CHAPTER XXIV. 



THE COAGULATION (CURDLING) 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 nitrogenous 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, accompanied 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, lactalbumen, and globulin. The first forms 

 about 80 per cent, of the total quantity, and the remainder is principally 

 lactalbumen (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 is in this case split up into two portions differing greatly 

 in amount, viz., lacto-protein, small in quantity, soluble, and remaining in the 

 whey, and the insoluble paracasein. The latter, therefore, forms the chief 

 constituent of the coagulum separated (" set ") in cheese-making by the aid of 

 rennet, and known as rennet curd (Ger. Bruch), or crude cheese. 



Casein, or paracasein, though the sole nitrogenous constituent of the coagu- 

 lum produced in any of these methods, is, however, by no means its sole 

 component, a number of other substances being precipitated and carried down 

 at the same time. If whole milk i.e. unskimmed milk is set for cheese, 

 almost the whole of the fat will be found in the curd, which will then subse- 

 quently produce rich cheese skim-cheese being the result in the converse case. 

 Along with the fat, the calcium phosphate contained (in suspension) in the milk 



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