No. 6. DEPARTMENT OF AGiaCL'ET[n;E. 5«8 



CHAl'TEK II. 



CONTAMINATION OF MILK. 



Milk, on standing, undergoes a variety of changes sooner or later, 

 many of which spoil it lor use as food. The most common and 

 extensive changes occurring in milk are due to fermentations. One 

 result of some fermentations is the production of bad llavors, but 

 these may be acquired also either by direct absorption from the 

 surrounding air or from the food consumed. To these different 

 sources of contamination we will now give some detailed attention. 



8. What is Fermeutation? 



The meaning of fermentation can best be made clear by illustra- 

 tion. When a fruit juice, like sweet cider, is allowed to stand at 

 ordinary temperature, it soon begins to undergo change©, large 

 quantities of bubbles of carbon dioxide gas come from the surface 

 of the liquid, and some alcohol is formed. In this case, the sugar 

 of the apple juice is changed into alcohol and carbon dioxide. If 

 this cider, containing alcohol, is allowed to stand long enough at 

 ordinary tem^jeratures, the alcohol disappears, being changed into 

 acetic acid and, we then have what we commonly call cider vinegar. 

 The process resulting in the change of the sugar in the cider into 

 alcohol and carbon dioxide is called a fermentation; and that re- 

 sulting in the change of alcohol ioto acetic acid or vinegar is alsc 

 a fermentation. 



When milk is freshly drawn, it is sweet, but, on standing a while 

 at ordinary temperature, it becomes sour. The change taking place 

 is the formation of lactic acid from milk sugar, and it is caused hj 

 certain germs. The souring of milk is another case of a fermenta- 

 tion. 



That which causes fermentation is called a ferment. As there 

 are many different kinds of fermentation, so there are many different 

 kinds of ferments. In the production of alcohol and carbon dioxide 

 from (lie sugar in cider, the ferment is yeast; in the fermentatioi 

 of vinegar from alcohol, certain kiiids of bacteria are the ferment 

 in the conversion of milk sugar into lactic acid, certain other bat 

 teria constitute the ferment. 



Ferniep.ts possess certain general characteristics in common, among 

 which may be mentioned the following: (a) A very small amount of 



