352 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



formation within the curd prevents matting while the curd is in the vat, 

 and may even render difficult the fusion of the particles under pressure. 

 The nature of the change which the curd undergoes at this stage in the 

 manufacture is not well understood, but probably is due to a com- 

 bination between the paracasein and the lactic acid, the resulting 

 compounds differing from the paracasein in physical properties and in 

 solubilities. 



RIPENING or CHEESE. Cheese in ripening undergoes profound 

 physical and chemical changes under the influence of a number of factors, 

 which for purposes of discussion may be divided into two groups: 

 those by which the content of soluble nitrogen in the cheese is increased 

 and the digestibility enhanced; and those which cause the formation of 

 flavoring substances. During the ripening of the cheese the maker 

 can do little toward the control of the factors which ultimately determine 

 its commercial value. As in butter, the flavor is the most important 

 characteristic of the ripened cheese and the most difficult to control. 



Theories of Cheese Ripening. Many theories have been advanced to 

 explain the changes that occur during the ripening process. Duclaux, 

 a French microbiologist, studied the bacterial flora of Cantal cheese by 

 aid of the crude methods available before the introduction of the gelatin- 

 plate method. By the use of the dilution method, using bouillon as the 

 nutrient medium, he isolated a number of kinds of spore-forming bacteria. 

 The organisms formed two enzymes, one a curdling enzyme related to 

 rennin, the other a proteolytic enzyme to which was given the name 

 casease. A chemical study of the by-products of the organisms, when 

 growing in milk, revealed a number of compounds that had previously 

 been found in ripe cheese, such as leucin, tyrosin, and the ammonia salts 

 of acetic, valeric and carbonic acids. The cultures often possessed a 

 cheese-like odor. These facts led Duclaux to believe this class of organ- 

 isms were responsible for the ripening of the hard cheese in question. 

 The generic name Tyrothrix was applied on account of the supposed 

 relation to cheese. This term is still found in current bacteriological 

 literature. The methods employed by Duclaux were such as favored 

 the growth of the liquefying, rather than the acid-forming bacteria. 

 To the latter more recent investigators have devoted attention. 



The theory that the proteolytic bacteria function in the ripening of 

 hard cheese has been more recently emphasized by Adametz. It is 

 sufficient to say that the number of spore-forming proteolytic bacteria 



