2 PURE CULTURES OF 



lie in the yeast cells themselves, and that these apparently 

 similar cells might possibly belong to different species. It 

 was from this starting point that my investigations on the 

 Saccharomycetes gradually developed. The practical results 

 to which they led were in the first place a new analytical 

 method, and the certain demonstration that some of the com- 

 monest and most serious diseases of beer, such as yeast 

 turbidity and objectionable changes in flavour, were caused not 

 by bacteria but by certain species of yeast. 



It was only after this had been proved by exact experi- 

 ments, both in the laboratory and on a large scale in the 

 brewery, that it was evident that it was not sufficient to 

 purify the yeast from bacteria and mould fungi, but that the 

 question must be treated from a quite different point of view. 

 And when I further showed that there are also different 

 species of good brewery yeast which produce beers of 

 different character, it also became clear that the pitching yeast 

 should consist only of a single species, namely, that best suited 

 to the brewery in question. 



A short preliminary account of how this reform was 

 effected and might be carried further, was published in the 

 'Zeitschrift f. d. ges. Brauwesen' for 1884. The first detailed 

 description, however, appeared in 1888, in the first German 

 edition of this work. In some respects it is as well that it 

 was not published earlier ; I had in the meantime gained 

 greater experience, and was thereby in a position to sum- 

 marise the most important publications which had appeared 

 in the interval, and which threw light on my work, partly 

 in support of it and partly against it. It would be out of 

 place here to quote these treatises ; they are published in the 

 journals relating to the fermentation industries, and especially 

 in the brewing journals, all the leading zymotechnologists 

 having added their contributions. A list of the most im- 

 portant authors will be found in the first German editions 

 of this book. 



