54 DAIRY ANALYSIS 



by 0.062 gives the percentage of boric acid (calculated 

 as H 3 B0 3 ). 



Formaldehyde. -Dilute a little milk with an equal 

 bulk of water in a test-tube ; pour carefully down the 

 side of the tube a little 90 per cent, commercial 

 sulphuric acid ; a bluish colour is developed at the 

 junction of the acid and milk in the presence of 

 formaldehyde. This blue colour may also be observed 

 during the estimation of fat by the Gerber process 



(P- 17)- 



Salicylic and Benzoic Acids. These acids give a violet 

 colour with ferric chloride, and are best tested for by ex- 

 tracting the nitrate produced by treating milk with 

 acid mercuric nitrate (p. 31) with ether, shaking the 

 ether with a little water to which a drop of phenol- 

 phthalein solution has been added, and dropping in 

 dilute caustic soda solution, till the water after shaking 

 just turns pink ; after discharging the pink colour 

 with very dilute acid and adding a little ferric chloride 

 solution, the presence of salicylic acid is shown by a 

 violet colour and of benzoic acid by a buff precipitate. 



Hydrogen Peroxide. Mix the sample with a little 

 fresh milk, and add a small amount of para-phenylene- 

 diamine or ortol ; a blue or red colour will be developed 

 if hydrogen peroxide be present. 



Reaction of Milk with Hydrogen Peroxide. 

 Fresh milk when treated with a little para-phenylene- 

 diamine or ortol (a photographic developer) and a 

 drop of hydrogen peroxide gives a deep blue (with the 

 diamine) or a brick-red (with ortol) coloration within 

 a few seconds. Milk heated above 80 C. remains white. 



The Cause of Poor Milk. The detection of added 

 water and of a deficiency of cream would be an obvious 

 explanation of the cause of milk being poor. If it is 

 normal in composition, but very white, and the fat 

 separated in the Gerber process is nearly free from 

 colour, this would show that the poverty of the 

 milk had been fallaciously inferred from its lack of 

 colour ; if this is not the case, the test given above 



