YEAST, or BARM, is a micro-organism of the vegetable kingdom and belongs in the order of the Gymnoascerr to the family of the SaccIsaramycetes, of which there are three classes, namely, Monospora, Saccharoonyces and Schisosaccharomyces. The main charac teristic of the first class, as its name indicates, is that it only develops one spore in the ascus. Thus far only one type, the Monospora Cus which is parasitic in daphiniader which destroys, has been determined. There are many representatives of the second class, while only a few of the third have up to the present time been identified. The second Saecharamyees — is generally what is understood under the term yeast, not only in the brewery but also in the distillery and the compressed yeast fabrication. Its structure is sery simple; it consists of a single cell, which has few distinguishing char acteristics. Each cell of the saccharomycrtes is more or less globular or ovoid in form and, just as every other vegetable cell, consists of a colorless, viscous substance (protoplasm). a cell-wall (membrane), and also a nucleus. In the protoplasm there are observed, according to the species of the plant cell, different num bers of spaces filled with cell-juice, which are termed vacuoles.
In young yeast cells the cell-wall, or mem brane, is very thin-0.4 to 0.9 to — and has almost the same refractive power as the proto plasm which it surrounds. The membrane is thickened through external influences, especially when the yeast-cells must develop in or upop a highly nutritious medium. An increase in the thickness of the cell-wall is also accom panied by greater impermeability and a corre sponding decrease in the fermentative capability of the cell. Through the action of acids or alkalies upon the yeast cell the layers of the cell-wall can be shown, which are two, and sometimes three, or even more in number. Chemically the membrane of the yeast cell con sists of cellulose (erythro and achroo-cellulose), pectinic compounds, and plasmatic components, for which reason it is very nitrogenous. The
cell contents consists of protoplasm with more or less glycogen. (According to Laurent there is 32.28 per cent glycogen in the dry substance.) In the young yeast cell the whole interior is filled with protoplasm; in a short while, how ever, apparently hollow spaces appear, which are filled with the reserve neces sary for the vitality of the cells. The nucleus furnishes the substances necessary for building up the cell, and, according to its chemical coin position, belongs to the albuminoids in the class of the proteids, and especially to the nucleic'. The form of the nucleus varies considerably; in a young cell it is spheroid, flattens out with increasing age to a disc whose periphery is no longer entire, but is lobed ; elliptical shapes were also found.
The nucleus is in some instances of a con siderable size; its diameter in some cases being one-third of that of the cell. Concerning the inner structure of the nucleus it was observed that the nucleus of the yeast cell has a pellicle as well as a nucleolus, which is spheroid, and is situated in approximately the centre of the nucleus, the space between the pellicle and the nucleolus being filled with nucleus plasma (Karyoefessnia), this having a netlike structure through which the nucleolus is suspended. One or more vacuoles were observed in several types of yeast in the Karyoplasma under certain coy ditions, especially when the cells were trans ferred into fresh nutrient solutions. With the beginning of the propagation of the cell a corresponding development of the nucleus takes place, in either one of two ways, that is, the direct division (Amitosis), or the indirect divi sion (Karyokinesis, Mitosis). The manner Is which the nucleus divides itself differs in the various types of yeast. The yeast cell is propa gated in one of two ways, either oxygen spore formation, budding, or through endogen spore formation, the real sporulation.