Lignin as natural radical scavenger. Effect of the obtaining and purification processes on the antioxidant behaviour of lignin

In the present work, the antioxidant activity of 14 lignin samples obtained from apple tree pruning was studied. Different solvents (water, soda–water 7.5% (w/w), ethanol–water 60:40 (v/v) and acetic acid–water 60:40 (v/v)) under different operation conditions were used in order to extract the lignin and different techniques (ultrafiltration, differential precipitation and lignin purification) were applied with the aim of improving the lignin properties. The obtained lignin samples were characterized by different analytical techniques evaluating their chemical structure (ATR-IR), thermal behaviour (TGA and DSC) and molecular weight (GPC). Further analyses (13C NMR and Folin–Ciocalteu method) were carried out in order to determine the purity and the total phenolics content present in the lignin samples. The antioxidant activity of the analyzed lignins was evaluated by the radical ABTS assay. The results revealed the high influence that the lignin obtaining process had on the lignin properties, determining its possible commercial application. Results also indicated that autohydrolysis, organosolv and some ultrafiltrated soda lignins showed antioxidant efficiency comparable with a powerful natural antioxidant, as catechin, and a commercial one, Trolox®.