152 WHAT IS THE PURE YEAST OF PASTEUR? 



deserves further investigation, and may be recommended to 

 the attention of zymotechnologists. 



In many of the breweries where my pure yeast system 

 has been introduced, the closed propagating apparatus men- 

 tioned above is now employed in which the yeast is developed 

 in a state of absolute purity. It is evident that there must 

 be no contamination whatever in the fermenting cylinder, and, 

 therefore, of course, no trace of wild yeast. If the apparatus 

 is handled with strict care, the yeast in it will remain pure for 

 an indefinite period. I know breweries in which the apparatus 

 has been in uninterrupted use for years without it being 

 necessary to introduce a fresh pure culture. But if the 

 apparatus is not properly handled, contamination may readily 

 occur. There is, however, no necessity to further discuss 

 this matter, as full information, both with regard to the 

 construction of the apparatus and its employment in the 

 brewery, will be found in the earlier part of this book. 



From what has been stated above, it is evident that the 

 yeast in the apparatus should, from time to time, be sub- 

 mitted to a rigorous test. For this purpose, samples of the 

 fermenting wort should be carefully drawn from the ferment- 

 ing cylinder ; this is best done towards the end of the 

 primary fermentation, for at this stage it is easier to detect 

 both bacteria and wild yeasts if they are present. From 

 these samples small portions are introduced into flasks con- 

 taining yeast extract, and these are exposed to a moderately 

 high temperature e.g. 25 C, in order to ascertain whether 

 bacteria are present. The remainder is allowed to stand 

 until the yeast has settled to the bottom ; the beer is then 

 poured off and average samples of the yeast introduced into 

 the tartaric acid and sugar solution. The further treatment is 

 carried out in the manner described in the fifth and sixth ex- 

 periments at the ordinary room-temperature or at 25 C. ; it 

 is sufficient to carry out three or at most four cultivations in 

 the sugar solution. After this treatment, the yeast is culti- 



