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The Best Mode of Melting Isinglass
1001. To melt a quarter of a pound of isinglass, take a little more than a pint of water, into which throw in the twelfth part of the white of an egg; beat the water well till it becomes white; throw the isinglass into that water, and lay it on the stove over a very slow fire. If it is kept covered, it will melt more easily. Take care it does not burn, for then it can never be made clear, besides it would have an unpleasant taste. For a larger quantity put more water, but not more white of egg. If the isinglass is required to be particularly clear, squeeze into it the juice of a lemon.
A base preparation rather than a finished dish: isinglass was the standard setting agent for jellies, creams and blancmanges before commercial gelatine sheets became common. The whole point of the method is clarity, and everything in it serves that end.
- The hinge: never let it boilThe hinge: keep the heat very low -- "a very slow fire" means the lowest heat you have, with the liquid steaming rather than bubbling. The original's own warning is against burning: scorched isinglass sticks to the pan, and as the recipe says, it can then never be made clear and takes on an unpleasant taste.
- What isinglass isDried collagen from the swim bladders of sturgeon and other fish. It is still sold, mainly to brewers and conservators, but for kitchen use modern leaf or powdered gelatine does the same job. Setting strength is not interchangeable ounce for ounce, so go by the gelatine packet rather than by this weight.
- The quantitiesA quarter pound is 4 oz, about 115 g, dissolved in a little over an imperial pint, so somewhere around 600 ml. That gives a very strong stock solution, of the kind a cook would draw on in small amounts rather than use as it stands.
- A twelfth of an egg whiteAround half a teaspoon. Beating it into cold water before the isinglass goes in disperses the albumen; as it heats it coagulates and traps the fine suspended matter that would otherwise fog the jelly. Do not scale it up with the water, as the original says.
- Lemon juiceThe acid helps the albumen bind and drop out cloudiness. Add it only if the finished jelly is meant to be tart or fruit-flavoured, since it will carry through to anything delicate such as a cream.
- Straining and keepingThe coagulated egg needs to be left behind, so pour the melted solution through a scalded cloth or fine sieve. It will set firm on cooling; keep it chilled and re-melt gently over hot water when needed.
Isinglass reached British kitchens largely by way of Russia, where sturgeon bladders were dried and pressed into the sheets and "staples" sold by grocers and druggists; the name is generally traced to a Dutch term for sturgeon bladder rather than to any glass. Through the eighteenth and nineteenth centuries it was a favoured setting agent for clear jellies and moulded creams, prized because, properly handled, it gave a brighter and less flavoursome set than calf's-foot stock, which nonetheless remained in use alongside it. Brewers used it for the same clarifying property, to fine beer. Instructions like this one, on melting rather than on any finished dish, are a reminder that a cook of this period bought her gelatine raw and had to make her own working solution -- a step the packaged leaf and powdered gelatines of the later nineteenth century quietly abolished.