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Magnum Bonum Plum Jam
The magnum bonum is the largest of our plums, and is excellent for jam and for other purposes. Choose the fruit when fully ripe. Take off the skins, remove the stones, and boil the plums gently for forty minutes, keeping them well stirred all the time to prevent their burning; add three pounds of sugar in lumps for every four pounds of fruit, and boil ten minutes longer. Three or four minutes before the jam is taken from the fire, add a quarter of the kernels blanched and sliced. Probable cost of plums, 6d. per dozen.
The jam itself is a plain long-boil preserve, but the finishing touch — blanched plum kernels stirred in at the end — is the reason this one should be read rather than cooked.
- The kernels are the problemPlum stones contain amygdalin, a cyanogenic glycoside that can release hydrogen cyanide when the kernel is crushed, chewed or wetted. Three or four minutes in boiling jam cannot be relied on to remove it, and the sliced kernels are then eaten. Use a current plum jam recipe; almond flavour in preserves is now usually taken from almond extract, which does not carry cyanogenic glycosides.
- How much is "a quarter of the kernels"The recipe gives no count. Magnum bonums are large plums, so four pounds might yield somewhere around thirty to forty stones, which would put "a quarter of the kernels" at roughly eight or nine sliced into the batch -- an estimate, not a figure from the book. Either way it is not a trace amount, and the recipe gives no way to judge how bitter, and therefore how amygdalin-rich, a particular kernel is.
- Sugar ratio and keepingThree pounds of sugar to four of fruit is about 43 per cent of the combined raw weight; the proportion in the finished jam is higher, since the fruit is boiled down before the sugar is added. It is still low by modern preserving standards, where roughly equal weights of fruit and sugar are usual to reach a sugar concentration high enough to inhibit mould and yeast. Victorian jams of this kind were commonly sealed under paper, and surface mould was expected to be skimmed off.
- "Sugar in lumps"Lump or loaf sugar was cut from a refined cone and was the cleanest sugar available; it was preferred for preserves because it clouded the syrup less. Nothing in the method depends on the shape.
- Skins off, forty minutesThe plums are skinned and stoned before cooking, and the long unsweetened boil is doing the work of reduction rather than a quick set with the skins in. Expect a soft, thick jam rather than a bright modern set.
- The price noteSixpence a dozen for plums places this in a British kitchen where fruit was priced by the dozen; the quantities assume fruit bought or picked in some volume.
The Magnum Bonum — Latin for "great good" — is the large yellow egg plum, known variously as White Magnum Bonum, Yellow Egg, or in France as Dame Aubert, and it was grown in English gardens for centuries as a culinary rather than a dessert plum: too coarse and acid to eat raw, superb stewed or preserved. The instruction to blanch and slice a portion of the kernels into the finished jam belongs to an older confectionery habit of using stone-fruit kernels as a cheap source of almond flavour — the same idea behind ratafia and noyau — a practice generally traced to the days before commercial almond essence, and one that fell away in most kitchens as the cyanogenic chemistry of the kernels became understood, though kernel flavouring persisted in some preserving and confectionery traditions.