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Isinglass, to Clarify
Allow one quart of fresh water, the beaten white of an egg, and two table-spoonfuls of lemon-juice to each quarter of a pound of isinglass; mix these ingredients thoroughly, and put them into a saucepan; take care that the isinglass does not burn at the bottom of the pan, and remove the scum as it rises. Strain, and put aside for use. The isinglass should be quite clear. A number of excellent jellies may be made by extracting the juice from fresh fruit and mixing with it a little isinglass, without boiling. The flavour and colour are both better than when the juice is boiled. The best isinglass will require half an hour's gentle boiling to dissolve it.
This is a component preparation rather than a finished dish: a clear setting stock to be flavoured and moulded later. The hinge is the clarification, and the fact that isinglass, unlike modern gelatin, is meant to be boiled.
- What isinglass isDried collagen from the swim bladders of sturgeon and, later, cheaper white fish such as hake and cod. It is still sold, mainly to brewers and conservators, but it is not a kitchen staple; leaf or powdered gelatin does the same work.
- Strength is not convertible on paperA quarter pound to a quart is a firm jelly stock, but isinglass varies enormously by grade and cut and has no bloom rating, so there is no honest gram-for-gram equivalent in gelatin. If you are working from a modern gelatin, follow that product's own ratio and treat this recipe as background.
- Do not boil gelatinThe half-hour gentle boil here is specific to isinglass, which needs prolonged heat to dissolve. Modern gelatin is bloomed in cold liquid and dissolved below a simmer; boiling it weakens the set noticeably.
- The egg white does the clearingBeaten white and acid are a raft: as the liquid heats, the coagulating albumen traps cloud and fine particles and carries them to the surface. Stir the white in while everything is cold, then stop stirring once it begins to set, or you break the raft up.
- Skim, then strain properlyTake the scum off as it forms rather than at the end. Strain through a jelly bag or several layers of scalded muslin and let it fall by gravity; squeezing pushes the trapped sediment straight back through.
- Two table-spoonfuls of lemon-juiceSpoon measures were not standardised in this period and varied, though a nineteenth-century table-spoon was broadly comparable to the modern one, so somewhere in the region of 30 ml. The acid helps the clarification but also slightly softens the set, which is why the quantity is small.
- The uncooked fruit jellyThe advice at the end is sound and still worth following: set raw pressed juice with dissolved gelatin cooled to blood heat rather than boiling the juice, and you keep the fresh flavour and colour. Note that raw pineapple, kiwi, papaya and fig contain proteases that will digest gelatin and prevent any set at all.
- KeepingClarified stock has no preservative in it. Refrigerate and use within a day or two, or it will sour like any other fish-derived stock.
Isinglass reached English kitchens by way of the Russian and Baltic trade, and the name is generally traced to the Dutch huisenblas, sturgeon bladder, rather than to glass of any kind. Before commercially standardised gelatin, a clear jelly meant either hours of boiling calves' feet or a purchase of isinglass, and the latter was prized precisely because it gave a colourless, almost tasteless set that showed off fruit juice and wine. The same material was, and in some breweries still is, used to fine beer, which is why isinglass appears in nineteenth-century price lists as a chemist's and brewer's commodity as much as a cook's. Its use reflects a period when a competent housekeeper was expected to prepare her own setting agent and judge its clarity by eye, and isinglass persisted in domestic and trade use even after branded packaged gelatine became available.