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Lemon Sponge
1066. Take half an ounce of isinglass, dissolve it in a little boiling water, then take the juice of eight lemons, and put sugar to your taste; whisk it together until it becomes a sponge, then wet the mould and put it in; when set turn it out. The mixture ought to be - nearly cold to whisk well.
A whisked lemon jelly rather than a baked cake: the setting agent is beaten into a foam as it thickens, so it sets full of air. Everything depends on catching the mixture at the right stage of cooling.
- The hinge: whisk at the syrupy stageWhisking a hot mixture achieves nothing, and a cold one has already set. Let it stand until it is just beginning to thicken at the edges of the bowl and moves sluggishly, then whisk hard until it has trebled in volume and holds a ribbon on the surface.
- IsinglassDried sturgeon swim bladder, once the standard clear setting agent. Half an ounce of it corresponds roughly to a scant half-ounce (12-14 g, about two standard envelopes) of powdered gelatine or six to eight leaves, set to soften in cold water first, then dissolved without boiling.
- Allow for the acidLemon juice weakens a gelatine set, so this is not the place to reduce the gelatine. Err on the generous side, or the foam will collapse into a froth and a puddle.
- Quantities to work withEight lemons give roughly 1 to 1 1/4 cups (250-300 ml) of juice. That much neat juice is very sharp; some later versions dilute it to about a pint (500 ml) total liquid with water and sweeten with 3/4 to 1 cup of sugar. Dissolve the sugar in the warm liquid before cooling.
- "Wet the mould"Rinsing the mould with cold water and pouring off the excess leaves a film that helps the set turn out cleanly. Allow for the whisked volume when choosing the mould, or divide between individual glasses. To unmould, dip the outside briefly in warm water.
- Setting timeThe original gives none. Refrigerate at least four hours, preferably overnight.
- Whisking equipmentWhat was ten to fifteen minutes of hand work is three to five minutes in a stand mixer on high. Use a bowl much larger than the volume of liquid suggests, since the mixture roughly trebles.
- YieldServes four to six. Serve the day it is made; whisked jellies begin to weep and separate after about 24 hours.
Lemon sponge belongs to the large nineteenth-century family of "shapes" -- jellies, creams and flummeries turned out of moulds and set on the table as a cold sweet. Isinglass, made from the swim bladders of sturgeon and other fish, was a favoured fine setting agent of the period, valued because it set clear and had almost no flavour of its own; commercial packet gelatine gradually displaced it in domestic kitchens over the later nineteenth century. Whisking the jelly as it cooled produced the pale, opaque, spongy texture that gave the dish its name.