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Fermentation of hemicellulose rich liquid fractions derived from steam pretreated softwoods Liu, Michael Li Yu

Abstract

One of the main challenges in the bioconversion of lignocellulosics into bioethanol is to maximize the recovery of hemicellulosic sugars while increasing ethanol production through fermentation of these sugars. Steam pretreatment of Douglas-fir (DF) and Lodgepole pine (LPP) at a severity factor of logR₀ = 3.64 resulted in water soluble fractions (WSFs) containing monomeric hexose sugars up to 86 g/L. The crude WSFs were not fermentable by four yeast strains: T₁ and T₂ (spent sulfite liquor adapted strains), Y1528 (haploid strain that preferentially ferment galactose first) and BY4742 (haploid laboratory strain). Dilution of fermentation inhibitors in crude WSFs led to appreciable improvements in their fermentability, especially by the SSL-adapted yeast strain T₂. The four yeast strains were tested against several model furan and phenolic compounds to examine their tolerance to these fermentation inhibitors. All four yeast strains produced comparable ethanol productivity when 3 g/L of HMF or 0.8 g/L of furfural were added to medium containing 2% glucose. However, T₁ and T₂ exhibited higher ethanol productivity compared to Y1528 and BY4742 when 5 g/L of 4-hydroxybenzoic acid and 5 g/L of vanillic acid were added to media as supplements. This provides evidence that the robustness of SSL-adapted T₁ and T₂ yeast strains probably originates from their tolerance to certain phenolic compounds. Overliming improved ethanol production from Douglas-fir 1 (DF1) WSF by T₂ from 1.7 g/L to 13 g/L. When DF1 WSF was spiked with glucose up to 100 g/L, it produced ethanol yields similar to that of the glucose reference fermentation media. Since it is not practical to spike WSF with glucose in an industrial process, we investigated the applicability of separate hydrolysis and fermentation (SHF) and hybrid hydrolysis and fermentation (HHF) with the whole slurry to achieve higher initial fermentable sugar concentration. SHF of combined WSF and hydrolysates recovered after enzymatic hydrolysis of water insoluble fraction (WIF) by T₂ produced up to 90% ethanol yield. HHF produced ethanol concentrations comparable to those of SHF with or without overliming. This result indicated that SHF and HHF of the whole slurry can help improve the fermentability of WSFs.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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