16 CITY MILK SUPPLY 



ble nutriment are considerably greater in some of the milks than in 

 others and the tendency is for the fat and protein to increase together 

 from the low solids milk to the high. However, they do not increase at 

 equal rates as the column headed protein fat ratio shows. All of these 

 things are important to those who produce and those who consume milk 

 and, therefore, affect the price at which it sells. 



The chemical constituents of milk that have been described make up 

 its nutrients and determine its food value. Until recently the quality 

 of milk was judged solely by the percentage of these components that it 

 carried but in this country about 1890 a new criterion of milk quality 

 was introduced. Questions began to be asked about its bacterial content. 

 Interest in this subject had been long in developing. In the period from 

 1857 to 1873 Pasteur, Lister and others had finally convinced people that 

 the souring of milk was due to lactic acid bacteria and by 1881 it began to 

 be generally accepted that infectious diseases are carried in milk. About 

 this time the specific germs of these maladies began to be isolated so 

 that the public became accustomed to the idea that such germs occasion- 

 ally got into milk but the demonstration that market milk contained 

 large numbers of germs of different sorts amazed people and led bacteri- 

 ologists to study the role they played. It was soon found that bacteria 

 did affect milk in various ways and that its flavor and keeping quality 

 might be improved by cleanly methods of dairying and by keeping milk 

 cold to prevent the multiplication of the germs that in one way and 

 another got into it. Clean and cold became the watchword of progressive 

 dairymen and of all those who were endeavoring to improve market milk. 

 The importance of bacteriology to the milk industry was further increased 

 by the discovery that pure cultures of bacteria might be used commer- 

 cially to improve the flavor of manufactured dairy products and to make 

 artificial buttermilk and other milk beverages which were steadily growing 

 in popularity. So, in order to get a correct conception of the city milk 

 problem, the relation of bacteria to dairying must be briefly considered. 



Bacteria in Milk. Milk as it comes from the udder generally con- 

 tains but few bacteria. They are germs that have worked their way up 

 the teat canal and have established themselves in the milk cistern and the 

 several ducts leading therefrom. Relatively few forms are able to ha- 

 bituate themselves to the specialized conditions of life that continued exist- 

 ence within the udder imposes. The organisms that do so, constitute a 

 very small but the only fixed part of the bacterial content of milk which, 

 except for these types, is composed of a mixture of divers microbes from 

 many sources. Since milk is an excellent culture medium, a large per- 

 centage of the microbes that chance to fall therein grow vigorously; 

 consequently it has no characteristic bacterial flora but rather one that 

 is a composite of all the germ life with which it has had contact. It is 

 true that certain forms such as Strept. lacticus somewhat constantly set 



