BACTERIA IN ARTS AND INDUSTRIES 95 



of the requisite amount of yeast. An ounce of this yeast con- 

 tains something like 5,000,000,000 cells, the chief organism 

 present being the Saccharomyces cerevisiae. As mentioned on 

 p. 18, "diseases" of beer (such as a disagreeable bitter 

 flavour, persistent turbidity, bad smell, and the like) may be 

 due to the presence of undesirable forms of Saccharomyces. 



For further details of this complicated process, special books 

 on brewing should be consulted. It is sufficient for our pur- 

 pose here to summarise the changes which occur as (1) the pro- 

 duction of fermentable sugars from the starch of the grain, 

 and (2) the fermentation by the yeast of the sugars so pro- 

 duced, with the formation of alcohol and other by-products. 



Whisky is a grain-spirit prepared from malted barley, the 

 fermented liquid being then distilled. 



Wines are alcohol-containing beverages prepared by suitable 

 fermenting the sugar present in the juices of various fruits 

 such as grapes, currants, apples, etc., or from rhubarb, or even 

 in some cases, from flowers such as the elder-flower or cowslip 

 grape-juice being, of course, the most important source. The 

 fermentation is due to special wine-yeasts, which may either 

 be present on the fruit naturally, or may be added artificially. 



Many other industrial processes are, in whole or in part, de- 

 pendent upon bacterial action, but those described above will 

 serve as typical examples. 



