Effect of microwave treatment on phenolic content and antioxidant activity of citrus mandarin pomace Khizar Hayat, Xiaoming Zhang * , Umar Farooq, Shabbar Abbas, Shuqin Xia, Chengsheng Jia, Fang Zhong, Jing Zhang State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, Lihu Road 1800, Wuxi, Jiangsu 214122, China article info Article history: Received 29 August 2009 Received in revised form 5 March 2010 Accepted 26 April 2010 Keywords: Microwave treatment Citrus mandarin pomace Phenolic content Antioxidant activity Kinnow Feutrell’s Early abstract The effects of microwave treatment on the phenolic compounds and antioxidant capacity of citrus man- darin pomace were evaluated. After treatment, methanol extracts of citrus pomace were prepared and the contents of phenolic acids (free and bound) and flavanol, flavanone and flavonol compounds (FCs) were determined by HPLC. Antioxidant capacity of pomace extracts was measured using DPPH radical- scavenging assay, hydroxyl radical-scavenging assay and reducing power. After microwave treatment, the free fraction of phenolic acids increased, whereas the bound fractions decreased and antioxidant activity was increased. The content of total FCs increased with power but, at longer irradiation time, it declined which meant that some FCs might be degraded. The results indicated that appropriate micro- wave treatment could be an efficient process to liberate and activate the bound phenolic compounds and to enhance the antioxidant activity of citrus mandarin pomace. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Antioxidants are both natural and synthetic compounds, able to scavenge free radicals and to inhibit oxidation processes. Currently, the growing interest to find cheap, renewable and abundant sources of antioxidant compounds has fostered research on vegeta- ble sources of residual origin from agricultural industries (Moure et al., 2001). The antioxidants from natural sources, with potential nutritional and therapeutic value, can be used for increasing the stability of foods by preventing lipid peroxidation (Zia-ur-Rehman, 2006). Pomace is the residue remaining when fruits are processed for juice, wine, or other products. Many studies have reported that fruit pomaces are rich sources of phenolic compounds and these byproducts, obtained from the juice and wine industry, might be useful raw materials for creating new value-added products (Zhou et al., 2009). Citrus is an important crop, mainly used in food indus- tries for fresh juice production, and peel and pomace are the main byproducts during its processing (Li, Smith, & Hossain, 2006a, 2006b). Citrus pomace has been used as a source for molasses, pectin, and limonene. Citrus pomace has also been widely studied, because it contains numerous biologically active compounds, including natural antioxidants such as phenolic acids and flavo- noids (Kim, Lee, Lee, Nam, & Lee, 2008b). Phenolic acids have attracted increasing attention for their anti- oxidant behaviour and beneficial health-promoting effects and they account for about one-third of the phenolic compounds in plant foods. It is assumed that many antioxidative phenolic com- pounds in plants are usually presented in a covalently-bound form (Xu, Ye, Chen, & Liu, 2007). Therefore, reliable and practical meth- ods for liberation of natural antioxidants from plant materials are of considerable interest. Microwave energy can potentiate the bio- availability of free pharmacologically active natural compounds by preventing the binding of polyphenols to the plant matrix (Gulati, Rawat, Singh, & Ravindranath, 2003). The internal temperature dis- tribution of a material subjected to conventional-heating depends on its thermal conductivity, whereas microwave-heating results in the heating of all the individual elements of a material instanta- neously. Consequently, heating time, using microwaves can be sig- nificantly reduced as compared to conventional-heating methods (Robinson et al., 2009). Several methods, such as heat treatment, far-infrared radiation, electron-beam irradiation, fermentation and protease treatment have been studied to liberate and activate low-molecular weight natural antioxidants (Jeong, Kim, & Kim, 2004; Kim, Bae, Kim, & Lee, 2008a; Kim et al., 2008b; Niwa, Kanoh, Kasama, & Neigishi, 1988; Xu et al., 2007). However, to the best of our knowledge, there has been no detailed report on the use of microwaves to liberate phenolic compounds in plant materials and particularly in citrus pomace. 0308-8146/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2010.04.060 * Corresponding author. Tel.: +86 510 85919106; fax: +86 510 85884496. E-mail addresses: khizaraura@hotmail.com (K. Hayat), xmzhang@jiangnan. edu.cn (X. Zhang). Food Chemistry 123 (2010) 423–429 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem