) plan. Most food has a water activity greater than 0.95 which supports the growth of bacteria, yeast and mould.

However, the water activity of salami is 0.82 and cooked beef is approximately 0.98. For example, salami and cooked beef have similar water content of approximately 60 per cent. It is possible to have products with the same water content but very different water activities. It is easy to assume that foods with higher water content will have a higher water activity than dry foods. This relationship must be determined for each specific food item. The relationship between water content and water activity is complex and related to the relative humidity of the food and its water content. Water content on its own is not enough information to determine food safety or predict product shelf life. Relationship between Water Content and Water Activity Water activity increases with temperature due to changes in the properties of water such as, the solubility of solutes such as salt and sugar, or the state of the food. a w= P/P 0įor example, a water activity of 0.90 means the vapour pressure is 90 per cent of that of pure water. It predicts whether water is likely to move from the food product into the cells of micro-organisms that may be present. Water activity (a w) is expressed as the ratio of the vapour pressure in a food (P) to the vapour pressure of pure water (P 0). If water activity decreases, micro-organisms with the ability to grow will also decrease. It determines the ability of micro-organisms to grow. Water activity is a measurement of the availability of water for biological reactions. Mw = moisture content on a wet per cent basis

It is usually expressed as a percentage of the total weight: Water content, or moisture content, is a measurement of the total water contained in a food. Available water can support the growth of bacteria, yeast and mould, which can affect the safety and quality of food.

Water activity in food preservation