COAGULATION OF MILK. 201 



cheese. It forms, to a certain extent, a rich medium for suitable 

 development, as desired, of the different kinds of micro-organisms 

 present. These organisms can be developed or suppressed, and the 

 growth of other kinds favoured. The most valued and the most 

 lasting kinds of cheeses are prepared from the coagulum. 



The curd is not elastic, is less insoluble in water than the 

 coagulum, and is sticky and greasy. Since, in accordance with the 

 method by which it is obtained, it possesses a strong acid reaction, 

 and contains luxuriantly growing lactic bacteria, it only forms a 

 suitable nutritive medium for a comparatively limited number of 

 bacteria and fission fungi, and offers, therefore, a much more 

 restricted basis for the manufacture of cheeses of different kinds. 

 In sour-milk cheeses, with few exceptions, the process of ripening 

 resembles in general the putrefactive process, and goes on from outside 

 to inside. In the case of the different rennet cheeses, on the other 

 hand, the process of ripening is essentially characterized as a process 

 of decomposition, or a process of fermentation, which goes on 

 throughout the , whole mass with different phenomena, and appears 

 as a highly complicated process, in which, in addition to bacteria, 

 moulds, and perhaps also fission fungi, take part. 



Coagulum and curd are distinguished from one another by the 

 fact that the former encloses the entire quantity of di- and tri-calcic 

 phosphates which are in suspension in the milk, while the latter 

 (the curd) only encloses a small quantity of calcic phosphate, since 

 a large portion of the suspended phosphate is dissolved by the lactic 

 acid which the separation of the curd gives rise to, and is, therefore, 

 not mechanically enclosed in the precipitate of the coagulation. 



The process of milk coagulation by means of acids may be simply 

 explained as follows : As has been pointed out in 5, the caseous matter 

 of the milk may be regarded as a chemical compound of casein or an 

 albuminoid (which plays the part of an acid), along with calcium oxide, 

 in the proportion of 100 parts of casein to 1*55 parts of calcium oxide. 

 From this compound of casein with lime, which is present in the milk as 

 a strongly coagulated colloidal mass, casein is separated, by the addition 

 of acids, in an insoluble from, i.e. in the form of a non-precipitable body. 

 This operation takes place in the souring of milk by acids. 



The rennet souring of milk does not admit of such simple explanation. 

 We know, it is true, a good deal regarding the accompanying conditions 

 under which it takes place, but with regard to the process itself little is 

 known. We know little more with certainty than that it is a process of 



