Improving Lignin Homogeneity and Functionality via Ethanolysis for Production of Antioxidants

As an antioxidant material, lignin has some structural disadvantages such as heterogeneity, high molecular weight, and low functionality. We propose a simple method based on ethanolysis pretreatment (120, 160, and 200 °C for 1 h) of lignin by which the structure, especially the molecular weight distribution and phenolic hydroxyl content (Phe-OH) of lignin, are improved. After reaction, over 80 wt % structurally modified lignin (solubilized lignin: SL) was recovered from raw lignin, and its homogeneity as well as Phe-OH content increased. The antioxidant activity of SLs was investigated by 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging ability. Interestingly, DPPH radical scavenging capacities of SL produced at 160 °C (IC50 = 0.016 mg/mL) was close to commercial antioxidants such as ferulic acid (IC50 = 0.010 mg/mL) and guaiacol (IC50 = 0.013 mg/mL), suggesting that ethanolysis pretreatment was effective in modifying the structure of lignin for use as antioxidant. We also performed experiments with lignin model compounds, which revealed that antioxidant activities of lignin were not only affected by homogeneity and functionality but also other structural characteristics, including H-, G-, and S-type composition and side chain characteristics