MAllSHALL ON THE AHKATION OF MILK. 189 



exit of the gases easy, the cause ot the odor (the condensed gases) will 

 be there, and be readily detected by the taste; and at 58° or 00° it will 

 remain there until tlu^ milk sours. The cowy flavor is most effectually 

 preserved when the milk is cooled in a close vessel shut out from the air, 

 and the heat absorbed away by an application of ice and cold water." 



Again : "The gas in milk varies both in quality and relative effect. 

 For instance, it is in the smallest amount when the cow is in good health 

 and quiet. It is more abundant when actively exercised, as when sharply 

 driven to the yard by dogs. It needs but little hurrying especially in 

 the morning, to make the effect apparent in cheese. It is different in 

 health and disease, and verv abundant and very infectious in cases of 

 fever. There is more in a state of debility than of strength and more 

 when pinched with cold than when comfortably warm." 



Arnold by these statements has conveyed the correct popular im- 

 pression, I believe, for these are the views held by the more intelligent 

 milk producers. His statements above carry us farther, inasmuch as 

 they indicate that there seems to be a direct relationship existing between 

 exercise and rest and the milk secreted by the animal, as well as that 

 relationship existing between the physiological functions and the secreted 

 milk as portrayed through the assimilation of food and its causa- 

 tive metabolism in the body. It is well known that certain foods may 

 influence the odor and flavor of the milk. For instance, ensilage fed before 

 milking may be detected in the milk, so too such foods as leeks, turnips 

 and other oderiferous materials \\\^j convey to the milk unpleasant odors 

 and flavors. This, in a general way, may be due to the production of 

 aromatic substances which by means of the functional changes in the 

 body find their way into the milk. This fact mav be illustrated bv the 

 ingestion of benzoic acid and the elimination of hippuric acid in the 

 urine. 



Incited by the chaotic condition of knowledge relevant to milk aeration, 

 we attempted to cast some light upon the perplexing problem, subtle too, 

 because it does not easily yield to attacks which are in range of our 

 present knowledge and methods. 



In our work on aeration we have been able to demonstrate that the 

 gases of milk undergo a change after leaving the cow. In milk taken 

 directly from the udder of the cow without exposure to the air, we have 

 found an average of 81.496 per cent of carbon-dioxide; after milking, this 

 amount has been considerably reduced, to an average per cent of 59.636 

 of carbon-dioxide; after aeration, the carbon-dioxide has fallen as low as 

 25.805 per cent. Correspondingly but not proportionally, the oxygen has 

 been increased from 2.42 per cent in unexposed milk, to 13.176 per cent 

 in milk after milking, to as high as 25.805 per cent after aeration. We 

 find, therefore, that the carbon-dioxide stands as a measure of great impor- 

 tance so far as the interchange of gases is concerned in milk undergoing 

 the usual manipulations of dairy work. It does not follow necessarily 

 that the carbon-dioxide itself has any significance beyond its value in air 

 analyses where it is more of an indicator than a vicious contaminating 

 factor. 



Another feature presents itself when considering the change taking 

 place during the milking process ; there is an interchange of gases going 

 on as the milk passes from the udder of the cow to the pail and further 

 as the milk undergoes a churning motion in the pail when the streams of 

 milk impinge against the surface. The gases in the air of the stable 



