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YEAST, POTATO (another way)
Boil potatoes of the mealy sort till they are thoroughly soft. Skin and mash them very smooth, and put as much hot water on them as will make a mash of the consistency of common beer yeast, but not thicker. Add to every pound of potatoes two ounces of treacle, and when just warm, stir in for every pound of potatoes two large spoonfuls of yeast. Keep it warm till it has done fermenting, and in twenty-four hours it will be fit for use. A pound of potatoes will make nearly a quart of yeast, which has been found to answer the purpose so well, as to render indistinguishable the bread made with it from bread made with brewer's yeast.
This is a leaven, not a dish: a home substitute for brewer's barm, made by feeding a live yeast a potato-and-treacle mash. It works, but a few points in the original are easy to get wrong.
- The hinge: temperature"When just warm" means blood heat or a little below, roughly 80-95F. Potato mash holds heat stubbornly in the centre, so stir it and check with a thermometer; much hotter than blood heat will damage the yeast you are adding, and at around 130F and above it is killed outright, leaving you with sour mush instead of a leaven.
- TreacleBritish treacle here means black treacle or a dark cane syrup, not golden syrup. Unsulphured molasses is the closest supermarket equivalent. Two ounces per pound of potatoes is about three tablespoons.
- "Two large spoonfuls of yeast"The starter culture is any live yeast: barm from a brewer, a spoonful of an established sourdough or previous batch of potato yeast, or in modern terms about a teaspoon of active dry yeast rehydrated first. A "large spoonful" in this period is roughly a modern tablespoon, so the original is asking for a generous inoculum.
- Mealy potatoesRussets or another high-starch baking potato. Waxy varieties will not mash to the thin, pourable consistency the recipe wants.
- Consistency"The consistency of common beer yeast, but not thicker" means a thin, pourable cream, about like heavy pouring batter. Err on the loose side; a thick mash traps gas and ferments unevenly.
- Fermenting and finishingKeep it at 75-85F, loosely covered so gas escapes. It will foam vigorously, then subside; when the froth has fallen and it smells beery rather than sweet, it is ready. The stated 24 hours is about right at that temperature and much longer in a cold kitchen.
- Keeping itThe book assumes prompt use. Refrigerate as soon as fermentation slows and use within a few days. Potato mash is low in acid, and a starter left warm and sweet for long periods can be taken over by spoilage bacteria: if it turns slimy, pink, or smells of cheese or vomit rather than beer, discard the batch rather than baking with it.
- Yield and use in breadA pound of potatoes gives close to a quart, as the recipe says. Quantities in use are not stated: begin with something under a cup of this liquid leaven for a two-loaf batch, reduce the water in the dough accordingly, and adjust from there. Expect a slower rise than commercial yeast, so judge by the dough, not the clock.
Before packaged yeast, home bakers depended on barm skimmed from brewing or from a baker who would sell it, and both were unreliable in a household without a brewery nearby. Potato yeast, which appears in English and American cookbooks throughout the nineteenth century in dozens of near-identical versions, solved that problem: cooked potato starch and a little sugar in the form of treacle give a yeast population enough food to multiply into a fresh, sweet leaven every few days. The claim at the end of this entry, that the bread is indistinguishable from brewer's-yeast bread, is the standard selling point in these recipes and hints at the reputation barm had for carrying bitterness from the hops. The reliable compressed and dried yeasts of the later nineteenth and twentieth centuries made the whole practice unnecessary, and it survives now mainly among bakers keeping potato-based starters by choice.