A remarkable influence of microwave extraction: Enhancement of antioxidant activity of extracted onion varieties Zill-e-Huma, Maryline Abert Vian ⇑ , Anne-Sylvie Fabiano-Tixier, Mohamed Elmaataoui, Olivier Dangles, Farid Chemat Université d’Avignon et des Pays de Vaucluse, INRA, UMR 408, UMR 406, F-84000 Avignon, France article info Article history: Received 5 July 2010 Received in revised form 7 December 2010 Accepted 26 January 2011 Available online 31 January 2011 Keywords: Microwave extraction Solvent free Onions Flavonoids Antioxidant activity abstract Four (red, yellow, white and grelot onion) varieties of Allium cepa, a rich source of quercetin (flavonol) glycosides, were studied for their total content of reducing compounds (TCRC), flavonol content and anti- oxidant activity evaluation. Extracts obtained by solvent free microwave hydrodiffusion and gravity (MHG) technique and conventional solvent extraction (CSE) were analysed with high performance liquid chromatography (HPLC) for quantification of flavonoids. Three different methods were selected for eval- uating the antioxidant capacity of the different onion varieties (after the determination of their phenolic content by the Folin–Ciocalteu method): the reduction of the stable DPPH (2,2-diphenyl-1-pic- rylhydrazyl) radical, the ORAC (oxygen radical absorbance capacity) method, and the inhibition of the AAPH-induced peroxidation of linoleic acid in SDS micelles. The highest antioxidant capacity was observed for red onion, followed by yellow, white and grelot onion. In spite of the low recovery of extract- able flavonoids (quercetin 3,4 0 -diglucoside, 4 0 -glucoside and 3-glucoside), MHG remained the preferred extraction method in comparison to the conventional method, as all the samples obtained under micro- wave-assisted extraction (MAE) exhibited the highest antioxidant activities in all the tests. Also the microscopic observations of extracted tissues showed that at cellular level, microwaves induced disrup- tions of vacuoles and cell walls thus promoting the effectiveness of this method. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Fruits and vegetables are an essential part of the human diet, partly because of their content in natural antioxidants, especially flavonoids, which may play a beneficial role in the maintenance of normal physiological functions (Huang, Wang, Eaves, Shikany, & Pace, 2007). In many in vitro experiments, dietary flavonoids having a catechol (1,2-dihydroxybenzene) group have been shown to inhibit oxidation of biomolecules by: (i) acting as free radical scavengers via donation of hydrogen atoms or electrons, (ii) bind- ing proteins and enzymes involved in the generation of reactive oxygen species (ROS), (iii) complexing transition metal ions able to catalyse ROS formation by redox cycling, and (iv) regenerating potent endogenous antioxidants, such as a-tocopherol (Cook & Samman, 1996). Moreover, uncontrolled oxidation of biomolecules (e.g., nonenzymatic lipid peroxidation) has been reported to take place during cell degeneration, tumour promotion, coronary heart diseases and some forms of cancer (Olinski, Gackowski, Rozalski, Foksinski, & Bialkowski, 2003; Trushina & McMurray, 2007). Importantly, it is now increasingly recognised that flavonoids may exert favourable health effect by regulating the expression of genes involved in the inflammatory response, the metabolism of carcinogens and the antioxidant defence (Havsteen, 2002). Quercetin is the most common flavonol aglycone. It is present (usually as glycosides) in a wide range of fruits and vegetables and particularly abundant in onion, which is one of the tasteful members of the genus Allium and a major source of antioxidants (sulphur compounds, flavonoids) (Corzo-Martinez, Corzo, & Villa- miel, 2007; Rhodes & Price, 1996; Stratil, Klejdus, & Kubán, 2006). Two major groups of flavonoids found in onions are antho- cyanins (cyanidin and peonidin glycosides) (Donner, Gao, & Mazza, 1997) and flavonols (quercetin, isorhamnetin, kaempferol and their glycosides) (Griffiths, Trueman, Crowther, Thomas, & Smith, 2002). The most abundant flavonols in onions are quercetin 4 0 -O-b-D-glu- coside and quercetin 3,4 0 -O-b-D-diglucoside, which account for more than 85% of the total flavonoid content (Bonaccorsi, Caristi, Gargiulli, & Leuzzi, 2008; Stratil et al., 2006). Moreover, red onions are richer in flavonols than yellow, and pink onions (Prakash, Singh, & Upadhyay, 2007). Red onions also display anthocyanins, which not only impart red colour but also participate in their strong antioxidant activity. In contrast to red-skinned onion, white onions contain only trace levels of flavonols (Aoyama & Yamamoto, 2007). Quercetin is usually found in higher concentration in the 0308-8146/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2011.01.112 ⇑ Corresponding author. E-mail address: maryline.vian@univ-avignon.fr (M.A. Vian). Food Chemistry 127 (2011) 1472–1480 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem