XVI. THE ANALYSIS OF MILK AND MILK PRODUCTS 855 



banzoic acid is left and cm be recognised by the odour of benzene 

 evolved on heating it with soda-lime. 



4. Formaldehyde in the form of " Formalin," which is a 40 per 

 cent solution in water of the real formaldehyde, O = OIL, is a very 

 efficient preservative. It can be detected by Hehner's test, which 

 consists in diluting the milk with an equal volume of water, placing 

 the mixture in a test tube, and pouring a little sulphuric acid (specific 

 gravity about 1-825) to which a drop of ferric chloride or other oxidising 

 agent has been added, down the sides of the tube, so as to form a layer 

 at the bottom If formaldehyde be present, a violet or blue colour 

 occurs at the surface of contact of the two layers. In this reaction 

 Hehner l finds that the casein of the milk takes part ; it cannot, there- 

 fore, be obtained with aqueous solutions of formaldehyde. 



5. Fluorides or Fluosilicates are also possessed of good antiseptic 

 properties. They can be detected, if present, in the ash of the milk by 

 the usual reaction for hydrofluoric acid its etching effect on glass 

 when it is liberated by the action of strong sulphuric acid. 



BUTTER. With samples of genuine butter, the chief differences 

 in chemical composition are in the proportions of water, salt and casein. 

 These are determined by drying a weighed quantity in a flat-bottomed 

 dish at 100 until it ceases to lose weight ; the loss gives the water 

 present. The residue is then extracted repeatedly with ether, the in- 

 soluble matter dned and weighed. The weight gives the amount of 

 casein and salt. The residue is then treated with hob water, filtered, 

 and the amount of chlorine determined by titration with standard silver 

 nitrate, using potassium chromate as indicator, in the usual way. 



The most difficult part of such analytical processes is the taking of 

 n, satisfactory sample ; the amount of water present usually varies very 

 much in different parts of the same mass of butter. Probably the best 

 way is to melt a considerable quantity of the butter, at as low a tem- 

 penture as possible, in a stoppered bottle and shake it continually until 

 it stiffens. 



It the butter is to be tested for possible adulteration with other 

 .animal fats, "oleomargarine," etc., the procedure is more complicated. 

 As already explained, butter fat differs from other natural glycerides in 

 containing considerable quantities of fatty acid radicals of low mole- 

 cular weight, e.g., butyric acid. Butyric acid and its neighbouring 

 homologues are soluble in water and volatile in steam, while the higher 

 fatty acids are both insoluble and non-volatile. Upon these facts most 

 of the methods of analysis are based. 



The method adopted (though numerous modifications in detail have 

 been introduced) is essentially the digestion of a known weight (gener- 

 ally 5 grammes) of butter with excess of caustic soda solution, whereby 

 ; glycerol and soaps are produced. The latter are then decomposed with 

 a slight excess of dilute sulphuric acid, thus liberating the fatty acids. 

 The liquid is then distilled until a certain proportion of it has passed 

 over, and the acidity of the distillate is then determined by means of 

 decinormal soda or baryta solution and phenolphthalein. 



'Analyst, 1896, 92; Jour. Chem. Soc., 1896, Abstracts, ii. 58:-}. 



