Phenolic Compounds From Selected Namibian Leafy Vegetables (NLV); Extraction And Characterization

ABSTRACT

The purpose of this project was to study the antioxidant potential of selected Namibian Leafy Vegetables (NLV), using Total Phenolic Content (TPC), Total Flavonoid contents (TFC) as well as Diphenyl-1-picrylhydrazyl (DPPH), 2,2-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and Reducing Power (RP) assays. The study also aimed at detecting and characterizing eight phenolic acids (4- hydroxybenzoic, trans-ferulic, vanillic, gallic, syringic, sinapic and para-coumaric acids) using Gas Chromatography-Mass Spectrometry (GC-MS). Thirteen leafy vegetables collected from Kavango East Region were identified as: Hibiscus sabdarifa, Aeschynomene nambalensis, Hibiscus cannbinus, Oxygonum delagoense, Amaranthus thunbergii, Amaranthus hypochondriacus, Vigna unguiculata, Cleome gynandra, Cucurbita maxima, Lagenaria siceraria, Aeschynomene fluitans, Citrullus lanatus, and Corchorus tridens. Organic extracts were prepared using aqueous methanol, ethanol and acetone solvents. Acid and Base hydrolyses were employed for extraction of bound phenolic acids which were analysed by GC-MS. The best yields for each assay were as follows: TPC: methanol: 333.8±2.7µgGAE/g in H. sabdarifa. TFC: methanol: 166.0±0.6µgQE/g in A. nambalensis. DPPH assay: methanol: 79.3±0.6% in H. cannbinus. ABTS assay: ethanol: 98.8±0.2% in H. sabdarifa. RP assay: ethanol: 1227.5±106.2% of L. siceraria. GC-MS detected phenolic acids in twenty extracts of Base Hydrolysed Samples (BHS), seven extracts of Acid Hydrolysed Samples (AHS) and thirteen extracts of Non-Hydrolysed Samples (NHS). BHS yielded the most phenolic acids among the three extraction methods, with highest yields of p-coumaric acid in A. nambalensis (53.1µg/g). iii Results obtained from the TPC, TFC and antioxidant assays showed that the selected NLVs are potential sources of phenolic compounds and antioxidants. GC-MS analysis indicate that this tool can be used for characterizing phenolic acids. This projects’ work has contributed to our understanding of the phytochemical constituents of the selected NLVs and further studies are recommended on the possible industrial application of the identified phenolic acids.