Water moves. Whether in the air, soil, concrete, pipes, riverbeds, plants, or even a plastic straw, water always seems to be on the go. It’s hard to keep out of places it’s not wanted (basements, for example), but also makes life worth living (running water is important to most folks). Water responds to forces, usually gravity or pump pressure, by moving from high to low potential. This action makes water flow from a tap, tumble down a waterfall, or even migrate from cheese to cracker. Here’s how it works.
In food, water activity measures the potential energy of water in a product. When two products come together with different potential energies, water wants to move from high to low energy areas. This spells trouble for food products like spice blends or layered cookies. Water uptake by a crispy cookie from its filling can make it soft. Spice blends can turn into rock-hard clumps in 10 or 15 minutes. How do you know if this will happen to your product? Measure the water activity of the different components.
Stopping moisture migration requires either a barrier, or reformulating your product. A barrier like plastic works great for packages of cheese and crackers, even if using the little red knife to apply “cheese” to cracker is a little demeaning. Reformulation can be seen on a product like Pop-Tarts® where the breading on the outside is the same water activity as the filling on the inside.