Yeast: How Bread Rises Quiz
Yeast at work
Yeast is a living thing: a tiny, single-celled fungus. A spoonful of baker's yeast holds billions of them, resting until they get warmth, water and food.
In bread dough, yeast feeds on sugars from the flour. As it does, it gives off bubbles of carbon dioxide gas, along with substances that give bread much of its flavor. This process is called fermentation.
Yeast works best when it's warm, around 25 to 35 °C. In the cold it slows right down, which is why some bakers let dough rise slowly in the fridge overnight for more flavor.
Gluten traps the gas
Wheat flour contains proteins that, when mixed with water and worked, link up into a stretchy network called gluten. Kneading the dough builds and strengthens that network.
The gluten traps the yeast's gas bubbles like thousands of tiny balloons, so the dough puffs up instead of letting the gas escape. Leaving dough to rise is called proofing.
Breads made without gluten, or with flours that have little of it, rise much less, which is why many gluten-free breads use other ingredients to hold the bubbles.
In the oven
In a hot oven the gas bubbles expand one last time, giving the bread a final rise called oven spring. The yeast dies at about 60 °C; as the dough gets hotter still, the gluten and starch set, fixing the bread's airy shape.
The crust turns golden brown thanks to the Maillard reaction, a chemical reaction between sugars and proteins that also browns toast and seared meat.
People have been making raised bread for thousands of years; the ancient Egyptians were already doing it. Sourdough uses wild yeast and bacteria instead of packaged yeast, and quick breads like soda bread use baking soda to make the gas instead.