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.

Phosphoric Acid Pre-treatment Improves Enzymatic Hydrolysis of Weedy Biomass (<em>Thysanolaena latifolia</em>) for Bioethanol Production

“…"The raw biomass of T. latifolia was pretreated according to the procedure outlined by Premjet et al" [25]. "A 300 mg dry sample was mixed with 24 mL of 70%, 75%, 80%, and 85% H 3 PO 4 (v/v) in a 50 mL centrifuge tube.…”

“…"Analysis was performed according to the procedure suggested by Premjet et al" [25]. "Both treated and untreated T. latifolia biomasses were rinsed three times with acetone and dried overnight at 25 °C.…”

“…"Enzymatic hydrolysis was performed according to the procedure described by Premjet et al" [25]: "Both treated and untreated T. latifolia samples underwent enzymatic hydrolysis. The reaction was carried out in a 50 mL Erlenmeyer flask containing 0.1 g of biomass (dry weight).…”

“…However, treating the feedstock with the maximum concentration (85% H3PO4) can not only damage the structure of the lignin-carbohydrate complex but also disrupt the organized hydrogen bond network that exists in crystalline cellulose. "Because of this structural alteration, the carbohydrate and lignin constituents dissolve more readily, leading to a substantial decrease in the recovery yields of solids, glucan, xylan, and total lignin, whereas arabinan is eliminated entirely" [23,25,[53][54][55][56][57]. These phenomena exhibit parallels to those reported in several studies conducted on different biomass sources, including Glycyrrhiza glabra [57], V. pusilla [14], Corchorus capsularis [58], poplar [30], Hibiscus cannabinus [26], Durio zibethinus [24], Hibiscus sabdariffa [25], wheat straw [55], and Luffa cylindrica [59].…”

“…"In particular, phosphoric acid (H3PO4) has shown great potential as a pretreatment agent because of its ability to break the linkages among lignocellulose components and disrupt the orderly hydrogen bonds in cellulose fibers under mild conditions" [7,22,23]. "Different types of biomasses, including agricultural residues [24][25][26][27], grasses [14,28,29], hardwood [30], and softwood [31], have shown a significant improvement in the yield efficiency of enzymatic hydrolysis following their treatment with H3PO4. "…”