Uses for Yeasts
Elsie Singruen
Consultant, Brewers Yeast Council
Chicago, Illinois
The term yeast designates a group of single-cell microorganisms
bordering between the plant and animal kingdom which usually
multiply by budding and, in some instances, by cell division. Generally,
yeasts are classified as plants, although they exhibit some character¬
istics which they share with other low forms of animal life. "Yeast"
is no botanical connotation; rather it is a description of the behavior
of these microorganisms during fermentation. Derived from the old
English word yearing, yeast means that which arouses or agitates.
True yeasts distinguish themselves from their next of plant kin—
the fungi—by their non-filamentous nature, and from the bacteria,
which are classified with the animals by their much larger size.
In the household of nature, yeasts are assigned the task of dis¬
integrating sugary fruits to free their seed kernels and provide them
with the nutrients necessary to initiate their growth. In accordance
with this their natural function, yeasts are endowed with a structure
and constituents which enable them to carry out their destiny. It is
well to keep this in mind, inasmuch as this purpose is closely related
to the behavior, composition, and properties of yeasts. It also serves
as a guide in yeast research.
The yeast cell consists of an outer membrane of selective permea-
ability and the protoplasm, which is capable of carrying out all func¬
tions of life and reproduction. Yeasts are found in nature on ripe
fruits or under fruit trees in winter. Their adaptability to their hosts
gives them specific characteristics which accounts for the great vari¬
ability of species. Although their true nature was not known to man
until a scant century ago, he learned early to cultivate these tiny
plants for various purposes by selecting suitable strains and keeping
their environment as constant as possible.
Most yeasts of industrial importance today belong to the genus
Saccharomyces cerevisiae, the strains of which are differentiated by their
ability to ferment various sugars into alcohol and carbon dioxide The
Saccharomycetes belong to the "perfect" yeasts which are able to pro¬
duce ascospores and thus have a complete life cycle.   Their preferred