Water activity and pet food are a natural match for each other. For starters, pet food with a water activity of above 0.7 can mold in the bag. Moisture content can’t predict this problem. That’s really the key to using water activity in pet food – “prediction”.
Things happen that make pet food go bad in the bag: vitamin degradation, loss of texture, microbial growth, and lipid oxidation. All of these reactions have something in common: they are related to water. But the amount of water in the product doesn’t matter – only the energy level (i.e. water activity) affects how quickly these reactions take place. This allows for a prediction of what will happen in a bagged pet food product rather than just making the product and hoping it turns out o.k.