How to Invent Everything: A Survival Guide for the Stranded Time Traveler

by Ryan North

Yeast are single-celled organisms found all over the world; they’re floating through the air you’re breathing in any period you can survive in, and you’re going to farm some particularly suited to whatever grains you have. Here’s how: First, blend together a mixture of your flour and water, with about twice as much flour as water. Cover it, put it someplace warm, and check it every twelve hours. You’ll be looking for bubbles, which are signs of fermentation: in other words, signs some wild yeast colonized your flour mixture and is now feeding on it. Once you’ve found fermentation, throw away half of your culture and replace what you removed with some fresh 2:1 flour/water mixture. This gives your remaining yeast new food, and provides evolutionary pressure on yeast that can feed off your flour quickly. You, my friend, are selectively breeding yeast that go crazy for whatever type of flour you’ve got on hand. After about a week you should have yeast that reliably generates a bubbly froth—not unlike the head of a beer—after each replenishment. Now you have a yeast farm!* Keep this culture going for as long as you please by feeding it more of your flour-and-water mixture every day (or once a week if you can keep it refrigerated; see Section 10.2.4).