264 MICRO-ORGANISMS AND FERMENTATION. 



to J. Wortmann. He states, as the general upshot of his 

 investigations, that the differences in the divers types of 

 genuine wine yeast are sometimes so great that they can be 

 detected merely through the chemical analysis of the products- 

 of fermentation or metabolism ; in other cases, however, they 

 are of such a kind that we can only convince ourselves directly 

 of their presence by their odour and flavour. Every type of 

 yeast shows some individual peculiarity more or less charac- 

 teristic in its action on any must, regardless of its nature or 

 origin. 



The number of yeast cells formed in a given must, apart 

 from the nutritive contents of the must, depends on the specific 

 power of propagation of the chosen type ; on the other hand,, 

 it is in itself independent of the origin of the particular must. 

 In any given must, whether it be an excellent or an indifferent 

 nutrient medium for the wine yeast, one yeast type will multiply 

 more freely than another. 



An extensive comparison of the amount of extract contained 

 in a number of wines fermented with three different yeast 

 species showed that in the same must the " Wiirzburger " 

 yeast consumed the smallest quantity of extract ; next came 

 the " Johannisberger," whilst the " Ahrweiler " yeast used 

 up the largest amount of extract, and, accordingly, left the 

 smallest residue in the wine. 



The specific activity of wine yeasts is clearly brought out 

 in the formation of glycerine, which has a predominant in- 

 fluence in determining the flavour of wine. The three species 

 mentioned above were compared in a large number of musts 

 of different origin, and, on the average, the Wiirzburger yeast 

 formed more glycerine than the other two ; of these, the 

 Johannisberger yeast was superior to the Ahrweiler, which,, 

 as already stated, ferments the extract most vigorously. 



The difference observed between the chemical activities 

 of these species was emphasised by the fact that the Wiirz- 

 burger yeast had multiplied most feebly. This example, 

 amongst others, shows that the alcoholic fermentation of 

 must is independent of the formation of glycerine. It is r 

 therefore, impossible to establish a definite relationship between 

 the contents of glycerine and alcohol in wine. 



