CONDENSED MILK 51 



secure prompt inversion of the sucrose without affecting the 

 lactose. Experiment shows that citric acid and invertase are 

 the most suitable agents. Stokes & Bodiner have worked out 

 the citric acid method substantially as follows : 



25 c.c. of the diluted sample are coagulated by addition of i 

 per cent, of citric acid, without heating, and made up to 200 c.c. 

 plus the volume of the precipitated fat and proteids (see p. 31). 

 The liquid portion, which now measures 200 c.c., is passed 

 through a dry filter. The reducing power with alkaline copper 

 solutions is determined at once upon 50 c.c. of this filtrate. To 

 another 50 c.c., i per cent, of citric acid is added, the solution 

 boiled at least 30 minutes, and the reducing power also deter- 

 mined. The increase over that of the first solution is due to the 

 invert-sugar formed by the action of the citric acid on the 

 sucrose. It is necessary to bear in mind that the reducing 

 equivalents of lactose and invert-sugar are not the same. 

 Volumetric method may be employed. 



The following method is based on the difference in polari- 

 metric reading before and after action of invertase. 75 c.c. of the 

 diluted milk are placed in a 100 c.c. flask, diluted to about 80 c.c., 

 heated to boiling, to correct birotation, cooled, and 10 c.c. of acid 

 mercuric nitrate solution added. The mixture is made up to 

 100 c.c., well shaken, filtered through a dry filter, and the polari- 

 metric reading taken at once. It will be the sum of the effect 

 of the two sugars. The volume of the sugar-containing liquid 

 is calculated by allowing for the precipitated proteids and fat, 

 as described on page 31. 



50 c.c. of the filtrate are placed in a flask marked at 55 c.c., 

 a piece of litmus paper dropped in, and the excess of nitric 

 acid cautiously neutralized by sodium hydroxid solution. The 

 liquid is then faintly acidified by a single drop of acetic acid 

 (it must not be alkaline) , a few drops of an alcoholic solution of 

 thymol are added, and then 2 c.c. of a solution of invertase, 

 prepared by grinding half a cake of ordinary compressed yeast 



