Explore the Science of Yeast | The Story of Yeast

What Is Yeast?

Yeast is a tiny form of fungi scientists refer to as "microorganisms". They are egg-shaped cells that 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.

A tiny organism with a long name
The scientific name for one species of yeast is SACCHAROMYCES CEREVISIAE, or sugar-eating fungus. This name is derived from the Latin word "cerevisiae", which means "brewer". A very long name for such a tiny organism! This strain 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 and ethyl alcohol, which are released by the yeast cells into the surrounding liquid. This is how alcoholic drinks are produced from starch containing flours. For example, barley flour is used for making beer and wheat, corn and other grains are used for making whiskey.

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.

Wine and bread making
In commercial fermentation of grape juice for the production of wine, the carbon dioxide gas escapes from the solution. Evidence of gas can be seen in the heavy foam caps in fermenting wine tanks.

In bread baking, when yeast ferments the sugars available from the flour and 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. Thus, the term "yeast-leavened breads" was added to the vocabulary of the world of baking.