THE RELATION OF MICEOORGANISMS TO MILK 365 



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 chartges 

 in the milk. 



Microbial Content of Milk 



When we recognize the extreme ease with which milk undergoes 

 bacterial changes, we are not surprised to find that ordinary milk, 

 when delivered to the consumer, contains relatively large numbers 

 of bacteria. The amount of care exercised in the production and 

 handling is a most important factor in determining the bacterial 

 contamination of milk. On this basis milk may be roughly divided 

 into three classes. 



Common Milk. — Age is one of the chief factors in determining 

 the germ content of milk. We are, therefore, not surprised to find 

 the milk in large cities having a much higher germ content than in 

 smaller cities and towns. The normal germ content of ordinary 

 milk as it is found in the cities may be shown by the following 

 tables. 



