BACTERIA IN FOODS 221 



cheese bacteria are not only favourable, but actually essen- 

 tial, for in its manufacture the casein of the milk has to be 

 separated from the other products by the use of rennet, and 

 is then collected in large masses and pressed, forming the 

 fresh cheese. In the course of time this undergoes ripening, 

 which develops the peculiar flavours characteristic of cheese, 

 and upon which its whole value depends. 



We have said that the casein is separated by the addition 

 of 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, however, that bacteria really play their role. After 

 this physical separation, 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 pro- 

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

 they are either removed or opposed the curing changes im- 

 mediately cease. If the milk be first sterilised, or if anti- 

 septics, like thymol, be added, the results are negative. It 

 is not yet known whether this peptonising 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 com- 

 mon groups of bacteria more or less constantly present in 



