MILK. 451 



a weighed dish gives the quantity of sugar plus some salts. Upon 

 igniting the milk-sugar a residue of salts is left, which is also weighed, 

 and this weight deducted from the first one. 



Casein. The residue now left (after treatment with ether and alco- 

 hol) contains chiefly casein with some albumin and salts. If any 

 casein should have been washed upon the filters accidentally, it has 

 to be transferred back to the dish, the contents of which are dried 

 and weighed. By burning off the casein and re weighing the dish 

 plus the salts, the quantity of the casein is determined. 



The remaining salts added to those previously obtained from the 

 alcoholic solution form the total ash or inorganic solids, an analysis of 

 which may be made according to the methods given heretofore. 



Casein may also be determined directly by precipitating it from 

 milk, by the addition of acetic acid and boiling. The precipitated 

 casein is filtered off, and has to be well washed, first with water, and 

 then with ether, as it contains most of the fat. 



Experiment 68. a. Determine the specific gravity of milk, cream and skimmed 

 milk by means of the lactometer (a urinometer answers the purpose). 



b. Acidulate some skimmed milk with acetic acid, notice the coagulation of 

 the casein, and separate it from the whey by filtering through paper or cloth, 

 using some pressure to expel most of the liquid. 



c. Test the casein by heating with nitric acid (yellow color), by using Mil- 

 Ion's reagent (purple-red color), by warming gently on the water-bath with con- 

 centrated hydrochloric acid (violet-colored solution), by warming gently with 

 water and a few drops of potassium hydroxide, when a solution is obtained 

 from which the casein is reprecipitated on neutralizing with acetic acid. 



d. Test the whey for milk-sugar by heating with Fehling's solution (red pre- 

 cipitate), by applying Moore's test, i. e., heating with potassium hydroxide 

 (brown color), and by heating with solution of picric acid and potassium 

 hydroxide (reddish-brown color). 



e. Determine the constituents of milk quantitatively by using the directions 

 given above. 



QUESTIONS. 531. Mention the five principal constituents of milk. 532. Give 

 the average composition of human and of cow's milk. 533. What compounds 

 constitute milk-ashes ? 534. What physical and what chemical changes does 

 milk suffer on standing? 535. What acid is formed in milk on standing, and 

 how does this acid act on the casein? 536. Describe the processes used for 

 preventing the decomposition of milk. What are their advantages and their 

 disadvantages? 537. Give approximately the quantities of the chief com- 

 ponents of cream, skimmed milk, butter, buttermilk, curd, whey, and cheese, 

 and state how these substances are obtained. 538. Why does the specific 

 gravity of milk not indicate its purity and richness? 539. Describe the 

 advantages of the combined use of the lactometer and creamometer in testing 

 milk. 540. Give a process for the complete quantitative analysis of milk. 



