Evaluation of recalcitrant features impacting enzymatic saccharification of diverse agricultural residues treated by steam explosion and dilute acid

Exploring agricultural biomass for biofuel production necessitates pretreatment as a prerequisite step. However, due the variability in recalcitrance among biomasses, choosing an optimum pretreatment methodology suitable for multiple feedstocks is challenging. To assess which parameters of pretreated biomass may serve as useful indicators of potential subsequent enzymatic saccharification, an insight into the structural alteration during pretreatment and its impact on the downstream process is essential. In this study, two pretreatment methods, dilute acid (DA) and steam explosion (SE) have been employed on three different biomasses viz. rice straw (RS), cotton stalk (CS) and mustard stalk (MS). The alteration in recalcitrant features of the pretreated residues was measured by chemical analysis, XRD, BET and FT-IR. FT-IR proved useful to measure the cellulose related properties viz. lateral order index (LOI) and hydrogen bond index (HBI) besides lignin related features, i.e. cross-linked lignin (CLL), lignin/cellulose (L/C) and syringyl/guaiacyl (S/G) ratio. The results show that S/G ratio, specific surface area and HBI of the pretreated residues had a positive correlation with enzymatic saccharification across different biomasses and pretreatment methodologies employed. On the other hand, lignin content, CLL, L/C ratio and LOI showed a negative correlation. However, the extent of xylan removal showed a positive correlation with the enzymatic saccharification only when a single pretreatment method was applied to different biomasses. The structure-activity correlation presented here would help to assess and predict the enzymatic saccharification while applying DA or SE pretreatment methods on different biomasses. This correlation could provide assistance in designing an optimum technology.