28 INTRODUCTORY — THE i 'ONSTITUENTS OP MILK. 



Biel has described syntonin as a uormal constituent of milk, 

 but the existence of this must be considered doubtful at present. 



Palm has stated that albumoses are found in milk ; this is 

 probably not wholly correct ; it is possible that traces of albu- 

 moses are formed during the decomposition to which milk is 

 prone, but no other observer has identified more than traces, 

 while Palm gives 1'5 per cent, as occurring in milk. True 

 peptone has been proved to be absent. Storch's researches 

 have been referred to (p. 2). Babcock has found very small 

 amounts of nuclein, but the presence of this has now been 

 disproved, and the same observer has with Russell separated 

 a proteolytic enzyme. There are also a peroxydase, a catalase, 

 and a reductase. 



From the above list of the various proteins described as existing 

 in milk we may select the four of whose existence we have the 

 strongest evidence ; these are casein, lactalbumin, lacto-globulin, 

 and Storch's mucoid ; the last two, however, are only found in 

 traces in milk, jind, practically, the proteins may be reduced to 

 the former two. The other compounds described, except the 

 enzymes whose proteinic nature is not fully established, are 

 hypothetical. 



The main reactions that distinguish the four proteins of milk 

 are as follows : — Casein is precipitated by saturating the solution 

 with sodium chloride, magnesium sulphate, and ammonium 

 sulphate ; globulin is soluble in a saturated solution of sodium 

 chloride, but is precipitated by magnesium and ammonium 

 sulphates ; albumin is soluble in saturated solutions of sodium 

 chloride and magnesium sulphate, but is precipitated by satura- 

 tion with ammonium sulphate, while Storch's mucoid is not in 

 solution ; albumin is, however, precipitated from a saturated 

 solution of magnesium sulphate by acidifying slightly, and 

 is redissolved by neutralisation of the solution. Casein and 

 globulin are precipitated by the addition of acid, while albumin 

 (and globulin, if much salt is present) is not so precipitated. 

 Casein has the remarkable property of being acted on by chymase, 

 the enzyme of rennet, with the formation of an insoluble pro- 

 duct ; albumin is coagulated by the action of heat, the raising 

 of the solution to about 70° C. under suitable conditions of 

 acidity being sufficient to precipitate a great portion. Casein 

 is gradually removed by filtration through paper, and com- 

 pletely through coarse porcelain; filtration through fine por- 

 celain removes all the proteins. Properties common to the 

 three proteins are solubility in alkalies, insolubility of their 

 copper, mercury, and other salts, insolubility in alcohol ; all are 

 precipitated by tannin and phospho-tungstic acid. 



Casein. — This protein, when pure, is a white amorphous 



