Organosolv Fractionation of Kraft Lignin: Unlocking Its Potential as a Multifunctional Active Ingredient in Sunscreens.

Lignin, with its aromatic and phenolic-rich structure, exhibits antioxidant, tyrosinase-inhibitory, and UV-protective properties, but the heterogeneity and dark color of kraft lignin hinder cosmetic use. Herein, a sequential ethanol-acetone organosolv fractionation was developed to improve homogeneity, purity, and appearance. Structural features of the fractions were analyzed by FTIR, XPS, and 2D-HSQC NMR techniques. The acetone-soluble lignin from hardwood kraft lignin (H-AS) showed a higher syringyl-to-guaiacyl (S/G) ratio, enriched methoxyl groups, and abundant C-O bonds, leading to superior antioxidant activity, tyrosinase inhibition, and broad-spectrum UV absorption. Incorporation of 5% H-AS into a commercial SPF 15 sunscreen increased its SPF to 33.7, while antioxidant and tyrosinase-inhibitory activities improved by 19 and 46% (IC₅₀-based), respectively. These findings highlight the structure-function relationship of fractionated kraft lignin and demonstrate its potential as a multifunctional active ingredient for sunscreen formulations, providing a new route for high-value utilization in cosmetics.