Removal of colorants and polysaccharides from raw cane sugar remelts by ultrafiltration

Interest in membrane filtration in the sugar industry is increasing, especially now that mineral membranes are commercially available. The main objective is to remove the colorants before the crystallization step. However, a literature review showed that the polysaccharides influence the level of colorants included in the sugar crystals, so that the main criteria in choosing a membrane are the decolorization ratio together with the removal of polysaccharides. The authors first demonstrated that a lot of precautions must be taken in the color analysis (e.g. use of the same 0.45 mm prefilter and to discard the first few mL of permeate) to obtain reliable decolorization ratios. Then considering the operating parameters in ultrafiltration (temperature, crossflow velocity and trans-membrane pressure) it was shown that the temperature is the most important parameter: the best decolorization ratio was obtained with the lowest temperature tested. The removal of polysaccharides and turbidity was, in every case, very good with ultrafiltration membranes. The increase in the concentration of the rejected molecules in the retentate, evaluated by the Volumetric Reduction Factor (VRF), did not modify the selectivity of the ultrafiltration membranes, but increased the fouling markedly as the time during which the flux was maintained was reduced. This last result confirmed the need to test the VRF as soon as possible in the laboratory, in order to test the usefulness of a filtration process.