THE PHYSICAL PROPERTIES OF MILK 61 



from 1,000,000 to 11,000,000. All kinds of milk contain fat globules 

 of different size, but the extremes are farther apart in some milks 

 than in others. In goat's milk, for example, the extremes lie 

 between 3 and -4.5 microns; in sheep's milk, between 2.75 and 22.4 

 microns, and in Jersey milk, between 4 and 12 microns. It has 

 been said that the chemical composition of the fat in the large 

 globules differs from that in the small ones, that the small glob- 

 ules contain more oleic and volatile acids than the large ones. 

 Conclusive evidence on this point is still lacking. 



The emulsion of fat in milk has been compared to an artificial 

 emulsion of oil. In the latter the globules are kept in suspension 

 and from coalescence by a thin covering of a gummy substance 

 which is held in position by molecular attraction. Similarlj^, the 

 fat globules in milk may be enveloped in casein, also held there 

 by molecular attraction. Any condition, such as the addition of 

 acid, alkali, or rennet, which affects the casein, would then release 

 the fat globules and cause them to coalesce. 



Another hypothesis, whose chief supporter is B^champ, as- 

 sumes that the globules are surrounded by an organized mem- 

 brane. The supposition is that these membranes are destroyed 

 by the violent agitation in butter making, so that fat separation 

 becomes complete. Attempts to demonstrate the presence of a 

 membrane have been made by dissolving the fat with some solvent 

 in the expectation of finding the empty membranes. Some rem- 

 nants have actually been found, but it has not been possible to 

 show conclusively that they are membranes. Or the fat globules 

 have been made to rise through a column of water in order to free 

 them from adhering milk plasma. These washed globules have 

 then been extracted with ether, and the substance left has been 

 thought to constitute the membrane. Mathews states that these 

 remnants give no biuret reaction and when hydrolyzed yield glyco- 

 coU which is not present either in casein or lactalbumin. Accord- 

 ing to this author the amount of nitrogen in these elements is 

 4.04 to 5.70 per cent., and he states: "It is clear that it is not a 

 homogeneous substance nor is it composed of casein." Storch 

 also beheved in the existence of a membrane, but thought it was 

 composed of a mucoid substance — mucoid protein. He isolated 

 this substance from cream and butter and concluded that it must 

 occur in milk. He treated milk with ammoniacal picrocarmin, 

 and after cream had collected washed it with water until all milk- 

 sugar had been removed. He then examined the globules under 

 a microscope and found them surrounded by a stained halo. 

 Furthermore, he was able to isolate a larger quantity of his mucoid 

 protein from buttermilk than from sweet cream, which fact led 

 him to believe that the churning had broken away the mucoid 



