234 PRACTICAL DAIRY BACTERIOLOGY 



order that the proper chemical changes be obtained. In the 

 manufacture of the Cheddar cheese the development of a proper 

 acidity in the curd is necessary to produce the proper texture 

 in the cheese, and, also, to bring about the proper ripening. If 

 the milk sugar is washed out of the cheese so that lactic acid 

 cannot be formed the ripening does not progress normally ; and 

 very badly ripened, badly flavored cheeses are the result. 1 This 

 is due to the fact that in such cheese the acid bacteria do not 

 grow, while the liquefiers develop abundantly. A study of the 

 chemistry of the ripening of cheeses has shown that the con- 

 version of the casein into the desired digested products occurs 

 only when lactic acid is present to combine with them in forming 

 certain new products. Thus the development of lactic acid in 

 the cheese is a necessary first step in the ripening, and inasmuch 

 as this lactic acid is dependent upon lactic bacteria, the ripening 

 is closely associated with these organisms. Weigmann believes 

 further that the lactic bacteria prepare the way for other species 

 that grow later and furnish the flavors; but this is not yet 

 proved. 



In the ripening of Swiss cheeses the relation of bacteria is 

 somewhat different. These cheeses, as already noticed, are 

 made from sweet milk rather than sour. The Swiss cheeses 

 when properly ripened contain large holes, a distinctive char- 

 acteristic of this peculiar type. These holes seem to be due to 

 the carbon dioxide gas produced by certain types of bacteria, 

 but it is not the common lactic acid bacteria that produce them, 

 for these do not give rise to gas. Nor does it seem to be the 

 common gas-producing lactic bacteria referred to on page 34; 

 for these, as noticed later, are apt to ruin cheeses. The normal 

 holes of the Swiss cheeses are due to a different species of 

 bacterium, which has recently been described. 2 But neverthe- 

 less the Swiss school of dairymen has learned that the ripening 



1 Babcock and Russell. Cent. f. Bact., II., ix., p. 757, 1902. 



2 Freudenreich and Jensen. Cent. f. Bact., II., xvii., p. 529, 1906. 



