CHAPTER XXIV. 



THE COAGULATION (C UK D LING) OF MILK. 



144. Acid Curdling* and Rennet Curdling. 



THE amount of nitrogenous constituents in cow's milk fluctuates 

 between 2.5 per cent, and 4.2 per cent, by weight, and is on the 

 average 3.5 per cent. The chemical composition of these nitro- 

 genous matters has not yet been satisfactorily determined, and 

 can only be touched upon here so far as is requisite and useful 

 for bacteriological purposes. More precise information, accom- 

 panied by copious bibliographical references, will be found in W. 

 FLEISCHMANN'S (I.) work on dairying. 



In refutation of the opinion expressed by DUCLAUX (VII.), 

 that only a single albuminoid body is present in normal cow's 

 milk, the Swedish chemist Olaf Hammarsten showed, in 1875, 

 that at least three such compounds can be distinguished therein, 

 viz., casein, lactalbumin, and globulin. The first forms about 80 

 per cent, of the total quantity, and the remainder is principally 

 lactalbumin (free from phosphorus), globulin being present in 

 but very small quantities; both of these latter are soluble in 

 water. The casein (containing phosphorus) is acid in character, and 

 consequently is not present in a free state in the milk, but occurs as 

 a salt of lime containing 1.55 parts of CaO per 100 parts of casein. 

 This compound of lime and casein is not dissolved in the milk, 

 but is held in suspension as a swollen, colloid, finely divided mass. 

 When the milk is acidified the casein is liberated, and being 

 insoluble and incapable of swelling is precipitated in fine flakes ; in 

 other words, the milk curdles. The acid may be either artificially 

 added or generated by fermentation in the milk itself ; in either 

 case the ensuing precipitate is known as acid curd (Ger. Quark}. 



Milk can also be curdled by another means, namely, by lab or 

 rennet, an enzyme secreted by special glands in the stomach of 

 many animals. This rennet is very plentiful in the stomach 

 of the calf, from which it is prepared by drying in the air and 

 leaving to stand for a few months, then comminuting the mass 

 and extracting with a weak (5 per cent.) solution of common salt. 



On adding a small portion of such a solution of rennet to 

 sweet, unboiled, lukewarm milk, the latter gradually curdles, the 

 coagulum thus formed being, however, not casein itself, but a 

 derivative of that substance. Hammarsten found that the casein 



