Bread rises because of yeast, a fungus made of a single cell. The yeast in bread dough is alive, and the kind used for baking is Saccharomyces cerevisiae, called baker's yeast. Yeast eats sugar and gives off a gas called carbon dioxide. Scientists call the process where yeast cells use up sugar and release gas fermentation. Yeast also gives off a second product as it eats sugar: an alcohol called ethanol.

Wheat flour has two proteins, glutenin and gliadin, that join to make gluten. 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, so the gas that yeast gives off stays inside it.

Sugar is food for yeast. Flour supplies sugar for yeast, but the starch in flour has to be broken down first. Dry yeast needs warm water to wake up, and dough rises best in a warm place, above 75 degrees Fahrenheit, until it doubles in size. Rising dough is covered, often with a clean, damp towel. Yeast dies when it gets too hot.

Wild yeast floats in the air and lives on flour and fruit. Sourdough bread rises with wild yeast and bacteria instead of yeast from a packet. A sourdough starter is kept going by adding fresh flour and water to it.

Baking soda and baking powder also make gas. They make carbon dioxide gas bubbles that help cakes and pancakes rise. Baking powder has an acid already mixed in with the baking soda, and baking soda starts to make gas almost as soon as it touches an acid. Double-acting baking powder makes gas at two different times. Harvard chemist Eben Horsford replaced cream of tartar in baking powder with an acid phosphate. Cream of tartar comes from the sediments of wine barrels.

When dough first goes into a hot oven it rises fast, which bakers call oven spring. Water in dough turns to steam in the oven, and the steam pushes the dough up. Children need a grown-up with them in the kitchen when they bake. A dough left to rise too long is over-fermented, so the gas cells can burst and the loaf falls, and the bread can turn out hard and tough. Heat sets the proteins in eggs, which turns a liquid foam of batter into a solid. A chemical reaction makes the crust of bread dark and toasty. Bread is done baking when it turns golden brown.

Words to know

• fermentation: yeast using sugar
• gluten: dough net
• ethanol: an alcohol
• starch: flour's sugar source
• sourdough: wild yeast bread