226 MILK 



2. Leach's Hydrochloric Acid Test. — "Commercial hydrochloric 

 acid (spec. grav. 1.2) containing 2 c.c. of 10 per cent, ferric chlorid 

 per liter is used as reagent. Add 10 c.c. of the acid reagent to an 

 equal volume of milk in a porcelain casserole, and heat slowly- 

 over a free flame nearly to boiling, holding the casserole by the 

 handle and giving it a rotary motion while heating to break the 

 curd. The presence of formaldehyd is indicated by a violet col- 

 oration, varying in depth with the amount present. In the ab- 

 sence of formaldehyd the solution slowly turns brown. By this 

 test 1 part of formaldehyd in 250,000 parts of milk is readily de- 

 tected before the milk sours. After souring the limit of dehcacy 

 proves to be about 1 part in 50,000." 



3. Confirmatory Tests with Distilled Milk {Leach). — Distil 100 

 to 200 c.c. of the milk and use the first 20 c.c. of the distillate for 

 testing. The following tests can be applied: 



(a) To a few drops of the distillate in a test-tube add 1 drop of 

 Schiff's reagent. A pink coloration appears in the presence of an 

 aldehyd. 



Preparation of Schiff's reagent : One gram of fuchsin is dissolved 

 in water and a mixture of 20 c.c. of a saturated solution of sodium 

 bisulphate and 10 c.c. of pure hydrochloric added, and the volume 

 made up to 1000 c.c. 



(6) Add to 5 c.c. of the milk distillate a few drops of a 1 per 

 cent, aqueous solution of resorcin or phenol, mix, and pour down 

 the side of a tube containing sulphuric acid. A rose-red zone is 

 formed at the junction of the liquids. Formaldehyd can be de- 

 tected 1 part to 200,000. If present in larger quantity than 1 

 part to 100,000 a white turbidity or precipitate is formed above 

 the colored zone. 



(c) Use 1 or 2 c.c. of the milk distillate in a test-tube. Add to 

 this 2 to 4 drops of the following reagent: One gram of phenyl- 

 hydrazin hydrochlorid and 1.5 grams sodium acetate dissolved- in 

 10 c.c. of water. A green coloration appears when formaldehyd is 

 present. 



{d) To a distillate from milk which has been acidified before 

 distillation to bind ammonia a few drops of Nessler's reagent are 

 added. A yellow color appears in presence of formaldehyd. If 

 much formaldehyd is present the color is dark and a grayish 

 precipitate may form upon standing. 



The following methods for detecting formadehyd in milk are 

 given by the Association of Official Agricultural Chemists: 



1. To 3 to 5 c.c. of milk add a lump of phenylhydrazin hydro- 

 chlorid about the size of a pea, from 2 to 4 drops (not more) of 

 a 5 to 10 per cent, solution of potassium ferrocyanid, and from 8 

 to 12 drops of an approximately 12 per cent, solution of sodium 



