Abstract
This study investigated the fermentability of hemicellulosic hydrolysates obtained by intensified steam explosion pretreatment of switchgrass (Panicum virgatum) and Eucalyptus globulus. Xylitol and carotenoids were the high-value molecules produced by fermentation. The intensified pretreatment allowed to process a large amount of biomass per unit of time and resulted in hydrolysates containing high amount of sugars, among of which, a significant fraction was in the form of oligomers (eucalyptus hydrolysate: 25.0 g/L of oligomers and 50.6 g/L of monomeric sugars; switchgrass hydrolysate: 18.9 g/L of oligomers and 39.6 g/L of monomeric sugars). To be used as fermentation media, a post-hydrolysis step was applied to increase the content of monomeric sugars in the hydrolysates. Then, a detoxification process was carried out to reduce the concentration of inhibitors present. Two evolved yeasts were used for fermentation: Kluyveromyces marxianus for xylitol production, and Rhodosporidium toruloides for carotenoids production. Results revealed that the hydrolysates produced by intensified steam explosion of switchgrass and eucalyptus present good fermentability and can be used to produce valuable compounds such as xylitol, after detoxification. K. marxianus presented better tolerance to inhibitory compounds still present in the detoxified hydrolysates (acetic acid up to 3.94 g/L and phenolic compounds up to 2.28 g/L) than R. toruloides, which favored the production of xylitol. Finally, the intensified pretreatment was found to be a potential strategy to obtain hydrolysates with high concentration of sugars, reducing the need of concentration in a subsequent step. Moreover, the detoxification strategy applied in this study allowed to recover valuable compounds from the hydrolysates, offering extra value to a biorefinery. Altogether, the findings of this study contribute to the advancement of a technology for valorization of hemicellulosic hydrolysates.
Highlights
Lignocellulosic biomass is an attractive feedstock for use on the production of valuable compounds, providing environmental, eco nomic, and social benefits (Dragone et al, 2020)
This study investigated the fermentability of hemicellulosic hydrolysates obtained by intensified steam explosion pretreatment of switchgrass (Panicum virgatum) and Eucalyptus globulus
The formation of these compounds could be minimized by optimizing the pretreatment conditions. This should be carefully evaluated since the use of a lower severity factor during the steam explosion pretreatment results in less formation of sugar degradation compounds, and promotes less deconstruction of the lignocellulose structure, which will affect the subsequent access of the cellulose fibers to the cellulase enzymes during the subsequent step of enzymatic hydrolysis (Pielhop et al, 2016; Auxenfans et al, 2017)
Summary
Lignocellulosic biomass is an attractive feedstock for use on the production of valuable compounds, providing environmental, eco nomic, and social benefits (Dragone et al, 2020). Using lignocellulosic biomass as a feedstock to produce second generation biofuels, for instance, avoids utilization of petroleum, thereby reducing greenhouse gas emissions (Zhao et al, 2021; Mussatto and Dragone, 2016). Hemicellulose, and lignin are the main components of lignocellulosic biomass structure. Deconstruction of this structure is necessary to obtain monomeric sugars, which can later be converted into fuels and chemicals by fermentation process. Several pretreatment processes have been proposed for biomass deconstruction, among of which, steam explosion has been the most used in industrial-scale to produce cellulosic ethanol as it is very efficient to disrupt the material structure exposing the cellulose fibers to enzymatic hydrolysis in the subsequent step (Duque et al, 2016; Mussatto and Dragone, 2016)
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