The Performing Seal

• Although most bakers tend to think of icing as a decorative element or a way to make the cake even richer and tastier, it also serves a very practical purpose. Icing makes an airtight seal around the cake's surface, just as if it was coated in plastic. This prevents the cake from losing its moisture to evaporation, keeping it moist and pleasurable. European bakers often take the extra step of drizzling the cake with flavored syrups or liqueurs before they're iced, knowing that the added moisture will infuse through the entire cake without evaporating.

Putting On Your Coat

• The light, delicate texture of most cakes means they're fragile, and crumbs can easily pull away from the surface as you spread your icing. To avoid having crumbs come to the surface, most bakers coat their cakes with a thin layer of icing, called a crumb coat. Thin your icing with a small amount of milk to loosen it and make it more spreadable, then use it to cover the cake's entire surface. You don't need much, just a thin layer like the butter on your sandwich. The smoother and more even it is, the easier it will be to make your final layer of icing smooth and even as well.

Why You Don't Wrap It

• Once your crumb coat is in place, the next step is to wait while it dries and forms a crust at the surface. This firmly immobilizes the crumbs, and makes it easier for you to spread your final coat of icing. If you're preparing your cake ahead of time, it might seem prudent to wrap it in plastic to keep it fresh until you finish decorating it. That isn't usually necessary because the crumb coat will keep the cake from drying. Even worse, wrapping your cake prevents the crumb coat from drying as it should. That means when you come back to your cake, you'll still need to wait for the crumb coat to set before you finish decorating.

And Why You Might

• Still, there are some specific reasons for wrapping a crumb-coated cake. If your cake has perishable fillings, leaving it out on the counter overnight isn't really an option. Neither is freezing a crumb-coated cake for later use without any protective wrappings. The key in either case is to let the crumb coat set before you wrap it up. When the icing has dried and crusted, sheath it carefully in two layers of plastic film wrap, or in wrap and a heavy-duty freezer bag. To minimize the risk of damage in your refrigerator or especially your freezer, put the wrapped cake inside a rigid food-storage container of the appropriate size. Let the cake come to room temperature for at least an hour before you continue decorating it.