FOOD 577 



Nil else siiould be permitted without permission from the Board of 

 Agriculture. 



Good firms turn out canned products which are a great boon to 

 most tropical residents. 



For quiilitatwe and quantitative tests for preservatives a larger 

 work should .be consulted. We venture to give here one simple 

 qualitative method for each principal preservative. 



Formaldehyde. 



Put 5 c.c. of milk in a test tube. 



Add to this 5 c.c. of the liquid to be tested. 



Pour carefully down the side 5 c.c. of concentrated sulphuric acid 

 so that it shall not mix with the diluted milk. 



A violet ring at the junction of the two liquids is positive for 

 formaldehyde. The delicacy of the test is about i part in 200,000. 



Note. — Pure sulphuric acid does not give the test. In commercial 

 acid some ferric salts are present which produce the test. Hence if 

 onlv pure sulphuric acid is to hand add a little ferric chloride. 



Benzoic Acid and Sodium Benzoate. 



This salt is very widely used in many varieties of foods. 



If the material is a liquid, acidify 25 c.c. with 5 c.c. of sulphuric 

 acid and shake with 25 c.c. of ether in a separatory funnel. 



If the substance is not fluid, dilute it or macerate it and obtain a 

 watery solution of it, strained and treated as above. 



The mixing must be thorough but not violent as emulsions may be 

 formed. 



Separate the other layer and evaporate it at a low temperature. 



If the original material contained much fat shake the ether with 

 ammonia, evaporate and test the residue. 



Crystalline scales mav be seen in the residue which give off a 

 characteristic odour when heated if much benzoic acid was present. 

 The residue must be tested as follows : — 



Add to the residue 2 to 3 c.c. of concentrated sulphuric acid. 



Heat until the acid gives off fumes strongly. 



The benzoic acid is changed to sulpho-benzoic acid, in the charring. 



Add ammonia nitrate in small portions while it is still fuming. 



The organic matter is oxidized and m-dinitrobenzoic acid formed. 



Let it cool. 



Add water and ammonia in excess, transfer to test tube. 



Pour down side of test tube a little freshly prepared colourless 

 ammonium sulphide. It must not be mixed. 



A red ring at the junction of the two fluids is positive for benzoic 

 acid and is due to ammonium m-diamidobenzoate. 



