84 MILK [vi 



acid. Another sub-class are those which produce no 

 acidity in milk, but which effect the coagulation of 

 the caseous matter, or which effect the fermentation 

 of the caseous matter without coagulation. This class 

 includes such bacteria as the potato bacilli, a group 

 of bacilli to which Duclaux has given the name 

 tyrothrix, and those bacteria which were formerly 

 known as "putrefactive" bacteria, since they 

 decompose albumin and produce an unpleasant 

 smell. 



Lastly, we have micro-organisms present in milk 

 other than bacteria, viz. yeasts and moulds. 



Before treating of the action of certain classes of 

 bacteria which effect the common kinds of fermenta- 

 tion going on in milk, such as its souring, it will be 

 convenient to refer to certain milk "faults" which 

 have been shown to be generally caused by the action 

 of bacterial life. Of these, the following are the most 

 important : 



Blue Milk. This fault consists in the develop- 

 ment in the milk, after a lapse of from twenty-foi 

 to seventy-two hours, according to the temperatui 

 of the milk, of patches on its surface of a blue 

 colour. It was the first kind of fermentation of 

 milk to be studied, having been investigated by 

 Foulkes in 1841. The formation of these blue patches 

 only takes place to any extent, at any rate after 

 the milk has assumed a slight, though distinct acid 



