Impact of conventional/non-conventional extraction methods on the untargeted phenolic profile of Moringa oleifera leaves

The impact of different extraction methods, namely maceration, homogenizer-assisted extraction, rapid solid-liquid dynamic extraction, microwave-assisted extraction and ultrasound-assisted extraction, on polyphenols of Moringa oleifera leaves was studied. The phenolic composition of alcoholic (methanol 100%) and hydroalcoholic (methanol/water 50:50, v/v) extracts was compared by using an untargeted metabolomics-based profiling approach followed by multivariate statistics. With this aim, ultra-high-pressure liquid chromatography coupled to a quadrupole-time-of-flight mass spectrometry was used to profile phenolic compounds under the different extraction conditions. Besides, the in vitro antioxidant activities of Moringa leaves were also investigated as ferric reducing antioxidant power (FRAP) and oxygen radical absorbance capacity (ORAC). The metabolomic approach allowed to putatively annotate 262 phenolic compounds. In particular, glycosidic forms of quercetin (i.e., quercetin 3-O-galactoside, quercetin 3-O-glucoside, and quercetin 4’-O-glucoside) were the most represented compounds among flavonoids. Furthermore, protocatechuic acid was found to be the most abundant hydroxybenzaldheyde derivative, while the isomeric forms of hydroxybenzoic acid characterized the phenolic acids class. Overall, the extractions in methanol 100% were found to be the most effective for phenolic compounds recovering, when compared with those in methanol/water (50:50, v/v). Homogenizer-assisted extraction of M. oleifera leaves using 100% methanol allowed extracting the highest amounts of polyphenols (35.19 mg/g) and produced the highest oxygen radical absorbance capacity (536.27 μmol Trolox Equivalents/g). The supervised orthogonal projection to latent structures discriminant analysis identified phenolic acids as the phenolic class mostly affected by the different extraction technologies. These findings demonstrate that each extraction method promoted the recovery of specific phenolic subclasses with different efficiencies.