Antioxidant,anti-inflammatory and DNA scission inhibitory activities of phenolic compounds in selected onion and potato varieties

Processing of onion and potato produces a large amount of discards, mainly skins. This study compared the antioxidant activity of skin and flesh phenolics of selected onion (Pearl, Red, Yellow and White) and potato (Purple, Innovator, Russet and Yellow) varieties. All tests were carried out separately for the soluble and insoluble-bound phenolic constituents of onion and potato samples. The potency of the phenolic extracts in the inhibition of radical-induced DNA scission, human low-density lipoprotein (LDL) cholesterol oxidation and LPS-stimulated cyclooxygenase-2 (COX-2) expression in J774A.1 mouse macrophage cells were monitored. Results showed that the soluble extracts had a higher phenolic content and antioxidant activity than the insoluble-bound extracts in most of the assays. Both onion and potato phenolics exhibited notable inhibition of LDL cholesterol oxidation, DNA scission and COX-2 expression at concentrations as low as 5 μg/mL. Pearl onion skin and Purple potato peel phenolics exhibited the highest activities among the tested onion and potato varieties, respectively.     

Antioxidative phenolic constituents of skins of onion varieties and their activities

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.     

Phenolic content and antioxidant activities of selected potato varieties and their processing by-products

The total content of free, esterified and bound phenolics of the peel and flesh of four potato varieties (Purple, Innovator, Russet and Yellow) were determined using the Folin–Ciocalteu method. The respective antioxidant activities of these potatoes and/or their skins were evaluated using several in vitro assays. The phenolic profiles of potato peel and flesh samples were determined using high-performance liquid chromatography with photodiode array detection (HPLC-DAD). Bound and esterified phenolics contributed as much or even more than the free phenolics to the antioxidant activity of the peels; extracts from Purple variety showing the highest activity. The peels of all varieties showed significantly (p < 0.05) higher phenolic content and antioxidant activities than their respective flesh. Chromatographic data showed differences in the amounts, but not in types, of phenolic compounds in the potato peel samples. Thus, potato processing discard may be used in food formulations and their extracts could potentially be employed as an effective source of antioxidants in food systems.