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Plum Jam
Have ready twelve pounds of large ripe plums peeled and divided into halves, crack their stones, blanch the kernels and pound them in a mortar. Put the parings and cracked stones into a pan with the juice of some very red beet-root, which has previously been pressed out in a mortar, and six pints of water. Boil this until it is reduced to half the original quantity, and then strain it through a fine wire sieve; put the fruit in a preserving-pan with the strained liquor and pounded kernels, and thirteen and one-half pounds of coarsely-crushed loaf sugar, place the pan over a moderate fire, and stir until it is reduced to a stiff jam, then turn it into pots and let it stand for several hours; when quite cold, sift into each jar a small quantity of caster-sugar, and cover with rounds of paper dipped in beaten egg, tie securely and put them away for future use.
This is a Victorian preserving formula that deliberately incorporates the kernels from inside the plum stones, and it also relies on a sealing method that no longer meets any standard for shelf-stable preserves. It should be read rather than made.
- The kernels are the problemPlum kernels contain amygdalin, which releases hydrogen cyanide when the kernel is crushed and wetted. The recipe calls for the kernels of twelve pounds of plums to be blanched, pounded and stirred into the pan with the fruit, where they cook only for as long as it takes the jam to stiffen. That is a large and entirely unmeasured dose of cyanogenic material distributed through a preserve that will be eaten a spoonful at a time over months.
- Boiling the stones does not fix itBoiling is not a reliable fix -- The stones are cracked and the kernels lifted out before the shells and parings go into the water, so that liquor carries little kernel material. Boiling does drive off some hydrogen cyanide, which is volatile, and heat destroys the enzyme that liberates it, but the pounded kernels are stirred in at the jam stage in an unmeasured quantity, and the method cannot be relied on to remove the hazard.
- Why nobody noticedKernel flavouring was standard in nineteenth-century confectionery and the quantities in a small tart or a bottle of ratafia were modest. Scaled to a twelve-pound batch of jam with all the kernels retained, this recipe sits well outside anything a present-day cook should attempt.
- The seal is not a sealRounds of paper dipped in beaten egg, tied over the jar rim, were a standard Victorian cover. They exclude dust and slow surface mould in a cold larder; they do not produce a vacuum or an anaerobic seal. Preserves kept this way often grew a skin of surface mould, which was commonly scraped off.
- Loaf sugar and caster sugarLoaf sugar was refined sugar sold in a hard cone and broken up as needed, equivalent to ordinary granulated. Caster sugar is the finer grind, sifted over the cold surface here as a drying layer to discourage mould.
- The beetrootThe pressed beet juice is purely a colourant, added to deepen a jam made from peeled plums that would otherwise set a dull straw colour. It contributes no flavour worth chasing and no setting power.
- If you want plum jamUse a current tested recipe with a measured sugar ratio and proper water-bath processing. Plum jam is one of the easiest preserves there is without any of this.
Oscar Tschirky's Cook Book of 1896 gathered a great deal of European household practice alongside the Waldorf's own service dishes, and this jam belongs squarely to the older tradition. The use of plum and apricot kernels as a flavouring is very old: the almond-like note they give is the basis of noyau, ratafia and the crème de noyaux still made from stone fruit pits, and confectioners of the period treated kernels as a free bonus lying inside the fruit they were already stoning. The beetroot juice belongs to the same era of frankly cosmetic cookery, when a preserve was expected to look jewel-coloured on the shelf and cooks reached for beet, cochineal or burnt sugar to get there. Blanching and pounding the kernels in a mortar, then simmering the cracked stones for a stock, is the sort of thrift-and-flavour move that reads as ingenious until you know what amygdalin is; the toxicity of stone-fruit kernels was understood in outline by the nineteenth century, but the dose in a domestic recipe was rarely considered.