168 CHARACTERIZATION AND IDENTIFICATION 



which do not coincide with those for sporulation, scum formation, 

 and which serve to distinguish between the yeasts. These tempera- 

 ture studies are, then, very useful in identifying yeasts. Below are 

 listed the results of temperature determinations for a few varieties 

 of yeasts cultivated in beer wort. 



Maximum Minimum 



Name of Yeast Temperature Temperature 



C. C. 



Saccharomyces cerevisiae 40 1 to 3 



Saccharomyces pastorianus . . 34 0.5 



Saccharomyces intermedium 40 0.5 



Saccharomyces validus 39-40 0. 5 



Saccharomyces turbidans 40 0.5 



Saccharomyces Marxianus 46-47 0. 5 



Willia anomala 37-38 1.5 



Saccharomyces Ludwigii 37-38 3-1 



Johannisberg yeast II 37-38 0. 5 



An inspection of this table is very instructive. One sees that 

 certain varieties are able to live at high temperatures (S. Marxianus 

 46-47); others, on the contrary, are not able to bud below 34 C. 

 It is also seen that a determination of the temperature limits enables 

 the separation between varieties of the same shape (S. pastorianus 

 and Saccharomyces intermedius) . It has been shown that when yeasts 

 are cultivated at temperatures approaching their maximum tempera- 

 tures, there is a tendency to take the shortest, or spherical, form 

 (Hansen and Klocker). 



Thermal Death Point Determinations 



By this is meant the amount of heat which is necessary to destroy 

 the yeast. This varies, of course, depending on whether it is tried 

 on vegetative cells or spores. It has been stated that the ascospores 

 are very resistant, more so than the vegetative cells. It is then 

 necessary to determine, for each variet}^ the thermal death point 

 for the spore and the vegetative cell. The temperatures which are 

 thus observed vary with the age of the culture and the condition of 

 the cell. A great many factors influence determinations of the thermal 

 death point of microorganisms. Some of these are the reaction of the 

 medium, the time of exposure, the presence of organic matter, the 

 presence of spores, etc. 



Temperature Limits and Optimum for Ascospore Formation 



It has been shown in a preceding chapter that temperature plays 

 an important role in this phenomenon. The investigations of Hansen 



