THE PRODUCTION OF ALCOHOL 



213 



at the top which is thick with yeast, whereas the latter either form a 

 thin layer only of yeast at the top, or else none at all. 



6. Cleansing. Finally, the action of yeast is checked by a process of 

 cleansing which we need not enter into. The finished beer varies in 

 specific gravity from 1 '002 to 1 - 030, and contains from 4 to 24 per 

 cent, of proof spirit. There will be more yeast at the close of fermen- 

 tation than at the beginning. A portion is kept aside and used for the 

 next fermentation, the remainder being sold or otherwise disposed of. 



As in former times no special precautions were taken with the fer- 

 mentation, it is not surprising that occasionally everything went wrong 

 because other organisms had got into the wort and got mixed up with 

 the yeast. Some of these intruders produced injurious products which 

 resulted in heavy financial losses. What was called yeast was often a 

 mixture of several organisms, and the brewer never knew when one of 

 the undesirable constituents was going to predominate in the wort and 

 spoil the fermentation. When Pasteur's doctrine that bacteria were 

 responsible for the diseases of fermented liquids became accepted, a cry 

 arose for the purification of brewer's yeast, as it was thought that 

 bacteria were responsible when a yeast fermentation went wrong. It 

 was, however, subsequently shown by the work of E. Chr. Hansen that 

 some of the worst diseases of fermented liquids were due to " foreign " 

 or " wild " yeasts that had gained access to the wort. A wide-reaching 

 reform was effected when Hansen separated the culture yeasts from 

 the wild yeasts, and eliminated the 

 latter from the breweries of Copen- 

 hagen. His researches further resulted 

 in the separation of the culture yeasts 

 into several species and races, the best 

 being adapted for fermentation. His 

 new system of employing pure cultures 

 spread quickly into different countries, 

 and was adopted not only by the 

 breweries, but by the spirit, pressed- 

 yeast, and wine industries. It is regret 

 table that Great Britain has not yet 

 taken full advantage of these dis- 

 coveries. For a detailed description of the species and races of 

 yeasts, the student may refer to Kloeker's Fermentation Organisms, in 

 which much useful information on this subject may be obtained. A 

 good example of a " wild yeast " is afforded by Saccharomyces Pas- 

 torianus I., discovered by Hansen in the air at the Alt-Carlsberg 



FIG. 120. Saccharomyces Pastorianus 

 I. (After Hansen.) 



