154 WHAT IS THE PURE YEAST OF PASTEUR? 



and finally, if the taste of the beer does not become impaired 

 in any other way than through flatness, there is every reason 

 to presume that the yeast was pure." The test affords a 

 good illustration of the standpoint of research at that time. 



If it is merely desired to ascertain whether bacteria and 

 film-forming species of Mycoderma are present, the method 

 is very useful. On the other hand, it is quite useless when 

 it is also a question as to whether the yeast cells present 

 belong to one or several species. The sedimentary yeast 

 may, in fact, consist of a mixture of a good beer yeast with 

 some of the worst species of disease yeasts, without the 

 possibility of these being discovered under the conditions 

 described. The microscopic examination does not suffice 

 in this case, and the same applies to the other characters. 

 This is seen even from theoretical considerations, and 

 tangible proof is obtained by direct experiment. Pasteur's 

 test can thus only be of service where it is exclusively a 

 question of bacteria and Mycoderma* 



The most definite statement of Pasteur's, with reference 

 to this question is in the * Bulletin de la Societe d'encourage- 

 ment pour 1'industrie nationale ' (Janvier 1887, p. 45), in which 

 he says " Hansen was the first to perceive that brewery yeast 

 must be pure not only as regards bacteria, the true disease 

 ferments, but that it has also to be freed from the wild 

 species of yeast." 



Pasteur's work and mine start from two different stand- 

 points. With Pasteur it was the bacteria which produce the 

 diseases of beer, and accordingly the problem with him was 

 to free the yeast from those minute organisms, and this he 

 attained by means of the method described. His object was 



* In some zymotechnic journals, the attention of brewers has recently been 

 called to Pasteur's test described above as a means for ascertaining how the 

 beer will turn out in practice. This is, however, a great mistake. The beer 

 produced in the flask is of a very different character from that produced in the 

 brewery, even when the yeast and the wort are the same. The fermentation 

 and the struggle between the organisms present take place under such different 

 conditions in the two cases that no comparison can be made. 



