Abstract
Earlier work had indicated that enzyme-mediated hydrolysis of xylooligomer-rich water-soluble streams (derived from steam pre-treated wheat straw) resulted in the effective production of xylose which was subsequently used to produce bio-glycol. In the work reported here, both the thermostability and recyclability of xylanases were significantly improved by covalent immobilizing the enzymes onto alginate beads. The immobilized xylanases showed a lower hydrolytic potential (~55% xylooligomer conversion) compared to the commercial xylanase cocktail HTec3 (~90% xylooligomer conversion) when used at the same protein loading concentration. This was likely due to the less efficient immobilization of key higher molecular weight enzymes (>75 kDa), such as β-xylosidases. However, enzyme immobilization could be improved by lowering the glutaraldehyde loading used to activate the alginate beads, resulting in improved hydrolysis efficacy (~65% xylooligomer conversion). Enzyme immobilization improved enzyme thermostability (endoxylanase and β-xylosidase activities were improved by 80% and 40%, respectively, after 24 h hydrolysis) and this allowed the immobilized enzymes to be reused/recycled for multiple rounds of hydrolysis (up to five times) without any significant reduction in their hydrolytic potential.
Highlights
One of the key challenges in establishing an effective “biorefinery” is to biochemically deconstruct the polysaccharides within the lignocellulosic substrate to the “sugar platform” which can subsequently be used to produce a range of chemicals and fuels [1,2]
The yield when immobilizing the commercial xylanase preparation HTec3 was approximately 65%, which was slightly lower than previous reports (Table 1) [8,9,10,11,12]
When the hydrolytic potential of the surface-immobilized enzymes was evaluated after addition to the xylooligomer-enriched water-soluble streams (~23 g xylooligomer per liter) [7], the hydrolysis yield of the xylooligomer was about 55% (Figure 1b)
Summary
One of the key challenges in establishing an effective “biorefinery” is to biochemically deconstruct the polysaccharides within the lignocellulosic substrate to the “sugar platform” which can subsequently be used to produce a range of chemicals and fuels [1,2]. Due to the recalcitrant nature of biomass, a physicochemical pre-treatment process is typically required to open up the tightly packed cell wall structure to facilitate the access of the enzymes to the target polysaccharides, cellulose and hemicellulose. In many of the industrial relevant pre-treatment processes (e.g., steam explosion, hydrothermal pre-treatment, diluted acid pre-treatment, etc.) that have been used, increased cellulose accessibility has been achieved by solubilizing a large portion of hemicellulose [3,4]. Most of this solubilized hemicellulose remains in an oligomeric form, which somewhat limits their further valorization [5,6]. In earlier work, hemicellulose enzymes were successfully used to hydrolyze these hemicellulose-derived oligomers to monomeric sugars, when compared with traditional acid hydrolysis processes [7].
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