Optimized membrane process to increase hemicellulosic ethanol production from pretreated rice straw by recombinant xylose-fermenting Saccharomyces cerevisiae

• Rice straw was pretreated by hot water.
• NF, dilution, UF, and enzymatic hydrolysis were optimized.
• Oligomeric and monomeric sugars were recovered.
• High and low molecular weight fermentation inhibitors were removed.
• Lignocellulosic ethanol was produced by xylose-fermenting Saccharomyces cerevisiae.

Oligomeric sugars in the liquid fraction of hot water-pretreated rice straw are more amenable to membrane process than monomeric sugars, as lower pressure is required. Following membrane process was employed: nanofiltration (NF) concentration → (dilution → NF concentration) × 2 times → enzymatic hydrolysis (EH) → ultrafiltration (UF) permeation [Implication: NF for recovery of oligomeric sugars, dilution and NF for removal of low molecular weight fermentation inhibitors, UF for removal of high molecular weight fermentation inhibitors and recovery of monomeric sugars after EH]. This process provided the liquid fraction containing 111.4 g L−1 of sugars, corresponding to 681.0 mM as monomeric sugars, from the original liquid fraction (181.1 mM monomeric sugars). Concentrations of low molecular weight fermentation inhibitors, acetic and formic acids, were decreased to 24% and 48%, respectively. Xylose-fermenting recombinant Saccharomyces cerevisiae produced 34.5 ± 2.2 g L−1 ethanol from the 0.8 times liquid fraction (76% of theoretical yield).