PREFACE 



IT is about twenty years since the serious study of the 

 bacteria in milk began ; for while, in an incidental way, certain 

 problems were investigated earlier, not until about 1890 was 

 it recognized that the subject had a large field of practical ap- 

 plications. The development of Dairy Bacteriology has been 

 due to the work of scientists on both sides of the Atlantic and 

 this country has contributed no less than Europe to our present 

 knowledge. 



As the study has progressed it has appeared that the bacteri- 

 ology of milk products has an intimate relation to two different 

 classes of problems. 



1. The problems arising in the dairy proper and affecting 

 many questions relating to milk, butter and cheese. It is no 

 exaggeration to say that the development of the knowledge and 

 application of dairy bacteriology has revolutionized all phases 

 of dairying, until all processes, from the milking to the final 

 consumption of the completed product, have been modified by 

 bacteriological knowledge. The time has come when it is no 

 longer possible to be a successful dairyman without a knowl- 

 edge of milk bacteria. To-day all courses planned for the study 

 of dairying devote more or less time to the subject, and the 

 student who fails to acquire some practical knowledge of the 

 relation of bacteria to milk products is not properly trained 

 to make a practical dairyman. 



2. The problems of the relation of milk bacteria to the public 

 health. The increasing proof of the distribution of disease 

 germs by milk, the accumulating evidence that the health of 

 the public, especially of children, is closely related to the 

 bacteria in milk, and the growing realization of the conditions 



