Abstract
Different biological approaches, highlighting the use of laccases, have been developed as environmentally friendly alternatives for improving the saccharification and fermentation stages of steam-pretreated lignocellulosic biomass. This work evaluates the use of a novel bacterial laccase (MetZyme) for enhancing the hydrolysability and fermentability of steam-exploded wheat straw. When the water insoluble solids (WIS) fraction was treated with laccase or alkali alone, a modest increase of about 5% in the sugar recovery yield (glucose and xylose) was observed in both treatments. Interestingly, the combination of alkali extraction and laccase treatment boosted enzymatic hydrolysis, increasing the glucose and xylose concentration in the hydrolysate by 21% and 30%, respectively. With regards to the fermentation stage, the whole pretreated slurry was subjected to laccase treatment, lowering the phenol content by up to 21%. This reduction allowed us to improve the fermentation performance of the thermotolerant yeast Kluyveromyces marxianus CECT 10875 during a simultaneous saccharification and fermentation (SSF) process. Hence, a shorter adaptation period and an increase in the cell viability—measured in terms of colony forming units (CFU/mL)—could be observed in laccase-treated slurries. These differences were even more evident when a presaccharification step was performed prior to SSF. Novel biocatalysts such as the bacterial laccase presented in this work could play a key role in the implementation of a cost-effective technology in future biorefineries.
Highlights
The transition towards a post-petroleum society for mitigating global climate change is currently led by the development and implementation of biorefineries
Sugar concentration was quantified by high-performance liquid chromatography (HPLC) in a Waters chromatograph equipped with a refractive index detector (Waters, Mildford, MA, USA)
In comparison to the cellulose content of the untreated wheat straw (40.5%), steam explosion increased the cellulose proportion of the water insoluble solids (WIS) fraction (53.5%) due to an extensive hemicellulose solubilization and degradation. This solubilization is evidenced by the lower proportion of the remaining hemicellulose (11.7%)
Summary
The transition towards a post-petroleum society for mitigating global climate change is currently led by the development and implementation of biorefineries. Biorefineries will be competitive, innovative and sustainable local industries for the production of plant- and waste-derived fuels, materials and chemicals. Due to its low costs and wide distribution, lignocellulosic biomass is the most promising feedstock to be used in biorefineries, and lignocellulose-derived fuels, including ethanol, the most significant product. Conversion methods, and process configurations have been studied for lignocellulosic ethanol production, with the biochemical route being the most promising option [1]. Lignocellulose is a complex matrix where a ‘skeleton’ polymer, cellulose, is coated by two ‘protective’. Fermentation 2016, 2, 11 polymers, hemicellulose and lignin. Biochemical conversion of lignocellulosic biomass includes a pretreatment step to open up the structure and increase biomass digestibility. Cellulose and hemicellulose polymers are subjected to an enzymatic saccharification process to obtain the fermentable sugars. The resulting sugars are converted into ethanol via microbial fermentation [1]
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