270 MICRO-ORGANISMS AND FERMENTATION. 



ammonia, etc. Their experiments were carried out in presence 

 of toluol to prevent an infection with bacteria. The process 

 of auto-digestion begins if yeast is kept for a long time at a 

 high temperature without nitrogenous food. 



In practice, auto-digestion may take place in the manu- 

 facture of pressed yeast where decomposition of the yeast is- 

 frequently encountered without the occurrence of any bacterial 

 infection. In this case, owing to lack of nourishment, the 

 cells gradually resolve their albuminoids, and doubtless a 

 proteolytic enzyme is simultaneously secreted. The yeast 

 mass is then more readily exposed to infection by bacteria. 

 Cells rich in glycogen appear to be less liable to such a decom- 

 position. 



Fermenting Power ; Fermentative Energy ; Raising Power. 



The work carried out by yeast can be distinguished under 

 three heads : The activity of the enzymes ; metabolism ; 

 synthesis of material. 



The activity of the enzymes is of a sugar splitting, hydro- 

 lysing, and proteolytic character. As proposed by Neumann 

 Wender, the fermenting power of yeast may be expressed in 

 terms of the quantity of sugar which is split up at a given 

 temperature in unit time by unit quantity of yeast. The 

 fermentative energy may be defined by determining the time 

 required within which a given quantity of sugar is decom- 

 posed by unit mass of yeast under special conditions. The 

 '' raising power " is a function of the carbon dioxide formed 

 by pressed yeast, whereby the dough is raised. Carbon dioxide 

 is developed during both true and auto-fermentation. A true 

 and practicable determination of the " raising power " of 

 baker's yeast can only be carried out in so far as it is possible 

 to prepare a kind of normal dough. 



The Biological Relationships of Yeast. 



The problem of the occurrence of yeast in nature was raised 

 as soon as its vegetable character had been established. The 

 first researches on this question were undertaken by Brefeld 

 in 1875, who arrived at the result that the yeasts are very 





