136 AGRICULTURAL BACTERIOLOGY 



trated with milk and its products. There are special rea- 

 sons why a detailed discussion of the action of microorgan- 

 isms on this food product is desirable. Milk is one of the 

 most important foods in the dietary of the American and 

 European people. It forms about one sixth of the food 

 of the population of this country, and for children a much 

 greater proportion of their nutriment. One milch-cow is 

 kept for each 4.5 persons. A large portion of the milk 

 consumed is used as raw milk, and hence its contamination 

 with disease-producing bacteria is of great importance. 

 Again, its preservation is a problem that is presented to 

 the producer and to the consumer daily; for, in the pro- 

 duction and handling of milk, it becomes seeded with great 

 numbers of bacteria, which find in it a most favorable place 

 for growth. The manufacture of butter and cheese was 

 originally carried out on the farm. Their preparation has 

 now been largely removed therefrom ; but the farmer is still 

 the producer of the raw material, the quality of which de- 

 termines the quality of the product. 



Factors governing decomposition. In the decomposition 

 of any substance the rapidity of the changes involved are 

 determined by the number of organisms from foreign 

 sources that are brought in contact with the material, and 

 by the rapidity with which growth occurs, since decompo- 

 sition processes can not occur without growth, no matter 

 how great the initial contamination of the food. The ques- 

 tion of food preservation, therefore, may be divided into 

 two divisions : first, the contamination of the food ; second, 

 the destruction of the microorganisms contained in the food 

 or the inhibition of their growth. 



The first is especially important with liquid foods, such 

 as milk, because the organisms can be uniformly incorpo- 

 rated with the liquid, and their growth will not be limited 

 to any one point, as in the case of solid foods. Again, when 



