What is Yeast?
Yeast are single-celled fungi. As fungi, they are related to the other fungi that people are more familiar with, including: edible mushrooms available at the supermarket, common baker's yeast used to leaven bread, molds that ripen blue cheese, and the molds that produce antibiotics for medical and veterinary use.
Yeast cells are egg-shaped and can only be seen with a microscope. It takes 20,000,000,000 (twenty billion) yeast cells to weigh one gram, or 1/28 of an ounce, of cake yeast.
A tiny organism with a long name
The scientific name for the yeast that baker's use is Saccharomyces Cerevisiae, or "sugar-eating fungus". A very long name for such a tiny organism! This species of yeast is very strong and capable of fermentation, the process that causes bread dough to rise.
A fungus with a sweet tooth
Yeast cells digest food to obtain energy for growth. Their favorite food is sugar in its various forms: sucrose (beet or cane sugar), fructose and glucose (found in honey, molasses, maple syrup and fruit), and maltose (derived from starch in flour).
The process, alcoholic fermentation, produces useful end products, carbon dioxide (gas) and ethyl alcohol. These end products are released by the yeast cells into the surrounding liquid in the dough. In bread baking, when yeast ferments the sugars available from the flour and/or from added sugar, the carbon dioxide gas cannot escape because the dough is elastic and stretchable. As a result of this expanding gas, the dough inflates, or rises. Thus, the term "yeast-leavened breads" was added to the vocabulary of the world of baking.
The ethyl alcohol (and other compounds) produced during fermentation produce the typical flavor and aroma of yeast-leavened breads.
Fermentation in nature
Fermentation occurs naturally in nature. For instance, many berries break open in late fall when they are overripe and full of sugar. Natural yeast organisms, so small they cannot be seen with the naked eye, lodge on the surface of these berries, which then become fermented and alcoholic.