18 



BULLETHnT 819, U. S. DEPARTMENT OF AGRICULTURE. 



RELATION TO TEMPERATURE. 



As has been stated previously, the pink yeast grows best at room 

 temperature (21° to 25° C), whereas no growth occurs in the 

 37° C. incubator. The yeast grows but sparsely at refrigerator 

 temperature (6° C), and with but slight pigment production, the 

 bright pink color appearing only after several days. 



Thekmai, Death Point. 



In view of these facts, it was assumed that the thermal death point 



of the. yeast would be low, so that in determining this temperature 



the start was made at 36° C. The method used was taken from the 



Standard Methods of Water Analysis of the American Public Health 



Association. A 48-hour-old culture grown at room temperature was 



used. 



Table 7. — Thermal death point of the pink yeast. 









Result. 











Result. 





Tem- 













Tem- 

 per- 









per- 































atvire. 



1 



2 



3 



4 



5 



6 



7 



ature. 



1 



2 



3 



4 



5 



6 



7 





day. 



days. 



days. 



days. 



days. 



days. 



days. 





day. 



days. 



days. 



days. 



days. 



days. 



days. 



"C. 

















"C. 

















48 



+ 



+ 



+ 



++ 



++ 



++ 



++ 



58 











+ 



4- 



4- 



+ 



-1- + 



50 











+ 



+ 



+ + 



+ + 



++ 



60 











-f- 



+ 



+ 



-f- 



-I--I- 



52 











+ 



+ 



- + 



+ 



+ 



62 











+ 



-1- 



-1- 



-1-1- 



+-f 



54 











+ 



+ 



+ 



+ 



+ 



64 































56 











+ 



+ 



+ . 



+ 



' + 



66 































In Table 7, + indicates that the yeast was living in the tubes 

 after exposure to the heat, and indicates that there was no growth. 



The temperature of 36° C, at which the experiment was started, 

 was very much too low, and the heat had no apparent effect on the 

 yeast up to 48° C, The temperatures below 48° C. are therefore 

 omitted from the table, as are the records of the control tubes in 

 which there was no growth. By raising the temperature of the bath 

 2 degrees at a time it has been found that the thermal death point 

 of the yeast is 64° C. There is apparently no relation between the 

 optimum temperature of the yeast and its thermal death point. 

 In working on the thermal death point of the true yeast used in 

 bread making. Wells (1917) found that the thermal death point 

 of yeast in bread making is approximately 68° C, slightly higher 

 than was found by the author to be the case with the i^ink yeast. 



RELATION TO REACTION OF MEDIUM. 



Experiments on the relation of the growth of the pink yeast to 

 the acidity or alkalinity of the culture medium were conducted to 

 determine the limits of alkalinity or acidity at which the yeast would 

 grow; also to find the optimum reaction. Dextrose broth was pre- 

 pared, and divided into 60 cc. portions. The various portions of 

 media were titrated, and the reactions corrected, so that amounts of 

 media were obtained with titre varying from — 9 per cent alkaline, 

 to -j- 10 per cent acid. 



