Several years ago, I was reading the US Food and Drug Administration technical guidance (as one does), and happened across the following: “If the water activity (aw) of food is controlled to 0.85 or less in the finished product, it is not subject to the regulations of 21 CFR Part 1081.” I hope you will agree that this is a very exciting sentence.Why? The term “activity” is, like fugacity, one of the ways we describe the thermodynamic behavior of compounds in mixtures. Was the FDA really talking about the ratio of component-to-standard-state-fugacities in a document aimed at clarifying packaging of cheese spread? Surprise, yes! The thermodynamic concept of aw is the bedrock “why?” of everything from the high salt and sugar content of processed food to chips going stale to the continually surprising fact that “creme-” filled baked goods are fine without refrigeration.