BACTERIA 



41 



that they do not ferment lactose to any appreciable extent. Even 

 though they may make milk slightly acid to begin with, they 

 always turn it alkaline at last. 



THE PROPIONIC ACID FERMENTATION 



The second fermentation process to be described is that in 

 which sugar or calcium lactate is converted into propionic, acetic 

 and carbonic acids. This fermentation is of particular interest in 

 dairy practice in connection with the formation of cavities or 

 " eyes " in cheese. The author's investigations on the ripening 

 of Emmental cheese have shown that the above-mentioned 

 products mostly appear at the stage at which the eyes are formed l ; 



FIG. 46. Bacterium acidi propionici (a). 

 X 1,000. 



FIG. 47. Bacterium acidi propionici (6). 

 Grown at 39 C. x 1,000. 



and Clark has shown that normal eyes contain nothing but carbon 

 dioxide 2 . The presence of the organisms in question can only be 

 demonstrated after several successive inoculations into peptone 

 broth containing calcium ' lactate instead of sugar. If such a 

 medium be innoculated with almost any kind of cheese, active 

 propionic acid fermentation will be found to have set in after 

 systematic cultivation for a week or two. 



Bacterium acidi propionici was first isolated by Freudenreich 

 and the author 3 . These bacteria are non-sporing, non-motile and 

 Gram-positive. Several species are known which differ consider- 

 ably in appearance. The species (a), which is mainly instrumental 



1 " Studier over de flygtige Syrer i Ost." Doctoral. thesis, published by 

 J. Gjellerup, Copenhagen. 



2 U.S. Dept. of Agriculture, Bureau of Animal Industry, Bull. 151. 1912. 



3 "Uber die im Emmenthalerkase stattfindende Propionsauregarung " 

 ("Landwirt. Jahrbuch der Schweiz," 1906, p. 320.) 



