EXPERIMENT STATION BULLETINS. 259 



AERATION OF MILK. 



Bulletin 201 — Department of Bacteriology and Hygiene. 



(A popular consideration of Special Bulletin 16.) 



CHARLES E. MARSHALL. 



So many values and advantages have been attributed to aeration by dairymen that it 

 has been thought pertinent and important to make an exhaustive and scientific study of 

 the subject, especially in its relation to the handling of milk. The full details of the 

 experiments appear in Special Bulletin No. 16 of this Station. This may be obtained on 

 application. 



In this popular and brief resume of some of the results obtained, I desire to 

 emphasize those only which have a direct bearing upon the care of milk and which may 

 be utilized to advantage. 



Aeration does have an influence upon milk, an influence which may be measured, but 

 its importance may not be so great as is sometimes assigned to the process. When milk 

 leaves the udder of the cow, the gas which is predominant, carbonic acid gas, begins to 

 pass from the milk and the gases of the air take its place. This is brought about by the 

 natural diffusion and solubility of gases. 



The fact that carbonic acid gas exists in such large percentage does not indicate that 

 this gas has any detrimental effect upon the individual consuming the milk directly 

 from the cow; this would not accord with known facts. As soon as the milk leaves 

 the udder of the cow it comes in contact with germ life; it is this germ life which is 

 controlled largely by the conditions of the milk: in it there are germs of many kinds, 

 some of which flourish readily where there are traces of oxygen only, and others where 

 there is an abundance of oxygen. These germs produce the various fermentations of 

 milk, consequently it makes a difference in the character of the fermentation whether 

 there is an abundant supply of oxygen or not. Bacteriologists have shown that when 

 there are only traces of oxygen present in a fermenting substance as milk there is 

 more likely to result from the fermentation toxic substances or products which are 

 really detrimental to the body. As milk comes from the udder there is little oxygen 

 and much carbon dioxide, but after the aerating process the oxygen materially increases 

 and the carbonic acid gas decreases. (See bulletin.) It therefore follows that aeration 

 favors those fermentations which produce no poisons because where there is a plentiful 

 supply of oxygen, toxic substances are not formed. 



As the milk passes from the teats of the cow to the pail it is exposed freely to the air. 

 At first thought, one might conclude that there is a free interchange of gas during the 

 milking process. This is partly true, the carbonic acid gas fortunately falls about 

 twenty per cent in amount. However, the process of aeration is difficult after a certain 

 reduction of carbon dioxide has taken place or a partial equilibrium of gases estab- 

 lished; and if a more perfect aeration is to be secured it is necessary that the milk 

 be brought intimately in contact with the air by creating the greatest possible surface 

 for the longest possible exposure. The more perfectly the drop can be shattered or a film 

 established the more satisfactory the aeration. 



If agitation of milk therefore aids aeration and if during the few moments imme- 

 diately after milking the inter-change of gases between the air and milk is greatest, 

 it follows that where milking is in process the air must be pure, otherwise the foulness 

 of the air will be incorporated in the milk. Milk absorbs gases. What must be the 

 condition of the air of a stable in which all sorts of fermentations are going on and in 

 which are odors of diverse kinds? These obnoxious substances are in the air and must 

 pass into the milk with the air. 



In addition to milking, frequently the milk is further agitated in the stable by passing 

 it from pail to pail or even straining it and also sometimes aerating it. Such methods 

 are rightly condemned and the reasons for such condemnation are conclusive. Milking, 

 and the handling of milk, should be carried on in pure air. It is sometimes the practice 

 of milk-producers to pour milk directly from the milking pail into a ten gallon can. 

 From the foregoing this must be considered reprehensible, for filling the can excludes the 



