8 INTEODUCTOBY — THE CONSTITUENTS OF MILK. 



by heat. Other proteins have been described in milk, but many 

 of them are only decomposition products of casein or albumin, 

 which were formed during the process adopted for the removal 

 of the other proteins. Evidence has been adduced of a third 

 protein in milk ; Storch's mucoid protein has already been 

 referred to. Bechamp has described a starch-liquefying 

 enzyme, and lately Babcock and Russell have separated a 

 proteolytic enzyme. There also exists an enzyme which 

 has the properties of a peroxydase, which may be identical with 

 those of Bechamp and Babcock and Russell. 



The casein in milk is not in a state of true solution ; it is 

 probably in the state described by Picton and Binder as " pseudo- 

 solution." They have shown that this state is due to the exist- 

 ence of particles in the solution not sufficiently large to settle 

 under the influence of gravity, but which will interfere with 

 the passage of light ; they can also be separated by a current 

 of electricity, or by passage of the solution through a porous 

 jar. They show also that there is no sharp dividing line between 

 crystalloids and colloids in solution, substances in pseudo-solu- 

 tion, and substances in suspension. In milk we have the four 

 states represented — the fat is in suspension, the casein in pseudo- 

 solution, the albumin in solution as a colloid, and the milk- 

 sugar in solution as a crystalloid ; these four states are 

 probably due to the size of the conglomerates of molecules or 

 particles. 



Salts. — The salts of milk are not yet fully studied ; the presence 

 of chlorides, phosphates, and sulphates of sodium, potassium, 

 calcium, and magnesium is generally admitted. Salts of organic 

 acids are also present; Henkel has described citric acid, and 

 Bechamp acetic acid, but this latter result is not universally 

 accepted. Bechamp also maintains that the casein and albumin 

 exist in milk as salts of alkalies ; there is much to recommend 

 this view. A solution greatly resembling milk can be prepared 

 in which casein undoubtedly exists combined with an alkali, 

 while it has not been found possible to dissolve casein to an 

 appreciable extent unless an alkali is present; milk does not 

 taste sour until an appreciable acidity has developed ; at about 

 the same pomt it curdles on heating ; it is proved that this is 

 due to the acid developed displacing the alkali from its compound 

 with casein. It is also found impossible to coagulate the albumin 

 m milk unless a certain amount of free acid is added and this 

 fact accords well with the theory of Bechamp. Soldner has also 

 adduced evidence in proof of this view. 



Besides the constituents enumerated above, milk contains 

 traces of other compounds; among these may be mentioned 

 urea and other bases, an odoriferous principle, and a colouring- 



