280 MICRO-ORGANISMS AND FERMENTATION. 



vidual cells, but also upon the removal of the products of 

 metabolism. The stirring up of the cells brought about by 

 air bubbles brings them constantly into contact with fresh 

 portions of the nutritive fluid. In the air-yeast factories this 

 fact is utilised, and a considerably higher yield of yeast is 

 obtained than in the older process (foam yeast). According 

 to Delbriick, it is found in practice that 100 parts of malt 

 yield 21 to 23 parts of pressed yeast in a non-aerated wort, 

 and 30 parts in an aerated wort. He further proved that the 

 higher yield was reached after four and a half hours' aeration. 



For brewery yeasts the aeration of the wort is of special 

 importance, as the clarification is dependent upon it. In a 

 badly-aerated wort the yeast does not readily settle out. 

 This circumstance must not be neglected even during the 

 growth of the pure culture in the flasks. 



Amongst the products of metabolism removed by aeration, 

 carbon dioxide deserves special mention, for it exercises a 

 restrictive effect on the multiplication of the yeast. This has 

 been proved by parallel experiments in open and closed vessels 

 under conditions otherwise identical. If, however, a com- 

 parison is made between the amount of alcohol formed in the 

 two vessels and the quantity of yeast produced, it will be found 

 that the individual cells in the closed vessel have produced a 

 larger quantity of alcohol than those in the open vessel. The 

 carbon dioxide appears, therefore, to exercise a favourable 

 influence on the fermentative power of yeast. 



The action of light on yeast cells is described in the general 

 review of the physiological properties of fungi. 



Variations in the Saccharomycetes. 



Hansen's numerous investigations proved that the Saccharo- 

 mycetes are affected in varying degree by external agents, 

 and that it is possible by suitable treatment to bring about 

 variations along different lines. Even the individual peculi- 

 arities of cells in a pure culture may be of importance in 

 this respect. Some of these changes are only evanescent. 

 By suitable cultivation they disappear, and the species returns 

 to its original condition. Others are more deeply seated, and 



