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Potato Yeast
Boil some good mealy potatoes until they are quite soft. Peel and weigh them, and while they are hot mash them till they are smooth. Mix them quickly with as much boiling water as will make the mash of the consistency of good beer yeast. Rub the puree through a sieve, and add for every pound of potatoes either two ounces of treacle or an ounce of brown sugar or honey. Boil the mixture until it is as thick as batter, and stir into it whilst it is warm two large spoonfuls of yeast for every pound of potatoes. Cover with a cloth, and keep in a warm place till it has done fermenting. In twenty-four hours it will be ready for use. It will be necessary to use twice as much of this yeast as of beer yeast to make bread. Sufficient, a pound of potatoes will make a quart of yeast.
This is a leaven, not a dish: a potato barm that stands in for the brewer's yeast a Victorian cook could once beg from the local brewery. It works, but the temperatures matter more than the book lets on.
- The hinge: temperature when you pitchThe mash is boiled thick and the yeast stirred in "whilst it is warm" — warm here means blood heat, around 95-100F. Much hotter and you risk damaging the culture, which is killed outright somewhere above about 120F; pitch into a scalding mash and you will get a sour, gluey potato paste instead of a working barm.
- Treacle and its substitutesTreacle is British molasses; use unsulphured molasses, or the stated ounce of brown sugar or honey per pound of potatoes. The sugar is food for the yeast, not sweetening, so the choice affects flavour only slightly.
- "Beer yeast"The two large spoonfuls of starter meant barm skimmed off fermenting ale — a live, liquid culture. A tablespoon of any active sourdough starter, or a scant half-teaspoon of instant dried yeast slurried in warm water, will do the same job of seeding the mash.
- "Two large spoonfuls" is vagueVictorian spoon measures were not standardised; read it as roughly two tablespoons of liquid barm. With modern dried yeast far less is needed, since dried yeast is many times more concentrated than skimmed brewery barm.
- Mealy potatoesFloury, high-starch varieties — russets or Maris Piper. Waxy potatoes will not break down into the smooth puree the recipe wants and give a sluggish ferment.
- The warm placeAbout 75-80F is right. A modern kitchen is often cooler than a Victorian one with a range going all day; an oven with only the light on works well.
- Yield and keepingA pound of potatoes gives roughly a quart. It is a live, unpreserved culture with little acidity to protect it: use it within a few days, refrigerated, and discard it if it smells cheesy, acetone-like or off rather than yeasty and sweet.
- Using it in breadThe note to use twice as much as beer yeast is sound — potato barm is weaker. Expect long, slow rises measured in hours, and judge by the dough's volume rather than by any clock time in an old bread recipe.
Before packaged yeast, a domestic baker's leaven came from whatever was fermenting nearby — usually barm skimmed from an ale brewery, which was free or nearly so in towns but unreliable in the country and liable to turn bitter with hops. Potato yeast, along with hop yeast and various "everlasting" barms, filled that gap: a potato mash, fed with treacle, sugar or honey, made a serviceable medium for keeping a culture alive from one baking to the next. Recipes of this kind fill the bread chapters of British and American household books throughout the nineteenth century and then vanish, as compressed yeast spread in the later nineteenth century and dried yeast, a mid-twentieth-century product, arrived cheap and shelf-stable. Such a leaven was treated as basic equipment — something a cook was expected to keep going, like a stock pot or a crock of dripping.