PRODUCED BY ALCOHOLIC FERMENTS. 187 



similar diseases is by no means small. All these disease 

 yeasts exhibit several characteristics, by means of which they 

 can be distinguished from brewery yeasts. Accurate ex- 

 periments have further shown that the disease yeasts gain 

 admission into the brewery from without, and that they are 

 not forms of development of the brewery yeast. 



Since the introduction of my pure yeast system into 

 breweries, favourable opportunities have occurred in practice 

 of making important observations in different directions. In 

 the two breweries, Old and New Carlsberg, I have thus for 

 many years studied the cultivation of pure yeast on the large 

 scale, and never have the brewery yeasts shown any sign of 

 developing forms like those of the disease yeasts mentioned ; 

 on the contrary, under the conditions obtaining in the brewery, 

 they always retained their specific characters. The theories 

 of the degeneration and transformation of yeast have thus, in 

 this respect, proved to be qtdte untenable. 



Just as all organisms are subject to variations, so this is 

 also evidently the case with the species of brewing yeasts. 

 As long as they are subject to the conditions obtaining in 

 the brewery, however, they only exhibit slight modifications, 

 and these are only of a temporary nature. When we regard 

 them from a biological point of view, we are inclined to look 

 upon them as quite insignificant ; for the practical brewer, 

 however, the matter is quite different. These changes can, 

 indeed, occur in a very disagreeable manner, and sometimes 

 cause an appreciable irregularity. In the course of a year 

 they pass like a wave through the brewery, and in most 

 cases we have no idea of their cause. 



The question of the variation of species of yeasts is thus 

 not only of great theoretical, but of equally great practical 

 interest. A review of the experimental investigations 

 which I carried out in this direction will be found on 

 pp. 92-102. These studies are being continued without 

 break in my laboratory. Any further discussion of them 



