XVI. THE ANALYSIS OF MILK AND MILK PRODUCTS 353 



tion, an acid solution of mercuric nitrate may be employed to precipi- 

 tate the proteids. 1 It is prepared by dissolving mercury in twice its 

 weight of strong nitric acid and diluting the solution with an equal 

 volume of water. This diluted solution will serve to clarify fifty 

 times its volume of milk. 



Many methods of conducting the determination of lactose are in 

 use. For details, a manual on quantitative analysis should be con- 

 sulted One method of performing the Fehling test may be briefly 

 described here. 



Fehling's solution is best prepared when required, by mixing equal 

 volumes of the following solutions : 



(1) A solution of 34*64 grammes of pure crystallised copper 

 sulphate in 500 c.c. of water. 



(2) A solution of 173 grammes of sodium potassium tartrate 

 (" Eochelle salt ") and 51 grammes of sodium hydrate in 500 c.c. of 

 water. 



The filtrate from the precipitated proteids is so diluted that it 

 occupies exactly 10 times the volume of the milk taken. 50 c.c. of 

 this filtrate are then taken, heated in a water bath, and mixed with a 

 previously boiled mixture of 30 c c. of the above copper sulphate 

 solution, 30 c.c. of the alkaline tartrate solution, and about 120 c.c. of 

 water. The mixture is kept on the water bath for 15 minutes and 

 filtered through a small filter paper or a Gooch's crucible. The pre- 

 cipitated cupreous oxide is thoroughly washed with boiling water, next 

 with alcohol, and finally with ether. The precipitate is then dried,. 

 transferred to a weighed porcelain crucible, and strongly ignited with 

 free access of air, so as to oxidise it completely to cupric oxide. The 

 amount of hydrated milk sugar corresponding to the weight of the 

 cupric oxide is then ascertained, best by reference to a table, or ap- 

 proximately, by multiplying the weight of the precipitate by 0'6024. 



Adulteration of Milk. The commonest adulterant is water. 

 Direct proof of the presence, in a sample of milk, of added water is. 

 very difficult, unless the water happens to contain some substance 

 not naturally present in milk, e.g., nitrates. In such cases the- 

 detection of nitrates in the milk, say by the reaction with diphenyl- 

 amine and sulphuric acid, becomes at once proof of the addition of 

 water, though in some cases this may be due to the small quan- 

 tities employed in rinsing out the milk cans, etc. The usual 

 way of estimating the amount of added water is from a deter- 

 mination of the amount of solids-not-fat. By assuming that this, 

 in genuine milk, never falls bslow 8 -5 per cent, the percentage of 

 added water is given by the expression 



100 - 



O'O 



in which S = percentage of solids-not-fat. This gives the probable 

 minimum amount of added water. 



1 Wiley, Amer. Chem. Jour., 6, 289. 



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