CHAPTER XIII. 



MILK. 



MILK is the most satisfactory individual food material elaborated 

 by nature. It contains the three nutrients, protein, fat and carbo- 

 hydrate and inorganic salts in such proportion as to render it a 

 very acceptable dietary constituent. It is a specific product of 

 the secretory activity of the mammary gland. It contains, as the 

 principal solids, tri-olein } tri-palmitin, tri-stearin, tri-butyrin, case- 

 mo gen, lactalbumin, lacto- globulin, lactose and calcium phosphate. 

 It also contains at least traces of lecithin, cholesterol, urea, creatine, 

 creatinine and the tri-glycerides of caproic, lauric and myristic acids. 

 Citric acid is also said to be present in milk in minute quantity. 

 Fresh milk is amphoteric in reaction to litmus, 1 but upon standing 

 for a sufficiently long time, unsterilized, it becomes acid in reaction, 

 due to the production of fermentation lactic acid, 



H OH 

 H-C-C-COOH, 



from the lactose contained in it. This is brought about through 

 bacterial activity. The white color is imparted to the milk partly 

 through the fine emulsion of the fat and partly through the medium 

 of the caseinogen in solution. The specific gravity of milk varies 

 somewhat, the average being about 1.030. Its freezing-point is 

 about 0.56 C. 



Fresh milk does not coagulate on being boiled but a film con- 

 sisting of a combination of caseinogen forms on the surface. If 

 the film be removed, thus allowing a fresh surface to come in 

 contact with the air, a new film will form indefinitely upon the 

 application of heat. Surface evaporation and the presence of fat 

 facilitate the formation of the film but are not essential (Rettger). 

 As Jamison and Hertz have shown, a similar film will form on 

 heating any protein solution containing fat or paraffin. If the 



1 Human milk as well as cow's milk. It is, however, acid to phenolphthalein. 



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