Antioxidative phenolic constituents of skins of onion varieties and their activities

• Antioxidant activity of skin and flesh of different coloured onions were studied.
• Phenolic content of onion skin was higher than flesh; Pearl skin containing the highest.
• Antioxidant assays showed a similar trend; Pearl and Red skin being the best.
• HPLC analysis showed quercetin glucoside, quercetin and kaempferol in onion extracts.

The antioxidant activity of phenolic constituent of skin and selected flesh of different coloured onions (Pearl, Red, Yellow and White) were determined. The green shoot obtained after sprouting of the red onion was also analyzed to study the changes in the phenolic constituents during germination. For the first time, all tests were carried out separately for the free, esterified and insoluble-bound phenolic constituents of onion samples. The content of phenolics extracted from onion skins was approximately six times higher than that of their flesh counterparts. Among onion varieties, pearl onion skin showed the highest phenolic content (26.4 mg quercetin eq/g freeze dried sample). A similar trend was observed for free radical scavenging activity of the tested samples. Extracts from edible part of onion showed lower activity in all antioxidant tests carried out. The HPLC–MS analysis showed that quercetin 3,4′-diglucoside, quercetin, and kaempferol were the predominant phenolics in all onion extracts tested.