210 AGRICULTURAL BACTERIOLOGY 



The cheese-maker can not, of course, test the milk of the 

 different patrons of his factory for gas-forming bacteria 

 by the methods employed by the bacteriologist. He can, 

 however, gain an idea of the <|ii;ility of the raw product by 

 making what is known ;is ;i curd test, in which ;i small por- 

 tion of cadi |>a1 ron's supply is curdled with rennet, after 

 which the curd is broken up so as to expel the whey, which 

 is then turned off. The small mass of curd is kepi for sev- 

 eral hours at temperatures that favor the growth of the 

 gas-forming bacteria, 100 to 104 F. The quality of the 

 milk is then determined by the appearance of the curd with 

 reference to Hie presence of gas-holes; the flavor and odor 

 are also noted. This rough qualitative test is of great serv- 

 ice to the cheese-maker in detecting the quality of the raw 

 material. All of the operations must be carried out with 

 due regard to contamination from outside sources, since it 

 is essential that the changes noted in the curd be caused 

 only by the bacteria in the milk, and not by those intro- 

 duced by the use of unclean utensils. 



The growth of mold, which occurs readily on the surface 

 of the cheese and which is objectionable on account of dis- 

 coloring the surface, can be easily prevented by dipping 

 the cheese in melted paraffin. The impervious layer thus 

 formed excludes the air, thereby preventing the growth of 

 mold spores. 



Swiss cheese. Swiss cheeso is made from sweet milk 

 that contains only small numbers ol' lactic bacteria. The 

 rennet used to curdle the milk is obtained from natural 

 "rennets," i.e., portions of dried calves' stomachs, which 

 are soaked in whey and kepi J 1 a temperature of from 

 80 to 95 F. for from twenty-four to thirty-six hours. 

 The whey contains numerous lactic bacteria, and on the 

 dried rennets there are always organisms of the Bact, 

 Bulgaricum type. This serves as a starter to inoculate the 



