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Plain Boiled Icing
1/4 cup Karo, Red Label 1 cup granulated sugar 1/2 cup boiling water Few grains cream of tartar 1/4 teaspoon desired flavoring Combine all the ingredients except flavoring, and boil until the mixture threads, 238 degrees F. Cool till tepid, flavor, beat until creamy and spread on the cake, using a knife which has been dipped in boiling water.
A sugar-syrup icing of the fondant family, made without egg white, so its whole character depends on hitting the right temperature and beating at the right moment. Everything else is secondary.
- The hinge: 238°F, then waitUse a candy thermometer and ignore the "threads" description, which in practice reads lower than soft-ball stage. Once the syrup reaches 238°F, take it off the heat and do not stir or beat it until it has cooled to about 110°F, or it will grain into a gritty mass.
- Karo Red LabelKaro sold its syrups by label colour: Blue Label was the crystal-white grade, while Red Label had a colour and flavour of its own and is probably nearer what is sold today as dark corn syrup. Either light or dark corn syrup will serve as a stand-in; the dark will tint the icing and push the molasses note forward.
- Why the corn syrup and cream of tartar are thereBoth work against crystallisation, by different routes: the corn syrup contributes glucose and dextrins that get in the way of sugar crystals forming, while the acid of the cream of tartar inverts some of the sucrose as it boils. Either helps keep the finished icing smooth rather than sandy, and "few grains" is probably something on the order of 1/8 teaspoon; an excess tends to leave the icing soft.
- Keep the pan sides cleanCover the pan for the first minute or two of boiling so steam washes sugar crystals off the sides, or wipe them down with a wet pastry brush. A single stray crystal dropped into the cooling syrup will seed the whole batch.
- BeatingBeat with a wooden spoon or a mixer on medium until the syrup turns opaque, thickens and just loses its gloss. It goes from pourable to unspreadable quickly; stop while it still flows off the spoon in a thick ribbon.
- The hot knifeDipping the spreader in boiling water melts the surface slightly and gives a smooth, glassy finish. Wipe the blade between passes so water does not pool on the cake.
- Weather mattersOn a humid day this style of icing stays tacky and may never fully set. Boiling the syrup a degree or two higher in damp weather is the usual remedy; better simply to make it on a dry day.
- YieldRoughly a cup of icing: enough to cover the top of a two-layer cake, or a single layer, or a sheet cake thinly. It sets with a thin crust and is best used the day it is made.
Boiled icings were a common American cake finish in the era of cooked sugar frostings: sugar, water and a little acid, cooked to a measured stage and beaten to opacity, with or without egg white to lighten it. Naming a brand and a label colour in the ingredient list, rather than simply calling for "syrup", suggests a text written to promote a product as much as to instruct: Karo appeared in the early 1900s and was sold in colour-coded grades, and corn-syrup booklets circulated widely to teach cooks that glucose syrup did jobs cane sugar alone could not. The chemistry it demonstrates is real enough, and still the reason bakers add a spoonful of corn syrup to caramel and fondant today.