210 MILK AND THE PUBLIC HEALTH 



CHAP. 



dish over the water-bath till all free ammonia has disappeared, 

 but before it is completely dry add a few drops of ferric chloride 

 solution. The characteristic flesh-coloured precipitate indicates 

 benzoic acid. Care should be taken not to add the ferric 

 chloride till all the ammonia has been driven off, otherwise a 

 precipitate of ferric hydrate is formed. 



Salicylic Acid. Take 100 c.c. of milk, mix with 100 c.c. 

 of water at 60 C., add 8 drops of acetic acid and 8 drops of 

 mercuric nitrate solution. Shake well. Filter and shake the 

 filtrate with ether in a separation funnel. The ether dissolves 

 out the salicylic acid. Evaporate the ether extract to dryness. 

 The residue is dissolved in a little alcohol and tested with 

 a drop of 1 per cent ferric chloride solution. The characteristic 

 violet colour indicates salicylic acid. 



Sodium Carbonate or Bicarbonate. Occasionally added, 

 since they mask the effects of bacterial action. Their presence 

 in appreciable amount is shown by the effervescence caused by 

 treating the ash from milk with dilute acid. 



Hydrogen Peroxide. Add to 10 c.c. of the milk 4 drops 

 of a 4 per cent alcoholic solution of benzidine and 2 drops of 

 acetic acid. If hydrogen peroxide is present the mixture 

 becomes blue. 



THE EXAMINATION OF CREAM AND SPECIAL FORMS OF MILK 



Condensed Milk. The varieties of condensed milk are 

 considered in Chapter XX., page 391. The analysis of con- 

 densed milk is mainly chemical, but a number of investigations 

 have been made of its bacterial content. The chemical 

 analysis is chiefly for three purposes : (a) estimation of the 

 milk-fat, total solids, and ash to determine the chemical 

 quality of the milk used in its preparation; (f) estimation 

 of the amount of cane-sugar, or occasionally of the glucose or 

 invert sugar which may rarely take its place ; (c) examination 

 and estimation of poisonous metals. For directions as to the 

 chemical analysis of condensed milk standard books on chemical 

 analysis must be consulted. 



The bacteriological examination of condensed milk should 

 be on the same lines as for ordinary milk, the sample being 

 first thoroughly mixed with a definite quantity of sterile water. 



