In choosing its nutriment yeast is very selective. Of the carbohydrates, glucose, maltose and those of C_{ 6}H_{ 12}O_{ 6} group are capable of direct fermentation, and are quickly and vigorously changed by yeast. In direct opposition, we find that cane sugar, beet sugar, as well as the starch of flour, are not fermentable until chemically changed. This change is brought about by yeast itself.
The plasma of the yeast contains an albuminous substance called Invertin. As explained above, the Invertin, by dialysis, is diffused out through the cell covering and changes cane sugar and sugars of the same class, as well as part of the flour starch, into fermentable sugar, known as invert sugars.
_Reproduction of Yeast._—During fermentation yeast nourishes and reproduces itself. The granulations of the living plasma divides itself, and with a portion of the plasma forms a small protuberance at one end of the cell; it then enters the neck, which is gradually developed by the contraction of the cell wall and forms a bud.
The neck finally closes, the budding daughter cell releases itself from the parent cell, and each are then an individual organism.
This operation is known as “budding.” Each parent cell is capable of giving off several buds in succession. The daughter cells in their turn reproduce in the same manner, and so with remarkable rapidity yeast cells multiply.
But yeast is also reproduced by spores termed “ascospores.”
In this case yeast cells do not throw out a bud, but the plasma divides itself into (usually) four portions called spores, each of which surrounds itself with a thin membrane.
These spores, when set free by the dissolution of the cellulose coverings of the parent cells, on account of their minuteness float away into the atmosphere. If by chance they drop into the proper medium, such as malt wort or flour barms, spontaneous fermentation sets in.
This is recognized by the fact of spontaneous fermentation frequently and easily occurring in the fermenting rooms of yeast factories and breweries, as innumerable quantities of spores are present in the atmosphere at all times.
_Pure Yeast Cultures._—By the manner in which yeast nourishes and reproduces itself, we acknowledge it to be a plant of exceedingly elemental structure.