356 THE ANALYSIS OF MILK AND MILK PRODUCTS. XV. 



Another way in which milk is impoverished is by the 

 removal of a portion of the fat, or, what comes to the same 

 thing, the admixture of skimmed or separated milk. The 

 amount of fat removed can obviously only be calculated if the 

 amount originally present in the milk be known. The usual 

 plan is to assume that genuine milk contains 3% fat, when the 

 percentage of the total fat removed is given by 



100 - ^. 



It is evident from the above figures that the calculation of 

 the amount of added water or proportion of fat removed from 

 a sample of milk is based upon pure assumptions as to the 

 real character of the original milk. When the enormous 

 variability shown by genuine milk is taken into account it will 

 be seen how unreliable are the results of such calculations. 

 The most that can be said of the results so obtained is that 

 they probably express the lower limits of the alleged sophisti- 

 cation. 



Detection of the Presence of Preservatives. 



1. Boric acid or borax. Easily detected in the ash of the 

 milk (best obtained by igniting the residue left on evaporating 

 the milk with lime water) by the well-known cherry-red colour 

 which is shown by turmeric paper when dipped into a dilute 

 hydrochloric acid solution of the ash and dried at 100. As a 

 confirmatory test the reddened paper should be moistened with 

 very dilute caustic soda solution, when a blue-black colour will 

 be produced. 



2. Salicylic acid. About 50 c.c. of the milk are mixed with 

 an acid solution of mercuric nitrate and the coagulated pro- 

 teids and fat filtered off. The filtrate is then shaken with 

 ether or (better) a mixture of ether and petroleum spirit, in 

 which the salicylic acid dissolves. The ethereal solution is 

 then evaporated and a drop of neutral ferric chloride solution 

 added to the residue ; a violet colouration then indicates the 

 presence of salicylic acid. 



3. Bcnzoic acid. The milk is made alkaline with lime water 

 and evaporated with calcium sulphate or pumice to dryness on 

 the water bath. The residue is then powdered, moistened 



