Yeast

Yeast taught the fermentation exactly, because it was a quiet marvel.

"I'm a fungus — a single cell, smaller than you can see — and I eat the sugars in the dough," Yeast said. "When I break that sugar down for energy without much air around me, I let off two things: a little alcohol, which mostly bakes away in the oven, and carbon dioxide gas. That gas is the point. It bubbles up inside the dough — and here's the clever part — the dough is stretchy, full of a springy protein called gluten, so instead of the gas escaping, it gets trapped in thousands of little pockets. Each pocket puffs the dough out a little. Multiply that by millions of me, working for a couple of warm hours, and the whole loaf swells." It patted the risen dough. "Fermentation makes the gas. The stretchy dough catches it. Between us, we make bread rise."

The older students heard the subtext: the lightness in a good thing often comes from the room left for air, not from piling more in.

Yeast told how it once mistook slowness for doing nothing.

"When I was new I fretted," Yeast admitted. "I'd get to work in the dough, and for an hour — nothing. The dough just sat there looking exactly as heavy as before. I was sure I was failing." It smiled. "But I wasn't. I was making bubbles the whole time — they were just too small and too few to see yet. The rising was happening under the surface long before it showed. If a baker had given up on me at minute ten and thrown the dough out, they'd have thrown away a perfect loaf that simply hadn't shown itself rising yet." Yeast dusted off its hands. "Slow isn't stopped. In the warm dark, patient and unseen, the change was already underway."

A student asked whether yeast was the same helpful trick as Lacto's souring.

"Cousins!" Yeast said, delighted. "We're both fermenters — both making energy from sugar without needing air — but our leftovers differ, and the leftover is the gift. Lacto's leftover is lactic acid, which sours and preserves. Mine is carbon dioxide gas, which lifts and lightens. Same family trick, two different by-products, two different foods — her tang, my rise. And sometimes we work side by side: in sourdough, Lacto's kind gives the sour flavor while my kind gives the loft, in the very same bowl." It grinned. "People have baked with us for thousands of years without knowing we were alive. They just knew that if you kept a bit of yesterday's lively dough and mixed it in, today's would rise too. They were keeping us alive, passing us hand to hand down the generations."