ANALYSIS OF YEAST. 313 



of the primary fermentation, published by Hansen in 1883, 

 it was shown that the young cells of wild yeasts are present 

 in largest amount during the last stages of primary fermenta- 

 tion and in the upper layers of the liquid. The samples taken 

 from the fermenting vessel for the analysis of yeast must, 

 therefore, be taken during the last few days of the primary 

 fermentation. If a dried or partially dried sample of yeast 

 is to be examined, it must first be transferred to wort, and one 

 or more fermentations must be completely carried out with it. 



The rule that wild yeasts develop only in the more advanced 

 stages of fermentation applies also to top-fermentation yeast 

 as used in breweries. This was shown by numerous analyses 

 of beer from Danish, English, French, and German breweries 

 carried out in the author's laboratory. As is well known, it 

 was this very appearance of wild yeast in English top-fer- 

 mentation breweries which gave rise to the erroneous view 

 that such species are necessary for conducting a normal 

 secondary fermentation. 



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 the conclusion of fermentation, samples are withdrawn, with 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 maintained at a temperature of 25 C., the object 

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

 aside to allow the yeast to settle, the beer is decanted, and an 

 average sample 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 culti- 

 vated 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.* 



* 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 im- 

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



