172 CHARACTERIZATION AND IDENTIFICATION 



arating species. Hansen has shown that scum formation is related 

 to the temperature. Certain temperature limits exist, minimum and 

 maximum. They vary with the species and are easy to determine. 

 When once determined, they are very useful for the separation of 

 species. 



The scums have different macroscopic characteristics, depending on 

 the variety and the temperature of culture; it may cover the surface 

 entirely, float about as a small island, or appear as a ring around the 

 walls of the container. In most cases the cells which make up the 

 scum are united end to end to give somewhat the appearance of a 

 rudimentary mycelium. In some of the bottom yeasts and the indus- 

 trial varieties, one may observe the presence of durable cells. On the 

 scum, then, one should determine the temperature limits, the optimum 

 temperature, the macroscopic appearance of the scum at different 

 temperatures and the microscopic appearance of the cells which make 

 up this scum at different temperatures. The relation of temperature 

 to scum formation is a very important characteristic. This determina- 

 tion furnishes important data. The following tables, prepared from 

 data secured by Hansen, are interesting. 



TEMPERATURES AT WHICH SCUM FORMATION TAKES PLACE WITH Six 

 VARIETIES AS DETERMINED BY HANSEN 



,, , T , Maximum Optimum Minimum 



Name of Yeast 



degrees degrees 



Saccharomyces cerevisiae. . . 33-34 20-22 6-7 



Pastorianus 26-28 26-28 3-5.8 



" intermedium 26-28 26-28 3-5 



" validus. 26-23 26-28 3-5 



" ettipsoideus 33-34 33-34 6-7 



turbidans 36-38 33-34 3-5 



TIMES AT WHICH SCUMS BEGIN TO APPEAR IN THE Six VARIETIES 

 STUDIED BY HANSEN 



(Time expressed in days) 



,, , ,, Maximum Optimum Minimum 



Name of Yeast , 



degrees degrees degrees 



Saccharomyces cerevisiae 9 to 18 7 to 10 2 to 3 l 



" Pastorianus 7 to 10 7 to 10 5 to 6 l 



" intermedius 7 to 10 7 to 10 5 to 6 l 



validus 7 to 10 7 to 10 5 toe 1 



" ellipsoideus 8 to 12 8 to 12 2 to 3 * 



" turbidans 8 to 12 3 to 4 5 to 6 l 



It is evident that Saccharomyces ellipsoideus is distinguished 

 from Saccharomyces turbidans by its maximum temperature. On the 



Months. 



