Yeast is a single-celled organism that bakers put into dough to make
it rise. It is also used in the production of beer, wine, and other
alcoholic beverages. The yeasts used commercially consist of
masses of the microscopic yeast organisms. There are about 600
species of yeasts, but only a few are used commercially.

In early times, people made bread, beer, and wine without
understanding the role yeasts played in their production. In the 1600's,
Dutch scientist Anton van Leeuwenhoek first observed yeast cells.
Then, in 1860, French scientist Louis Pasteur confirmed that live
yeast organisms caused the fermentation of wine and beer.

Yeasts belong to a group of simple organisms known as fungi, which
exist almost everywhere in nature, including the air. Yeasts reproduce
rapidly, and they grow especially well in substances containing sugar.
Yeast cells reproduce by fission (splitting in two) or by budding. In
budding, part of the cell wall of the yeast swells and forms a new
growth called a bud. The bud then breaks off and becomes an
independent cell.

How yeast is used. Yeast fungi lack chlorophyll, the green matter that
green plants use to make their own food. Therefore, yeasts must rely
on other sources for food. They feed on sugar from a variety of natural
sources, including fruit, grain, and nectar, and also from molasses.
Yeast cells produce chemicals called enzymes, or ferments, that break
down their food. Some species of yeast break down sugar into
alcohol and carbon dioxide. This process, called fermentation, plays
an important part in making bread, beer, and wine.

In Breadmaking, a commercial yeast called baker's yeast is used as a
leaven, a substance that makes dough rise. Bread dough is made by
mixing such basic ingredients as flour, water or milk, salt, and yeast.

Since sugar is needed for fermentation, bakers add to dough certain
enzymes that convert some of the starch in the flour into sugar. In
addition, bakers may hasten fermentation by adding sugar to the
dough. The yeast then breaks down the sugar into alcohol and carbon
dioxide gas. The bubbles of this gas are trapped by a substance in the
dough called gluten. As the gas expands, the gluten stretches, causing
the dough to rise. The alcohol produced by fermentation evaporates
in baking. Baking also destroys the yeast.

The yeast used in winemaking acts on the sugar in grapes and other
fruits to produce alcohol and carbon dioxide gas through fermentation.
In most wines, the gas is allowed to escape into the air. But in some
champagnes and other sparkling wines, the gas is retained to provide
the beverages' characteristic bubbles.

Another type of commercial yeast, called brewer's yeast, cannot act
directly on the grain used in the brewing of beer. Brewers must first
convert the starch in the grain into sugar by means of a process called
malting. The yeast is then added to convert the sugar to alcohol. Gas
formed during fermentation is pumped off the beer and later added
back to the beer to carbonate it.

Other uses of yeast fungi include the production of a dietary
supplement called single cell protein (SCP). Some species of yeasts
produce large amounts of a particular vitamin and are used in the
commercial production of that vitamin. Other species, such as the
yeasts used in brewing, can absorb and store vitamins from their food.
People may eat these yeasts as vitamin supplements. Certain kinds
of yeast fungi can produce large amounts of such useful substances
as fat, glycerol, industrial alcohol, and various enzymes. The yeasts
are used in the commercial production of these substances.

How yeast is made. Before the commercial production of yeast in the
1880's, yeast fungi from the air leavened the bread that people baked. Homemakers prepared a dough and left it uncovered, and yeasts
landed on it and began the fermentation process. Later, excess yeast
from the beer and winemaking industries was used in Breadmaking.
This yeast is called barm. When the production of baker's yeast first
became an industry, manufacturers grew yeast fungi on malted grain.

Today, baker's yeast is produced on molasses, which consists mostly
of sugar. Baker's yeast is manufactured in two forms--as a moist,
compressed cake and as dried grains. Cakes of yeast are made up
of live, active yeast cells. The yeast cells in dried yeast are alive but
not active. Dried yeast must be mixed with warm water before the
yeast fungi can grow. Yeast cakes must be refrigerated, but they spoil
after about six weeks. Dried yeast need not be refrigerated, but it lasts
longer under refrigeration.

Arthur J. Ashe, III, Ph.D., Prof. of Chemistry, Univ. of Michigan.