Influence of heat pretreatment on ultrafiltration of a solution containing hemicelluloses extracted from wheat bran

• Heat pretreatment reduced viscosity.
• Heat pretreatment increased flux.
• Heat pretreatment improved fractionation performance.
• Heat pretreatment does not reduce the influence of fouling substances.

Hemicelluloses extracted from wheat bran can be concentrated and purified by ultrafiltration. However, the viscosity of the process solution is rather high (about 5 mPa s at 80 °C and a shear rate of 100 s−1), which results in a relatively low flux. The solution was subjected to heat pretreatment in an attempt to decrease the viscosity and increase the flux. Pretreatment of the solution at 80 °C for 72 h was found to reduce the viscosity by 25%. The flux during ultrafiltration with a 10 kDa ceramic membrane increased from 48 l/m2 h, for the untreated solution, to 62 l/m2 h. The increase in flux was lower than expected, and this was attributed to the formation of a fouling layer of acid-insoluble compounds on the membrane. Both hemicelluloses and lignin were partly degraded. The retention of lignin was reduced from about 30% to 18%, whereas the retention of hemicelluloses was above 95%, and was not affected by heat pretreatment. The decrease in lignin retention after heat pretreatment results in improved separation of hemicelluloses and lignin, facilitating the purification of hemicelluloses.