Abstract
The aim present study was to measure the quercetin and quercetin glucoside content in onion cultivars with respect to pre and postharvest treatments including lifting time, drip irrigation, curing and forced curing on quercetin content in nineteen onions (Allium cepa L.) cultivars were studied. The quercetin contents ranged from 0.011 to 4.109 mg/100 mg dry weight basis among nineteen studied onion varieties. The highest amount of quercetin was detected in red varieties ('Colossus‘), lowest in white varieties ('ISI 2120'), meanwhile yellow variety T-M-8-4 exhibited higher quercetin content compared to other yellow and white varieties. The amount of quercetin glucosides content in different parts of onion follows an increasing order from inner to outer scales. It has been observed that, outer scales of onions contain 3.5 times higher quercetin glucosides than the inner ones. There is no direct relation between the numbers of drip irrigation on the contents of quercetins; compared to the control group (with regular irrigation after every week). Similarly, Lifting time had minor effects on quercetin content among the selected onion cultivars. Cultivar differences in quercetin content were significant but not consistent during the time of this study. During forced curing of skin, as well as the peeled onion, the total quercetin content of cured onion decreased as compared to non-cured control samples. This study indicates that, the onion genotypes provided to be the best way to increase quercetin content in onion bulb.
Highlights
Onions have been widely used throughout the world as a flavoring vegetable as well as a traditional and folk medicine
There are about 20 flavonols found in onion species and the main flavonol glucosides present in onions (Allium cepa) are the monoglucoside quercetin 4’-O-glucoside (OMG) and the diglucoside quercetin 3,4’-O-diglucoside (QDG) which account for 85% to 90% (Beesk et al, 2010; Bonaccorsi et al, 2008; Yoo et al, 2010) of total flavonoids and up to 97% of total quercetin (Yoo et al, 2010) in onion
Quercetin glucosides (QMG and QDG) were present in high amounts in the inner (QDG) and middle parts (QMG) of the onion bulb whereas quercetin and QMG dominated in the outer scale (Beesk et al, 2010) These results coincide with the finding of the activities of region specific quercetin glucosyltransferases activities reported in onions (Bonaccorsi et al, 2008; Yoo et al, 2010) where the glucosyltransferase activity was found higher in the inner than in the outer scales
Summary
Onions have been widely used throughout the world as a flavoring vegetable as well as a traditional and folk medicine. Onion extracts and the isolated bioactive compounds from onion has wide range of biological effects including antioxidant, antiplatelet, antidiabetic, anticarcinogenic, antiasthmatic, antimicrobial, anti-inflammatory and antibiotic effects on human health (Griffiths et al, 2002; Sharma, et al, 2015). The bioactive flavonols which are rich in onions are having potential antioxidant activity and found in high amount in many foods and vegetables. Onions contains 16 different kinds of flavonols in different forms including quercetin and its aglycones and glycosylated derivatives, isorhamnetin and kaempferol (Beesk et al., 2010). The flavonols are often found in high concentrations in the skin of most the onions, where they impart their yellow/brown colour, unless concealed by the red pigment, such as anthocyanin. The concentration of quercetin was found to be as high as 2.1% w/w (Nile and Park, 2013). Compared to yellow onions the white skinned onions, garlic, and leek contains very low amount
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