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

Albishi, Tasahil (2012) Phenolic constituents and antioxidant activities of selected onion and potato varieties and their processing by-products. Masters thesis, Memorial University of Newfoundland.

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Abstract

Potato and onion processing produces a large an1ount of discards, tnainly peels and skins. This study cotnpared the antioxidant activity of phenolic constituents of skin and flesh of di fferent coloured onions (pearl skin, red skin, yellow skin, white skin, red fl esh, sprouted red flesh) and potatoes (purple, russet, innovator, yellow). Phenolic constituents were separated into free, esterified and bond fractions. The bound fraction was extracted into diethyl ether after consecutive alkaline hydrolysis. The contribution of free phenolics toward total phenolic content was significantly (p<0.05) higher than the esterified and bound for onion skin extracts tested then those in the flesh. Phenolics were present n1ainly in the free fonn in both onion skin and fl esh. The content of flavonoids extracted from onion and potato skins was approxin1ately six titnes higher than that of their fl esh counterparts. An1ong onion varieties, pearl onion skin showed the highest phenolic content (26.4 mg quercetin equivalents/g freeze dried sample). Similarly, purple potato peels had the highest phenolic content (13.85 mg gallic acid equivalents/g freeze dried sample). The phenolic compounds in potatoes were predotninantly present in the bound fonn in the peels of both lnnovator and Russet varieties ( 45.95-51.07o/o) while free and esterified phenolics were predotninant in purple and yellow varieties. Red onion skin was most effective in inhibiting DNA strand scission at 94.45o/o, and that of purple potato peel was 91.02o/o. Sitnilar trends were observed for inhibition of LDL cholesterol oxidation and free radical scavenging activities of satnples tested. HPLC-MS analysis showed that quercetin, quercetin 3-glucoside, and kaempferol were the predominant phenolics in all onion extracts, while chlorogenic, caffeic, p-coumaric and ferulic acids were predotninant in potato peels.

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