abstract
Hardwood Spent Sulphite Liquor (HSSL) is a by-product from acidic sulphite pulping of wood, rich in pentoses, which can be fermented by the yeast Scheffersomyces stipitis to produce second generation bioethanol. Unlike Saccharomyces cerevisiae, S. stipitis had the advantage of fermenting both hexoses and pentoses. However, S. stipitis is quite sensitive to inhibitors present in HSSL, namely acetic acid. For this reason, before sugars fermentation to ethanol a prior step of HSSL detoxification is required. This detoxification can be performed by Paecilomyces variotii, a filamentous fungus able to use some of the toxic compounds found in HSSL as carbon source. Furthermore, the biomass of P. variotii can be also used in animal feed as single cell protein. This work aimed to investigate the ability of ethanol production by yeast S. stipitis in defined medium containing HSSL with and without detoxification by P. variotii. This work aimed to investigate the ability of ethanol production by yeast S. stipitis in defined medium containing HSSL with and without detoxification by P. variotii. Firstly, HSSL was subjected to chemical pre-treatment and then detoxified with P. variotii. The detoxification step was performed in a sequential batch reactor with three cycles. Two assays were performed, one with an inoculum of 70 mg of biomass/L and another one with an inoculum of 110 mg of biomass/L. Then, bioethanol production by S. stipitis was studied in four assays: one with defined Verduyn medium; another one with HSSL without detoxification; another one with detoxified HSSL with sugar supplementation; and one other with detoxified HSSL without sugar supplementation. P. variotii proved to be able to detoxify HSSL with 17 g/L of acetic acid without consuming xylose. It was also observed that although it was possible to speed up the uptake of acetic acid by increasing the initial concentration of inocula, the increase in the acetic acid uptake rate was not very significant. As regards to the S. stipitis assays while in the assay with Verduyn medium the high fermentative efficiency of this yeast was shown, in assay with HSSL without detoxification the presence of inhibitory compounds was demonstrated. Finally, in the assays with detoxified HSSL with and without sugar supplementation yields of ethanol/substrate of 0.30 and 0.31 g/g were obtained, respectively. This showed that the initial sugar concentration did not affect the yield. The ethanol production success indicates that the P. variotii has efficiently consumed the inhibitory compounds of the HSSL.
authors
T. M. Henriques, S. R. Pereira, L. S. Serafim, A. M. R. B. Xavier
our authors
Tiago Manuel dos Santos Henriques
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