136 MICRO-ORGANISMS AND FERMENTATION. 



analysis can be performed with mixtures such as ordinary 

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

 time. 



When the object of the analysis is to more accurately 

 characterise the different species present in the sample, a 

 number of cells are isolated by fractionation, and each of the 

 growths obtained is separately examined. 



In an investigation on bottom-yeast during the different 

 stages of the primary fermentation, published by Hansen in 

 1883, it was shown that as a rule the wild yeasts are present 

 in largest amount during the last stages of primary fermen- 

 tation in the upper layers of the liquid. The samples of the 

 liquid which are taken from the fermenting vessel for the 

 analysis of the yeast, must therefore, as stated above, be taken 

 during the last days of the primary fermentation. If much 

 time elapses before an analysis is commenced, the yeast must 

 be introduced into wort, and one or more fermentations carried 

 out ; and this applies whether the yeast to be examined was 

 in a dry or a liquid state. 1 



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 link in the system will, 



1 The observation mentioned above with reference to low-fermen- 

 tation 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 liters capacity ; by counting 

 the cells and by means of cultures the relative proportions of the 

 different species were determined. It is found, however, from the 

 experiments hitherto conducted by Vuylsteke that the rule mentioned 

 is not of general application in the case of mixtures of high-fermentation 

 yeasts with wild yeasts. In some experiments with mixtures of Sacch. 

 ce?'evisi<e I., Hansen, and Sacch, Pastorianus /., Hansen, the wild yeast 

 was found to have increased towards the end of the primary fermenta- 

 tion, whilst in other experiments a diminution of the wild yeast was 

 observed. On the other hand, all the experiments with mixtures of 

 Sacch. cerevisiee /., and Sacch. Past. 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. 



