What is the water activity of pure water?

Water activity is a measure of the availability of water for microbial growth and chemical reactions in a substance. It is expressed on a scale from 0 to 1, where pure water has a water activity value of 1.0. This means that in pure water, there is complete availability of water molecules, without any solute molecules that would bind to water and reduce its availability for biological processes.

In contrast, solutions or mixtures with solutes typically have water activity values that are less than 1.0, because the presence of solutes reduces the amount of free, available water. Therefore, pure water is uniquely positioned at the maximum water activity value of 1.0, indicating that it is the optimal environment for biological activity, given that it provides the most free water molecules.

This value is critical for understanding how different environments can affect microbial growth and the stability of various biological materials.