Understanding the structural and chemical changes of plant biomass following steam explosion pretreatment

Abstract

BackgroundBiorefining of lignocellulosic biomass has become one of the most valuable alternatives for the production of multi-products such as biofuels. Pretreatment is a prerequisite to increase the enzymatic conversion of the recalcitrant lignocellulose. However, there is still considerable debate regarding the key features of biomass impacting the cellulase accessibility. In this study, we evaluate the structural and chemical features of three different representative biomasses (Miscanthus × giganteus, poplar and wheat straw), before and after steam explosion pretreatment at increasing severities, by monitoring chemical analysis, SEM, FTIR and 2D NMR.ResultsRegardless the biomass type, combined steam explosion pretreatment with dilute sulfuric acid impregnation resulted in significant improvement of the cellulose conversion. Chemical analyses revealed that the pretreatment selectively degraded the hemicellulosic fraction and associated cross-linking ferulic acids. As a result, the pretreated residues contained mostly cellulosic glucose and lignin. In addition, the pretreatment directly affected the cellulose crystallinity but these variations were dependent upon the biomass type. Important chemical modifications also occurred in lignin since the β-O-4′ aryl-ether linkages were found to be homolytically cleaved, followed by some recoupling/recondensation to β-β′ and β-5′ linkages, regardless the biomass type. Finally, 2D NMR analysis of the whole biomass showed that the pretreatment preferentially degraded the syringyl-type lignin fractions in miscanthus and wheat straw while it was not affected in the pretreated poplar samples.ConclusionsOur findings provide an enhanced understanding of parameters impacting biomass recalcitrance, which can be easily generalized to both woody and non-woody biomass species. Results indeed suggest that the hemicellulose removal accompanied by the significant reduction in the cross-linking phenolic acids and the redistribution of lignin are strongly correlated with the enzymatic saccharification, by loosening the cell wall structure thus allowing easier cellulase accessibility. By contrast, we have shown that the changes in the syringyl/guaiacyl ratio and the cellulose crystallinity do not seem to be relevant factors in assessing the enzymatic digestibility. Some biomass type-dependent and easily measurable FTIR factors are highly correlated to saccharification.

Bioprocessing of tea oil fruit hull with acetic acid organosolv pretreatment in combination with alkaline H2O2

“…Like other lignocellulosic substrate, the hydrolyzability of the original TOFH is very poor, hence a low enzymatic hydrolysis even with a high cellulase loading, because of the recalcitrant structure [26, 28, 29]. The substrates undergoing AAO-/AHP-based pretreatment have presented an improved hydrolyzability to a different extent depending on the substrate variety, pretreatment process, hydrolytic enzymes, and hydrolytic conditions [14, 15, 22, 26].…”

“…Many researchers have argued that the main chemical composition (cellulose, lignin, and hemicellulose) and physical structure (surface area, average size, and the crystallinity) of lignocellulose biomass are available to represent the inherent recalcitrance of substrate to enzymatic hydrolysis [29, 31–33]. Consequently, structural features of substrates at different pretreatment stages were depicted in the ensuing work.…”

Process intensification strategies for enhanced holocellulose solubilization: Beneficiation of pineapple peel waste for cleaner butanol production

“…Auxenfans et al (2017) also highlighted the re-polymerized/re-accumulated lignin affects enzymatic hydrolysis leading to lower glucose release. In case of steam exploded residue; elevated LOI and HBI levels (Table 3) indicated increased crystallinity with highly ordered cellulose structure which further decreased the enzymatic efficiency (Auxenfans et al, 2017;Ayeni and Daramola, 2017). Overall, the enzymatic efficiency was greater for residues treated at relatively lower temperature under varied enhancer and followed the order: phenol > sulfuric acid > oxalic acid > hydrogen peroxide >1% phenol + sulfuric acid >1% hydrogen peroxide + sulfuric acid >1% oxalic acid + sulfuric acid.…”

“…Besides, HBI of pretreated sample was significantly higher than untreated sample. It may be due to cellulose chain is being in highly organized form and arranged in crystalline structure (Auxenfans et al, 2017). FTIR also provides information on syringol (S) to guaiacol (G) ratio at wavenumber (1/cm) of 1260 and 1330 which represent the lignin in biomass.…”

Utilization of Microalgal Biofractions for Bioethanol, Higher Alcohols, and Biodiesel Production: A Review

“…The steam explosion method provides accessibility to the degradation of cellulose. Steam explosion is increasingly considered to be one of the most efficient, eco-friendly and cost-effective processes for commercial application and thus, it has been widely tested at the pilot scale for various biomasses [52]. This method consists of heat treatment (160-270 • C) to biomass under high-pressure steam (20-50 bar) Energies 2017, 10, 2110 7 of 19 for a short duration (a few minutes); then, the reaction is stopped when the pressure conditions reach atmospheric conditions.…”