I’ve attempted your Lemon Layer Cake (see related recipe) a few times. The fluffy white icing always looks thick and stable going onto the cake, but after a short while it turns soft and runs down the sides. What am I doing wrong?
The frosting for this cake is a variation on the classic seven-minute icing. Egg whites, sugar, lemon juice, and corn syrup are mixed together and then warmed gently over simmering water. The mixture is then whipped to stiff peaks before being applied to the cake, where it should remain stable for a full three days.
We wondered if your problem might stem from underheating the egg white mixture before whipping it. We created two batches of icing, heating the egg white mixture in one to the 160 degrees specified in the recipe and the other to just 140 degrees. An hour after whipping both batches, the 140-degree icing was a soupy mess, while the 160-degree sample was picture-perfect.
Here’s why: As egg whites and sugar are agitated, the tightly coiled egg proteins temporarily unravel and cross-link with each other, allowing air to be trapped inside the matrix. Heating the egg whites before whipping causes the proteins to permanently unwind and cross-link, so the icing holds its shape—but only if the eggs are heated to a high-enough temperature of 160 degrees. If the mixture has not been sufficiently heated, the network isn’t as stable and will eventually break down, turning the icing runny.
To ensure that your icing lasts as long as the cake, make sure to stick your thermometer deep into the center of the egg white mixture. Verify the temperature by stirring the mixture and taking a second reading.