Original Article
Identification and quantification of flavonoids in traditional cultivars of red and white onions at harvest

https://doi.org/10.1016/j.jfca.2009.08.013Get rights and content

Abstract

Onions are rich in different types of phenolics, mainly flavonols, and in red varieties anthocyanins are also present. This is significant because these classes of phenolics are antioxidants and hence may impart important functional properties to onions. The aim of the present work was to simultaneously determine flavonol and anthocyanin concentrations in different onion varieties, two white (Branca da Póvoa and the hybrid SK409) and three red (landrace Vermelha da Póvoa, a selected line of Vermelha da Póvoa and Red Creole). Flavonols (quercetin 7,4-diglucoside, quercetin 3,4-diglucoside, isorhamnetin 3,4-diglucoside, quercetin 3-glucoside, quercetin 4-glucoside and isorhamnetin 4-glucoside) were the predominant polyphenolic compounds. White cultivars had the lowest total flavonol content, with values of 89.3 ± 38.5 and 101.0 ± 18.9 mg quercetin/kg fresh weight for Branca da Póvoa and the hybrid SK409, respectively. The red onions had the highest levels of flavonols, especially the selected population of Vermelha da Póvoa and Red Creole, with values of 280.2 ± 41.5 and 304.3 ± 81.2 mg quercetin/kg fresh weight, respectively. Red onions are not only richer in flavonols, but also contain anthocyanins. Four anthocyanins (cyanidin 3-glucoside, cyanidin 3-laminaribioside, cyanidin 3-(6″-malonylglucoside), and cyanidin 3-malonylaminaribioside) were quantified in all red onions, with Red Creole presenting the highest concentration (28.6 ± 8 mg cyanidin/kg fresh weight). Red onions may be recommended for their major potential functional properties. A distinct gradient in total flavonoid content was found between the outer, central and inner edible scales and along the longitudinal axis of the bulb. Differences in flavonol levels between small- and large-sized onions were also found. All of these factors are of paramount importance for sampling and characterizing onions with regard to flavonoids.

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

References (43)

  • A. Crozier et al.

    Quantitative analysis of the flavonoid content of commercial tomatoes, onions, lettuce, and celery

    Journal of Agricultural and Food Chemistry

    (1997)
  • G.L. Fenwick et al.

    The genus Allium—Part 1

    Critical Review in Food Science and Nutrition

    (1985)
  • T. Fossen et al.

    Characteristics anthocyanin patterns from onions and other Allium spp.

    Journal of Food Science

    (1996)
  • C. García-Viguera et al.

    The use of acetone as an extraction solvent for anthocyanins from strawberry fruit

    Phytochemical Analysis

    (1998)
  • L. Gennaro et al.

    Flavonoid and carbohydrate contents in tropea red onions: effects of homelike peeling and storage

    Journal of Agricultural and Food Chemistry

    (2002)
  • M.M. Giusti et al.

    Molar absorptivity and color characteristics of acylated and non-acylated pelargonidin-based anthocyanins

    Journal of Agricultural and Food Chemistry

    (1999)
  • G. Griffiths et al.

    Onions—a global benefit to health

    Phytotherapy Research

    (2002)
  • J.B. Harborne et al.

    The flavonoids in leaves of diploid Triticum species (Gramineae)

    Plant Systematics and Evolution

    (1986)
  • M.G.L. Hertog et al.

    Optimization of a quantitative HPLC determination of potentially anticarcinogenic flavonoids in vegetables and fruits

    Journal of Agricultural and Food Chemistry

    (1992)
  • S. Hirota et al.

    Tissue and spatial distribution of flavonol and peroxidase in onion bulbs and stability of flavonol glucosides during boiling of the scales

    Journal of Agricultural and Food Chemistry

    (1998)
  • S. Hirota et al.

    Distribution of flavonols and enzymes participating in the metabolism in onion bulbs: mechanism of accumulation of quercetin and its glucosides in the abaxial epidermis

    Food Science and Technology Research

    (1999)
  • Cited by (187)

    View all citing articles on Scopus
    View full text