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THE RELATION OF MICROORGANISMS TO MILK 389 



Cotton filters are more efficient than cheese cloth and in some 

 cases the keeping quality of the milk may be improved by this process. 



Aeration. — ^This is the process of exposing the milk to the atmos- 

 phere by allowing it to run over the surface of the aerator in a very 

 thin film. 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 atmos- 

 phere 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 atmos- 

 phere contains dust the number of organisms 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 proc- 

 ess 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 con- 

 taining 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 

 beKef 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 or clarifier 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 imdetermined value as yet from the standpoint of the 

 bacterial content and the keeping quaUty of the milk. In spite of the 

 fact that the separator slime is very rich in bacteria, the milk and 



