THE RELATION OF MICROORGANISMS TO MILK. 293 



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

 nounced 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, perhaps 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 phe- 

 nomena 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 fermentation 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 the microorganisms. 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. 



When we recognize the extreme ease with which milk undergoes 

 bacteria] 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 infection 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. 



