256 HISTORY OF SACCHAROMYCETES IN NATURE. 



animals, to the primary breeding-places (the sweet, juicy fruits), 

 and thence in turn by the aid of the same factors either to new 

 primary breeding-places, where extensive reproduction occurs, or 

 else to a more modest existence for an inderterminate period in a 

 secondary breeding-place. In the fruit season an important part 

 in the conveyance from one primary breeding-place to another 

 (often over long distances) is played by birds; and, in addition, 

 cells find their way from primary breeding-places to the soil in 

 the excrement of these animals. At present the Hansen theory, 

 especially since his investigations on the secondary breeding- 

 places, affords a natural explanation of all the observations 

 hitherto made. 



The importance of these experiments to the practice of brewing 

 consists chiefly in the light they have thrown on the habitats of 

 wild yeasts, and on the way in which these yeasts can find their 

 way into the brewery. Thus it is evident that atmospheric dust 

 at all times of the year may contain cells of true Saccharotnycetes 

 and also those of wild yeasts. The soil of fruit gardens and vine- 

 yards constitutes the chief source of danger, especially during the 

 season of ripe fruit. In the air analyses referred to above it was 

 found that the greatest risk of infection by yeast cells in the 

 brewery is greatest, for Denmark, in the months of August and 

 September. As a rule these cells find their way into the brewery 

 vid the cooler, but they may also enter the fermenting-room 

 direct. Where the conditions allow the cooler to be abolished, 

 this should be done. Nevertheless, the risk of infection has been 

 greatly diminished by the introduction of pure-culture yeast, the 

 more or less enfeebled wild yeasts gaining access to the wort on 

 the cooler being generally suppressed by the pure yeast in the 

 fermenting vessel. 



