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
(see 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.