BACTERIA AND ENZYMES IN AGRICULTURE 277 



aroma in different cases, is not very perfectly understood. 

 Obviously the conditions admit of the production of esters, 

 by combination of different organic acids with various 

 alcoholic groups in a great variety of ways. 



The chemistry of cheese making is more complex than the 

 chemistry of butter making ; we have here, in addition to the 

 activity of lactic organisms, to consider more especially 

 proteolytic changes in the curd, which is the starting point of 

 cheese. Reference has already been made to the chemical 

 changes which take place when the clotting enzyme rennet 

 is added to milk. The first process in the manufacture of 

 cheese consists in throwing out the curd or casein by means of 

 rennet. Milk can also be curdled by the activity of acid- 

 forming organisms, and this method is actually employed in 

 the preparation of certain home-made cheeses ; but in this case 

 the curd is of a different composition, the whole of the albumin 

 of the milk being thrown out. The curd produced by rennet, 

 as we have seen, consists of casein together with calcium 

 phosphate ; the curd will also carry down with it fat particles, 

 and will retain, of course, a certain proportion of the whey, 

 i.e., the liquid left after separation of the curd, and which 

 contains the soluble constituents of the milk. Curd thus 

 prepared is tasteless, and in order to be converted into 

 cheese has to undergo a ripening process. In the process 

 of cheese making the curd is granulated, placed in cloths, and 

 the whey pressed out ; the pressed curd is then set aside to 

 ripen. 



The ripening process, in the case of cheese, is brought 

 about, not only by bacteria, but also in certain cases by 

 moulds. The former are chiefly concerned in the ripening of 

 hard cheese, which may take place at a fairly high tempera- 

 ture, at which the activity of moulds will be inhibited. The 

 activity of moulds is concerned more with soft cheeses, which 

 are allowed to ripen at a lower temperature. 



As in the case of butter, so in the case of cheese, if the 



