Flavonoid antioxidants and ascorbic acid in onion (Allium cepa)
1999
Lachman, J. | Orsak, M. | Pivec, V. (Ceska Zemedelska Univ., Prague-Suchdol (Czech Republic). Agronomicka Fakulta)
One of the richest sources of flavonoids in human diet is common onion (Allium cepa) and shallot (Allium ascaloni). The most manifested flavonol quercetin is present in both bound and free form. In onion were found quercetin di- and triglycosides. Very highly are manifested quercetin-monoglycosides. There are also present kaempferol-glycosides in minor amounts. Another type of flavonol-isorhamnetin - is present only in yellow and red cultivars of onion in free form and bound form in glycosides. As a minor compound is referred taxifolin-4'-O-beta-D-glucopyranoside of dihydroflavonol type. Highest contents of flavonoids represent red onion cultivars that contained further anthocyanins. Newly are referred malonylesters of some of glycosides. Yellow and red onion cultivars contain 60-1,000 mg/kg flavonoids. The other groups of polyphenolic compounds contained in onion are phenolcarboxylic acids. Significantly is also present amino acid tyrosine. Content of flavonoids and other polyphenolic antioxidants is influenced by many extrinsic and intrinsic factors such as cultivars differences, locality and year of cultivation, growth stage and storage conditions, ways of processing. Red and yellow cultivars contain higher levels of these compounds in comparison with white onions. There are differences between the resistant and susceptible cultivars in their polyphenol content suggesting that these compounds play an important role in defence mechanism.
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