Balance of Lignin Light-Color and UV-Shielding Properties: Pyruvic Acid Fractionation for Green Sunscreen Formulations

Light-color and UV-shielding properties were both critical factors of lignin-based sunscreens. Lignin’s conjugated structure provides effective UV-shielding characteristics that also impart coloration, making it challenging to achieve a balance between the two. The color requirements of sunscreens often require a compromise in the UV-shielding properties of lignin. To resolve this issue, a sustainable and efficient method was proposed by utilizing biocompatible pyruvic acid to fractionate the Camellia oleifera shell, resulting in the extraction of a light-colored lignin with exceptional UV absorption at 290–400 nm. Incorporating 1% lignin into sunscreens led to a significant increase in the sun protection factor (SPF) of the resulting product (from 18.19 and 32.72 to 29.79 and 49.52, respectively), with no staining on skin. The characteristic of the lignin structure demonstrates that the light-colored lignin can be largely attributed to the G group lignin swiftly dissolving in pyruvic acid without notable condensation. The enhanced UV-shielding properties of lignin arise from the cleavage of the β-O-4 bond, resulting in the production of more phenolic hydroxyl groups and the esterification of pyruvic acid with the hydroxyl groups of the side chains.