358 Bacteria in Relation to Country Life 



As modern times are approached, a steady improve- 

 ment in the art of milk -production and treatment is 

 noticeable. Progress in dairy husbandry was most 

 marked in the production of different varieties of cheese, 

 and some of the dairy industries then established still 

 retain their prominence. But while there was undoubted 

 progress in more than one direction in the seventeenth 

 and eighteenth centuries, it was not until the nineteenth 

 century that the true nature of the changes in milk 

 was understood. The teachings of bacteriology revealed 

 to us the relation of bacteria to the keeping quality of 

 milk and made possible, thereby, a wonderful develop- 

 ment of dairy methods. Sanitary production of milk, 

 the significance of cleanliness in the barn and dairy, 

 pasteurization and sterilization, the use of pure cul- 

 tures in the ripening of cream, are well appreciated today, 

 but were scarcely known a generation ago. 



Milk a medium for bacterial development. Because of 

 its composition, milk offers an excellent medium for the 

 development of all sorts of microorganisms. It contains 

 sugar, a food constituent suitable for many bacteria; 

 protein substances capable of serving as nourishment for 

 a great number of species; and fats especially acceptable 

 to various molds. Because of these substances, the bac- 

 teria that gain access to the milk multiply with extreme 

 rapidity and cause chemical changes in its composition. 

 Souring is but one of these changes. There are many 

 others that may destroy its value as food, or may even 

 lead to the accumulation in it of substances injurious to 

 health. 



Without bacteria or other microorganisms milk under- 



