186 NORMAL MILK : ITS ADULTERATIONS, ETC. 



alkaline, ashed, and the ash dissolved in a little acid ; calcium 

 chloride is added, and the solution made alkaline with ammonia ; 

 the precipitate is collected, burnt, and extracted with acetic 

 acid, and the test made on the insoluble portion. 



Formaldehyde, which has been introduced of late years, is 

 now frequently employed as a milk preservative. 



It is generally added as a 1 per cent, solution in water, which 

 is made by diluting the 40 per cent, solution known as " For- 

 malin," " Formal," " Formol," or " Formine." A very large 

 number of reactions for this substance have been worked out. 

 The most easily applied test is that due to Hehner, which is 

 best carried out as follows : — The milk is diluted with an equal 

 volume of water, and a little 91 per cent, sulphuric acid run 

 in so that it forms a layer at the bottom. In the presence of 

 formaldehyde a violet-blue colour appears at the junction of 

 the two liquids, and the colour is permanent for two or three 

 days. This test will detect, easily, 1 part of formaldehyde in 

 200,000 of milk. Milk, in the absence of formaldehyde, gives a 

 slight greenish tinge at the junction of the two liquids, and on 

 standing a brownish colour is developed, not at the junction of 

 the two liquids, but lower down in the acid. 



Leonard and Smith's test for formaldehyde consists in heating 

 a little milk with 3 to 5 times its volume of concentrated hydro- 

 chloric acid ; a fine violet colour is produced in the presence of 

 formaldehyde (O'OOOl per cent, to O'l per cent.). The presence 

 of a trace of ferric chloride in the hydrochloric acid is 

 essential. 



These tests are not absolutely characteristic of formaldehyde, 

 and are not given in the presence of large amounts of this body. 

 It is a reaction of the tryptophane of the casein with formalde- 

 hyde, and certain other aldehydes — e.g., vanillin — give similar 

 colours. Leonard has pointed out that pure acids give no reaction, 

 but the presence of an oxidising agent is necessary ; he found 

 that a trace of ferric chloride gave the best results ; it is better 

 to use commercial acid than a purer form, as the necessary 

 oxidising agent is present. 



As a confirmatory test, some of the milk may be curdled by 

 dilute sulphuric acid and a little Schiff's reagent — a solution of 

 rosanihne bleached by sulphurous acid — added to the filtrate in 

 a test tube, which is corked and allowed to stand. In the pre- 

 sence of an aldehyde a violet-pink colour is produced after a 

 short time. Excess of sulphurous acid must be avoided in 

 preparing the reagent, or the test may fail with small amounts. 



There are many confirmatory tests, which are best applied to 

 the clear solution obtained by distilling the filtrate obtained by 

 curdling the milk with sulphuric acid. Smith and Leonard have 



