186 VARIATION OF SPECIES 



out by Will * and Jorgensen. 2 Under industrial conditions, degenera- 

 tions of yeasts which have been pure when used have often been 

 obtained. A yeast which may always have given good results 

 may, of a sudden, give a beer with evident defects. The fermentation 

 is too slow or too rapid, or the beer takes on an abnormal taste. A 

 degeneration of the yeast has taken place. Will has noticed that this 

 may occur in the cells of scum yeasts more than in those of bottom 

 yeasts. According to Will, the yeast may be regenerated by repeated 

 culturing. On the other hand Jorgensen has arrived at similar con- 

 clusions. It seems then, that the cells in scum yeasts are more liable 

 to degeneration. 



We shall now consider the results secured by Hansen 3 on the 

 transformation of bottom yeasts into top yeasts. It has been pointed 

 out that, from the industrial viewpoint, yeasts are divided into two 

 groups, bottom and top. The first are those which only produce fer- 

 mentations at the higher temperatures, the second class produce 

 fermentations at the lower temperatures. Hansen had noticed that 

 certain yeasts of the bottom type, after having been cultivated for 

 a period of time at low temperatures are able to induce top fermenta- 

 tions. He then searched for an explanation of this observation, 

 taking Saccharomyces turbidans which is well known as a bottom yeast. 



He inoculated a trace of this yeast into flasks containing beer 

 wort and left them for from 3 to 5 months at a temperature of 5 C. 

 after which he transferred some cells from these flasks to others at 

 more favorable temperatures. The yeast thus obtained produced 

 a top fermentation. Of 130 cells which he examined, none produced a 

 bottom fermentation. Hansen has thus obtained the transformation 

 of a yeast, which is normally a bottom yeast into a top yeast by simply 

 keeping it at a temperature of 5 C. What is the cause of this trans- 

 formation? In order to seek an explanation, Hansen analyzed the 

 properties of the cells of a culture of Saccharomyces turbidans which 

 were subject to this transformation. He examined 100 cells and found 

 that one-half offered characteristics of a top yeast and one-half had 

 the characteristics of a bottom yeast. He extended his observations 

 by inoculating cells from mixed industrial yeasts into separate flasks 

 and incubating at 5 C. At the end of from 3 to 4 months, the cells 

 of the bottom yeasts had given no growth while, on the other hand, 

 the cells from top yeasts had given evidence of development. The 



1 Will, H. Zeitschr. f. d. ges. Brau. 21, 1898. 



2 Jorgensen, A. Untersuchungen iiber das Ausarten der Brauereihefe. Zeit. 

 f. d. ges. Brau. 21, 1898. ' 



3 Hansen, E. C. Oberhefe und Unterhefe. Studien iiber Variation. Cent. 

 Bakt. 15, 1905; 18, 1907. 



