466 COMPOSITION OF MILK. 



formed from the milk-sugar in the milk by the action of bacterium lacticum [which 

 is introduced from without (Pasteur, Cohn, Lister)]. It changes the neutral alka- 

 line phosphate into the acid phosphate, takes the casein from the calcium phosphate, 

 and precipitates the casein (p. 373). The ferment may be isolated by means of 

 alcohol. 



Rennet, which contains a special ferment, coagulates milk with an alkaline 

 reaction (sweet whey). This ferment decomposes the casein into the precipitated 

 cheese, and also into the slightly soluble whey-albumin (Hammarsten, KOster), so 

 that the coagulation by rennet is a process quite distinct from the coagulation of 

 milk by the gastric and pancreatic juices. When the milk is coagulated we obtain 

 the curd, consisting of casein with some milk-globules entangled in it ; the whey 

 contains some soluble albumin and fat, and the great proportion of the salts and 

 milk-sugar, together with lactic acid. 



Boiling (by killing all the lower organisms), sodium bicarbonate (T^), ammonia, 

 salicylic acid (^nnr), glycerine, and ethereal oil of mustard prevent the spon- 

 taneous coagulation. Fresh milk makes tincture of guaiacum blue, but boiled 

 milk does not do so (Schacht, C. Arnold). When milk is exposed to the air for a 

 long time, it gives off COg, and absorbs ; the fats are increased (? owing to the 

 development of fungi in the milk), and so are the alcoholic and ethereal extracts, 

 from the decomposition of the casein (Hoppe-Seyler, Kemmerich). According to 

 Schmidt-Miilheim, some of the casein becomes converted into peptone, but this 

 occurs only in unboiled milk. 



Composition, 100 parts of milk contain 



Human. Cow. Goat. Ass. 



Water 87 '24 90 '58 86 '23 86 '85 89 '01 



Solids, 9-4212-39 13'77 13'52 10'99 



Casein, 2'91 3'92\ , . Q0 .,, \ 3'23 2'53 1 ,.- 7 



Albumin J l ~ 221 / 0-50 1-26 J 



Butter, 2-67 4-30 4*50 4'34 T85 



Milk-sugar, . . . 3'15- 6'09 4'93 3'78 



Salts, 0-14 0-28 0'6 0'65 



Human milk contains less albumin, which is more soluble than the albumin in 

 the milk of animals. 



Colostrum contains much serum -albumin, and very little casein, while all the 

 other substances, and especially the fats, are more abundant. 



Gases. Pfltiger and Setschenow found in 100 vols. of milk 5 '01-7 '60 C0 2 ; 

 0-09-0-32 0; 0'70-1'41 N, according to volume. Only part of the C0 2 is expelled 

 by phosphoric acid. 



Salts. The potash salts (as in blood and muscle) are more abundant than the 

 soda compounds, while there is a considerable amount of calcium phosphate, 

 which is necessary for forming the bones of the infant. Wildenstein found in 100 

 parts of the ash of human milk sodium chloride, 10'73; potassium chloride, 26*33; 

 potash, 21-44; lime, 18 '78; magnesia, 0'87; phosphoric acid, 19; ferric phosphate, 

 0'21 ; sulphuric acid, 2 '64 ; silica traces. The amount of salts present is affected 

 by the salts of the food. 



Conditions Influencing the Composition. The more frequently the breasts 

 are emptied, the richer the milk becomes in casern. The last milk obtained at any 

 time is always richer in butter, as it comes from the most distant part of the 

 gland viz. , the acini (Reiset, Heynsius, Forster, de Leon). Some substances are 



