Effect of far-infrared radiation drying of citrus press-cakes on free radical scavenging and antioxidant activities Mahinda Senevirathne a , Soo-Hyun Kim b , Yong-Deog Kim c , Chang-Kyung Oh d , Myung-Cheol Oh d , Chang-Bum Ahn a , Jae-Young Je a , Won-Woo Lee e , You-Jin Jeon e, * a Division of Food Science and Aqualife Medicine, Chonnam National University, Yeosu 550-749, Republic of Korea b Department of Food Bioengineering, Jeju National University, Jeju 690-756, Republic of Korea c Jeju Special Self-Governing Province Development Corp., Jeju 690-961, Republic of Korea d Department of Tourism Hotel Culinary Art, Jeju College of Technology, Jeju 690-714, Republic of Korea e School of Marine Biomedical Science, Jeju National University, Jeju 690-756, Republic of Korea article info Article history: Received 18 July 2009 Received in revised form 22 September 2009 Accepted 3 October 2009 Available online 13 October 2009 Keywords: Citrus press-cakes Far-infrared radiation drying Antioxidant activity Flavonoid Phenolic content abstract In this study, far-infrared radiation (FIR) drying was used to convert wet citrus press-cakes (CPCs) into dried form at different temperatures (40, 50, 60, 70, and 80 °C). The dried samples were evaluated for fla- vonoid composition by HPLC and antioxidant activities by various in vitro assays. Compared with CPCs prepared by freeze drying (FD), all FIR-dried CPCs showed slightly lower extraction yield and total phe- nolic content, but almost equal total flavonoid contents and antioxidant activities. However, CPCs dried by FIR showed significantly lower drying times. Therefore, FIR drying is effective and economical method. CPCs dried by FIR at 80 °C showed higher antioxidant activities in several assays, and needed very short drying time. Hence, FIR-80 sample was more potential than other FIR-dried samples. The data confirmed that FIR drying is an effective process to convert wet CPCs into dried form which shows strong antioxi- dant activities. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Flavonoids are a group of phytochemicals which distributed through out the plant kingdom and exhibit a wide variety of bio- logical activities. The highest amount of flavonoids in citrus fruit occurs in the peel (Maurer et al., 1950; Kanes et al., 1993). Hence, citrus press-cakes (CPCs) released from the citrus processing plants are a rich source of naturally occurring flavonoids as CPCs repre- sents large amount of peel (Sinclair, 1943). Further, citrus peel is particularly a rich source of flavanones and flavanone glycosides which are rare in other plants (Horowitz, 1961) as well as poly- methoxylated flavones (e.g. nobiletin and tangeritin). Previous studies demonstrate that citrus flavonoids promote scavenging of free radicals and metal chelation (Igor et al., 1989; Morel et al., 1994). Hesperidin, a flavanone, is an effective antiox- idant since it is able to quench the oxygen free radicals which are involved in cancer (Berkarda et al., 1998; Koyuncu et al., 1999). Also, flavonoids have exhibited unique cardioprotective effect by the ability of their capacity to inhibit LDL oxidation (Kondo et al., 1996). In the previous studies we showed that different flavonoids in the extracts from CPCs dried by far-infrared radiation (FIR) dry- ing and high speed drying (HSD) were very good free radical scav- engers (Senevirathne et al., 2008, 2009). Therefore, flavonoids can exercise their antioxidant activities either as antiradical, anti-lipo- peroxidation or metal chelating agents (Bombardelli and Morazz- oni, 1993). Most of the antioxidative phenolic compounds in plants are fre- quently available as a covalently bound form with insoluble poly- mers (Peleg et al., 1991). It is necessary to introduce an effective processing method to liberate the bound compound to obtain the natural antioxidants from plants (Gil-Izquierdo et al., 2002, 2003). Several methods have been applied to liberate the low molecular weight natural antioxidants from the plant materials such as heat treatment, far-infrared radiation, fermentation and protease treatment (Lee et al., 2003; Niwa et al., 1997; Nicoli et al., 1997). The removal of water from the agricultural or food materials is very energy consumption process. The efficiency of the drying of waste materials is an important economic consideration with re- spect to both energy and time. Therefore, in this study, FIR drying was used to transform wet CPCs into dried form. During FIR drying, the energy in the form of electromagnetic waves is absorbed di- rectly by the sample without loss to the environment leading to considerable energy savings (Sandu, 1986; Ratti and Mujumdar, 1995). Fasina et al. (1996) showed that infrared heating changes 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.10.006 * Corresponding author. Tel.: +82 64 754 3475; fax: +82 64 756 3493. E-mail address: youjinj@jejunu.ac.kr (Y.-J. Jeon). Journal of Food Engineering 97 (2010) 168–176 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng