166 THE ESTIMATION OF SUGARS. 



Griinhut and Riiber have shown that the estimation of milk- 

 sugar and cane sugar in condensed milk by reduction with 

 Fehling's solution does not give accurate results ; the figures 

 are always high for milk-sugar, and the cane sugar correspondingly 

 low. 



By polarisation before and after inversion, using Herzfeld's 

 formula for cane sugar, they obtain satisfactory results. 



Stokes-Bodmer Method. Stokes and Bodmer prefer esti- 

 mating the milk-sugar by titration with Pavy's solution (Feh- 

 ling's solution can be substituted for this), inverting the cane 

 sugar by boiling with 2 per cent, of citric acid for ten minutes, 

 and then estimating the combined milk-sugar and resulting 

 mixture of glucose and fructose by titration ; the difference 

 between the two figures will be due to the products of hydrolysis 

 of cane sugar. In this case it is advisable to standardise the 

 solution on a mixture of milk-sugar and inverted cane sugar in 

 about the same proportions as found in the milk. The deter- 

 minations may also be made gravimetrically. 



Watts and Tempany have proved that in the presence of milk 

 constituents cane sugar is not inverted completely by boiling 

 for 10 minutes with citric acid, and recommend that the time of 

 heating should be continued for 40 minutes. 



By Invertase. The best method of estimating cane sugar 

 depends on the hydrolysis of cane sugar by invertase, the enzyme 

 of yeast. This is carried out as follows : Estimate the rotation 

 due to milk and cane sugar by polarisation of the solution 

 obtained by precipitation with mercuric nitrate (100 c.c. of milk 

 should be taken). 25 c.c. of the solution are placed in a flask, 

 a drop or two of phenol- phthalein added, and dilute caustic 

 soda solution run in till neutral. This solution is filtered into 

 a 50 c.c. flask, and the precipitate washed with water till the 

 filtrate and washings measure about 45 c.c. 0-05 gramme of 

 invertase, or 1 gramme of yeast, is added, together with a drop 

 of acetic acid and a few drops of toluene, and the whole made 

 up to 50 c.c. The flask is corked and allowed to remain at about 

 55 (131 F.) for five hours. A little alumina cream is added 

 and the whole made up to 55 c.c., filtered, and polarised ; the 

 temperature at which the solution is polarised should be noted. 



55 



The reading should be multiplied by ^= = 2-2 ; the reading due 



00 



to cane sugar is found by the formula 



142-66- 



