Onions: A Source of Unique Dietary Flavonoids

Abstract

Onion bulbs (Allium cepa L.) are among the richest sources of dietary flavonoids and contribute to a large extent to the overall intake of flavonoids. This review includes a compilation of the existing qualitative and quantitative information about flavonoids reported to occur in onion bulbs, including NMR spectroscopic evidence used for structural characterization. In addition, a summary is given to index onion cultivars according to their content of flavonoids measured as quercetin. Only compounds belonging to the flavonols, the anthocyanins, and the dihydroflavonols have been reported to occur in onion bulbs. Yellow onions contain 270-1187 mg of flavonols per kilogram of fresh weight (FW), whereas red onions contain 415-1917 mg of flavonols per kilogram of FW. Flavonols are the predominant pigments of onions. At least 25 different flavonols have been characterized, and quercetin derivatives are the most important ones in all onion cultivars. Their glycosyl moieties are almost exclusively glucose, which is mainly attached to the 4', 3, and/or 7-positions of the aglycones. Quercetin 4'-glucoside and quercetin 3,4'-diglucoside are in most cases reported as the main flavonols in recent literature. Analogous derivatives of kaempferol and isorhamnetin have been identified as minor pigments. Recent reports indicate that the outer dry layers of onion bulbs contain oligomeric structures of quercetin in addition to condensation products of quercetin and protocatechuic acid. The anthocyanins of red onions are mainly cyanidin glucosides acylated with malonic acid or nonacylated. Some of these pigments facilitate unique structural features like 4'-glycosylation and unusual substitution patterns of sugar moieties. Altogether at least 25 different anthocyanins have been reported from red onions, including two novel 5-carboxypyranocyanidin-derivatives. The quantitative content of anthocyanins in some red onion cultivars has been reported to be approximately 10% of the total flavonoid content or 39-240 mg kg (-1) FW. The dihydroflavonol taxifolin and its 3-, 7-, and 4'-glucosides have been identified in onions. Although the structural diversity of dihydroflavonols characterized from onions is restricted compared with the wide structural assortment of flavonols and anthocyanins identified, they may occur at high concentrations in some cultivars. From bulbs of the cultivar "Tropea", 5.9 mg of taxifolin 7-glucoside and 98.1 mg of taxifolin have been isolated per kilogram of FW.

On farm and fresh produce management

“…It has also been suggested that long day onion cultivars of Rijnsburger type from Northern Europe have higher levels of quercetin glucosides than short day onions of North American and Japanese origin [161]. In a study on 16 European onion varieties levels of quercetin glycosides in the edible parts were highest in the red onion cv.…”

Handbook of Plant Food Phytochemicals

“…In red onions anthocyanins are also present. These are primarily cyanadin glucosides acylated with malonic acid or non-acylated [39,133,161].…”

“…Levels of flavonols vary considerably between onion cultivars with reported levels ranging from 2549 quercetin equivalents per kg fresh weight (FW) in the red onion cultivar Karmen to <1 quercetin equivalents in the white onion cultivar Contessa [161]. A study by Lombard et al (2005) examined the total flavonol content in 5 onion varieties and found significantly higher concentrations in red skinned onion varieties compared to yellow varieties [37].…”

“…The main flavonols found in onion are quercetin, quercetin 4'-glucoside, quercetin 3,4'diglucoside, kaempferol and kaempferol glucosides. Isorhamnetin and rutin have also been identified in some cultivars [97,161]. Quercetin has been demonstrated to inhibit growth of tumours cells containing type II estrogen binding sites including breast, colon, ovarian, leukaemia, gastrointestinal and meningioma cancer cells (reviewed in [129]).…”

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

“…Onions are a unique source of dietary flavonoids and other bioactive substances (Corzo-Martínez et al 2007;Griffiths et al 2002;Lanzotti 2006;Slimestad et al 2007;Rodriguez et al 2009Rodriguez et al , 2010 and there has been in vivo evidence for the hypolipidemic and antioxidant effects of onion consumption (Park et al 2007;Vidyavati et al 2010). It is estimated that more than 450,000 tonnes of onion solid waste is produced each year in Europe (Moure et al 2001).…”