DECOMPOSITION OF MILK. 13 



isms present, by which the milk sugar is converted 

 principally into lactic acid, and the proteids partly de- 

 composed and partly coagulated. The liquid becomes 

 sour and the fat is enclosed in the coagulated casein. 



In the initial stages of decomposition the proteids 

 frequently undergo transformations into highly poison- 

 ous benzene derivatives, among which diazobenzene, 

 commonly known as tyrotoxicon, is the most important. 

 This body is the cause of the violent poisonous effects 

 occasionally produced by ice cream and other articles 

 of food into the preparation of which milk enters. 



Boiling produces coagulation of the albumin, some 

 alteration of the sugar, and developes a greater fac- 

 ility of coalescence on the part of the fat globules. 

 Microbes and enzymes are destroyed. The scum 

 which appears on the surface of boihng milk is com- 

 posed largely of casein. Its formation is due probably to' 

 the more rapid evaporation at the surface of the liquid; 



Precipitation of the casein in the form of a curd, en- 

 closing the fat, occurs promptly on the addition of 

 rennet or free acids. 



Partial freezing produces a concentration of the milk 

 solids in the part re^maining liquid, while the solid por- 

 tion is deficient in them. The normal condition can, 

 therefore, be restored only by thawing the entire mass, 

 and mixing thoroughly. 



When milk is allowed to stand, some of the fat rises 

 gradually, and forms a rich layer, constituting cream. 

 The proportion of cream depends on several condi- 

 tions. The amount formed in a given time cannot be 



