The yeast used in baking bread, as well as in brewing bear, is a species of fungus. It's possible that the discovery of yeast as an agent for making bread rise occurred when naturally occurring fungus contaminated flour. These single-celled fungi need water and sugars to survive. When they come into contact with the flour and water used to make bread, they begin a series of reactions which result in the bread rising.

When activated by being brought into contact with air, water and flour, yeast to ferment the sugars present in the flour. As the microorganisms digest the sugar, they produce a number of waste products. One of these is carbon-dioxide gas. If the yeast are digesting in an anaerobic environment -- that is, an environment without air -- they also produce ethanol. In breadmaking, however, only small amounts of ethanol are produced, and these evaporate during baking.

As the yeast continues to metabolize sugar and release carbon dioxide, the bread dough gradually rises. This occurs because the dough, if properly kneaded, consists of a network of long, stretchy gluten strands. Instead of parting to let the gas out, the flexible dough traps pockets of carbon dioxide, inflating as it does so and creating small voids within the dough. Once the dough has risen, it's ready to go in the oven.

Heating the dough leads to several important chemical changes. One of the most important is that the rising temperature of the oven kills the yeast, stopping the fermentation process. The gluten strands in the dough also harden, preserving the shape of the gas bubbles left by the fermenting yeast. When the bread comes out of the oven, the holes left by the yeast's byproducts are preserved in the texture of the bread.