BACTERIA AND CHEESE-MAKING. 263 



it. Concerning the macroscopic impurities of milk, see 

 p. 31. The microscopic impurities are numerous and indi- 

 cate contamination of dust or dung-particles in the stable. 

 In the making of certain kinds of Dutch cheese it is con- 

 sidered of primary importance that the milk be removed 

 from the stable as quickly as possible, and that it be coag- 

 ulated while still warm from the cow. Those acquainted 

 with the care and cleanliness maintained in Dutch stables 

 will know that these cheeses are prepared from a compara- 

 tively pure milk. 



Eennet and cheese-color are first added to the milk. 

 The former coagulates the casein, entangling in it all ma- 

 croscopic impurities along with microscopic ones adhering 

 to them, and also retaining the bacteria suspended in the 

 milk. The bacteria thus inclosed in the coagulated casein 

 come from three different sources, viz. : (1) from the milk 

 and its impurities; (2) from the rennet which, especially if 

 prepared from calves' stomachs at the cheese-factory, usually 

 contains an immense number of bacteria (rennet extract 

 contains a small number of bacteria*); and (3) from 

 the cheese-vat and the air. In the beginning these bac- 

 teria seem to play an insignificant part, and in some 

 kinds of cheese the increase takes place but slowly. We 



* The number of bacteria in rennet extract has been determined 

 by Baumann and Pammel. The former (loc. cit.) found in two 

 samples of rennet tablets 39,250 bacteria per cc. (strength 1 : 200,000) 

 and 32,400 bacteria per cc. (strength 1 : 32,000) ; a sample of Danish 

 fluid extract contained 1,407,600 bacteria per cc. (strength 1 : 5000). 

 Pammel and students working with him found in five determina- 

 tions of the bacteria content of fluid rennet extracts from 409,500 

 to 4,019,200 bacteria per cubic centimeter. W. 



