Yeast is a happy accident. It is all around you, all the time. It’s on the surfaces you touch every day. It’s on the packages you get delivered and on the skin of those you come in contact with. It’s in the air. You can also buy it in the supermarket (sometimes).
But the road from “all around you, all the time” to supermarket shelves encompasses most of human history. Somehow, we coaxed an invisible creature out of the world around us and into our food. In its naturally occurring form, yeast is a single-celled fungus that pops up everywhere. If you’ve seen the sourdough starters that have bubbled and risen throughout social media feeds, you’re seeing natural yeast at work, gnawing on water and flour and giving off carbon dioxide that causes that mixture to grow.