Yeast is the primary ingredient that makes bread rise. Yeast is so important to baking bread that it has been cultivated for many, many years. In scientific terms the process that is happening when yeast makes bread rise is called fermentation.

You might be more familiar with the fermentation process as it applies to the making of beer and wine. The end result is quite different from a loaf of bread, but the process is similar in that yeast performs the magic.

Yeast is a single-celled organism that has a life cycle which includes a dormant state. This is why we can buy little packets of dry yeast in the grocery store. In its dormant state yeast is not active. Until the yeast is given the proper conditions to grow, it will stay in its dormant state. It can't wait forever for the right conditions, so it does have expiration dates. Look for the expiration date before using yeast in a recipe for bread or beer. Don't waste your ingredients on yeast that is beyond its expiration date. It just won't work as expected.

Yeast is brought out of its dormant state, called activating the yeast, by placing it in a warm liquid. Once yeast finds itself in warm and moist conditions it will start to grow. Like any living organism yeast needs food and shelter to live and grow. Providing yeast with warmth and moisture is a kind of shelter from extreme conditions that could otherwise kill the yeast. Temperatures over 140 degrees Fahrenheit will kill the yeast, while cold temperatures will slow its growth down to nil. Refrigerator temperatures will keep yeast in its dormant state.

Sugars are food for yeast. In the case of making wine the sugars in grape juice or apple juice are food for yeast. Sugars, which are the building blocks of carbohydrates, are also present in the barley mash used to produce beer. The dough used to make bread also contains sugars as food for the yeast.

Once yeast find itself in the right conditions to grow, it will multiply or undergo fermentation. In the case of producing wine and beer yeast ferments to produce alcohol. In other words yeast feeds on the sugars in grape juice to make wine. It feeds on sugars in barley mash to produce beer.

The fermentation process in making bread causes the dough to rise. As yeast ferments to produce carbon dioxide gas, the gas gets trapped in the dough and this causes the dough to rise. Most of the rising of the bread is accomplished in the early part of making the bread. Once the yeast is exposed to high oven temperatures, it will stop producing gas and then die off.