The non-saccharomycetes Hansen divides into three groups; such which ferment solu tions of dextrose and invert sugar, as S. Rouxii, S. apiculatus; such which separate out no su crase but ferment saccharose, maltose and dex trose, as Monilia candida, and finally such which ferment saccharose, glycose, galactose and lac tose, the lactose yeasts. In practice the division into culture and wild yeasts would be appro priate. The yeasts can also be safely divided according to their power of attenuation, that is, the decrease in the density of the nutrient me dium due to fermentation. The attenuation reached by a yeast in a diastase-free beer wort has been agreed upon as the limit, and in ac cordance therewith the brewers' yeasts are graded, and we have the Frohberg type with high attenuation, and the Saaz with low attenu ation. Van Laer has defined a new type, Logos, so that now we have the types Saaz Frohberg and Frohberg-Logos, with all inter mediate grades. The numerous wine yeasts vary according to their resistibility against heat and aads; the amounts of sugar produced by them, and the amount of fruitesters, which they impart to the liquid. The distillers' yeasts, mostly top yeasts, are known by their resistance against acids, and foreign ferments, through their fermentative capability and the amount of alcohol developed.
No one can deny that yeast induces alcoholic fermentation, but how this is accomplished is still a matter of controversy. Liebig and other chemists maintain that fermentation is called forth by a ferment contained in the yeast cell, while Pasteur deems it the necessary conse quence of life without oxygen. Naegeli claims it to be a transferring of the motion of mole cules of protoplasm of the yeast to the ferment ing liquid. Eduard Buchner and Rudolf Rapp of Munich discovered in the yeast cell a fermen tation-inciting enzyme — zymase — which, how ever, can only induce fermentation once after the cell is completely destroyed. This discovery, which is of much scientific import, has not yet been made applicable in practical arts.
Besides the culture yeasts, there are a num ber of wild yeasts, which can produce great disturbances in the liquids infected by them. In order to avoid such infection by wild yeasts and also by bacteria, Hansen experimented and succeeded in preparing a pure yeast culture, that is, a culture originating from a single cell. Such a culture is not only free from all wild yeasts and bacteria, but also can and does re main unchanged as all cells are of the same nature. A reliable fermentation process can only be obtained by the use of one single type.
Such pure cultures are variously prepared; the oldest and best known method is that of Hansen, which has recently been modified by Dr. Lindner of Berlin as the droplet-culture. Both investigators employ for the further devel opment of the young vigorous cultures the propagation apparatus, which when once sup plied with an absolutely pure culture can be used for a long time if proper care is observed. Upon this pure-culture method a new brewing process is based which permits of entirely sterile work from boiling the beer to the putting it on the market. This process, invented by Nathan,
has been called by him in honor of Hansen, the senior in the art of pure culture, the Han sena process. Another method to obtain yeast of a high degree of purity is that in which a mixture of yeast is exposed to definite condi tions of temperature, aeration, kind of nutrition, action of light, pressure and motion that one yeast of definite properties develops more and better than the other yeasts in the mixture. If, further, the corresponding layer is removed from the yeast and propagated under the afore said conditions, a pure yeast of a uniform char acter is finally obtained This process, founded by Delbrueck of Berlin upon his scientific obser vations in American breweries, was further elaborated by him. An idea of the importance and value of pure yeast culture can be had fruit the fact that almost all the larger breweries and distilleries employ apparatus for propagation and endeavor through observing the utmost care to protect their yeast against infection. But yeast is not only of great import to the brewer and distiller, but also to the baker, as the ity of his wares depends upon the quality off used, and in consequence thereof a special industry, that is, the manufacture of compressed yeast, has been introduced. Compressed yeast is chiefly made from distillers' yeasts; but in some instances also, beer yeasts, which must be purified and disembittered, are utilized. The price of compressed yeast in comparison to beer yeast is considerably high, but the baker has the advantage that by using the former his wares obtain a larger size than when the latter is used The process of fermentation with compressed yeast develops more slowly than with beer yeast, hence the dough requires more time to ripen. Much progress has been made in the compressed yeast fabrication and diverse methods are employed in its manufacture_ Many operations must be carefully and accu rately observed in order to obtain a salable product. In the main these operations are: (I) Preparation of a fluid in which the yeast can properly develop, either under simultaneous for mation of alcohol or without the formation of much alcohol; (2) preparation of a mass, in which there is a corresponding amount of rig orous seed-yeast, which is added to the above mentioned fluid (preparation of the mother yeast) ; (3) bringing together of the nutrient medium with the stock yeast or °setting': (11 reproduction of the yeast with or without simul taneous fermentation; (5) separation of the newly formed yeast from the fluid by skimming. etc.; (6) washing the yeast or removing the last traces of the nutrient medium; (7) pressing of the washed and separated yeast mass; (8) form ing the compressed yeast into cakes of suiuthk size; (9) packing of the cakes for the trade and for shipping.
For some time yeast, especially beer yeast. has been used in medicine, and has been suc cessfully employed partly fresh and partly dried. in tablet form in cases of furunculosis, acme nen, acne, psoriasis, and other skin diseases dyspepsia and partly in diabetes. See Baru-rec.