This study was designed to determine the types of phenolics in palm kernel cake (PKC) and optimise the extraction of phenolics. The phenolics of PKC were extracted in free methanol-soluble and cell wallbound forms. Optimisation of phenolic extraction from PKC was carried out by investigating the effects of pretreatment, different solvents (water, methanol, ethanol and acetone), extraction temperatures (25°C, 60°C and 80°C) and extraction durations (15, 30, 60 and 120 minutes). The results indicated that 70.44% (500.9 μg/g of dry weight) of the phenolics in PKC were in free methanol-soluble form whereas 29.56% (210.2 μg/g of dry weight) was cell wall-bound phenolics. The optimum phenolics extraction conditions for PKC suggested in this study are by using water with 1:10 material to solvent ratio, stirring at 250 rpm, 25°C for 30 minutes. The ultra-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry (UPLC-ESI-MS/MS) analysis showed that p-hydroxybenzoic and protocetechuic acids were the predominant phenolics in methanol extract. Meanwhile, p-hydroxybenzoic, protocatechuic as well as shikimic acids appeared to be the predominant phenolics in water extract.
Keywords: Palm kernel cake (PKC), phenolic extraction, free soluble phenolics, bound phenolics