Why does bread rise? Yeast helps bread dough rise. Yeast is a fungus with one cell, and the yeast in dough is alive. It eats sugar. It gives off a gas called carbon dioxide.

Scientists call the process where yeast cells use up sugar and release gas fermentation. Yeast also makes an alcohol called ethanol as it eats sugar. Wild yeast floats in the air and lives on flour and fruit. Sourdough rises with wild yeast and bacteria. Dry yeast needs warm water to wake up.

Dough rises best in a warm place, above 75 degrees Fahrenheit. Bread dough is set there to rise until it doubles in size. Salt slows down yeast, and yeast dies when it gets too hot. A dough left to rise too long is over-fermented. The gas cells can burst so the loaf falls, and the bread can turn out hard and tough.

Kneading means pushing, pulling and stretching the dough, and kneading makes the gluten in dough stretchy. Gluten works like a net that traps the gas in dough, and rising dough is covered, often with a clean, damp towel.

When dough first goes into a hot oven, it rises fast. Bakers call this oven spring. Water in the dough turns to steam, and the steam pushes the dough up. Children need a grown-up with them in the kitchen when they bake. A chemical reaction makes the crust dark and toasty. Bread is done when it turns golden brown. Fat in dough makes baked goods tender.

Yeast is not the only way to lift a bake. Baking soda and baking powder make carbon dioxide gas bubbles that help cakes and pancakes rise. Baking powder has an acid already mixed in with the baking soda. Baking soda starts to make gas almost as soon as it touches an acid. Double-acting baking powder makes gas at two different times. In a cake, heat sets the proteins in eggs, which turns a liquid foam of batter into a solid.

Words to know

• yeast: a living fungus
• fermentation: yeast using sugar
• gluten: dough net
• kneading: push and stretch
• ethanol: an alcohol