346 FAT AND PLASMA OF MILK. 



are dissolved, and if ether be added, the milk becomes clear and transparent, as the ether dis- 

 solves out all the fatty particles in the solution. Ether cannot extract the fat from cow s milk 

 until acetic acid or caustic potash is added to liberate the fats from their envelopes ; but shak- 

 ing with ether is sufficient to extract the fats from human milk. Some observers deny that an 

 envelope of casein exists, and according to them milk is a simple emulsion, kept emulsionised 

 owing to the colloid swollen-up casein in the milk plasma. The treatment of milk with potash 

 and ether makes the casein unable any longer to preserve the emulsion (Soxhlet). 



The fats of the milk-globules are the triglycerides of stearic, palmitic, oleic (very- 

 little), myristic, arachinic (butinic), capric, caprylic, caproic, and butyric acid, with 

 traces of acetic and formic acids and cholesterin. 



Butter. When milk is beaten or stirred for a long time (*.*., churned), the fat of the milk- 

 globules is ultimately obtained in the form of butter, owing to the rupture of the envelopes of 

 casein. Butter is soluble in alcohol and ether, and it is clarified by heat (60 C. ), or by washing 

 in water at 40 C. "When allowed to stand exposed to the air, it first becomes sour, owing to 

 the formation of lactic-acid, and afterwards rancid, owing to the glycerine of the neutral fats 

 beiug decomposed by fungi into acrolein and formic acid, while the volatile fatty acids give it 

 its rancid odour. 



The milk plasma, obtained by filtration through a clay filter or membranes, is a 

 clear, slightly opalescent fluid, and contains casein ( 249, III., 3), some serum- 

 albumin ( 32), peptone (0-13 per cent.), nuclein, and a trace of diastatic ferment 

 (in human milk). 



The presence of other peculiar chemical bodies, e.g., lactoprotein, globulin, albumose, galactin, 

 &c, is disputed by some chemists. 



When milk is boiled the albumin coagulates, while the surface also becomes 

 covered with a thin scum or layer of casein, which has become insoluble [the rest 

 of the milk remaining fluid]. 



Casein. When milk is filtered through fresh animal membranes or through a clay filter, the 

 casein does not pass through. Precipitation. It is precipitated by adding crystals of MgSO, 

 to saturation. [If to milk twice its volume of a saturated solution of NaCl and crystals of NaCI 

 be added, and the whole shaken thoroughly, casein is precipitated, and carries down with it 

 fat, so that the clear filtrate contains the lactose, salts, and coagulable proteids.] 



The plasma contains milk-sugar ( 252) ; a carbohydrate resembling dextrin, 

 (1 lactic acid), lecithin, urea, extractives, kreatin, sarkin, (potassic sulphocyanide in 

 cow's milk), sodic and potassic chlorides, alkaline phosphates, calcium and magnesium 

 sulphates, alkaline carbonates, traces of iron, fluorine, and silica, C0 2 , N, and O. 



The coagulation of milk depends upon the coagulation of its casein. In milk, casein is com- 

 bined with calcium phosphate, which keeps it in solution}; acids which act on the calcium 

 phosphate cause coagulation of the casein (acetic and tartaric acids in excess redissolve it). All 

 acids do not coagulate human milk. It is coagulated by two or more drops of hydrochloric acid 

 (O'l per cent.) or acetic acid (0*2 per cent.). The spontaneous coagulation of milk after it has 

 stood for a time, especially in a warm place, is due to the production of lactic acid, which is 

 formed from the milk-sugar in the milk by the action of bacillus acidi lactici [which is intro- 

 duced from without] ( 184, I.). It changes the neutral alkaline phosphate into the acid 

 phosphate, takes the casein from the calcium phosphate, and precipitates the casein. The sugar 

 is decomposed into lactic acid and C0 2 . 



Rennet ( 250, 9, d, 166, II.) 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, so that the coagulation by rennet is a process quite distinct from the coagulation 

 of milk by the gastric and pancreatic juices, [and also from the precipitation produced by acids. 

 The presence of calcium phosphate seems to be necessary for the complete action of the rennet 

 (Hammarsten)]. 



[Experiments. "Warm a little milk to 40 C, and add a few drops of commercial rennet, 

 setting aside the mixture in a warm place; a solid coagulum is soon formed, and by and by the 

 whey separates irpm it. If the milk be previously diluted with water, no coagulum is formed ; 

 and if the rennet be boiled before, it like other ferments is destroyed. A solution of rennet 

 may be prepared by extracting the fourth stomach of the calf with glycerine. [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.] 



[A milk-coagulating ferment is found in certain plants (artichokes, figs, Carica papaya), and 

 causes milk to coagulate in neutral or alkaline solutions. It is also found in the small intestine 



