XV. 



MILK AND MILK PEODUCTS 



317 



Hammarsten has proved that in the absence of calcium phosphate or 

 other salts of the alkaline earths, rennet will not curdle milk. 



Under ordinary circumstances, rennet acts best at about 35 and is 

 killed or destroyed at 70. 



The albumin of milk closely resembles serum albumin of blood. 

 It is in complete solution in milk but is coagulated at 72, or by satura- 

 tion with sodium sulphate at 30, or ammonium sulphate at ordinary 

 temperatures, but not by magnesium sulphate at 40. It is also pre- 

 cipitated by copper, mercury, or lead salts, by tannin and by alcohol. 



Its composition, according to Sebelien, 



Carbon . 

 Hydrogen 

 Oxygen . 

 Nitrogen . 

 Sulphur . 



IS 



Per cent. 



52-19 



7-18 



22-90 to 23-13 



15-77 

 1-73 to 1-96 



It differs in composition from casein in containing no phosphorus, 

 more carbon, and more than twice as much sulphur. 



Many other proteids have been described as occurring in cows' 

 milk, but some doubt as to whether they are not formed by the action 

 of the reagents employed upon the casein or albumin has been ex- 

 pressed. As an illustration of the complexity of the method adopted 

 for the separation and preparation of some of the proteids, the follow- 

 ing account of two proteids analysed and described by Storch may 

 be given. 2 Skimmed milk was mixed with three times its volume of 

 saturated solution of sodium sulphate and a few drops of egg albumin 

 and heated to 100. The coagulated casein was filtered off, and to 

 the filtrate more sodium sulphate solution, a trace of acetic acid and 

 solid sodium sulphate in excess were added, when a substance, A, 

 was precipitated ; to the filtrate strong acetic acid was added, when 

 another precipitate, B, was formed, and the liquid was then found to 

 be free from proteids. 



Substance A (the yield of which was about 2 per cent of the milk) 

 was found to contain calcium and to be soluble in water, from which 

 acetic acid reprecipitates it free from calcium and insoluble in water, 

 though soluble in alkalies. Substance B was free from calcium and 

 insoluble in water ; its amount corresponded to about O3 per cent of 

 the milk. A was coagulated by rennet, B was not. On analysis of 

 the purified substances the following figures were obtained : 



1 Jour. Soc. Chem. Ind., 1886, 387. 



2 Jour. Chem. Soc., 1897, Abstracts, ii. 420; 1900, Abstracts, i. 266. 



