76 MILK PRODUCTS 
and made up to 200 c.c. plus the volume of the 
precipitated fat and proteins (see page 38). 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 so c.c. of this filtrate. To another 
50 c.c., 1% of citric acid is added, the solution 
boiled at least thirty minutes, and the reducing 
power also determined. 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 methods may be 
employed. 
The following method is based on the difference 
in polarimetric reading before and after action of 
invertase. 75 c.c. of the diluted milk are placed 
in a too-c.c. flask, diluted to about 80 c.c., 
heated to boiling, to correct birotation, cooled, 
and 1o c.c. of acid mercuric nitrate solution 
(page 37) added. The mixture is made up to 100 
c.c., well shaken, filtered through a dry filter, and 
the polarimetric reading taken at once. It will 
be the sum of the effect of the two sugars. The 
volume of the sugar-containing liquid is calcu- 
lated by allowing for the precipitated proteins 
and fat, as described on page 38. 
50 c.c. of the filtrate are placed in a flask 
