ARS | Publication request: Structure and radical scavenging activity relationships of pyrolytic lignins

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 30, 2012
Publication Date: November 30, 2012
Citation: Nsimba, R.Y., West, N., Boateng, A.A. 2012. Structure and radical scavenging activity relationships of pyrolytic lignins. Journal of Agricultural and Food Chemistry. 60:12525-12530.

Interpretive Summary: Recently, the search for bioactive compounds, called antioxidants, that stop chemical reactions that alter foodstuffs, beverages and materials during storage has gained growing interest. These types of compounds have also been found to have human health benefits. In this work, pyrolytic lignins (phenol-based polymers) have been isolated from the liquid product obtained from the fast pyrolysis of a lignin rich wood by-product. Fast pyrolysis is a process where solid biomass materials are converted to liquid products via rapid heating in the absence of oxygen. We have investigated efficacy of these materials for antioxidant activity using the most commonly used in vitro methods. Obtained results reveal that the antioxidant activity of isolated pyrolytic lignins is higher than that of non-thermally treated lignin. This suggests the pyrolysis process converts lignins to more potent antioxidants that might be used against unwanted reactions occurring in agricultural products or composite materials. Consequently, pyrolytic lignins could be used for preservation of food and other materials subject to chemical degradation during storage. This information will be important to those considering producing valuable coproducts from pyrolysis oils particularly new types of antioxidant materials.

Technical Abstract: This work deals with antioxidant properties of pyrolytic lignins against two free radicals, the 1,1-diphenyl-2-picrylhydrazyl and the 2,2'-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid). Pyrolytic lignins produced by the thermal pyrolysis of the Etek lignin were extracted from the liquid pyrolysis product and fractionated using solvents of different polarities. Main functional groups linked to the lignin skeleton have been characterized by 1H NMR and 19F NMR upon acetylation and trifluoromethylation, respectively. Their radical scavenging activity against targeted free radicals was evaluated in vitro and it was correlated to the content of studied functional groups. In contrast to the extracted Etek lignin, we also demonstrate that thermal pyrolysis produces lignin adducts which have higher scavenging efficacy than the non-thermally altered lignin and even higher than quercetin, a well known phenolic antioxidant. The phenyl hydroxyl and methoxyl groups appear to be the main lignin structural factors contributing to the overall scavenging properties against the DPPH and ABTS free radicals. Also, these results show that there is no correlation between the content of aliphatic hydroxyl and carbonyl groups and the antioxidant activity.