MILK. 447 



peptone, kreatin, leucin, etc. The principles which give to milk its 

 peculiar odor have not yet been conclusively pointed out. 



The gaseous constituents of milk are mainly carbon dioxide, oxygen, 

 and nitrogen. 100 volumes of milk contain of carbon dioxide 7.6, 

 of oxygen 0.1, of nitrogen 0.7 volumes. 



Changes in milk. Soon after milk leaves the animal system 

 changes take place which are either of a physical or chemical nature. 

 The first change in milk, when allowed to stand for a few hours, is 

 a separation of the suspended fat globules toward the upper part of 

 the liquid, which gradually becomes loaded with fat, forming a dis- 

 tinct layer over the liquid. This upper layer having a slightly 

 yellowish color (cream color) is cream, the watery liquid below hav- 

 ing a bluish-white color is skimmed milk. 



Another change taking place in milk (rarely after a few hours, 

 but generally after a day or a few days) is the coagulation of casein, 

 which takes place both in the cream and in the skimmed milk, con- 

 verting the whole into a thick, semi-liquid mass, which gradually 

 separates into a solid white curd, and a thin, transparent milk-serum 

 or whey. 



The coagulation of the casein is caused by lactic acid, produced by 

 the so-called lactic fermentation of lactose. The ferments causing 

 this fermentation are undoubtedly floating in the air, as it is possible 

 to prevent the decomposition of milk-sugar for a considerable length 

 of time by taking proper precautions for destroying and excluding 

 them. Simultaneously with the coagulation of milk the alkaline 

 reaction becomes acid and the sweet taste gradually more and more 

 sour. 



These changes in milk can, to some extent, be artificially produced, 

 hindered, and controlled. Thus, the casein may be precipitated by 

 the addition of rennet or acetic acid (or any mineral acid) and heating. 

 The decomposition of the milk-sugar and with it the " curdling " may 

 be prevented 1, by chemical treatment with alkaline salts or anti- 

 septics ; 2, by physical treatment, such as cooling or icing, boiling 

 and aeration ; 3, by condensation or evaporation, with or without the 

 addition of a preservative agent. All these systems of preservation, 

 however, are subject to serious disadvantages because they either inter- 

 fere with the natural constitution and properties of the milk, or 

 because they serve their purpose for too limited a time. 



The addition of alkalies such as lime-water, sodium carbonate or 

 bicarbonate, does not prevent the lactic fermentation, but prevents 



