108 PHYSIOLOGY OF YEASTS (Continued) 



interesting data on the longevity of yeasts. He used 25 pure cul- 

 tures grown in 10 per cent cane sugar solutions without renewal. 

 Fifteen of these yeasts retained their vitality for 10J years, while 

 nine of them died after eight and one-half years. One remained alive. 

 Gayon and Dubourg 1 made an investigation which bears indirectly 

 on this subject. They examined wines made in 1810, 1818, 1819, 1832, 

 1836 and 1846 and found living yeasts capable of causing alcoholic 

 fermentation. 



ACTION OF PHYSICAL AGENTS ON THE YEASTS 



Temperature 



Moist yeasts die generally between 50 and 55 C. some being able 

 to withstand 60 (Hansen and Kayser 2 ). In the dry state, they re- 

 sist more elevated temperatures. Certain are able to withstand, with- 

 out perishing, a temperature of from 100 to 110, others from 115 to 

 120. The ascospores are much more resistant to heat and generally 

 withstand a temperature which is about 5 higher than the vegetative 

 cell. (Kayser.) On the other hand, the investigations of Pictet and 

 Young indicate that the yeasts are capable of resisting very intense 

 cold. These investigators have submitted yeasts to temperatures of 

 130 below zero for 24 hours without killing them. Doemus has stated 

 that the yeast of Frohberg could resist temperatures of 150 for 

 from 5 to 20 minutes. Cochran and Perkins 3 investigated the effect 

 of high temperatures on yeast. They heated their yeasts in a syrup 

 and found that 58 C. for 30 minutes did not stop fermentation; 65 C. 

 for the same length 'of time caused a devitalization of the yeast so 

 that fermentation was reduced. The yeasts were killed at 70 C. 

 Wells 4 found that the thermal death point in yeasts is considerably 

 effected by the presence of certain substances. Starch and sugars 

 were found to raise it. He states that the approximate thermal death 

 point in bread is about 68 C. It is quite well known, however, that 

 bread may contain living cells since, during the baking process, the 

 temperature is not high enough to kill all of the yeasts. 



1 Gayon and Dubourg. Experiments on the vitality of yeasts. Rev. vit. 

 38, 5-8. 



2 Kayser, E. Action de la chaleur sur les levures. Ann. Inst. Past. 3, 1889. 

 s Cochran, C. B. and Perkins, J. H. The effect of high temperatures on 



yeast. Jour. Ind. Eng. Chem. 6 (1914) 480. 



4 Wells, E. P. The thermal death point in yeast. Vermont Agricultural Ex- 

 periment Station, Bull. 203, 1917. 



