THE RELATION OF MICROORGANISMS TO MILK 



455 



The effect of straining upon the keeping quality is shown in the 

 following experiments where the milk was strained through the same 

 form of strainer mentioned above and the samples kept at constant 

 temperature of 21 until coagulation. 



EFFECT OF STRAINING UPON KEEPING QUALITY OF MILK 



It will be seen that in no case was the keeping quality of these 

 samples increased by the straining process while in some cases it 

 was materially injured. 



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- 



