The Iodometric Estimation of Sugars

Abstract

IN the course of a series of investigations of the changes that take place in the nature and amount of the carbohydrates of apples during the process of ripening, both in ordinary storage and in cold storage, it has been found that the usual method of estimating fructose by means of the polarimeter is very unsatisfactory. In the first place, the optical activity of apple juice is very low; the sugars present appear to consist mainly of saccharose, glucose, and fructose, the last being in considerable excess, with the result that the juice gives a very small polarimetric reading. Moreover, the juice is always coloured, and is frequently so viscous that it is necessary to dilute it very considerably before it is possible to clear the solution by means of basic lead acetate and alumina cream. The resulting solution seldom contains more than 05 % or 1 % of sugars, giving in a 2 decimetre tube a maximum reading of one degree. Obviously the error involved in calculating the proportion of each of the sugars present from such small readings is very large, It becomes clear then that some other method is required.