Short‐Time Hydrothermal Treatment of Poplar Wood for the Production of a Lignin‐Derived Polyphenol Antioxidant

Abstract

Artificial antioxidants are synthesized from fossil sources and are now widely used in the polymer, food, and cosmetics industries. The gradual depletion of fossil resources makes it practically significant and necessary to produce green antioxidants from renewable lignocellulosic resources. Herein, short‐time hydrothermal (STH) treatment was developed for production of lignin‐derived polyphenol antioxidants (LPAs) from poplar wood under conditions of high temperature and high pressure. LPA yields from 21.5 to 37.6 % on the basis of lignin in untreated wood were obtained by STH treatments as result of lignin depolymerization at 190–200 °C and 10 MPa in 5–8 min. Depolymerization reactions were confirmed by the much lower molecular weight of LPA (1076 g mol−1) than that of native lignin (4094 g mol−1). NMR spectroscopy revealed the structural features of lignin in the isolated LPA, namely syringyl and guaiacyl units with well‐preserved interunit linkages. A Folin–Ciocalteu assay indicated that each LPA molecule contained 5.4 phenolic hydroxyl groups on average, much more than other technical lignins. The remarkable antioxidant ability of LPA was verified by the radical‐scavenging index of 53.5–67.3, much higher than 0.2–11.1 of the commercial antioxidants butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA). STH treatment only requires water and heat for production of high‐value antioxidant, which provides a green and sustainable method for the utilization of lignocelluloses.

Protective Effects of Lignin-Carbohydrate Complexes from Wheat Stalk against Bisphenol a Neurotoxicity in Zebrafish via Oxidative Stress

“…Specifically, at 72 hpf, the hatching rate of zebrafish embryos decreased from 82.5% to 60% following treatment with BPA (500 µg•L −1 ), but this rate increased to 84.67% and 76.67% following further treatment with LCC-WS-A and LCC-WS-B (1 mg•L −1 ), respectively (Figure 2C,D). As the morphological and molecular characteristics of tissues and organs in zebrafish are similar to those in other vertebrates, including humans, they are regarded as a suitable animal model for evaluating the toxicity and safety of chemical compounds [19,21]. As shown in this study, BPA was toxic to zebrafish embryos at the hatching stage, and this toxicity was alleviated by the cotreatment of LCCs and BPA, albeit without significant differences.…”

“…To obtain LCCs with a lower proportion of carbohydrates as the control group for LCC-WS-A to evaluate the oxidative ability, LCC-WS-A was subjected to a hydrothermal treatment in a stainless steel tube furnace. The stainless steel was heated to 180 • C for 10 min (including heating up time of 3 min) with a tube furnace, as described by Zhang et al (2020) [21]. The treated LCC-WS-A (LCC-WS-B) was washed by deionized water to remove the degradation product and then freeze-dried to obtain the LCC-WS-B solid.…”

“…In the present study, LCC-WS-A was the carbohydrate-rich LCC characterized by its higher carbohydrate content than lignin content. Following hydrothermal treatment, galactan and mannan were degraded owing to their branched structure in the xylan skeleton of LCC-WS-A, resulting in a lignin-rich LCC (LCC-WS-B) [21]. Although some carbohydrates were degraded during the hydrothermal treatment, xylan was the major sugar in both LCC-WS-A (87% of total carbohydrates) and LCC-WS-B (90% of total carbohydrates).…”

Possible mechanisms for the generation of phenyl glycoside‐type lignin–carbohydrate linkages in lignification with monolignol glucosides

“…We further investigated the anomeric signals observed in the spectra of cell wall fractions. In line with previous studies of various plant cell wall samples (Balakshin et al ., 2011; Yuan et al ., 2011; Wen et al ., 2012; Miyagawa et al ., 2013, 2014; Du et al ., 2014; You et al ., 2015; del Río et al ., 2016; Giummarella et al ., 2016, 2019b; Yang et al ., 2016; Yao et al ., 2016; Zhao et al ., 2017; Huang et al ., 2018; Yue et al ., 2018; Rencoret et al ., 2019; Zhang et al ., 2020), we observed a number of anomeric signals possibly arising from the PG‐type LC linkages in the suspected region ( δ C / δ H , 105–95/5.5–4.5) of the LCC‐AcOH spectra (Figure 7), whereas no clear signals were detected in this particular region of the pMWL spectra (Figure 5). Although many of the anomeric signals attributed to PG‐type LC linkages did not match precisely with the PG signals observed in the DHPs that incorporated monolignol glucosides, the LCC‐AcOH spectra of all the plant species tested displayed, albeit generally at low levels, conspicuous signals [ PG G/1′ at δ C / δ H : 100.5/4.85 (pine); 100.3/4.87 (eucalyptus); 100.4/4.87 (acacia); 100.3/4.85 (poplar); 100.2/4.85 (bamboo); Figure 7] that were well matched with the signals from guaiacyl PG‐type LC linkages in the spectra of CA/CN‐DHP and CN/BGL‐DHP incorporating coniferin ( PG G/1′ at δ C / δ H : 100.7–100.2/4.88–4.82; Figure 3).…”

“…The PG‐type LCCs, which contain glycoside linkages between the reducing ends of carbohydrates and the phenolic end groups of lignin polymers, have been implicated in lignocellulose preparations from various plant origins. These suggestions are primarily based on the observation that cell wall HSQC NMR spectra show characteristic correlation signals adjacent to the signals observed in reference spectra of various synthetic model compounds containing PG‐type LC linkages (Balakshin et al ., 2011; Yuan et al ., 2011; Wen et al ., 2012; Miyagawa et al ., 2013, 2014; Du et al ., 2014; You et al ., 2015; del Río et al ., 2016; Giummarella et al ., 2016, 2019b; Yang et al ., 2016; Yao et al ., 2016; Zhao et al ., 2017; Huang et al ., 2018; Yue et al ., 2018; Rencoret et al ., 2019; Zhang et al ., 2020). The putative PG‐type LC linkage signals reported in the literature do not always exhibit a precise match to synthetic model compound spectra, however (Miyagawa et al ., 2014).…”