Critical Conditions for Improved Fermentability During Overliming of Acid Hydrolysates from Spruce

Biochemistry, Division for Chemistry, Karlstad University, SE-651 88 Karlstad, Sweden.
Applied Biochemistry and Biotechnology (Impact Factor: 1.74). 02/2005; 121-124(1):1031-44. DOI: 10.1385/ABAB:124:1-3:1031
Source: PubMed


Bioethanol can be produced from wood via acid hydrolysis, but detoxification is needed to achieve good fermentability. Overliming was investigated in a factorial designed experiment, in which pH and temperature were varied. Degradation of inhibitory furan aldehydes was more extensive compared to monosaccharides. Too harsh conditions led to massive degradation of sugars and formation of inhibiting acids and phenols. The ethanol productivity and yield after optimal overliming reached levels exceeding reference fermentations of pure glucose. A novel metric, the balanced ethanol yield, which takes both ethanol production and losses of fermentable sugars into account, was introduced and showed the optimal conditions within the investigated range. The findings allow process technical and economical considerations to govern the choice of conditions for overliming.

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    • "Feedstocks with high content of acetylated xylan, typically agricultural residues and hardwood, give higher concentrations of aliphatic acids than softwood. The total content of aliphatic acids in softwood hydrolysates is often below 100 mM and consequently beneficial for the ethanol yield rather than harmful [48,49]. "
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    • "Detoxification of dilute acid lignocellulosic hydrolysates by treatment with Ca(OH) 2 before fermentation to ethanol, is reported by many researchers (Horváth et al. 2005; Alriksson 2006,; Mohageghi et al. 2006). Larsson et al. (1999) performed a comparison of 12 different detoxification methods for treatment of dilute acid spruce hydrolysate prior to fermentation by Saccharomyces cerevisiae, It is reported that Ca(OH) 2 treatment at pH 10 is one of the best methods with regard to improvement of ethanol productivity. "
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    • "While the removal of inhibitory compounds has led to more efficient product formation, the challenge of detoxification is the increased processing costs [e.g., for fermentation of a pentose-rich hydrolysate with recombinant E. coli, it was estimated that detoxification via overliming constituted 22% of the total ethanol production cost (Von Sivers and Zacchi, 1995)] and additional operations within the biorefinery that inevitably leads to loss of the valuable sugars, or even additional inhibitory compound formation. For example, overliming can result in major sugar degradation (Horváth et al., 2005), with subsequent reduction in fermentation yields. Sugar losses also were observed within an ion exchange process (26% sugar loss) and after treatment with T. reesei to degrade phenolics (35% sugar loss) (Larsson et al., 1999). "
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