Yeast causes bread dough to rise and strengthens it. Yeast cells reproduce when exposed to the correct combination of flour, temperature and moisture, causing bread to rise. Combining water, flour and yeast converts the mixture into a simple sugar that exudes liquid, releasing carbon dioxide and alcohol into bubbles inside of the dough.
Yeast, a type of sac fungi and reproduces asexually, forms a swelling on the surface of the bread. A piece of cell nucleus enters the swelling and forms a wall, creating a new cell. This type of reproduction takes around 20 minutes in bread dough.
Proteins in flour combine with water to form gluten with an elastic consistency. Gluten holds in the gas and strengthens the dough. Kneading dough creates more gluten as the water and proteins join and make it more elastic. Alcohol in the bread dough evaporates during the baking process. Beer and some wines incorporate the same process used to make bread dough rise.
Commercial yeast grows in a mixture of mineral salts and sugar. When formed, it is skimmed from the top. Compressed yeast contains washed yeast mixed with starch. Pressing the yeast removes some of the moisture, which produces yeast cakes. Dry yeast has most of the moisture removed.