A premium cake maker was formulating a recipe for a cream filled cake. The components of the recipe were frosting (about 7% moisture), cream filling (12%) and cake (15%).
The maker worried that the different water contents of the ingredients might cause moisture migration during shipment and on the shelf—stale cake, rubbery frosting, liquefied cream filling bleeding into the cake. Wary of a creating a second-rate product that would ruin the brand, he was determined to formulate to assure high quality over the full shelf life span.
Tricky Problem: Moisture Migration
Intuitively, it seemed like the cake would dry out over time as it lost water to both the frosting and the filling. But moisture migration is a tricky thing, and before starting an extended trial-and-error development process for the cake, the formulators decided to have us produce isotherms for each ingredient.
They were surprised to discover that the frosting—the driest ingredient—had the highest water activity. Water activities of the cream and the cake were pretty similar—0.66 and 0.61 respectively.
Predicting aw of the Final Product
We transformed the isotherms to chi plots to determine what the water activity of the final product would be after it came to equilibrium (for more information about this model, see the Dry Ingredient Mixing section in Fundamentals of Isotherms).
This gave values to what the isotherm had already predicted—as the cake came to equilibrium inside the package, the dry frosting would get drier and the cake and cream would both wet up. The water activity of the final concoction would be 0.67, a microbially safe value for the cake.
Avoiding Unpleasant Surprises
The cake maker went on to bake and taste test the cake at equilibrium water activity (0.67). The isotherm’s “predict before you mix” convenience helped him avoid the surprise and frustration of the cook-and-look method.
Chi plot of isotherm data for snack cake components showing linear fits to data.