Enzymatic response to structural and chemical transformations in Hibiscus sabdariffa var. altissima bark and core during phosphoric acid pretreatment

Abstract

To investigate the potential of Hibiscus sabdariffa var. altissima (Thai kenaf) biomass as a feedstock for bioethanol production, Thai kenaf bark and core were pretreated at a moderate temperature with different phosphoric acid (H3PO4) concentrations. It was revealed that there was a higher glucan content in the Thai kenaf bark (57.97% ± 0.36%) compared with that in the core (43.10% ± 0.15%). The H3PO4 pretreatment resulted in a reduction in the lignin content and total removal of hemicellulose. This exposed the cellulose to attack by cellulase enzymes and resulted in an increased enzymatic digestibility. A high glucose concentration (GC; 7.02 g/L) and hydrolysis efficiency (HE; 95.79%) were achieved with 75% H3PO4 for the bark after 72 h of enzymatic hydrolysis. However, these values were not that different from those of the 70% H3PO4-pretreated bark (6.89 g/L and 95.43%, respectively). Nevertheless, the Thai kenaf core pretreated with 75% H3PO4 recorded a higher GC (6.30 g/L) and HE (91.67%) after 72 h of enzymatic hydrolysis. The scanning electron microscopy and X-ray diffraction analyses revealed the destruction of the surface structure and an increase in the porosity and crystallinity index of the Thai kenaf biomass, which corresponded to an increased enzymatic digestibility.