Most frosting hardens over cake. In fact, a professional looking cake depends upon it. At times, though, your frosting might become too firm in the refrigerator, so gentle microwaving or additional liquid might be required to soften the frosting and whip it back into shape. While softening hard frosting is simple enough, knowing whether it will form a hardened sugar crust ideal for piping is another matter. Fat type, stabilization and refrigeration play crucial roles in forming a sugar crust. Additionally, the correct sugar-to-fat ratio also helps frosting harden over cake.
Sugar to Fat
Frosting that has a higher ratio of sugar to fat, approximately 2 pounds of confectioners sugar to every 2 cups of butter or shortening, will develop a sugar crust. Therefore, even if a frosting has been softened by means of microwave heat, it will still form a crust over the cake. Professional bakers rely on the hard, crisp surface that forms a crumb coat on the cake surface prior to applying the final frosting. Additionally, a sugar crust on the final frosted layer will hold piped decorations and allow for a more stable surface if removing piping errors is required.
Keep It Cool
Gentle heat and additional milk, cream or water reconstitutes hardened frosting into a soft state ideal for frosting a cake. Yet when adding summer heat and humidity into the mix, your frosting and decorations may droop over time, never hardening enough to hold their shape. Rather than adding additional cream, milk or water to soften the frosting, gently heat the frosting in the microwave for about 10 seconds and whip. Once the cake is frosted, store it in the refrigerator, covered, allowing the fat to solidify and the sugar to crystallize and form a stable and smooth crust.
Butter vs Shortening
Butter begins to melt at 88 to 98 degrees Fahrenheit, and solid shortening begins to melt around 106 F. Many professional bakers choose shortening instead of butter for their butter-cream frosting because of the higher melting point of shortening. Softened frosting made with shortening offers more stability when it comes to hardening on a cake. However, butter just tastes better than shortening, so many bakers use a one-to-one ratio of butter to shortening for taste and stability.
Stabilizing Your Frosting
Both powdered meringue and cornstarch add body and stabilization to the frosting. Warm rooms and summer heat may contribute to soft frosting that won’t remain stable. Adding a stabilizer, such as cornstarch or meringue powder, to softened frosting will help harden the frosting even in warmer conditions. Add approximately 2 tablespoons of cornstarch or meringue powder to every 4 cups of confectioner’s sugar and 1 cup of butter or shortening.