The chief micro-organism of the vinous fer mentation is S. ellipsoideus. The elliptical cells of this species are found on the grapes and in the air of grape-growing districts, and fermen tation is thus set up spontaneously without the actual addition of yeast. Recently pure cul tures of the wine-yeast have been prepared and used with very good results in the fermenta tion of grape-juice. E. C. Hansen, an able Danish investigator, has added greatly to our knowledge of the chief yeast-fungi, and has prepared and thoroughly examined several pure cultures of the three most important species, S. cerevizier, S. pastorianus and S. ellipsoideus. He has described six kinds in all, two belong ing to the last species, three to the second and the sixth to the first, the distinctions being founded on the temperature of spore-forma tion and the conditions under which a surface film forms in a liquid medium. Hansen has also cultivated two pure forms of brewers' yeast, known as Carlsberg No. I and No. 2. Both are bottom yeasts, but the beer produced by No. 1 has less carbon dioxide than that of No.
2, and is used chiefly for bottling, the No. 2 beer being preferred for export.
Of the fermentations set up by bacteria only some of the most important can be referred to here. (1) Acetic Acid Fermentation. In this fermentation alcohol becomes oxidized into acetic acid, the total reaction being represent able by the equation: The microbe concerned in the fermentation, if allowed to continue its action, will oxidize the acetic acid into carbon dioxide and water. Two
bacilli, namely, Bacillus aceti and B. pasteuri anus, have been described as taking part in this important fermentation, which is the basis of the commercial manufacture of vinegar from wine. (2) Lactic Fermentation. The souring of milk which has stood for some time is due to this fermentation, by which the milk-sugar becomes transformed into lactic acid through the agency of bacteria. Carbon dioxide is an invariable product of the fermentation. (3) Butyric Fermentation, often set up in milk which has undergone the lactic fermentation. (4) Nitrification of Ammonia, a process of vital importance in nature, by which organic nitrogen becomes fixed in soils as nitrate and nitrites. The process of decay and putrefac tion, with the production of ptomaines, also be long to this section of the subject of fermenta tion. See ALCOHOL; BREWING AND MALTING; DISTILLED LIQUORS; ENZYMES; VINEGAR; WINE.
Bibliography.--Bayliss, W. M., 'The Nature of Enzyme Action' (London, 1911); Conn, H. W., 'Bacteria, Yeasts, and Molds in the Home' (Boston 1903) ; Fowler, G. J., 'An Introduction to Bacteriological and Enzyme Chemistry' (London 1911) ; Harden, A., 'Alcoholic Fer mentation' (London 1911); Jorgensen, A., 'Micro-organisms and Fermentation' (London 1911).