124 BACTERIA 



flask from movement, and protect it, not from air, which is 

 necessary, but from falling particles in the air. In a short 

 time small colonies appear, which coalesce and form patches, 

 then a film or membrane which covers the liquid and attaches 

 itself to the sides of the flask. By the differences in the 

 films and the temperatures at which they form it is possible 

 to obtain something of a basis for classification. The further 

 advances in a yeast culture and in our knowledge of the agen- 

 cies of fermentation have, however, tended to show that no 

 strict dividing lines can be drawn. Hansen's researches have, 

 notwithstanding, been of the greatest moment to the whole 

 industry of fermentation. What has been found true in 

 bacteriology has also been demonstrated in fermentation, 

 namely, that though many yeasts differ but little in struct- 

 ure and behaviour, they may produce very different products 

 and possess very different properties. Industrial cultivation 

 of these finer differences in fermentative action has to a large 

 extent revolutionised the brewing industry. 



The formation of films is not a peculiarity of certain 

 species, but must be regarded as a phenomenon occurring 

 somewhat commonly amongst yeasts. The requisites are 

 a suitable medium, a yeast cell, a free, still surface, direct 

 access of air, and a favourable temperature. The wort loses 

 colour, and becomes pale yellow. Microscopic differences 

 soon appear between the sedimentary yeast and the film 

 yeast of the same species, the latter growing out into long 

 mycelial forms, the character of which depends in part upon 

 the temperature. This often varies from 3 to 38 C. 



A fourth point helpful in diagnosis is the temperature 

 which proves to be the thermal death-point. Sac char omyces 

 cerevisice is killed by an exposure to 54 C. for five minutes, 

 and 62 C. kills the spores. As a rule, yeasts can resist a 

 considerably higher temperature when in a dry state than 

 in the presence of moisture. 



Lastly, yeasts may be cultivated on solid media. Hansen 



