Purification of oligosaccharides by nanofiltration

Nanofiltration (NF) of a model sugar solution and a commercial galacto-oligosaccharide mixture has been studied with a cross-flow filtration unit in an attempt to investigate the significance of the operating parameters and evaluate a possible purification process. The pressure, feed concentration and filtration temperature effects were studied in experiments carried out in full recycle mode of operation. The rejection factors of the sugar components present in the solutions increased with increasing pressure due to increased solvent flux and also compaction of the membranes (e.g. rejection factor increased from 0.10 to 0.42 for fructose), with this effect being greater for the low molecular weight sugars in the solutions. Increasing the total sugar concentration of the feed caused a decrease to the rejections of the sugars (e.g. from 0.58 to 0.43 for glucose). Increasing the filtration temperature caused a decrease to the observed rejections of the low molecular weight sugars of the solutions (e.g. from 0.72 to 0.48 for fructose). Continuous diafiltration (CD) purification using NF-CA-50 membranes (at 25 °C) and DS-5-DL membranes (at 60 °C), gave yield values of 14–18% for the monosaccharide, 59–89% for the disaccharide and 81–98% for the trisaccharide (oligosaccharide), respectively. The study clearly demonstrates the potential of cross-flow nanofiltration in the purification of oligosaccharide from mixtures containing contaminant monosaccharides.