Light-colored lignin isolated from poplar by ultrasound-assisted ethanol extraction: Structural features and anti-ultraviolet and anti-oxidation activities

Ethanol extraction of lignocellulosic biomass under mild conditions has re-attracted considerable attention since a light-colored lignin with low molecular weight can be obtained. Unfortunately, the yield of isolated lignin for the ethanol extraction process is very limited. To enhance the extraction efficiency of lignin from poplar, ultrasound was used to assist the ethanol extraction and the isolated lignin was analyzed in terms of structure features and functional properties. As expected, increasing the ultrasonic power and ultrasonic time led to higher lignin yields during the ultrasound-assisted ethanol extraction (UAEE) process. The chemical structure of the isolated lignin (UAE-L) was almost completely retained, the C-O (mainly β-O-4) linkages in the lignin only exhibited a slight decrease, while the C-C (β-β, β-5, and β-1) linkages were nearly unchanged. The UAE-L was light in color and had more phenolic hydroxyl groups, less carbohydrate impurities, lower molecular weight and narrower polydispersity as compared to the poplar MWL. The UAE-L could scavenge more 2,2-diphenyl-1-picrylhydrazyl radicals (DPPH·), displaying a stronger antioxidant activity. Moreover, the UAE-L showed good UV-absorbing properties in both UVA and UVB regions. These results indicated that the UAE-L has great potential for replacing the commercial synthesized antioxidant and anti-UV ingredients. The overall scheme of the isolation process of lignin from poplar by ultrasonic-assisted ethanol extraction (UAEE). [Display omitted] •Ultrasound was employed to enhance ethanol extract of lignin.•Light-colored lignin with abundant phenolic -OH and low molecular weight was obtained.•Lignin unit with less ortho- methoxy substitutions was preferentially released.•UAE-L showed good antioxidant activity and broad-spectrum UV-absorption capacity.