A thick layer of frosting acts as an airtight seal that keeps cupcakes moist and prevents them from going stale.

The process of bread or cake going stale is scientifically known as starch retrogradation. This occurs when the amylose and amylopectin molecules in the flour, which gelatinized during baking, begin to expel water and reform into a hard, crystalline structure. By applying a dense fat-based barrier like buttercream, bakers physically block the evaporation of water from the crumb to the surrounding air.A study published in the Journal of Food Science highlights that moisture migration is the primary driver of texture degradation in baked goods. Frosting functions as an edible sealant with low water vapor permeability. This is particularly effective with high-fat frostings such as Swiss meringue buttercream or chocolate ganache, which provide a more hydrophobic surface than simple sugar glazes.To maximize this effect, the frosting must be applied so it touches the edges of the cupcake liner or the sides of the cake. This technique is often used by professional pastry chefs to extend the shelf life of products without adding chemical preservatives. Research into food preservation suggests that maintaining a high relative humidity within the cake structure can delay the onset of staleness by up to 72 hours compared to exposed sponge.