Nanofiltration mediated process for ethanol fermentation of biomass hydrolysates by wild-type yeasts

Wild type ethanologenic microorganisms display very low ethanol yields on xylose in lignocellulosic hydrolysate due to low tolerance to ethanol and other metabolic inhibitors present in hydrolysates. The major thrust of research on xylose conversion has been towards the development of recombinant microorganisms capable of simultaneous uptake of glucose and xylose. These genetically modified ethanologenic strains also suffer from low yields, low productivities and genetic instability. Nanofiltration and membrane assisted cell recycle are process engineering strategies which are being described in the current study for the first time to increase ethanol yield and productivity from lignocellulosic hydrolysates using wild type strains. Various nanofiltration membranes have been evaluated for high pentose-hexose separation and high permeate flux. Operating conditions viz. temperature and transmembrane pressure have been optimized for achieving a maximal xylose to glucose separation factor of 3.4. The hydrolysate achieved by enzymatic saccharification of chemically pretreated biomass is subjected to nanofiltration and thereby split into two streams. The retentate stream is enriched in glucose while the permeate stream is enriched in xylose and depleted in metabolic inhibitors. The retentate and permeate steams are fermented by Saccharomyces cerevisae and Pichia stipitis respectively to obtain maximum yields. The retentate stream can be subjected to diafiltration by the fermented permeate from the first cycle to further increase the recovery of xylose and thus the overall ethanol yield from xylose. The permeate stream can be subjected to an additional nanofiltration based concentration step to further increase the ethanol titer. Overall yields between 0.39 and 0.43 have been achieved on validation of the process with hydrolysate of sorghum bagasse.