WHAT IS THE PURE YEAST OF PASTEUR? 151 



the present day without resort to antiseptics, and the way 

 to do so is to maintain strict cleanliness and order, and to 

 employ a good pure culture yeast in the fermenting vessels. 



When it is a question as to whether wild yeasts are 

 present or not in a certain pitching yeast, the method which 

 I indicated some years ago is employed, that is, culture tests 

 for spore formation are carried out at 25 and 15 C. Under 

 these conditions, the wild yeasts form spores earlier than 

 the brewery yeasts, and there is a further difference in the 

 appearance of the spores. As was mentioned above, it is 

 possible by means of this method to detect as little as 

 o 5 per cent, of wild yeast in a brewery yeast. Neverthe- 

 less, the detection of such a small admixture is no easy 

 matter, and requires a good deal of practice. It has been 

 shown, however, from the above that cultivation in a 10 per 

 cent solution of cane sugar containing 4 per cent, of tartaric 

 acid, affords an excellent means of ascertaining the presence 

 or absence of wild yeasts in a pitching yeast. In all the 

 cases examined, the brewery yeast was suppressed by the 

 wild yeasts when the latter were present at the commence- 

 ment, and this occurred both with high and low brewery 

 yeasts. My experiments in this direction were so numerous 

 that the rule which they enabled me to establish is certainly 

 one of wide application ; it is, however, highly probable that 

 exceptions may be found by further research. Every method 

 is known to have its limits, and this is especially the case with 

 biological methods. For the practical analysis of the yeast in 

 the fermenting vessels, this test is, however, too sensitive, 

 for, as we have seen, my experiments proved that a good 

 brewery yeast showed, by this test, a high degree of con- 

 tamination with wild yeast. Its employment might, therefore, 

 lead to a pitching yeast being condemned, which for practical 

 purposes is really a good yeast Whether the method could 

 be so modified as to render it applicable to the above 

 purpose, has not been investigated. The question, however, 



