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Boiled Frosting
One cup of granulated sugar; one-fourth cup of boiling water; white of one egg; one-half teaspoon of vanilla. Place the sugar and water in a small saucepan set in another containing boiling water, and boil for six minutes. Do not stir the sugar at all, or it will granulate. Beat the egg stiff, and gradually add to it the boiling syrup, pouring the latter in a thin stream on the egg, and stirring rapidly. Beat for five minutes after the last has been added, and flavor to taste.
This is an Italian meringue in all but name, and the flaw is in the vessel: a pan set inside another pan of boiling water cannot take sugar syrup past the boiling point of water, which is well short of what this technique needs.
- The double boiler is the problemSet in boiling water, the syrup tops out around 212F no matter how long it cooks. A boiled frosting depends on syrup concentrated to the soft-ball stage, roughly 238-240F, which is what both sets the frosting and heats the egg white enough to be safe. Six minutes over water will not get there.
- The egg white is never cookedBecause the syrup cannot exceed the boiling point of water, and loses further heat as it is poured in a thin stream into a beaten white, the finished frosting is essentially raw egg white, and a frosted cake was ordinarily kept at room temperature. The role of Salmonella in shell eggs was not understood when this was written.
- It will not set eitherA syrup at 212F is still mostly water. The likely result is a thin, weepy foam that slides off the cake and stays tacky rather than drying to a firm crust.
- "Do not stir" is soundThe one instruction worth keeping is the warning against stirring. Agitating a boiling sugar syrup seeds crystallisation and turns the frosting grainy. Modern practice adds washing down the pan sides with a wet brush.
- QuantityOne cup of sugar and one white gives roughly enough to cover a single 8-inch layer, or the top of a small loaf cake.
- If you want this frostingUse a current recipe that specifies a candy thermometer and either a syrup temperature in the soft-ball range or pasteurised egg white.
Boiled frosting, also called boiled icing or white mountain frosting, was the standard American cake covering through the second half of the nineteenth century and well into the twentieth, before buttercream became the default. It is economical: sugar, water and a single egg white will cover a cake, and confectioner's sugar was neither as fine nor as reliably available as it is now. The Butterick Publishing Company, better known for dress patterns, issued its cook book as part of a broad line of domestic manuals aimed at the same middle-class housekeeper who bought its patterns, and the boiled frosting recipe reflects that audience: no thermometer is assumed, and the double boiler is called for. The technique of pouring hot syrup into beaten whites is the same one that produces Italian meringue and, taken further, nougat and divinity.