A packaged cupcake has cake with 9 % moisture content and icing with 13.5% moisture content. What will happen to water in the cupcake as it sits on the shelf?
The answer is...who knows?
The moisture content values make this a trick question, because moisture content doesn't predict the direction of moisture migration. Water moves towards equilibrium, from high energy towards lower energy. Before you can solve the question, you need to know the energy status of the water in each component.
In this case, even though the frosting has a higher water content, it has a lower energy status. Icing has a high sugar content, and sugar "binds" water chemically, lowering its energy and inhibiting its movement. Water in the cupcake is not as tightly "bound," so the cupcake has a higher energy status. That means that over time the cupcake will dry out as water moves out of the cupcake and into the icing. Watch this video to see water activity in action.
Food creates many similar story problems. Will this protein bar mold on the shelf? Will these nuts break during shipment? Where's the sweet spot between dry enough to be safe and moist enough to be tasty? The best way to answer these kinds of safety and quality questions is by measuring the energy status, or water activity, of the water in the product.
If the energy status, or water activity, of a food is below a certain level, certain microbes simply can't grow in the product. Water activity is a measure of microbial susceptibility accepted by the U.S. Department of Agriculture and a key component in many hazard analysis and critical control point plans.
USDA guidelines state: "A potentially hazardous food does not include . . . a food with a water activity value of 0.85 or less."
Safe water activity levels are the same for every food.
A simple chart shows water activities below which certain molds will not grow. And there's just one chart, whether you are measuring the water activity of brie or of dog food.
For a strict scientific definition of water activity, read Water Activity Defined >>