The Aw of a food is directly proportional to the level of free water it contains. As microbial growth is adversely affected even with a slight reduction in Aw, it has been used, where possible, to retard and prevent growth mainly of spoilage bacteria in food. It is achieved either by removing or binding the free water. At lower Aw,microbial cells can lose viability and become injured with time. However, this alone cannot be used to ensure safety of food from pathogens. Although as a single method it has limited application in food preservation, along with other methods, such as low temperature or low pH, it can be used to extend the shelf life of food very effectively. The effectiveness of low pH, either alone or with other methods, in controlling microorganisms in food is discussed in Chapter 35