Spanish Blancmange

A cold cream set with isinglass, flavoured only with lemon. The whole thing turns on temperature: the setting liquid and the cream must meet lukewarm, or you get either strings of rubber or a mould that never firms.

• Isinglass to gelatineIsinglass is dried sturgeon swim bladder, sold in leaves or shreds and now largely confined to brewing. Substitute powdered gelatine at about 1 oz (four standard quarter-ounce envelopes) for the roughly three pints of liquid here, which gives a firm set that will stand unmoulded; drop to three-quarters of an ounce if you want it softer on the spoon.
• Loaf sugar and the lemon rindLoaf sugar was refined sugar broken from a cone; ordinary granulated is the equivalent, half a pound being about one cup plus two tablespoons. Rubbing the rind against the sugar lumps draws the peel oils into the crystals — the same principle as an oleo-saccharum — so scrub the whole lemon against granulated sugar in a bowl, or zest it finely into the sugar and rub with your fingers.
• The curdling riskLemon juice going into cream will thicken and can visibly split it. Add the juice slowly with the sugar and water already mixed in, stir constantly, and keep everything cold at that stage; the acidity is what gives this pudding its slight tang.
• "Lukewarm" means lukewarmGelatine dissolved in a quarter-pint of water will be hot; poured straight into cold cream it sets in threads on contact. Let it cool to blood heat, around 100F, then pour it into the cream mixture in a thin stream while stirring, not the other way round.
• Setting timeThe recipe says only "turn out when cold." Allow at least six hours in the refrigerator, and overnight for a large mould. Dip the mould briefly in hot water and invert onto a wetted plate.
• YieldTwo pints of cream, half a pint of water and a quarter-pint more with the gelatine make close to three pints of finished blancmange — a two-pint-plus mould, serving eight to ten. It is very rich; small portions with fresh fruit suit modern palates better.