The effect of filtration and centrifugation on raw sugar polarisation analysis

Abstract

In raw sugar analysis, the apparent sucrose content is determined as its polarisation value, a measure of the rotation of plane-polarised light passing through a solution of the sugar, which is directly proportional to sucrose concentration when the impurity level is low.

Before taking a reading with a polarimeter, a “normal” raw sugar solution (26·000 g per 100 ml) is clarified by addition of basic lead acetate solution to precipitate impurities, which are removed by simple filtration; about 50 ml of filtrate are needed for analysis. When the solution is filtered, the sucrose concentration increases because of preferential absorption of water by the filter-paper. Initially in the current study, the magnitude of the absorption has been examined, and it has been determined that the first 10 ml of filtrate should be discarded to minimise this effect.

Differences of 1 part in 40,000 in the polarisation value of sugar solutions, however, can now be detected by using modern photo-electric polarimeters and the refined analytical techniques described in this paper. This compares with 1 part in 2000 when using older visual instruments and standard techniques. Therefore, an insight into evaporation and humidity effects has also been gained, and a comparison of filtration and centrifugation, an alternative method of clarification, made.

Centrifugation has practical advantages over filtration in laboratories in which large numbers of analyses are performed. In addition, it is not subject to preferential absorption and evaporation errors, and currently appears to give the best estimate of polarisation value, as defined and specified by the International Commission for Uniform Methods of Sugar Analysis (ICUMSA).