Depolymerization behaviors of naked oat stem during autohydrolysis in mild subcritical water Ⅱ: Correlative changes in tissue structure, cell morphology, and chemical composition.

Abstract

Green, efficient depolymerization of lignocellulose is a prerequisite and the key in high-value utilization of its components. Understanding of the changes in morphological chemistry of the cell wall, related to the biomass recalcitrance, is critical for optimizing the processing conditions of cell wall deconstruction. Thus, herein, based on our previous studies, subcritical water autohydrolysis of naked oat stems was conducted at 190 °C and 1.18 MPa for 0–60 min. The depolymerization behaviors of tissues and cell walls were monitored dynamically at 10 min intervals. The micro-and ultrastructural changes of autohydrolyzed residue sections were analyzed. Parenchyma tissue commenced dissociation after 10 min at 190 ℃, whereas mechanical tissue dissociation occurred after 40 min at 190 ℃, along with interlayer separation of the secondary walls of sclerenchyma fibers. The autofluorescence changes of lignin showed that the order of regional lignin removal in tissues was from sieve tubes and companion cells to parenchyma cells, and finally to sclerenchyma fibers and vessel elements. The turning point of tissue and cell wall structural depolymerization of naked oat stems under subcritical water autohydrolysis conditions occurred after 40 min at 190 ℃ and 1.18 MPa. Under this condition, the degree of tissue dissociation was as high as 80%, while the fiber cell morphology remained intact. Dynamic changes in the tissue and cell wall structures and cell morphologies of naked oat stems maintained at 190 ℃ for various durations were captured by differentiating the use of the optical microscope, fluorescence microscope, and scanning electron microscope. The extensive, detailed assay data yield novel insights into the dynamic depolymerization mechanism of lignocellulosic composite cell walls under subcritical water autohydrolysis conditions.