METHODS OF STUDYING THE YEASTS 165 



the best method for preserving yeasts consists in cultivating them 

 in a 10 per cent sucrose solution with acid. The sucrose does 

 not ferment and is used very slowly. Of 42 yeasts subjected to this 

 method only two have been encountered which did not withstand 

 such a solution. S. Ludwigii did not keep longer than 2 years, 

 often 6, and S. Monacensis has not survived more tha*n two or 

 three years. The others have been kept for from 13 to 17 years. 

 Generally speaking, the method is a satisfactory one. The Hansen 

 flask is generally used. (Fig. 70.) Jorgensen has sug- 

 gested a modification of this flask which prevents 

 evaporation. 



Will * has proposed another method which consists 

 of drying the yeast and mixing it with powdered silica, 

 plaster of Paris and carbon, the whole being dried at 

 40 and sealed hermetically. By this method certain 

 yeasts have been kept for 9 years; Hansen has shown 



that the yeasts form ascospores during this period. _ H 



We have seen that desiccation is unfavorable to yeasts sen's Flask 

 and the cells form ascospores. * r Culturing 



In other investigations, Will 2 has shown that the 

 preservation of the yeasts depends upon three factors, 1, the quantity 

 of yeast, 2, the composition of the medium, and 3, the temperature. 



1 Will, H. Einige Beobachtungen iiber die Lebendauer getrockneter Hefe. 

 Zeit. f. d. Ges. Brau., 19 and 27, 1896-1904. Beobachtungen an Hefekonserva- 

 tion. Cent. Bakt. 24, 1909. 



2 Will, H. Beobachtungen iiber Vorkommen leben- und vermehrungsfahigen 

 zellen in sehr alten Wiirze Kulturen von untergarigen Bierhefen. Cent. Bakt. 

 44, 1916. 



