100 BACTERIA AND FERMENTATION 



cells. It is now vigorously shaken, and the contents are divided 

 into twenty portions of 1 c.c. each, and added to twenty tubes of 

 sterilised water. It is highly probable that half of these tubes 

 have received one cell each. In the course of a few days it can be 

 seen how far the culture is pure. If only one colony is present, 

 the culture is a pure one, and as this grows we obtain an absolutely 

 pure culture in necessary quantity. Even when the gelatine plate 

 method is used, it is desirable to start with a single cell (Hansen). 

 The advantage of Hansen's yeast method over Koch's bacterial plate 

 method is that it has a certain definite starting-point. This is 

 obviously impossible when dealing with such microscopic particles 

 as the bacteria proper. 



A third point in the differentiation of yeast species is the question 

 of films. Hansen set to work, after having obtained pure cultures 

 and ascospores, to examine films appearing on the surface of liquids 

 undergoing fermentation. The object of this was to ascertain 

 whether all yeasts produced the same mycelial growth on the surface 

 of the fermenting fluid. To produce these films the process is as 

 follows : Drop on to the surface of sterilised wort in a flask a very 

 small quantity of a pure culture of yeast ; secure the 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, and finally 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 yeast culture and in our knowledge of the 

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

 strict dividing lines can be drawn. Hansen's researches have, not- 

 withstanding, 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 structure 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. 



