Yeast and Bread

A tiny fungus with a big job

Have you ever wondered why a slice of bread is soft and full of little holes? The secret is a fungus so small you cannot see just one on its own. It is called yeast.

Yeast is a fungus, just like mushrooms and molds. But instead of growing a cap and stem, yeast lives as single tiny cells. A spoonful of baking yeast holds billions of these cells. The kind bakers use has a scientific name: Saccharomyces cerevisiae, which means "sugar-eating fungus." That name is a big clue about what happens next.

How yeast makes dough rise

When you mix flour, water, and yeast into dough, the yeast wakes up and starts to feed. Its favorite food is sugar. Flour contains starch, and tiny helpers in the flour called enzymes break some of that starch into sugar the yeast can eat.

As the yeast eats the sugar, it does something called fermentation — a way of getting energy from food without using oxygen. Fermentation gives off a gas called carbon dioxide (the same gas you breathe out).

Dough is soft and stretchy, so the carbon dioxide cannot escape easily. Instead, the gas gets trapped and forms thousands of tiny bubbles. The bubbles push the dough outward, and slowly the lump of dough puffs up and grows bigger. This is why bakers say dough "rises."

From dough to bread

When the risen dough goes into a hot oven, two things happen. The trapped gas bubbles get even bigger in the heat, giving one last push. Then the dough bakes firm around each bubble, freezing the holes in place. When you cut the loaf open, those holes are the fingerprints the yeast left behind.

The oven's heat also kills the yeast, so the fungus stops working once the bread is baked. Fermentation does make a small amount of alcohol too, but the heat of baking makes it evaporate away, leaving a warm, tasty smell in the kitchen.

Why this changed history

People have used yeast to raise bread for thousands of years. Long ago, no one knew a living fungus was doing the work — they just knew that saving a bit of old dough helped the next batch rise. Light, fluffy bread became one of the world's most important foods, and it all depends on a fungus too small to see.

Baking tip: yeast for baking is safe and sold in shops. But never taste or use wild fungus or mold you find growing on food or outdoors. If you bake bread at home, always do it with an adult, especially near a hot oven.