Valorization of Lignin–Carbohydrate Complexes from Hydrolysates of Norway Spruce: Efficient Separation, Structural Characterization, and Antioxidant Activity

Hot-water extraction has shown great potential in a green biorefinery; however, efficient separation of lignin and carbohydrates is still a big challenge, especially to lignin–carbohydrate complexes (LCCs). The aim of the present study was to develop a facile approach for efficiently fractionating LCCs from hot-water extraction liquor of spruce wood to understand their comprehensive structure and thus to achieve the valorization of fractions. Approximately 93% of the hemicelluloses were first recovered by a selective adsorptive resin from hot-water hydrolysates. The lignin-rich fractions obtained from desorption of the adsorbed compounds were further subjected to a gradient dialysis procedure toward functional fractionation. Linear relationship between lignin and uronic acids as well as xylose contents in graded fractions of all hydrolysates was observed. Further characterization of LCCs by two-dimensional nuclear magnetic resonance and pyrolysis-gas chromatography-mass spectrometry showed the large difference of their structures in each graded fraction and suggested ester bonds as the main types of chemical linkages between lignin and carbohydrate. Furthermore, the antioxidant properties of those LCCs as well as hemicellulose fractions will facilitate their applications to a broader spectrum such as cosmetics, pharmaceuticals, and functional packaging.