MILK. 125 



MILK. 



General properties and composition. Milk consists of fluid (milk 

 plasma) in which are suspended innumerable minute globules of fat. 1 It 

 is therefore an emulsion, and its white colour is produced, as in other 

 emulsions, by reflection from the surface of the numerous globules. 

 The specific gravity of cow's and of human milk is about the same, 

 namely, 1028 to 1034 2 It is increased by the removal of the lightest 

 constituent, the cream. Among the milk globules are smaller particles 

 of proteid matter (caseinogen or nuclein ?). 3 



The statement is still often made that each fat globule in milk is 

 surrounded by a thin membrane of caseinogen the so-called haptogen 

 membrane, 4 and it was considered that it was the rupture of these 

 membranes during the process of churning that enabled the fat globules 

 to run together to form butter. The evidence on which this idea has 

 rested is of a threefold nature : 



1. If the milk is filtered through a cell of porous earthenware, the 

 filtrate is free, not only from fat, but also from caseinogen. 



2. The mass of milk globules, after having been well washed within 

 the filter, gives the reactions for caseinogen. 5 



3. If ether is added to the milk, without previous addition of 

 caustic potash or acetic acid (these were supposed to dissolve or 

 break up the proteid envelope), the fat is dissolved out with great 

 difficulty. 



But it is now generally held with Quincke, 6 who made experiments 

 with oil and mucilage, that each fat globule by molecular attraction 

 is surrounded by a more closely adherent layer of caseinogen solution 

 (or rather milk plasma), and not by a membrane. How then can 

 one explain the three facts just adduced in favour of the membrane 

 theory ? 



1. If milk is filtered through porous earthenware, it is naturally free 

 from caseinogen ; blood serum filtered in the same way is proteid free. 

 The molecules of proteid are too big to go through the pores of the filter ; 

 there is no necessity, therefore, to suppose that the caseinogen is in a 

 solid condition in the milk. 



2. For the same reason, no amount of washing would wash the 

 caseinogen through, and so naturally the milk globules would give the 

 reactions of the proteid with which they are contaminated. Further, 

 Hoppe-Seyler 7 has shown that cream yields the same percentage of 

 casein as the layers of milk below it. 



3. The addition of reagents such as acetic acid (and rennet) enables 

 the fat to pass into solution more easily, not because they are solvents 

 of proteid, for they are not, but because they alter the relations 

 between the surface tensions of fat globules and milk plasma, and so 



1 For the measurement and examination of the fat globules, see Fleischmann, " Das 

 Molkereiwesen," Braunschweig, 1876-9, S. 206; F. W. Woll, "Wisconsin Exper. Stat. 

 Agric. So.," 1892, vol. vi. 



2 For observations on the specific gravity of human milk, see Monti, Arch. f. Kinderh., 

 Stuttgart, Bd. xiii. 



3 Kehrer, Arch. f. Gyimck., Berlin, Bd. ii. S. 1 ; D. F. Harris, Proc. Roy. Soc. Edin., 

 1896, p. 72. 



4 Ascherson, Arch. f. Anat. u. PhysioZ., Leipzig, 1840, S. 53. 



5 Radenhausen and Danilewsky, Forsch. a. d. Geb. d. Vichhaltimg, Bremen, 1880, Heft 9. 

 (i Arch.f. d. (jes. PhysioL, Bonn, 1879, Bd. xix. S. 129. 



7 "Physiol. Chem.,"S. 728. 



