Have you ever wondered how the bread you eat gets to be so light and fluffy? It's all thanks to a simple, single-celled fungus known as yeast--the same fungus used in the fermenting processes of beer and wine. Without it, bread would be hard and flat. The ingredients in bread are simply flour, salt, sugar and yeast, with other ingredients added for specially flavored bread. It's the yeast, though, that is responsible for bread rising and for its distinct texture. You can buy yeast in two forms: fresh or dried. If you choose fresh yeast, then it is alive when you buy it and it needs to be refrigerated because it has a very short shelf life. If you plan on saving some of what you buy, get dried yeast, which is only activated when exposed to heat. Yeast is killed when exposed to heat, so you don't have to worry about live fungus when you eat bread.
How Yeast Reacts with Sugar
Yeast devours sugar, then releases carbon dioxide bubbles and small amounts of ethyl alcohol. When the dough is kneaded and placed in an oven, the heat from the oven makes the yeast work overtime, which increases the rate at which carbon dioxide is produced.
How Bread Rises
The released carbon dioxide is responsible for bread rising. Yeast cells will continue to create carbon dioxide before dying out, which happens quickly after dough is placed in an oven, so it is important to have enough yeast in the bread recipe to create a significant rise. On the other hand, too much yeast produces too much carbon dioxide, creating airy bread, such as typical white bread. When bread is baked, the heat puts yeast in a highly excited state, creating carbon dioxide very quickly and causing the bread to rise in the oven. The high temperature also kills the yeast quickly, which is why toward the end of the baking process, carbon dioxide production ceases and the bread may actually fall a bit. Salt and animal fats make it difficult for yeast to do their work, which is why bread that calls for butter does not rise as much as bread without butter, and why extra sugar is added whenever the bread recipe calls for salt. The butter adds taste and a different texture to bread, while the extra sugar means the yeast creates the needed amount of carbon dioxide.