Lacto and Yeast

"I go first, in a way," said Lacto. "I ferment the sugar and my leftover is lactic acid. That acid does two things in this bowl. It makes the sour — that clean tang that makes sourdough taste like sourdough. And it makes the dough a little acidic, which keeps unwelcome microbes out and even helps the dough's texture." She stirred her part in. "If you had only me, you'd get a tangy, well-guarded dough — but a flat one. Delicious and dense. Tasty, and heavy as a stone. I bring the flavor and the safety. I do not bring the lightness."

"Which is where I come in," said Yeast. "I ferment the very same sugar, but my leftover is carbon-dioxide gas. My gas bubbles up and gets caught in the stretchy dough, thousands of pockets of it, and the loaf swells and turns light." Yeast puffed a little in demonstration. "But if you had only me — just the lift, no tang — you'd get a pale, airy, rather plain bread. Light as a cloud and about as interesting. I bring the rise. I do not bring the flavor, or the keeping. Between the two of us: her tang and safety, my air and lift. The sourdough is both, and it is better than either of us could make alone."

The student frowned, working it out. "So you're not doing the same job. You're doing opposite halves of one job."

"Now you have it," they said together, delighted. "We're both fermenters — both making energy from sugar without air, both cousins in the same family trick. But the leftover is the gift, and our leftovers are different: acid and gas, tang and lift." Lacto tipped her head. "People fermented food with the two of us for thousands of years before anyone knew we were alive. They just knew that a bit of yesterday's lively starter, kept warm and fed, would sour and raise today's dough. They were keeping both of us alive at once, in the same jar, passing us down the generations together."