Original ArticleIdentification and quantification of flavonoids in traditional cultivars of red and white onions at harvest
Introduction
Onions (Allium cepa L.) are one of the world's oldest cultivated vegetables (Fenwick and Hanley, 1985) and are the second most produced vegetable crop after tomatoes (Griffiths et al., 2002). Over the past 15 years, the total surface area dedicated to onion crops in the world has doubled, now reaching 3.07 million ha with a production of 53.6 Mt (FAOSTAT data, 2004). In Portugal, onion is the third vegetable in consumption (13 kg per capita and year), after potato and cabbage, and the landrace cultivars Branca da Póvoa and Vermelha da Póvoa are predominant in the northwestern region of the country. Onions contain high levels of non-nutrient antioxidant compounds (phenolics) which have protective effects against different degenerative pathologies such as cardiovascular and neurological diseases, cancer and other dysfunctions based on oxidative stress (Griffiths et al., 2002).
Flavonoids (C6–C3–C6) are major phenolics in onions and can be classified in different subclasses (flavones, flavanones, flavonols, isoflavones, flavanonols, flavanols, chalcones and anthocyanins) according to the degree of unsaturation and degree of oxidation of the 3-carbon skeleton. Subclasses of flavonoids can be further differentiated on the basis of the number and nature of substituent groups attached to the rings (Robards and Antolovich, 1997). Flavonols and anthocyanins are the main subclasses of flavonoids present in onions, the latter being found only in red onions. Many of these compounds are glycosylated, and some of these glycosyl derivatives are esterified with aromatic or aliphatic acids whose combinations yield a large variety of compounds (Slimestad et al., 2007). Aromatic acylation increases the stability of anthocyanins by intramolecular stacking of anthocyanins with polyphenols. Malonylation is one of the most common forms of aliphatic acylation of anthocyanins and is important for enhancing pigment solubility in water, protecting glycosyl moiety from enzymatic degradation, and stabilizing anthocyanin structures, what contributes to keep the colour hue. Some bioactive properties of anthocyanins, including antioxidant activity, have been shown to be strongly modulated by acylation (Suzuki et al., 2004). Within a vegetable family, the quality and quantity of the phenolic pool may change with the cultivar, growth stage, and environmental conditions.
The aim of this work is the characterization of the phenolic composition (anthocyanins and flavonols) mainly of two regional landrace cultivars of Portuguese onions (Branca da Póvoa and Vermelha da Póvoa), and to compare them with common commercial cultivars (the White Hybrid SK409 and Red Creole) and with a selected population of Vermelha da Póvoa. A methodology based on high performance liquid chromatography method coupled with diode array detection (HPLC/DAD) was developed to simultaneously determine flavonols and anthocyanins. In order to make a representative sampling, the spatial distribution of these flavonoids in an onion bulb and the correlation between onion size and flavonoid content was also studied.
Section snippets
Sample collection
Five onion (Allium cepa L.) cultivars were studied, two white (the landrace open pollinated Branca da Póvoa and the hybrid SK409) and three red (Vermelha da Póvoa, Red Creole and a line of Vermelha da Póvoa selected for uniform bulb morphology). The onion cultivars were grown under the same conditions in a farm located in a traditional onion-growing region in northwestern Portugal (Póvoa do Varzim, at 41°22′57″N and 8°45′46″W). Onions were harvested between July and August 2004 and left on the
Performance of the analytical procedure
Flavonols and anthocyanins are soluble in polar solvents, with the glucosides more soluble in water and the aglycones more soluble in alcohols. These phenolics are commonly extracted from plant materials with water–alcohol (methanol or ethanol) mixtures acidified with mineral acids (Harborne et al., 1986, Hertog et al., 1992, Fossen et al., 1996, Robards and Antolovich, 1997, García-Viguera et al., 1998, Revilla et al., 1998, Da Costa et al., 2000, Mozetic and Trebse, 2004) to prevent the
Conclusions
Eight flavonols and eight anthocyanins were isolated from the edible part of the onion varieties tested. Major flavonols were identified as quercetin 3,4-diglucoside and quercetin 4-glucoside, and the major anthocyanins as cyanidin 3-(6″-malonylglucoside) and cyanidin 3-glucoside. Significant differences in total flavonoid concentrations were observed among the red and white varieties, but were not observed between native and foreign varieties. The content of total flavonoids is considerably
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