Boiled Icing
2 egg whites 1 cup sugar 1/4 cup water Put the sugar and water into a dish which has a closely fitting cover, and set it on the stove to boil. Have the egg whites in a bowl ready to beat. Boil the sugar and water with the cover off the dish till a long, hairlike thread hangs from the spoon when dipped into the sirup and lifted, or to 244 degree F. Set the dish off the stove and put the cover on tightly while you beat the whites stiff. Then pour the hot sirup in a small stream into the whites, beating continuously, and beat till it becomes cool enough and stiff enough to spread on the cake without running off.
If properly made, this will be stiff enough to stay in place, but not too stiff to spread smoothly, even if it should take some few minutes to spread it on the cake.
One egg white is generally used for one cup of sugar, and the sirup boiled to 238 degree F., but by using two egg whites and boiling the sirup a little thicker, an icing is made which will not become too hard to spread before it is all spread on the cake. It makes a smoother and nicer frosting.
This is an Italian meringue frosting, and the temperature given in the original is the whole recipe. A thermometer removes all the guesswork the thread test was invented to handle.
- The hinge: 244FTake the syrup to 244F (firm-ball) as written, not lower. The book explains its own reasoning clearly: two whites plus a slightly thicker syrup gives an icing that stays spreadable for several minutes instead of setting up under the spatula.
- Why the lid goes onThe recipe boils uncovered and only covers the pan once it is off the heat, while you beat the whites. Trapped steam condensing on the sides will wash down stray sugar crystals, which may help guard against a grainy icing; it also keeps the syrup hot. A pastry brush dipped in water does much the same job.
- The thread testThis book treats a long, hairlike thread hanging from the lifted spoon as its indicator for 244F (and for 238F in the one-white version). Note that 'thread stage' in standard candy terms usually means a cooler syrup, so treat the description as this author's rule of thumb; use it as a cross-check if you like, but trust the thermometer.
- Pouring in the syrupWith a stand mixer, run it on medium and pour the syrup down the inside of the bowl in a thin stream, avoiding the whisk itself, which will fling hot sugar to the sides where it hardens. By hand this needs two people.
- Beating to coolExpect 5 to 8 minutes of beating after the syrup is in. The bowl should feel no more than barely warm before you stop; icing spread while still hot will slump.
- Egg safetyThe hot syrup partly cooks the whites as it is beaten in, so this is not a straight raw-white frosting. It is not, however, a guaranteed pasteurisation: the finished meringue may not hold 160F throughout, so use pasteurised whites if you are serving anyone vulnerable.
- HumidityBoiled icing is sensitive to damp air. On a humid day it takes longer to set and may stay tacky; if the kitchen is steamy, consider buttercream instead.
- YieldTwo whites and a cup of sugar make roughly 3 cups of icing: enough to cover a two-layer 8-inch cake fairly thinly, or to top a dozen or so cupcakes.