Antioxidative Compounds from the Outer Scales of Onion

Abstract

Antioxidative compounds were isolated from the methanol extract of dry outer scales of onion (Allium cepa L.). Nine phenolic compounds (1 − 9) were finally obtained by reversed-phase high-performance liquid chromatography, and their structures were elucidated by NMR and mass spectrometry analyses. They were the six known compounds, protocatechuic acid (1), 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone (2), quercetin 4‘-O-β-d-glucopyranoside (3), quercetin (5), 4‘-O-β-d-glucopyranoside of quercetin dimer (7), and quercetin dimer (8), and three novel compounds, condensation products of quercetin with protocatechuic acid (4), adduct of quercetin with quercetin 4‘-O-β-d-glucopyranoside (6), and quercetin trimer (9). These phenolic compounds were tested for their antioxidant properties using autoxidation of methyl linoleate in bulk phase or free radical initiated peroxidation of soybean phosphatidylcholine in liposomes. The flavonoid compounds having o-dihydroxy substituent in the B-ring were shown to be effective antioxidants against nonenzymic lipid peroxidation. Keywords: antioxidant; flavonoid; quercetin; lipid peroxidation; onion; Allium cepa

Kinetic modelling for polyphenol extraction from onion (Allium cepa) solid wastes using acidified water/ethanol mixture

“…5) in accordance with previously reported data (LY et al, 2005). In a similar fashion, compound (9) was identifi ed as a quercetin dimer (LY et al, 2005;GÜLŞEN et al, 2007a). Compounds (1) and (9) have been demonstrated to derive from quercetin through oxidative cleavage by onion peroxidase (OSMAN et al, 2008).…”

“…These compounds were assigned to adducts of quercetin with quercetin 4´-O-glucoside (Fig. 5) in accordance with previously reported data (LY et al, 2005). In a similar fashion, compound (9) was identifi ed as a quercetin dimer (LY et al, 2005;GÜLŞEN et al, 2007a).…”

“…The outer dry layers of onion bulbs contain a peculiar variety of polyphenols, many of them considered to derive from quercetin and quercetin glucosides, upon the action of peroxidase (LY et al, 2005;RAMOS et al, 2006;KHIARI & MAKRIS, 2012). The chromatogram of the extract obtained at 40 °C, which was monitored at 290 nm, revealed the presence of 5 major peaks accompanied by several minor substances.…”

“…The rejected parts of the onion bulb (outer dry layers and the apical parts) have been shown to contain a wide spectrum of polyphenolic components (LY et al, 2005;RAMOS et al, 2006). Most of these substances do not occur in the edible part of the onion bulb, but they represent oxidation products of quercetin and its glucosides.…”

Stability and transformation of major flavonols in onion (Allium cepa) solid wastes

“…It is estimated that more than 450,000 tonnes of onion solid waste is produced each year in Europe (Moure et al 2001). The non-edible portions of the onion bulb, which are considered as waste of onion consumption and processing, contain phenolics with very peculiar structures, not encountered in the edible parts (Furusawa et al 2003;Ly et al 2005;Ramos et al 2006). However, recent studies on the recovery of flavonols from onion solid wastes clearly showed that mainly quercetin and to a lesser extent, quercetin glucosides are the major phenolics in onion trimmings and the outer dry layers, and account for strong antioxidant effects (Beesk et al 2010;Khiari et al 2008;Kiassos et al 2009;Suh et al 1999).…”