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Potato Yeast
Have in readiness a kettle containing three quarts of boiling water; peel and thoroughly wash three large potatoes and place them into a basin to soak. Next put half a teacupful of flour into a bowl with an equal amount of sugar and a tablespoonful of salt; over these grate the potatoes quickly and stir the whole well with a wooden spoon so that it will not become of a dark color, which it will attain if not done very quickly. Pour a pint of the boiling water over the grater into the bowl, stirring it into the mixture, and add enough more of the boiling water to make the mixture of the consistency of thin starch. Should the preparation refuse to thicken put in a double boiler over the fire and stir it constantly until it does thicken; then pour it through a fine sieve into a bowl and let it cool. When lukewarm all through stir in a breakfast cupful of yeast, place a light covering over the bowl and set in a warm but not hot place and let it remain there until it is light and covered with a white foam. When it commences to rise beat it well and when it has thoroughly risen pour it off into wide-mouthed earthen jars, let it remain for twelve hours, then cover the jars over tightly and put in a cool place. Half a pint or so of the yeast should be put into a glass jar for making the succeeding lot of potato yeast. The jars should always be well shaken before the yeast is used and should be kept in a cool place.
This is a barm — a liquid leaven kept alive from batch to batch, propagated on grated raw potato and sugar. It presumes you already have a working yeast of some kind to start it, and that you will bake often enough to keep it going.
- The hinge: temperature at pitching"Lukewarm all through" is somewhere around blood heat, roughly 95-105F. Yeast is not actually killed until well above that, nearer 130-140F, but pitching into a mixture that is still noticeably hot will weaken or kill it, and the batch may sour instead of rising.
- Old cup measuresA teacupful is roughly 3/4 cup; a breakfast cupful is about 1 cup to 1 1/4 cups. So: about 6 tablespoons flour, the same of sugar, 1 tablespoon salt, three large potatoes, and a good cup of active yeast or previously made potato yeast.
- You need a starterThe "cupful of yeast" is not optional and is not wild yeast; it is a cup of a live leaven. A previous batch of this same potato yeast is what the recipe assumes, and an active, well-fed sourdough starter or packet yeast will also start one.
- Grate fast, and whyRaw grated potato browns within minutes through enzymatic oxidation (polyphenol oxidase). Grating directly into the flour-sugar-salt mixture and stirring at once coats the shreds and limits their contact with air; the recipe's insistence on speed is about color, as it says itself.
- "Consistency of thin starch"Think heavy cream: pourable, faintly glossy, not gluey. The boiling water is presumably meant to gelatinize the potato starch on contact; if the mixture refuses to thicken, the recipe's double-boiler step finishes the job.
- Cool it strainedThe fine sieve takes out the coarse potato fiber. Cool the strained liquid promptly, uncovered or loosely covered, rather than letting it stand warm.
- Do not seal earlyThe twelve hours before the lids go on let carbon dioxide escape. A tightly closed jar of actively fermenting barm will build pressure and can push its lid or crack; glass canning jars with the ring only finger-tight are safer than the earthen jars specified.
- Storage and life span"A cool place" meant a cellar or icebox; use the refrigerator. The book gives no keeping time; expect roughly a week or so of usefulness, refreshed by making a new batch from the reserved half-pint. Shake before measuring, since the starch settles hard. Discard any batch that smells of acetone, cheese, or rot, or that grows colored mold rather than white foam.
- How much to useThe book does not say. As a rough starting point, about a cup of this liquid yeast to several cups of flour, and it works slowly -- plan on a long or overnight rise rather than an hour.
Potato yeast, or potato barm, was standard equipment in American households through most of the nineteenth century, when bakers' yeast was neither reliably available nor reliably alive. The potato supplies gelatinized starch that the yeast can work on and the sugar gives it an immediate meal; keeping back half a pint to inoculate the next batch is the same logic that keeps a sourdough culture going. Commercial compressed yeast, sold in the United States from about 1868 onward, gradually made the practice less necessary, though home potato yeasts kept appearing in cookbooks well into the twentieth century — worth printing because many readers still baked from a jar in the cellar rather than a cake bought at the grocer's.