32 MICRO-ORGANISMS AND FERMENTATION. 



of this evidently depends upon the fact that it is very 

 difficult, and often quite impossible, to separate all germs 

 of bacteria and other organisms from each other by simply 

 shaking the gelatine mixture. This test proves therefore 

 that the plate-culture involves very material errors. 



Holm has subjected the method to a thorough analysis 

 (1891), and has experimented with a considerable number 

 of yeast-species, absolutely pure cultures of which were 

 prepared by the above-mentioned method of HanserCs. 

 The result of 23 series of experiments with different 

 mixtures was, that only in a single case were 100 colonies 

 developed from 100 cells; that is to say, all the colonies 

 were pure cultures. In all the other series the method 

 proved faulty. In the most unfavourable case 100 colonies 

 were yielded by 135 cells, and the average number obtained 

 was 100 colonies from 108 cells. This proves the plate 

 method to be faulty also in the case of yeast. 



Thus, the advantage of Han-sen's method over Koctis 

 for the pure cultivation of yeast is, that it has a certain 

 starting-point. Even if the plate-cultures are repeated 

 several times, one can never be certain whether the desired 

 result has been attained or not. With regard to the 

 bacteria, however, it is generally impossible to secure a 

 starting-point from one individual cell. In such cases 

 Koch? 8 plate-culture is still the best method we have. 



8. COUNTING THE YEAST-CELLS. 



In the yeast and spirit manufactures it is of impor- 

 tance to determine the multiplying capacity of the yeast- 

 cells during the growth of the yeast. This must naturally be 

 effected by a direct counting of the number of cells which 

 occur in a determinate volume of the liquid at different 

 stages of the fermentation. Experiments having this object 

 in view have been undertaken especially by Delbruck, 

 Durst, Hansen, Hay duck, and Pedersen, whilst Fitz has 

 applied the method of counting to bacteria. 



