EXPERIMENT STATION BULLETINS. 193 



When the modern scientific dairy methods are compared with the 

 methods of ten and twenty years ago, the significance of bacteriology 

 becomes apparent. Answers to the questions of the causes of various 

 abnormal conditions have been given. The souring of milk and its 

 putrefaction, the appearance of color, the presence of a bitter taste 

 and ropiness have all been explained. Cream is now ripened by pure 

 cultures, cheese is made in accordance with the laws of fungal and 

 bacterial growths and the commercial requirements demand Pasteurized 

 milk. 



c 



MILK AS A FOOD FOR BACTERIA. 



The chemical composition of milk appears to the bacteriologist not 

 by the number of components involved but rather by the value each 

 component possesses as a food for bacteria. Considering the analysis 

 of milk in this light and remembering what was said in Bulletin 139, 

 p. 68, regarding the food requirements of bacteria, the component parts 

 of milk may be resolved into three classes: the nitrogen class, the carbo- 

 hydrate or sugar class and the fat class. The igorganic salts are 

 essential but subsidiary to our purpose. 



The first class is represented by casein, the parent substance of cheese. 

 It is this which provides the bacteria with the necessary nitrogen for 

 their development. Owing to the various actions bacteria have upon 

 casein, it affords us a means of ascertaining how different species attack 

 its molecule and thus aids us in determining species. In some cases 

 there is a process instituted similar to putrefaction and again there 

 is simply a digestion of the casein. With other species there is 

 no change whatever in the casein. Whatever change is undergone by 

 the action of bacteria, it furnishes them with nitrogenous food. Sugar 

 of milk (lactose) is the main component of the second class. Con- 

 taining carbon, hydrogen and oxygen, it offers three food elements to 

 bacteria. The atoms of carbon, hydrogen and oxygen making up a 

 molecule of sugar are sometimes torn asunder; lactic acid which usually 

 produces the souring and curdling of milk, alcohol which is utilized in 

 the manufacture of certain alcoholic milk drinks and carbon dioxide 

 gas are the resulting fragments of the destroyed molecule of sugar. 



In the third case but one element has been mentioned, the fat. Upon 

 this substance bacteria have little effect, except indirectly. The bacteria 

 do not seem to feed upon it or change it in any manner.- The changes that 

 appear to take place in cream and butter are usually traceable to other 

 compounds than fat. So seldom is fat acted upon by bacteria that 'it 

 is almost disregarded by bacteriologists. 



In milk, nitrogenous food is in abundance, carbon, hydrogen, oxygen 

 and inorganic salts are not lacking, consequently milk is an excellent 

 food for bacteria. 



MILK IN THE UDDER. 



Milk in its normal condition in the udder is free from all forms of 

 bacteria. This is an old belief that has been established repeatedly by 

 numerous investigators in Europe and elsewhere. Some have ventured 



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