58 MILK HYGIENE 



It sometimes happens that milk coagulates quickly 

 without souring; the coagulum is quite firm, the taste 

 sweetish, often slightly unpleasant. In this case, the 

 lactic acid fermentation has ceased, while the bacteria 

 referred to in paragraph d have grown freely. Left 

 to itself, such milk will quickly become putrid. 



VIII. CHANGES IN MILK AT HIGH TEMPERATURES 



Wlien milk is boiled in an open vessel, it becomes 

 covered with a coating of coagulated casein ; after this is 

 removed, a new membrane is immediately formed. If 

 the milk is constantly stirred, the membrane is not 

 formed. The origin of this coating is not entirely clear, 

 but it is possible that it comes from a partial drying of 

 a superficial layer of milk. 



Viscosity is diminished upon boiling or long pasteur- 

 ization, so that boiled milk appears to be thinner than 

 raw milk. This is expressed by Babcock by the fol- 

 lowing figures relating to two samples of milk tested : 

 265 raw, and 250 pasteurized; 275 raw, and 257 boiled. 

 The specific gravity of milk changes so little upon heat- 

 ing as to be practicallj' of no account. 



Lactose is not changed when simply boiled, but heat- 

 ing at a higher temperature causes a burnt condition 

 (caramel formation) and partial separation into lactic 

 acid, causing the milk to become slightly acid. 



Albumin and globulin coagulate upon heating the 

 milk for a short time to 70° to 80° C, and it is probable 

 that casein also undergoes a change, but it has scarcely 

 any significance and has been studied but little up to 

 this time. But if milk possesses a certain degree of 

 acidity, heating causes coagulation of the casein. Boiled 

 milk is not influenced at all, or only very slightly, by 

 rennet ferment; the reason for this is not to be found 

 in change in the casein, but lies in the partial transfer- 



