Buttercream icing is a classic for frosting and decorating cakes for any occasion. There are four basic kinds of buttercream -- two meringue types that use only egg whites, French buttercream using egg yolks and whole egg buttercream. But most bakers today rely on adaptations of buttercream's core ingredients of butter, sugar, liquid and eggs. Temperature and the ingredients in your buttercream dictate how it should be stored.
Most buttercream icings made with butter and egg whites or egg substitutes -- called meringue buttercreams – perform best at room temperature. They can be kept out for up to three days – either in a bowl or on a cake -- covered or in a cool, dry place. Butter begins to separate at temperatures between 88 and 98 degrees Fahrenheit. This is not a health concern, but on a hot day, your buttercream can slide off the cake if the fat separates from the whey. Buttercream made with either 100 percent or partial shortening stands up to heat better with a melting point around 106 F, which is why buttercream made with shortening is a popular choice for decorating.
Confectioner’s sugar is a main ingredient in buttercream icing and is made from either cane sugar or sugar beets. Humid climates can negatively affect the texture of buttercream icing left at room temperature. Using confectioner’s sugar made from sugar cane ensures consistent quality.
Some recipes for buttercream call for a tablespoon or two of milk or cream, while others use water to make a sugar syrup. Because of the large amount of sugar, the U.S. Food Safety and Inspection Services say that buttercream frosting made with small amounts of milk are safe to leave out at room temperature for two to three days. Buttercream made with cream cheese, however, is not safe to leave out and should be refrigerated.
Both Swiss and Italian meringue (or mousseline) buttercream is made by whipping egg whites and adding to a sugared syrup that has been cooked to 240 F. This technique yields a stable buttercream suitable for cake decorating and can be stored at room temperature for two to three days. According to Food Safety.gov, frosting made this way is safe to consume, though you can purchase pasteurized egg whites or substitute meringue powder if Salmonella risk is a concern. French buttercream, sometimes called neoclassic buttercream, is made with egg yolks or whole eggs and should be refrigerated because it can separate if kept at room temperature for longer than six hours.