134 THE COLOURLESS PLANT CELL. THE YEAST PLANT 



for twelve to fourteen days while the process of fermenta- 

 tion takes place. The carbon dioxide produced rises to 

 the top, so that the surface of the fermenting wort 

 becomes covered with a thick layer of froth. Much of 

 the carbon dioxide escapes, but being heavier than air 

 lies on the surface of the vat, so that workmen have 

 sometimes been asphyxiated by approaching the vats 

 incautiously. The temperature of the wort rises, owing 

 to the great liberation of energy in fermentation, but 

 is kept down to 12 C. by currents of cold air directed 

 on to the vats. When the fermentation has reached 

 the desired point the beer is run into specially lined 

 storage casks or tanks and kept at i C, for three months, 

 when it is ready for bottling. 



The whole of the installation is kept scrupulously 

 clean, or infection of the wort with " wild " yeasts and 

 bacteria would occur, undesired fermentations would 

 take place, and the beer spoiled. 



The kind and quality of beer produced depends not 

 only on the kind of yeast used and the time during 

 which fermentation proceeds, but also on the kind and 

 quality of the malt and hops (and these again on the 

 strains of barley and hops, and on the soil and climate 

 in which they are grown), for beer is a complex solution 

 of many substances derived from different bodies con- 

 tained in the malt and hops, besides the sugar, which 

 is the main substance fermented. 



Wine -Making. In wine-making the fermentable 

 liquid (must) is grape juice, which is pressed from the 

 overripe grapes in a wine-press. The yeasts which 

 cause the fermentation in the must are always present 

 on the overripe grapes, and do not have to be added 

 as is done in the making of beer. As in the case of 

 beer, but to a far higher degree, the kind and quality 



