Explainer #12
How does yeast make bread rise?
Yeast is a tiny living fungus, and one spoonful holds billions of them. In bread dough they eat sugar from the flour and give off gas, the same gas as the fizz in soda, plus a little alcohol. Kneading turns flour and water into a net of stretchy strands that traps the gas in tiny bubbles. Over an hour or two the bubbles grow and the dough rises. In the oven the heat makes the gas swell even more, the yeast dies, the alcohol boils off, and the dough sets with the bubbles frozen in place as holes.
The principle
Yeast is alive. Baker’s yeast is a tiny fungus made of single cells, far too small to see one by one. A spoonful of yeast holds billions of them. In the video they are drawn hugely bigger.
Step 1: Yeast eats sugar. Flour holds a little sugar of its own. Once flour is mixed with water, natural helpers in the flour start breaking some of it down into more sugar. The yeast takes in that sugar as food.
Step 2: It breathes out gas. Inside the dough there is hardly any air, so the yeast turns the sugar into gas and a little alcohol. The gas is the same one that makes the fizz in soda.
Step 3: Stretchy dough traps gas. When you knead flour and water, some of the flour links up into long, stretchy strands that form a net through the dough. The gas can’t slip through that net, so it gets caught in tiny bubbles, like lots of little balloons.
Step 4: Bubbles grow, dough rises. As the yeast keeps making gas, the bubbles fill up and swell, and together they push the whole dough up. In a warm kitchen this takes about one to two hours, and the dough often grows to about twice its size. The video speeds this up a lot.
Step 5: The oven puffs and sets it. In the hot oven, the gas in the bubbles warms up and swells even more, so the loaf puffs up in the first minutes. Soon it gets too hot for the yeast, and it dies. The alcohol and some of the water boil off as steam. Then the dough firms up around the bubbles, and they stay behind as the holes you see in a slice of bread.
Common myth
Bread contains the yeast's alcohol
Almost none of it stays. The yeast does make a little alcohol, but the inside of a loaf gets close to the boiling point of water in the oven, which is far hotter than alcohol needs to boil away. Nearly all of it leaves with the steam, and that is part of the lovely smell of baking bread.How this video was made: Rendered 3D animation, simplified and drawn flat. The yeast, sugar and gas are drawn many thousands of times bigger, the rise is sped up from hours to seconds, and the stretchy strands are shown as a simple net.
