Antioxidant and antimicrobial activities of lignin-derived products from all steam-exploded palm oil mill lignocellulosic biomass waste

Background
Steam explosion pretreatment has been proven to be an effective treatment for breaking down the recalcitrant character of lignin–carbohydrate complexes (LCC) in lignocellulosic biomass. This study investigated the production of lignin-derived products from steam-exploded palm oil mill lignocellulosic biomass waste (POMLBW), that is, empty fruit bunches (EFB), kernel shells (KS), and kernel fibers (KF), also known as mesocarp fibers. Steam explosions cause lignin depolymerization, which forms various polyphenols. The low average molecular weight of the steam-exploded lignin-derived products and their antioxidant activities could potentially enhance their antimicrobial activities.
Methods
POMLBW was steam-exploded with various degrees of severity factors (R0: 4.03, 4.91, 5.12, 5.35, and 5.65). Steam-exploded POMLBW produces lignin-derived products such as low-molecular-weight lignin (LML) and water-soluble lignin (WSL). Antioxidant activity was evaluated using 0.5 mM 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging assay. Polyphenol content was evaluated using the Folin–Ciocalteu method. The antimicrobial activity was evaluated using an agar diffusion assay with Gram-positive and Gram-negative bacteria, and the thermal characteristics were evaluated using differential scanning calorimetry and thermogravimetric analysis.
Results
WSL and LML resulted in high radical scavenging activity (RSA) of approximately 95% and 80%, with 0.25 g/L and 0.5 g/L of EC50, where the polyphenol amount was 242–448 mg/g (catechin eq.) and 20–117 mg/g (catechin eq.) under all LML and WSL conditions, respectively. The steam-exploded POMLBW had an average molecular weight of 1589–2832 Da, and this condition, including high RSA and polyphenol amounts, was responsible for the high antimicrobial activities of LML against both Gram-positive (Salmonella enterica, Pseudomonas aeruginosa, and Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria. Additionally, the thermal properties investigations revealed that the glass transition temperature was 80–90 °C (Tg), the melting temperature (Tm) was 338–362 °C, and the start temperature was 101–128 °C at the beginning of mass loss.
Conclusions
These results show that the lignin-derived product from steam-exploded POMLBW has the potential for antioxidant (LML and WSL) and antimicrobial (LML) applications with good thermal resistance.

© The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.