256 MICRO-ORGANISMS AND FERMENTATION. 



normal conditions. It has been found, however (by the experi- 

 ments of HOLM and POULSEN), that it is possible, by HANSEN'S 

 analytical method, to detect with certainty the presence of 

 1-2 00th part of wild yeast. 



From numerous analyses carried out by this method, it has 

 been shown that the rules which were formerly generally 

 accepted for judging a sound fermentation do not suffice for 

 determining the presence of disease-germs, since both the head 

 of the liquid, and the attenuation, breaking, and brightening 

 may be satisfactory in spite of the yeast being strongly con- 

 taminated with disease-germs. 



The question how long a pure culture will remain in its 

 original good state can evidently not be answered in a general 

 way. HANSEN found that different races differ in their power 

 of resisting infection ; likewise the length of time during which 

 a yeast will remain pure and good will vary for the same 

 species in dissimilar fermenting rooms. We also know that 

 the season plays an important part, and that the time of year 

 when wild yeasts, bacteria, and moulds are most abundant in 

 the air, is especially dangerous. Infection is also known to 

 occur at other times of the year, especially from utensils, etc.; 

 disease-germs often gain admission to the brewery through the 

 open coolers; cask sediments form another source of con- 

 tamination. Most frequently, however, brewers and distillers 

 introduce disease-germs into the fermenting vessels with the 

 pitching-yeast obtained from other factories. As contaminations 

 often appear suddenly and show their pernicious effects only at 

 an advanced stage of the secondary fermentation, it is evident 

 that a brewery may supply contaminated pitching-yeast without 

 having the slightest suspicion of its impurity. A vast number 

 of cases examined in the author's laboratory bear evidence to 

 this fact. It must therefore be emphasised that real security is 

 only procurable if an absolutely pure culture is introduced. A 

 necessary adjunct to the system is the biological control of the 

 fermentations as adopted in all rationally-conducted factories. 

 The analysis will always indicate infection long before it has 

 become dangerous, so that a new, pure cultivation of the same 

 yeast can be introduced in good time. A still greater certainty 

 is attained by the continuous working of the yeast-propagating 



