Iodimetric determination of reducing sugars in the apple

Abstract

IT was pointed out by Judd [1920] that the polarimetric method in combination with the copper reduction method for the determination of fructose and glucose in mixtures of these sugars was unsuitable for the analysis of apple extracts, not only because of the low optical rotation of the extracts, but because they are always coloured and frequently viscous. Evans [1928] investigated various methods of clearing the extracts and found that considerable dilution was necessary for satisfactory clarification. In order to obtain a sufficiently large reading on the polarimeter the cleared solutions had to be evaporated under reduced pressure to a suitable concentration, making the analysis very tedious. Both these workers recommended the use of the iodimetric method [Baker and Hulton, 1920; Cajori, 1922; Willstatter and Schiidel, 1918] in combination with copper reduction for the estimation of fructose and glucose in the apple. This method is based on the oxidation of the sugars by alkaline iodine, and some disagreement existed as to the extent of the oxidation until Hinton and Macara [1924] reinvestigated the reaction. They established the fact that glucose was quantitatively oxidised to gluconic acid in 10 minutes at room temperature, provided not more than half the iodine added was reduced and the ratio of iodine to sodium hydroxide in terms of normal concentration was 1: 1P25. They also found that fructose was slightly oxidised at 17.50, the amount of oxidation varying with concentration, amount of alkali present, time of reaction and temperature. HIinton and Macara attribute the discrepant results obtained by earlier workers to the use of impure samples of sugars. Evans [1928] repeated Hinton and Macara's work on the oxidation of glucose at 17.50 and also made observations at 5°. He found that oxidation of glucose was complete in 45 minutes at 50 and obtained the same iodine value at both temperatures, namely 1-402 g. of iodine reduced per g. glucose. This is slightly lower than the theoretical value 1-410 obtained by Hinton and Macara. Evans obtained somewhat higher values than Hinton and Macara for the amounts of iodine reduced per g. fructose in spite of the lower