If you head to the dictionary to sort out the difference between frosting and icing, it might not be of much help. Merriam-Webster defines icing as “a sweet flavored usually creamy mixture used to coat baked goods … called also frosting.” Its definition for frosting? “Icing.”
From a culinary standpoint, however, the two sweet coatings aren’t identical. As The Pioneer Woman explains, frosting is typically thicker and fluffier, thanks to its most seminal ingredient: fat. This is often butter, cream, or cream cheese—which explains why you probably hear people refer to “buttercream frosting” and “cream cheese frosting” more often than they say “buttercream icing” or “cream cheese icing.” Because frosting is thicker, it’ll stay in whatever shape you decide to spread it in. That quality makes it ideal for using on cakes and cupcakes, as well as between cake layers.
Icing, by contrast, is more runny, less fluffy, and sometimes translucent (especially before it dries). While frosting is defined by its inclusion of fat, icing has a bigger focus on sugar—usually powdered sugar combined with water (and other ingredients). It’s great for decorating cookies, since you can pipe out a thin layer that’ll harden once dry. As Allrecipes points out, not all icing is runny. Rolled fondant—which is commonly found on wedding cakes and other specialty cakes—is a thick mixture of sugar, water, and corn syrup (and sometimes gelatin, among other things) which you can roll and cut much like you would with a dough. Because of its sugar-and-water base, it’s still considered icing.
So where does glaze fit in? It’s also a combination of sugar and liquid, so it’s more closely related to icing than frosting. It’s the thinnest of the three—so runny that it’s poured over a dessert, rather than piped or spread. According to Real Simple, glazes may get stiffer when dry, but they don’t typically harden like an icing would. They’re great for drizzling over things like pound cakes and doughnuts.
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