Biotreatment optimization of rice straw hydrolyzates for ethanolic fermentation with Scheffersomyces stipitis

In the current study, the potential application of baker's yeast as biological agent for the detoxification of rice straw hemicellulosic hydrolyzate containing high initial D-xylose content has been evaluated with the goal of improving ethanol production by Scheffersomyces stipitis. As required, various biodetoxification conditions in terms of treatment time, cell density and pH were assessed by measuring ethanol yield (YP/S) and ethanol volumetric productivity (QP) when the treated hydrolyzate was fermented by S. stipitis. Our results showed that baker's yeast is able to reduce the toxicity of hydrolyzate with only 6 h biotreatment. Interestingly, the maximum ethanol production from biotreated hydrolyzate was not correlated with the complete removal of furan and phenolic compounds, but when acetic acid was reduced from the medium. Under selected biotreatment conditions (5.0 g dm−3 baker's yeast concentration at pH 3.0 for 6 h), the fermentative performance of S. stipitis was noticeably favored in bench top bioreactor, i.e., fermentable sugars were completely consumed with production of 23.0 g dm−3 ethanol after 44 h (YP/S  = 0.24 g g−1 and QP = 0.52 g dm−3 h−1). Based on our results, baker's yeast may be considered a promising detoxification method for application in biorefineries, especially because its failure to consume D-xylose, which is the major sugar in these media, besides it is recognized as safe (GRAS status) and largely available commercially. Certainly, this bioprocess could be an important step toward processing lignocellulosic biomass for the development of second-generation ethanol production.