THE RELATION OF MICROORGANISMS TO MILK 429 



strong odor, such as silage or foul air, resulting from lack of ventila- 

 tion in the stable at milking time, these odors will be taken up by the 

 milk with surprising rapidity. If placed in an ice chest with fresh 

 strawberries or pineapple, or foods like cabbage or turnips, the milk 

 will very quickly absorb the odor of these foods. The absorption 

 of any foreign odor gives to milk a decidedly disagreeable taste. This 

 is true even when the odor which is absorbed is pleasant in itself as 

 in the case of strawberries or pineapples. When the "off" flavors are 

 due to absorption they are strongest at the outset and become less 

 pronounced as the milk becomes older, especially if it is subjected to 

 some method of aeration. 



CHANGES DUE TO MICROORGANISMS 



While absorption of foreign odors is not uncommon, probably 

 most of the undesirable flavors, found in milk when it reaches the 

 consumer, are caused not by absorption but by the growth of 

 microorganisms in the milk. In this class the changes are slight at 

 first and increase with the age of the milk. Changes of this sort 

 include the common phenomena of souring and curdling, the so- 

 called sweet curdling, ropy or slimy milk, bitter flavors, gassy milk 

 and a large variety of changes usually known as barny or cowy odors 

 and flavors. If milk could be kept free from microorganisms, it 

 might be kept for some time without showing perceptible changes in 

 appearance or taste. No other food product will undergo fermenta- 

 tion changes as rapidly as milk because it is an ideal culture medium 

 for the growth of most kinds of microorganisms, especially bacteria 

 and yeasts. Not only does milk contain the needed food elements but, 

 being in liquid form, they are easily available for the use of micro- 

 organisms. The proteins and milk sugar are most easily attacked 

 and it is the breaking down of these which causes most of the changes 

 in the milk. 



MICROBIAL CONTENT OF MILK 



The amount of care exercised in the production and handling is 

 a most important factor in determining the bacterial contamina- 

 tion of milk. On this basis milk may be roughly divided into three 

 classes. 



