GENERAL CHEMISTRY OF MILK 103 



The following table has been compiled according to Marshall's 

 results : 



THE GASES IN MILK 



Carbon Residual gas 



dioxid, Oxygen, (nitrogen), 



Condition of milk. per cent. per cent. per cent. 



Unexposed milk 81.496 2.42 16.535 



After milking 59.636 13.176 27.17 



After aeration over glass 40.572 20.586 38.842 



After aeration over tin 35.832 20.55 44.618 



After aeration over copper 42.328 17.256 40.416 



After aeration through glass wool and copper sieves. . 25.805 25.805 50.8775 



This table shows that there is gas before the milk leaves the 

 udder, since Marshall's experiments were such as to exclude the 

 air during milking. After the milk has left the udder there is a 

 diminution of carbon dioxid, while there is an increase of oxygen. 



Marshall thinks it possible that the carbon dioxid is actually 

 present in loose combination, in part at least, and not altogether 

 as free gas. This view is based on the fact that when the gas is 

 removed from milk the last quantity is difficult to extract, and 

 that the effect which the original carbon dioxid in milk has upon 

 chemical indicators differs from the effect of impregnated carbon 

 dioxid. However, carbon dioxid is easily soluble in water, and 

 the last remnant is difficult to remove from .distilled water, espe- 

 cially in the presence of salts, such as sodium carbonate, other 

 carbonates, and acid sodium phosphate. This question must, 

 therefore, remain undecided at present. 



When sterilized milk was exposed to air Marshall found no 

 evidence of absorption of gas after six weeks. 



Since carbon dioxid decreases and oxygen increases when milk 

 comes in contact with the air, there must be a rapid interchange 

 of gases during milking, a process which may be considered a sort 

 of aeration. If oxygen is absorbed it is reasonable to assume that 

 stable odors are also absorbed. By subsequent straining and 

 aerating an equilibrium of gas content with the gases of the air 

 is established, so that the gas content in milk is similar to the gas 

 content of the air, whatever that may be. Tainted air will, there- 

 fore, impart its taint to the milk, while the milk will give up some 

 of its normal gas to the air. 



Lactic acid fermentation in milk is favored by the presence of 

 oxygen, while absence of this gas favors anaerobic action on the 

 protein and possibly the formation of toxic products. 



Some of Marshall's most important conclusions from his in- 

 vestigations are these: 



1. Milk drawn from the udder of a cow contains a high per- 

 centage of carbon dioxid and a low percentage of oxygen. 



2. During the milking process the percentage of carbon dioxid 

 contained in the gases pumped from milk dropped considerably. 



