Flavonol glucosides

The major flavonol glucosides in the edible portion of the onion include quercetin 3,4'-O-diglucoside (QDG) and quercetin 4'-O-monoglucoside (QMG) (Price and Rhodes, 1996). These compounds were reasonably resistant to degradation during drying, storage and processing of onion bulbs, with a loss of 25% due to boiling or frying, and 50% of QMG was lost during the initial drying process at 28°C for 10 days (cvs 'Red Baron' and 'Cross Bow') (Price et al., 1997). The loss during drying may be due in part to mobilization of QMG towards the drying skins, which in this case were removed for the extraction process. Even after sprouting, little change in the flavonol glucosides occurred in the edible portion of the onion (Price et al., 1997). Hirota et al. (1999) and Takahama and Hirota (2000) described the formation of compounds found in brown onion skins, in particular 3,4-dihydro-benzoic acid and 2,4,6-trihydroxyphenylgly-oxylic acid. These compounds are thought to be created by the enhanced peroxidase-dependent oxidation of quercetin during the drying-down processes, which produce the protective skins of onion bulbs. 3,4-Dihydrobenzoic acid is an antifungal agent.

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