l68 FERMENTATION IN MILK 



this ferment, the most pronounced effect being produced by calcic 

 phosphate. Indeed, it is generally held that clotting of milk will 

 not take place in the absence of this salt. Hammarsten explains 

 this by saying that the ferment induces the conversion of casein into 

 tyrein, and the calcic phosphate causes the latter to separate out 

 as a clot. The character of the clot depends also upon the source 

 of the milk. Cow's milk coagulated by rennet yields a more solid 

 clot than human milk, which clots loosely and with a flocculent 

 consistence. This is due, of course, to the fact that cow's milk 

 contains much more casein and calcium than human milk. Rennet 

 will clot boiled milk but not so firmly, on account of the precipita- 

 tion in boiled milk of the soluble calcium salts (Soldner). 



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 a derivative of casein. 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 \n^o\\2\A^ 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 (Hammarsten). Casein or paracasein, 

 though the sole nitrogenous constituent of the coagulum produced 

 in any of these methods, is, however, by no means its sole com- 

 ponent, a number of other substances being precipitated 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 subsequently produce rich cheese^ skim milk cheese being the 

 result in the converse case. Along with the fat, the calcium phos- 

 phate contained (in suspension) in the milk will also be thrown 

 down only in the case of rennet coagulation, not in the curd pro- 

 duced by acidification (Lafar). 



Lafar has also pointed out that not only are fat and calcium 

 phosphate carried down by the coagulum, but also a large number 

 of the organisms present in the milk, so that the clot is relatively 

 as rich in organisms as the milk from which it was precipitated. 



Acid curd differs from that produced by rennet, both in the 

 method of production and also in composition. Being devoid of 

 flavour, both kinds are, however, unsuitable for food ; their con- 

 version into a form in which they both stimulate the appetite 

 and are also themselves more readily digestible, is the task of the 

 cheese-maker's art. 



But rennet is not alone a product of animal metabolism. 



