What is the ideal water activity level for bacterial growth in food?

Bacterial growth in food is most conducive at a water activity level of 0.85 or higher. Water activity (aw) is a measure of the available water in a food product that bacteria and other microorganisms need to thrive.

When the water activity level is at or above 0.85, it provides sufficient moisture for pathogens like Salmonella, E. coli, and Listeria to grow and multiply. Levels below this threshold, such as 0.5, 0.75 to 0.80, and even 1.0 can limit or control bacterial proliferation, making these conditions less favorable for growth. For example, while 1.0 represents pure water, it sets no restriction on the nutrients for microbial growth; thus, the ideal threshold starts at 0.85 to ensure a high risk for bacterial growth in food safety contexts.