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 Burface 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/ 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, 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 ' 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 Fleischmamr, " Das 
Molkereiwesen," Braunschweig, 1876-9, S. 200; F. W. Woll, ''Wisconsin Exper. Stat. 
Agric. Sc.," 1892, vol. vi. 
- For observations ou the specific gravity of human milk, see Monti, Arch. f. Kinderh., 
Stuttgart, Bd. xiii. 
3 Kehrer, Arch. f. Gynatk., Berlin, Bd. ii. S. 1 ; D. F. Harris, Proc. Boy. Soc. Ed in.. 
1896, p. 72. 
4 Ascherson, Arch./. Anal. u. Physiol., Leipzig, 1810, S. 53. 
3 Ptadenhausen and Danilewsky, Forsch. a. d, Geb. d. J' 'ichhaltuny ; Bremen, 1880, Heft 9. 
6 Arch. f. d. ges. Physiol., Bonn, 1879, Bd. xix. S. 129. 
7 "Physiol. Chem.,''S. 728. 
