RIPENING PROCESSES OF CHEESES 129 



The active principle in rennet is a proteolytic enzyme Chymosin, 

 the action of which does not cease with the coagulation of the milk 

 and the contraction of the casein, but continues in the cheese 

 with the formation of soluble proteins. The author's researches 

 have established that cheese rennet exerts a powerful solvent 

 action on the proteins of milk, and that this action is promoted by 

 the addition of small amounts of acid l . American researches 2 

 have shown that the ripening of cheese is accelerated by increasing 

 the amount of rennet used. As pepsin is also present in rennet, 

 it was formerly supposed that the solvent action on the proteins 

 was exclusively due to this enzyme. Careful investigation, 

 however, has conclusively shown that chymosin itself is a proteo- 

 lytic enzyme which can act in the presence of smaller amounts of 

 acid than pepsin 3 . The addition of pepsin to the milk used for 

 cheese making appears to have no influence on the ripening 

 process. Trypsin, on the other hand, has a decided influence on 

 cheese, but may easily cause a bitter taste 4 . In this connection, 

 mention may be made of galactase, a proteolytic enzyme which, 

 according to researches by Babcock and Russel 5 , is a normal 

 constituent of milk, and which just after its discovery in 

 1897 was assumed to play a most important part in the ripen- 

 ing of cheese. Subsequently it transpired that galactase does 

 not play any notable part in the ripening of soft cheeses, 

 in which, however, it is particularly plentiful 6 ; and the fact 

 that this enzyme is not indispensable to the ripening of hard 

 cheeses is amply demonstrated by the fact that good cheese 

 has been produced on a large scale from milk which has" been 

 heated to over 80 C., at which temperature the enzyme is 

 destroyed. 



As regards the action of lactic acid, this is of interest not only in 

 the making of sour milk cheeses, but also in the making of rennet 

 cheeses. As is well-known, casein occurs in milk as a calcium 

 salt, a dicalcium casemate (the calcium compounds of the proteins 

 usually form milky solutions in water), and when casein is precipi- 

 tated by acid it is not due to transformation into paracasein as 

 occurs with rennet, but simply to the abstraction of lime by the 

 acid. At the same time a little lactoglobulin is precipitated. 

 The greater the percentage of casein in the milk, the more acid 



1 " Landwirtschaftliches Jahrbuch der Schweiz," 1904, p. 404, and 

 1907, p. 97. 



2 Seventeenth Eeport of Wisconsin Agricultural Experiment Station, 

 Madison, 1900. 



3 Petry, " Wiener Klinisclie Wochenschrift," 1906, p. 143. 



4 Orla Jensen, " Nyt Tidskrift for Fysik og Kemi," 1897, p. 92, and 

 " Landwirtschaftliches Jahrbuch der Schweiz," 1901, p. 197. 



5 Wisconsin Agric. Expt. Station, Bull. 14, 15 and 19. 



6 Orla Jensen, " Centralblatt f. Bakt.," 2 Abt., 1900, Bd. VI., p. 793. 



D.B. 9 



