148 WHAT IS THE PURE YEAST OF PASTEUR f 



as demanded by the last objections raised by Velten, and a 

 yeast mixture was made use of in which the proportion of 

 disease yeasts was extremely small. My assistant, Mr. 

 Nielsen, assisted me in these experiments. 



Experiments VI L and VIII. In these new experiments I employed 

 the ordinary pitching yeast of a low-fermentation brewery in which the 

 fermentations were perfectly normal and the beer of which was in every 

 respect satisfactory. This brewery was provided with yeast from a pure 

 yeast propagating apparatus, containing an absolutely pure culture of one 

 of the species of bottom yeasts which yield spores with great difficulty. 

 In the ordinary gypsum block cultures of this yeast, the cells developed 

 either no spores at all, or, at most, very few after six to seven days at 

 25 C. The spores found had further the characteristic appearance of 

 the spores of a culture yeast, and could be distinguished microscopically 

 from the spores of a wild yeast, My method for the analysis of brewery 

 yeast could, therefore, be employed with safety. In the samples which I 

 examined in this way, I was unable to discover traces of wild yeast, and 

 the microscopic examination likewise indicated that the whole of the 

 yeast consisted only of the low-fermentation species of the brewery. 

 Holm and Poulsen have submitted my method to a thorough examination, 

 and have found that by its means it is possible to detect as little as 0*5 

 per cent, of wild yeast when admixed with a brewery yeast (' Compte- 

 rendu des travaux du laboratoire de Carlsberg,' 2 vol. 4 and 5 livr. 

 1886-88). The examination of the pitching yeast under discussion 

 showed then that it contained either no wild yeast at all, or, at any rate, 

 only extremely minute traces. As I took this yeast as the starting point 

 for my analysis, it is evident that the objection raised by Velten that my 

 yeast mixture had an abnormal composition such as never occurs in 

 practice, does not apply in this case. The cultivation of the yeast in a 

 solution of cane sugar and tartaric acid was carried out in the manner 

 described in the fifth and sixth experiments. Two series of experiments 

 were conducted with this brewery yeast, one at the ordinary room-tem- 

 perature (as a rule 17 C. during the day, and 10 C. at night), and the 

 other at a constant temperature of about 9 C. 



In the first series the composition of the yeast had not only changed 

 after four and five cultivations in the sugar solution, but even in the third 

 cultivation, and to such an extent that after cultivation in wort a growth 

 was obtained which consisted in the main of wild yeast. The ordinary 

 gypsum block cultures showed, after three to four days at 25 C., a very 

 abundant development of wild yeast cells with spores of a highly typical 

 appearance, and the microscopical examination of the newly formed 

 yeast likewise showed that most of the cells had the same appearance 

 as those of the ellipsoideus and Pastorianus groups. In the second 



