64 Report of the Department of Bacteriology of the 



INTRODUCTION. 



As soon as American cheddar cheese and its method of manu- 

 facture were studied from a biological standpoint it was found 

 that acid-forming bacteria thrive in the curd to such an ex- 

 tent as to make up more than 90 per ct. of the total flora of the 

 cheese. It was natural to assume that the growth of this large 

 number of bacteria, amounting to some millions in each gram of 

 the curd, could not fail to have an influence upon the final result 

 which we speak of as ripened cheese. Students of the problem 

 have even gone so far as to hold that the lactic-acid bacteria are 

 the principal, if not the only, cause of cheese ripening. 



Scientific opinion is by no means unanimous as to the causes 

 which bring about the ripening of cheese. Two other theories 

 aside from the one just mentioned are supported by considerable 

 evidence. According to certain students the ripening is brought 

 about by a different class of bacteria characterized by the elabora- 

 tion of enzymes capable of attacking and dissolving the coagulated 

 casein. A third explanation assumes that the breaking down of 

 the casein is largely due to the activity of enzymes secreted by the 

 cow with the milk. 



These wide differences of opinion concerning a problem whicii 

 has been so long an object of study are the result of our inability 

 to follow the changes going on within the cheese mass. 

 While the problem is essentially one of applied biology, an exact 

 knowledge of physiological chemistry, coupled with a proper ap- 

 preciation of commercial quality is necessary for its solution. 

 Since a sufficient knowledge of these three lines of human effort 

 was not possessed by one man the study of the cause of the ripen- 

 ing of cheese was assigned by the Station Director to the coopera- 

 tive activity of the Dairy Expert, the Chemist and the Bacteri- 

 ologist. As a result of this cooperative effort a considerable 

 advance has been made in our understanding of the problem of 

 cheese ripening and results on a number of points have already 



