Visualizing lignin coalescence and migration through maize cell walls following thermochemical pretreatment

TLDR

Plant cell walls consist mainly of cellulose, hemicellulose, lignin, and pectin, with lignin acting as a barrier to enzymatic saccharification, prompting pretreatment strategies to remove it. The study aims to elucidate the fate of lignin that remains in solids after dilute acid pretreatment. Researchers performed a structural investigation tracking lignin distribution on and within biomass cell walls. SEM and TEM imaging revealed lignin‑containing droplets that coalesce into molten bodies at high pretreatment temperatures, migrate within and out of cell walls, and redeposit on the surface, a process that may be as critical as lignin removal for improving biomass digestibility. © 2008 Wiley Periodicals, Inc.

Abstract

Abstract Plant cell walls are composed primarily of cellulose, hemicelluloses, lignins, and pectins. Of these components, lignins exhibit unique chemistry and physiological functions. Although lignins can be used as a product feedstock or as a fuel, lignins are also generally seen as a barrier to efficient enzymatic breakdown of biomass to sugars. Indeed, many pretreatment strategies focus on removing a significant fraction of lignin from biomass to better enable saccharification. In order to better understand the fate of biomass lignins that remain with the solids following dilute acid pretreatment, we undertook a structural investigation to track lignins on and in biomass cell walls. SEM and TEM imaging revealed a range of droplet morphologies that appear on and within cell walls of pretreated biomass; as well as the specific ultrastructural regions that accumulate the droplets. These droplets were shown to contain lignin by FTIR, NMR, antibody labeling, and cytochemical staining. We provide evidence supporting the idea that thermochemical pretreatments reaching temperatures above the range for lignin phase transition cause lignins to coalesce into larger molten bodies that migrate within and out of the cell wall, and can redeposit on the surface of plant cell walls. This decompartmentalization and relocalization of lignins is likely to be at least as important as lignin removal in the quest to improve the digestibility of biomass for sugars and fuels production. © 2008 Wiley Periodicals, Inc.