208 MICRO-ORGANISMS AND FERMENTATION. 



for the practical analysis of breiuery yeast (page 134), by 

 means of which it is possible to ensure in time against 

 foreign yeasts prevailing. It was previously proved by his 

 experiments on a large scale that the forms which produce 

 yeast turbidity may be present to the extent of one part 

 in forty-one of the pitching-yeast, and the species (Sacch. 

 Pastorianus I.) which produces a disagreeable odour and 

 an objectionable bitter taste to the extent of one part in 

 twenty-two, without exercising any injurious influence, pro- 

 vided the brewing operations are conducted under normal 

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

 ments of Holm and Poulsen\ that it is possible, by Hanserts 

 analytical method, to detect with certainty the presence of 

 1 -200th 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 contaminated with disease-germs. 



The question how long will a pure culture 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, &c. ; disease-germs often gain admis- 

 sion to the brewery through the open coolers ; cask sediments 

 form another source of contamination. Most frequently, 

 however, brewers introduce disease-germs into the fermenting 

 vessels with the pitching-yeast which they obtain from other 



