Identification and quantification of flavonoids in leafy by-products of Chinese jujube: Comparison between young and old leaves

Abstract

Jujube leaves are by-products of jujube plants, which contain various flavonoid compounds. The identification and quantification of these flavonoids from 14 cultivars of jujube, cultivated in Cangzhou, Hebei, China was explored in this study. Altogether, nine flavonoids were obtained using ethanol as the extraction solvent and using preparative reversed phase liquid chromatography. The identified nine compounds were quercetin-3-O-robinobioside, rutin, 3′,5′-Di-C-β-D-glucosylphloretin, kaempferol-3-O-rutinoside, quercetin-3-O-rhamnoside, quercetin-3-O-α-L-arabinosyl- (1→2)-α-L-rhamnoside, quercetin-3-O-α-L-rhamnopyranosyl-2″-(6′′′-p-coumaroyl)-β-D-glucoside, 3′,5′-Di-C-β-D-glucosyl-4-O-p-coumaroyl- phloretin, and quercetin-3-O-β-D- glucoside. Among them, three compounds, namely quercetin-3-O-rhamnoside, quercetin-3-O-α-L-rhamnopyranosyl-2″-(6′′′-p-coumaroyl)-β-D-glucoside, and 5′-Di-C-β-D-glucosyl-4-O-p-coumaroyl-phloretin were detected as the novel compounds in jujube leaves from Cangzhou, Hebei, China. All the detected compounds were confirmed through HPLC, LC-MS, ¹H-NMR, and ¹³C-NMR. The quantification analysis through HPLC chromatography revealed that young leaves contained much higher amounts of these compounds than the old counterpart. In terms of dominant major flavonoids, quercetin-3-O-robinobioside, rutin and 3′,5′-Di-C-β-D-glucosylphloretin in young leaves, while quercetin-3-O-robinobioside and rutin in old leaves took the accounts. It is concluded that jujube leaves can be utilized as the source of such flavonoids, which can be used by incorporating such leaves into mainstream food products for the valorization of these underutilized by-products.