228 CULTIVATION AND REPRODUCTION OF YEAST. 



the temperature throughout remains considerably below the 

 optimum reproduction temperature. The usual amount of pitch- 

 ing yeast, namely, about one part by volume of thick barm per 

 2000 of wort, causes fermentation to start quickly ; but when, 

 from any cause, the brewer is restricted to the use of a smaller 

 proportion of yeast, he does not allow this to act on the whole 

 mash at once, but first puts it through a reproduction process in 

 a portion of the wort that has been cooled to i5-2o C., and is 

 maintained at that temperature. Reproduction ensues rapidly 

 and abundantly, so that in a few hours the culture, which is 

 beginning to throw up a " head," can be used for pitching the rest 

 of the wort at the usual temperature. 



Particulars on the influence of temperature on the reproductive 

 capacity of yeast have also been published by A. L. STERN (III.), 

 who, cultivating Burton yeast in a mineral salt nutrient solution 

 treated with dextrose and asparagin, obtained the maximum 

 weight of yeast crop at a temperature between 21 and 25 C. 



The influence of the constitution of the nutrient medium on 

 the reproductive capacity has been established by a series of 

 observations. With regard to the source of nitrogen, F. HESS (I.) 

 was able to prove the superiority of yeast water over mineral salt 

 solutions containing sufficient asparagin or peptone to furnish an 

 approximately equivalent quantity of nitrogen (9.0, 8.2, and 8.5 

 mgrms. per TOO c.c.) and sugar, the following results having been 

 obtained in each 100 c.c. of the liquid during 28 days, the number 

 of cells being referred to each cell in the original sowing (20 cells 

 per c.c.) : 



A similar result was obtained by C. SOLDAN (I.); and the 

 reports of P. THOMAS (I.) and A. L. STERN (III.) have already 

 been mentioned in vol. ii. on p. 214. A certain influence is also 

 exerted by the nature of the source of carbon, as has been shown 

 by Soldan, the Saaz, Frohberg and Logos yeasts exhibiting the 

 highest reproductive capacity when maltose was added, as source 

 of carbon, to nutrient media consisting of mineral-salt solutions 

 containing asparagin or yeast water. Dextrose furnished a medium 

 crop, and saccharose gave the lowest. 



The influence of the concentration of the medium on the repro- 



