i 3 4 THE CHEMISTRY OF THE TISSUES AND ORGANS. 



acids contain equal quantities of caproic, capiylic, and capric acids, and 

 the merest traces of butyric acid. The principal acids present, as is 

 usual in animal fat, are palmitic, stearic, and oleic acids, and one or 

 more acids of lower molecular weight, including myristic acid. The 

 melting point of the mixture of acids is 37 to 39, and of the fat itself 

 30 to 31 C. 



The proteids of milk. The proteids which occur in milk are 

 three in number. The most abundant and most important of these is 

 caseinogen. It is this proteid which is acted upon by rennet, and 

 converted into casein or cheese. 1 The other two proteids are only 

 present in small quantities; they are called lactoglobulin and lact- 

 albumin. Proteoses and peptone were described in milk by many of 

 the older workers. This was due tt> the use of faulty methods of 

 analysis (see p. 41). 2 



Coagulation of milk. When milk is allowed to stand at the 

 ordinary temperature exposed to the air, the chief change it undergoes 

 is the lactic acid fermentation. The acid formed precipitates a part of 

 the caseinogen, but this is a different thing from the conversion of 

 caseinogen into casein. Sometimes, however, certain aerobic bacterial 

 growths act like rennet in causing a true curd. Certain of the higher 

 plants (Ficus, etc.) also curdle milk. 



The agency by which the clot is most readily formed is that of 

 rennet. This is a ferment secreted by the stomach, and is usually obtained 

 from the stomach of sucking animals, like the calf. The pancreatic 

 juice also has a curdling action on milk (see p. 137), and extracts of 

 many tissues (such as testis, liver, lung, muscle) have a feeble action of 

 the same nature. 3 



Hammarsten 4 and, later, Friedberg 5 showed conclusively that the 

 active principle of rennet is not pepsin ; that it requires for its efficient 

 action the presence of calcium salts, of which the phosphate is the one 

 which is mostly present in the milk, and that it will act in a weakly 

 acid, neutral, or alkaline solution. It acts most readily at 40 C., and 

 is destroyed at 70 C. The ferment itself in the rennet extracts is 

 termed chymosin by Friedberg, and rennin by Foster. 6 



When rennet is added to cows' milk the result is a coherent clot or 

 curd, which expresses a clear yellowish fluid, the whey. The curd 

 contains the fat entangled with the casein ; the whey contains the other 

 proteids, sugar, and salts of the milk. In human milk the curd is 

 usually composed of smaller flocculi, and a similar flocculent coagulation 

 can be produced in cows' milk by previously boiling it, or by diluting it. 

 Lime water, soda water, or barley water are generally used as diluents 

 for this purpose. 



The coagulation of milk is somewhat analogous to that of blood, 

 and the analogy is accentuated by the fact that in both cases calcium 



1 The utility of this nomenclature is at once apparent when casein and caseinogen are 

 contrasted with fibrin and fibrinogen, myosin and myosinogen, even although the analogy 

 is not complete in details. Hammarsten, however, prefers to call the proteid in milk, 

 casein ; while the coagulated proteid he terms, after Schulze and Rose (Landw. 

 Versuchs. Stat., Berlin, Bd. xxxi.), paracasein. 



2 For a critical article on the estimation of the various proteids in milk, see Schlossmann, 

 Ztschr. f. physiol. Chem., Strassburg, 1896, Bd. xxii. S. 197. 



3 Edmunds, Journ. Physiol, Cambridge and London, 1896, vol. xix. p. 465 r 

 4 Jahresb. il. d. Fortschr. d. Thier-Chem., Wiesbaden, 1874, S. 135.. 



5 Journ. Am. Chem. Soc., N. Y., 1888, p. 15. 



6 "Text-book," 5th edition, p. 519. 



