ALCOHOLIC FERMENTS. 155 



analysis of yeast by means of ascospore-formation gives ample 

 information as to the presence of these disease ferments. 



This method likewise possesses the advantage that the 

 analysis can be performed with mixtures similar to ordinary 

 pitching -yeast, and that it can be performed in a short time. 



When the object of the analysis is to characterise with 

 greater accuracy the different species present in. the sample, a 

 number of cells are isolated by fr action ation, and each of the 

 growths obtained is separately examined. 



In an investigation of bottom-yeast in the different stages 

 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 the yeast must, 

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

 fermentation. If considerable time has elapsed before an 

 analysis is commenced, the yeast must be introduced into 

 wort, and one or more complete fermentations carried out ; 

 and this applies to yeast supplied in either a dry or a liquid 

 state. 1 



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 



1 This observation regarding low-fermentation yeast has been confirmed by 

 J. VUYLSTEKE'S experiments, in which fermentations were carried out with 

 mixtures of different Saccharomycetes in cylindrical glass vessels of about two 

 litres capacity ; by counting the cells and by means of cultures, the relative 

 proportions of the different species were determined. VUYLSTEKE'S experi- 

 ments have shown that when his conditions are adopted this rule is not gene- 

 rally applicable to the case of mixtures of high -fermentation yeasts with wild 

 yeasts. In some experiments with mixtures of Sacch. cerevisice I. (HANSEN) 

 and Saccit. Pavtorianus I. (HANSEN), the wild yeast was found to have 

 increased towards the end of the primary fermentation, whilst in other 

 experiments a diminution of the wild yeast was observed. On the other 

 hand, all the experiments with mixtures of Sacch. cerevince I. and Sacch. 

 Pa*t. III. showed that the impurity was greater in the upper layers of 

 the liquid at the end of the primary fermentation than at the commencement, 

 just as in the case of bottom-fermentation. 



