While most people assume that moisture loss is the main culprit in a stale product, that’s actually not the case. When a product bakes, the starch in the recipe absorbs water and swells. This does a lot for the final texture and quality of the baked good, but it doesn’t last forever. Over time, the water begins to break free from the starch, and the starch reverts back into its natural crystal form, creating the texture we associate with stale bread.
Volatile molecules, which are often the building blocks for the flavor in our food, will begin to evaporate over time, ruining and dulling the flavor of the product. This, like the starch recrystallization, is an inevitable chemical reaction and can’t be stopped. But water loss still is a factor in product staling, and for that some things can be done.
The biggest water-related factors that cause a product to go stale are the moisture content of the recipe and the baking method. Obviously a recipe with more water in it is going to have more moisture, but too much water can ruin the texture of a nice cake or cookie, so simply adding more water is not going to work. Adding sugar is a great way to add moisture, because sugar will absorb water and hold onto it during baking. Often times fat will be added in a recipe to give the perception of moisture, which is why many bakeries add shortening or oil to their cakes. Emulsifiers like soy lecithin will also aid in retaining water.
As for baking method, it’s pretty straightforward: the longer you bake a product, the more moisture you’re going to lose. The heat from the oven causes water in your recipe to turn to steam and rise up and out of the product. You want some of this reaction since it helps your product rise, but too much and you lose water that keeps your baked good moist. Bottom line is don’t over bake your cakes and cookie! Removing them from the oven and letting carry over cooking – heat retained from baking continues to cook your product even out of the oven – finish the job is a good way to avoid too much moisture loss.