Influence of air and nitrogen sparging on flux during ultrafiltration of hemicelluloses extracted from wheat bran

Ultrafiltration can be used to concentrate arabinoxylan isolated from wheat bran. Prefiltration with diatomaceous earth and air sparging during ultrafiltration of the alkaline hemicellulose solution were both found to increase the flux, while nitrogen sparging had no effect. The flux of the untreated solution was 51 L/m2 h, while the flux after prefiltration of the solution with diatomaceous earth was 62 L/m2 h. After 5 h of air sparging during ultrafiltration of the prefiltered solution the flux was 230 L/m2 h. However, sparging the prefiltered solution with nitrogen gas had no effect on the flux. This shows that the increase in flux was not due to the reduction of concentration polarization by sparging per se. Size-exclusion chromatography showed that sparging with air caused a reduction in the size of hemicelluloses >100 kDa to 10–100 kDa, which is believed to be due to oxidative degradation of hemicelluloses. This size reduction of the hemicelluloses is likely the main cause of the increased flux. The retention of hemicelluloses during ultrafiltration with a ceramic membrane with a nominal cut-off of 10 kDa was 96% in all experiments, except at the end of the experiment with the prefiltered, air-sparged solution, when the retention decreased to 93% due to the size reduction of the hemicelluloses. Air sparging seems to be an efficient flux enhancing method with limited negative influence on the molecular size of hemicelluloses.