CHEESE-MAKING 247 



rennet, which has the power of coagulating the casein. But this 

 precipitation may also be accomplished by allowing acid to develop 

 in the milk until the casein is precipitated, as in Some sour-milk or 

 cottage cheeses. The former method is, of course, the usual one in 

 practice. It has been suggested that the bacteria contained in the 

 rennet exert a considerable influence on the cheese, but this, although 

 rennet contains bacteria, is hardly established. It is not here, how- 

 ever, that bacteria really play their rdle. After this physical separa- 

 tion, when the cheese is pressed and set aside, is the period for the 

 commencement of the ripening process. 



That bacteria perform the major part of this ripening process, 

 and are essential to it, is proved by the fact that when they are 

 either removed or opposed the curing changes immediately cease. ' If 

 the milk be first sterilised (Freudenreich), or if antiseptics, like 

 thymol, be added (Adametz), the results are negative. It is not yet 

 known whether this ripeniug process is due to the influence of a! 

 single organism or not. The probability, however, is that it is to be 

 ascribed to the action of that group of bacteria known as the lactic 

 acid organisms. Nor is it yet known whether the peptonisation of 

 the casein and the production of the flavour are the results of one or 

 more species. Freudenreich believes them to be due to two different 

 forms. 



However that may be, we meet with at least four common groups 

 of bacteria more or less constantly present in cheese-ripening, either 

 in the early or late stages. First, there are the lactic acid bacteria, by 

 far the largest group, and the one common feature of which is the 

 production by fermentation of lactic acid ; secondly, there are the 

 casein-digesting iacteria, present in relatively small numbers ; thirdly, 

 the gas^roducing iacteria, which give to cheese its honeycombed 

 appearance ; lastly, an indifferent or miscellaneous group of extrane- 

 ous bacteria, which were in the milk at the outset of cheese-making, 

 or are intruders from the air or rennet. All these four groups may 

 bring about a variety of changes, beneficial and otherwise, in the 

 cheese-making. 



Eussell divides the ripening process into three divisions :■ — 



1. Period of Initial Bacterial Decline in Cheese. — Where the green' 

 cheeses were examined immediately after removing from the press, 

 it was usually found that a diminution in numbers of bacteria had 

 taken place. This period of decline lasts but a short time, not 

 beyond the second day. Lower temperature and expulsion of the 

 whey would account for this general decline in all species of 

 bacteria. 



2. Period of Bacterial Increase. — Soon after the cheese is removed 

 from the press a most noteworthy change takes place in green cheese. 

 A very rapid increase of bacteria occurs, confined almost exclusively 



