Continuous co-production of ethanol and xylitol from rice straw hydrolysate in a membrane bioreactor.

Abstract

The present study was set to develop a robust and economic biorefinery process for continuous co-production of ethanol and xylitol from rice straw in a membrane bioreactor. Acid pretreatment, enzymatic hydrolysis, detoxification, yeast strains selection, single and co-culture batch fermentation, and finally continuous co-fermentation were optimized. The combination of diluted acid pretreatment (3.5%) and enzymatic conversion (1:10 enzyme (63 floating-point unit (FPU)/mL)/biomass ratio) resulted in the maximum sugar yield (81% conversion). By concentrating the hydrolysates, sugars level increased by threefold while that of furfural reduced by 50% (0.56 to 0.28g/L). Combined application of active carbon and resin led to complete removal of furfural, hydroxyl methyl furfural, and acetic acid. The strains Saccharomyces cerevisiae NCIM 3090 with 66.4g/L ethanol production and Candida tropicalis NCIM 3119 with 9.9g/L xylitol production were selected. The maximum concentrations of ethanol and xylitol in the single cultures were recorded at 31.5g/L (0.42g/g yield) and 26.5g/L (0.58g/g yield), respectively. In the batch co-culture system, the ethanol and xylitol productions were 33.4g/L (0.44g/g yield) and 25.1g/L (0.55g/g yield), respectively. The maximum ethanol and xylitol volumetric productivity values in the batch co-culture system were 65 and 58% after 25 and 60h, but were improved in the continuous co-culture mode and reached 80% (55g/L) and 68% (31g/L) at the dilution rate of 0.03L per hour, respectively. Hence, the continuous co-production strategy developed in this study could be recommended for producing value-added products from this hugely generated lignocellulosic waste.