156 



FOODS 



COMPOSITION OF Cows' MILK MODIFIED FOR VARIOUS AGES (Rotch). 



Percentages. 



The milk of the ass, common in some Oriental lands, is very similar 

 to human milk save in its small proportion of fat. 



In general, milk is an opaque, bluish-white or yellowish-white liquid, 

 of a specific gravity of from 1027 to 1035, the larger the quantity of 

 cream the lower, of course, being the density of the milk. It is the most 

 perfect emulsion known, consisting of a plasma holding in permanent 

 suspension innumerable globules of fat (butter), each from 2 to 5 mmm. 

 in diameter. The plasma, easily separated from the oil by dialysis, is a 

 somewhat opalescent, transparent fluid. It contains as proteids caseino- 

 gen, lactalbumin, an albumose-like substance (sometimes called lacto- 

 protein), some nuclein, and (in human milk) a small amount of a diastatic 

 enzyme, referred to above. The caseinogen is allied to the alkali albu- 

 mins, and contains iron ; it does not coagulate on heating unless the milk 

 becomes acid ; it seems to act as the emulsifying agent of the milk. The 

 plasma also contains lactose, lecithin, cholesterin, traces of lactic acid 

 (from the natural fermentation of the lactose), and of kreatinin and 

 urea, besides the important salts, calcium, magnesium, sodium, and 

 potassium phosphates, chlorides, sulphates, and carbonates, with traces of 

 iron, fluorine, silicon, oxygen, nitrogen, and carbonic dioxide. Of these, 

 calcium phosphate is the most abundant. The newborn body being very 

 rich in iron, the milk contains little, so little in fact, as Bunge has pointed 

 out, that infants weaned too late are apt to suffer from anemia due to lack 

 of this iron. The cream or fat of milk consists of the triglycerides of 

 palmitic, stearic, and oleic acids, together with small quantities of butyric 

 acid (giving butter its flavor), and of others in still smaller traces, such as 

 caproic, myristic, and formic acids. Olein is most abundant (about 

 three-sevenths) and plamitin next (about one- third), while the stearin is 

 present in the proportion of about one-sixth; these are all about as 

 in body-fat. A pigment, lipochrome, gives butter its characteristic 

 yellowness. 



Colostrum is the first milk secreted during and for a few days follow- 

 ing parturition. The mammary acini have then been rapidly prolifer- 

 ating, and it is natural that the first milk produced should be loaded 



