THE RELATION OF MICROORGANISMS TO MILK. 317 



If milk has been produced under such conditions that it has absorbed 

 foreign odors, this process may be of value in getting rid of the absorbed 

 odors, but from the bacterial standpoint the process of aerating is not 

 desirable, since it gives one more opportunity for the milk to become 

 contaminated with organisms from the atmosphere and from the aerator 

 itself. It is possible to aerate milk under such conditions that the germ 

 content will not be increased, but if aeration takes place in the cow stable 

 or other place where the atmosphere contains dust the number of organ- 

 isms will be greater after aeration than before, the amount of increase being 

 proportional to the sanitary conditions under which the aeration is done. 

 It is even possible that the milk may absorb foreign odors during the 

 process of aeration and be of poorer quality than it was before. It is 

 thought by many that the process of aeration is necessary in order to get 

 rid of the so-called animal odors commonly found in milk. These odors 

 are, however, not normal to the milk but are absorbed from the foul air 

 in the stables or other sources. This is shown by the fact that some of the 

 very finest quality of certified milk is bottled while still containing the 

 animal heat with the least possible exposure to the air, tightly sealed 

 at once and plunged into ice water. Such milk contains no suggestion of 

 animal odor. Aeration may be of value in removing undesirable odors 

 from milk which is not produced under good sanitary conditions, if done 

 in an atmosphere free from all dust and odors, but it is not necessary for 

 milk of good quality. The common belief that aeration is valuable is 

 probably due to the fact that most aerators .are coolers as well, and the 

 beneficial results are due to the cooling and not the aeration. 



CENTRIFUGAL SEPARATION. It is a common practice in some dairies 

 to pass the milk through a centrifugal separator to remove any dirt which 

 it may contain. This operation is effective for the removal of much of 

 the insoluble dirt which may be in the milk, but it is of very doubtful 

 value from the standpoint of the bacterial content and the keeping quality 

 of the milk. In spite of the fact that the separator slime is very rich in 

 bacteria, the milk and cream as they come from the separator will nor- 

 mally show larger bacterial counts in agar and gelatin plates than will the 

 milk before it enters the separator. The usual effect upon the germ con- 

 tent of passing milk through a separator may be seen in the following 

 experiments: 



