Compositional Chemical Property Analysis and Evaluation of Liquid Smoke Produced from Microwave-assisted Pyrolysis of Mixtures of Oil Palm Solid Waste

Abstract

Microwave-assisted pyrolysis (MAP) was employed to valorise oil palm solid waste, namely empty fruit bunches (EFBs), kernel shells (KSs), and mesocarp fibres (MFs), into liquid smoke at 300 and 400 °C. Unlike conventional pyrolysis systems, which rely on slow, external heating and often yield broad, less selective chemical profiles; MAP offers rapid, volumetric heating and non-thermal effects that enhance product specificity and energy efficiency. This study investigates how MAP temperature and binary blending ratios (EFB-to-KS and EFB-to-MF) influence liquid smoke yield, chemical composition, and antioxidant capacity. Liquid smoke yields were significantly affected by temperature in EFB-KS mixtures, with higher yields at 400 °C, while EFB–MF mixtures showed yield stability across conditions. Gas chromatography–mass spectrometry (GC-MS) analysis revealed phenol as the dominant compound across all samples, with compound diversity and antioxidant activity varying by feedstock. KS-rich mixtures favoured catechol and cresol formation, MF-rich mixtures produced cyclopentenones and carboxylic acids, and EFB-rich mixtures yielded more carbonyl-containing compounds. Antioxidant capacities, measured via DPPH assay, were highest in KS-derived liquid smoke due to its catechol content, while EFB-rich samples exhibited lower activity. Principal component analysis (PCA) was applied to GC-MS data to elucidate the chemical transformation pathways, revealing distinct degradation routes for cellulose, hemicellulose, and lignin under MAP conditions. These routes were further supported by compound clustering in PCA loading plots, highlighting the influence of temperature and biomass composition on product speciation. This study demonstrates the innovative integration of MAP with oil palm waste valorisation, offering a sustainable alternative to wood-based pyrolysis. By tailoring feedstock ratios and operating temperatures, MAP enables the targeted production of high-quality liquid smoke with enhanced antioxidant functionality, contributing to environmentally friendly food preservation and agricultural applications.