Antioxidative phenolic constituents of skins of onion varieties and their activities Tasahil Albishi a , Jenny A. John a , Abdulrahman S. Al-Khalifa b , Fereidoon Shahidi a,c, * a Department of Biochemistry, Memorial University of Newfoundland, St. John’s, NL, Canada A1B 3X9 b Department of Food Science and Nutrition, King Saud University (KSU), Riyadh, Saudi Arabia c Distinguished Visiting Professor, KSU, Saudi Arabia ARTICLE INFO Article history: Received 14 February 2013 Received in revised form 11 March 2013 Accepted 1 April 2013 Available online 1 July 2013 Keywords: Onion Green shoots Phenolic content Free radical scavenging activity HPLC–MS ABSTRACT The antioxidant activity of phenolic constituent of skin and selected flesh of different col- oured 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 high- est 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 0 -diglucoside, quercetin, and kaempferol were the predominant phenolics in all onion extracts tested. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Onions (Allium cepa), one of the most widely consumed vegeta- bles, are classified based on their colour into yellow, red and white and based on their taste as sweet and non-sweet (Shahidi & Naczk, 2004). Fresh and dehydrated onions are widely used in human diet as a source of nutrients, spicy garnish and non- nutritive health promoting compounds (Lee, Lee, Choi, Lee, & Levin, 2008). Onions contain a number of phytochemicals and their consumption has long been associated with health pro- motion and disease risk reduction; decreasing the incidence of cancers in several tissues, preventing vascular and heart dis- eases, neurodegenerative disorders and cataract formation (Kaur, Joshi, & Kapoor, 2009). Among phytochemicals with health benefit, flavonoids, fructans and organosulphur com- pounds in onions are considered important contributing fac- tors to the overall antioxidant activity of the diet (Ames, Shigenagaand, & Hagen, 1993; Paganga, Miller, & Rice-Evans, 1999; Rice-Evans, Miller, & Paganga, 1997). Epidemiological studies about major sources of antioxidant intake highlighted the importance of onions for high levels of flavonols (Hertog, Feskens, Hollman, Katan, & Kromhout, 1993; Suh, Lee, Cho, Kim, & Chung, 1999). Despite the high inter- est that onions have generated in the field of antioxidants, few studies have addressed the need to evaluate the antioxidant capacity of different species of onion. Red, yellow, and white onions are, in fact, known to contain a large amount of flavo- nols; the majority being glucose derivatives of quercetin and keampferol (Lee & Mitchel, 2011; Rhodes & Price, 1996; Sellappan & Akoh, 2002; Shim, Yi, & Kim, 2011). Onion skins 1756-4646/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jff.2013.04.002 * Corresponding author at: Department of Biochemistry, Memorial University of Newfoundland, St. John’s, NL, Canada A1B 3X9. E-mail address: fshahidi@mun.ca (F. Shahidi). JOURNAL OF FUNCTIONAL FOODS 5 (2013) 1191 – 1203 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jff