THE CHEMICAL CONSTITUENTS OF MILK 307 



division by condensation of their vapours, or by any other 

 means, they do not at once solidify when cooled to temperatures 

 considerably below their freezing-points. The term " super- 

 fusion " has been employed to describe this state, and it is 

 believed that the liquid condition of the fat in milk is merely 

 a particular case of this phenomenon. 



It is perhaps more difficult to explain why the liquid fat 

 globules do not immediately coalesce in the milk, and why 

 they do so on churning. It was formerly supposed that each 

 globule was enclosed in a substantial protein membrane, which 

 prevents actual contact of the fat in one globule with that of 

 another, and the formation of butter was attributed to the 

 rupture of this membrane by the mechanical violence of the 

 churning process. This hypothesis is not only unsupported 

 by reliable evidence, but also it is not entirely consistent with 

 the known facts, and is now generally discredited. 



The more modern view is that the fat, globules are not 

 enclosed in a substantial membrane, but are surrounded by a 

 mucoid film, comparable, in some respects, to that of a soap 

 bubble. Storch, the author of this theory, claims to have 

 stained the film, and to have isolated and analysed it. Accord- 

 ing to his estimate, it consists of ninety-four parts of water and 

 six parts of protein. Further, important evidence has also 

 been adduced, but it is not considered conclusive. 



There is no actual necessity to premise the existence of 

 either a membrane or a film. The globules, being so very 

 small, must have great surface tension, and this alone would 

 explain why they do not coalesce. All the observations of 

 Storch and others are capable of explanation in a different way. 

 Many authors hold that the condition of fat in milk is not 

 essentially different from that of the fat in other, artificially 

 prepared, emulsions or that the only differences observed can 

 be accounted for by the smaller size of the fat globules in 

 milk, and the properties of the emulsifying medium. This, 

 however, does not seem to afford an adequate explanation of 

 the difficulty of dissolving out the fat by shaking the milk 

 with ether. 



Milk fat is not a homogeneous substance. Like other 



