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Syrup Clarified with Isinglass
Allow half a pint of water to each pound of sugar, and a quarter of an ounce of Russian isinglass to every six pounds. Pour the water over the sugar, and let it stand till dissolved. Dissolve the isinglass in boiling water, and add it to the syrup. Let the mixture cool, then put it on the fire, and as the scum rises remove it. When quite clear it will be ready for use. Syrup thus prepared may be used for preserving fruit.
A clarifying method rather than a dish: the isinglass coagulates on heating and carries suspended impurities to the surface as scum, which is skimmed off. With modern refined sugar the whole operation is essentially obsolete, but the proportions still tell you how to build a preserving syrup.
- The measuresCassell's is British, so a pint is 20 fl oz: half a pint to the pound of sugar is about 1 1/4 US cups liquid to 1 lb sugar. A quarter ounce of isinglass to six pounds of sugar is roughly 7 g to 2.7 kg — a very small amount, and precision matters less than dissolving it fully.
- What isinglass isDried collagen from the swim bladders of fish, sturgeon in particular; "Russian isinglass" was the premium grade, traded out of the Caspian sturgeon fishery. It is still sold for fining beer and wine, though leaf gelatine behaves similarly in this role.
- Why refined sugar makes this pointlessThe method exists because nineteenth-century loaf and moist sugars carried molasses residues, plant debris and mineral matter that clouded a syrup. Modern granulated sugar is 99.9 per cent sucrose and dissolves water-clear, so there is nothing for the isinglass to collect.
- The hinge: cool before heatingThe instruction to let the mixture cool before putting it on the fire matters: the isinglass is dispersed while cold so that it can coagulate into a raft as the syrup warms slowly. Adding it to already-hot syrup would give it less chance to spread through the liquid first.
- Skim, do not boil hardBring the pan up slowly and take the scum as it forms. A rolling boil breaks the coagulated raft apart and stirs the impurities back through the syrup, which undoes the work.
- Strength of the finished syrupHalf a pint of water to a pound of sugar gives a heavy syrup, in the region of 60 per cent sugar — appropriate for preserving fruit, where the syrup must draw water out of the fruit rather than dilute into it. It is too sweet for sorbet or for poaching delicate fruit without thinning.
- Yield and keepingSix pounds of sugar with three pints of water yields roughly six imperial pints, about 3.4 litres, of finished syrup. Store it refrigerated in a clean sealed jar; a heavy syrup keeps for weeks, but discard it at any sign of cloudiness, film or fermentation.
- If you avoid fish productsIsinglass is an animal product and the syrup is not vegetarian. Egg white was a common alternative in period cookery books, whisked into the cold syrup and skimmed the same way.
Isinglass reached British kitchens as an import, chiefly through the Russia trade, and the "Russian" grade named here was the best regarded — sturgeon bladders from the Volga and Caspian fisheries, dried and sold in sheets or shreds. Before gelatine was manufactured cheaply, isinglass was the household setting agent for jellies and blancmanges as well as the standard fining for beer, wine and cider, and its use to clear a sugar syrup follows the same logic as fining a cask. That a general cookery dictionary should devote an entry to clarifying syrup at all says something about the sugar of the period: cooks bought loaf sugar to be broken and pounded, or moist sugars still carrying molasses, and a cloudy syrup would have spoiled the look of bottled fruit in a glass jar.