66 Dairy Bacteriology. 



" sweet curdling. " In some cases the curdled casein may 

 remain intact; in others it steadily diminishes in volume, 

 a turbid and somewhat colored watery fluid separating 

 from it. In this stage, the milk is said to have "wheyed 

 off.' 1 



The physical appearance of milk undergoing these fer- 

 mentations is materiall}* different from that where normal 

 souring occurs. The curd of sour milk is hard and breaks 

 with a fractured surface, while the coagulum in these 

 cases is soft and somewhat slimy. In the later stages of 

 the digestive process, the milk assumes a watery appear- 

 ance. 



These fermentations assume two phases: 1. Curdling 

 followed by a subsequent digestion of the casein; 2. 

 Digestion or peptonization of the casein without any ap- 

 parent previous coagulation. 



A great variety of bacteria are able to participate in 

 these changes, particularly those belonging to the class 

 represented by the hay and potato bacilli. This group of 

 bacteria as a rule are able to liquefy gelatin, a fermenta- 

 tive change of a similar nature to the digestion of the 

 casein. 1 In fact they are frequently referred to as 

 casein- ferments. The characteristic of these fermenta- 

 tions is the production of certain unorganized ferments 

 or enzymes that have the power of acting on the proteid 

 molecule independent of vital activity. The two ferments 

 that are best known are the rennet or curdling enzyme, 

 and the tryptic or digesting enzyme. As a rule any or- 

 ganism that possesses the digestive power, first -causes a 

 coagulation of the casein in a manner comparable to ren- 

 net. Conn 2 has separated this enzyme in a relatively 

 pure condition, and Fermi 3 has isolated the digestive 



1 Sterling, Cent. f. Bakt., II. Abt.,1: 473. 



2 Conn, 5th Rep't. Storrs' Expt. Stat., 196, 1892. 



3 Fermi, Arch. f. Hyg., 14: 1, 1892. 



