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Yeast, Potato
Boil any quantity of good potatoes in their jackets. When done enough, peel, weigh, and mash them quickly, and with each pound mix a quart of boiling water. Rub the mixture through a hair sieve, and add an ounce of honey or brown sugar to each pound of potatoes. Boil the mixture till it is as thick as batter; pour it out, and when lukewarm, stir into it a large table-spoonful of solid yeast to each quart of water. Cover the mixture with a cloth; it will be ready for use in twenty-four hours. Double the quantity of this yeast as of beer yeast will be needed. The next time yeast has to be made, use a bottle of this preparation (first pouring off the thin liquid from the top) instead of fresh yeast. Fresh yeast will, however, be required every two months.
This is not a dish but a leaven — a home-propagated liquid yeast for bread, of the kind kept going in kitchens before reliable packaged yeast. It works, but the vocabulary and the keeping instructions need translating.
- What "solid yeast" meansIn this period solid or compressed yeast is fresh cake yeast, sold moist and crumbly; brewers' "beer yeast" was the liquid barm skimmed off fermenting ale. A tablespoon of fresh yeast is roughly half an ounce, equivalent to something like 1½ teaspoons of modern instant dried yeast.
- The proportionsOne pound of peeled boiled potato to one quart (4 cups) of boiling water, plus 1 oz honey or brown sugar per pound of potato. A pound of potato is about two large russets.
- Hair sieveA woven horsehair sieve, used because it did not react with acids or discolour food. A fine-mesh nylon or stainless sieve does the same job; press the hot potato through with a ladle, or use a food mill.
- "Lukewarm" is the hingeStirring the yeast into liquid that is still hot will kill it. Wait until the batter is at blood heat, no more than about 100F, before adding the yeast; potato batter is thick and holds heat far longer than the surface suggests.
- The 24-hour riseTwenty-four hours assumes a cool larder. In a warm modern kitchen at 70F or above it is likely to be active and foaming well before that, and may smell sharp and acetic if left the full day.
- Do not seal it tightThe instruction to cover with a cloth is deliberate — an active ferment produces a great deal of carbon dioxide. If you bottle it for keeping, use a loose cap or a jar with the lid resting on, not screwed down.
- Substituting it in bread"Double the quantity of this yeast as of beer yeast" means where a recipe calls for a given measure of liquid barm, use twice as much of this. Reduce the water in your dough by the volume of starter you add, or the dough will be slack.
- Keeping it goingPouring off the thin liquid from the top and using the sediment is the same principle as feeding a sourdough starter. The two-month limit may reflect real drift: a domestic ferment kept going by backslopping gradually picks up lactic and acetic bacteria, and the loaves get sour and sluggish.
- YieldA pound of potato and a quart of water gives somewhat less than a quart once boiled down to batter thickness — enough for several large loaves, with a bottle held back for the next batch.
Potato yeast belongs to a family of domestic leavens — hop yeast, salt-rising bread, milk emptyings, sugar-and-flour barms — that households relied on before dependable commercial yeast reached the grocer. Bakers and brewers had long shared barm, the frothy skim from fermenting beer, but barm was perishable, unpredictable in strength, and simply not available to cooks far from a brewery, which is why cookbooks of the middle and later nineteenth century print several rival methods for making yeast at home. Potato appealed because the boiled starch is readily fermentable, cheap, and gives a smooth batter that holds a colony better than plain flour and water; the honey or brown sugar is there to give the yeast an immediate meal. It is infrastructure — the thing you had to have before you could bake at all. Compressed fresh yeast, sold in reliable cakes from the later part of the century onward, made instructions like these steadily obsolete, and starters of this kind survived mainly in rural kitchens and, later, in the revival of sourdough baking.