156 MICRO-ORGANISMS AND FERMENTATION. 



known, it was this very appearance of wild yeast in English 

 top- fermentation breweries which gave rise to the erroneous 

 view that such species are necessary for conducting a normal 

 secondary fermentation. The assertions advanced in other 

 quarters that wild yeasts do not usually occur in any con- 

 siderable quantity in top-fermentation breweries can only be 

 attributed to incorrect observations. 



It is evident, however, that, valuable as the analysis of 

 yeast is, it must always remain of secondary importance in the 

 brewery ; the most important factor, whatever the conditions, 

 will be the employment of a pure cultivation of a selected 

 species of yeast. 



2. The analysis of the yeast in the propagating apparatus, 

 which must be absolutely pure, is carried out as follows: At 

 the conclusion of fermentation, samples are withdrawn, with 

 every precaution, into Pasteur flasks or into the Hansen flasks 

 employed for sending out yeast samples ; from these, small 

 quantities are introduced into flasks containing neutral or 

 slightly alkaline yeast water or yeast-water dextrose, and 

 these are maintained at a temperature of 25 C., the object 

 being to test the yeast for bacteria. The remainder is set 

 aside for the yeast to settle, the beer is decanted, and a 

 sample portion of the sediment is introduced into a cane- 

 sugar solution containing 1 to 4 per cent, of tartaric acid. After 

 three or four cultivations in such a solution it is further 

 cultivated a few times in beer-wort, and then tested for 

 spore-formation. The smallest traces of wild yeast in the 

 apparatus are brought into a state of vigorous development 

 by this treatment (see Chapter I., Physiological Methods 1 ). 



(c) The Formation of Films. By studying the formation 

 of films, HANSEN has discovered an entirely fresh set of 

 characteristics for the Saccharomycetes. Earlier information 

 by various writers not being in accordance with facts, the 

 credit is due to HANSEN of having opened up this field of 

 research. 



1 It is evident that this method is not available for the analysis of ordinary 

 yeast, because the cultivation in the tartaric solution will cause the wild yeast- 

 cells to increase very considerably in number, and consequently render it 

 impossible for the analyst to judge of the degree of contamination. 



