158 MICRO-ORGANISMS AND FERMENTATION. 



set up a vigorous fermentation, and again rise with the bubbles 

 of carbon dioxide to the surface, where they enter upon a new 

 phase of development. If sterilised lager beer is infected with 

 this fungus, no fermentation sets in, and only a thin film 

 resembling dust is developed ; under other conditions the 

 fungus forms a white, floury, or woolly layer, as in the case 

 of O'idium. 



The true Saccharomycetes also form films, which, however, 

 differ somewhat from those just mentioned; this is also the case 

 with some of HANSEN'S Tonda and with Saccharomyces apicu- 

 latus. From these observations it is evident that the formation 

 of films is not a peculiarity of certain species, but must be 

 regarded as a phenomenon common to all micro-organisms. 



In the case of the Saccharomycetcs this phenomenon generally 

 occurs in the following manner : If cultures in wort are left 

 undisturbed for a longer or shorter time at the ordinary room 

 temperature, small specks of yeast gradually appear on the 

 surface of the liquid, after the termination of the primary 

 fermentation ; these can afterwards coalesce to figures of differ- 

 ent forms and sizes isolated patches the upper surfaces of 

 which are flat and the under surfaces arched. Finally, they 

 unite to form a coherent and generally a light greyish-yellow, 

 glutinous film, which may extend to the walls of the glass 

 vessel, forming a complete ring. Such a perfect film-formation 

 only occurs after the primary fermentation is at an end. If 

 the flask is shaken, fragments of the film are detached and 

 sink to the bottom; and in this way a complete layer may 

 gradually collect at the bottom, whilst the film is continually 

 renewed, and assumes a mottled appearance owing to the 

 fresher portions being thin and dark, whilst the older parts are 

 thick and light. 



The conditions under which a film can be formed are a free, 

 still surface, with direct access of air ; and a vigorous film- 

 formation presupposes an abundant supply of air. The 

 function of film-formation is thus subject to the same con- 

 ditions as obtain in the case of endogenous spore-formation. 



Simultaneously with the formation of a film, a decoloration 

 of the wort takes place, the latter turning to a pale yellow 

 colour This reaction takes place most quickly at higher 



