Philippine Journal of Science 140 (2): 145-152, December 2011 ISSN 0031 - 7683 Date Received: 04 Jun 2010 Key Words: antioxidant, phenolic content, radical scavenging activity, reducing power, yam *Corresponding author: djanna.cornago@up.edu.ph djanna_cornago@yahoo.com 145 Djanna F. Cornago 1* , Rowena Grace O. Rumbaoa 2 , and Inacrist M. Geronimo 1 Philippine Yam (Dioscorea spp.) Tubers Phenolic Content and Antioxidant Capacity 1 Institute of Chemistry, College of Science, University of the Philippines, Diliman, Quezon City 2 Department of Food Science and Nutrition, College of Home Economics, University of the Philippines, Diliman, Quezon City Five Philippine varieties of purple yam or ube (Dioscorea alata) — Daking, Kimabajo, Rapang- rapang, Sampero, and Shiket, and two varieties of lesser yam or tugui (Dioscorea esculenta)— Highland and Lowland, were analyzed in the study for phenolic content and antioxidant activity. The total phenolic content of the samples ranged from 69.9 to 421.8 mg gallic acid equivalent (GAE)/100 g dry weight. EC 50 values were 1.7-14.8, 6.2-31.7, and 17.5-35.1 mg/mL for radical scavenging activity, reducing power and iron chelating capacity, respectively. Total antioxidant activity by ferric thiocyanate method at 50 mg/mL was between 92.0-95.6%. All samples had better radical scavenging activity and reducing power on a µg analyte basis than α-tocopherol. Significant correlation was observed between total phenolic content and DPPH radical scavenging activity (R=-0.7664, p<0.05) and reducing power (R=-0.8083, p<0.05) but none between total antioxidant activity and phenolic content (0.1378, p>0.05), for both purple yam and tugui. Significant correlation between total phenolic content and iron-chelating capacity was observed only for the tugui varieties(R= -0.9859, p<0.05). INTRODUCTION Researches have established that root crop extracts, specifically yam (Bhandari & Kawabata 2004; Chung et al. 2008; Hsu et al. 2003), potato (Al-Saikhan et al. 1995; Reyes 2005; Rumbaoa et al. 2009b), sweet potato (Huang et al. 2005a; Rumbaoa et al. 2009a; Teow et al. 2007), yacon (Yan et al. 1999), cassava and taro (Lako et al. 2007), exhibit antioxidant activity. Antioxidant activity of root crops has been attributed to well-known phytochemicals such as α-tocopherol, ascorbic acid and β-carotene (Kalt 2005). However, recent researches have focused on polyphenolic compounds, which are mainly responsible for antioxidant activity as shown in studies of in vitro models of lipid oxidation (Vinson et al. 1998). Yam (Dioscorea spp.) belongs to the high antioxidant activity but low phenolic content group in the study by Kaur & Kapoor (2002) on Asian vegetables. Halvorsen et al. (2002) ranks yam as having the 7 th highest antioxidant concentration among 11 roots and tubers analyzed using Ferric Ion Reducing Antioxidant Power (FRAP) assay. Ozo et al. (1984) identified cyanidin-3-glucoside, (+)-catechin and the procyanidin dimers ‘B-1’ and ‘B-3’ as the phenolic constituents of yam. In addition, Bhandari & Kawabata (2004) reported that yam contains chlorogenic acid. There are approximately 600 species of yam (Ozo et al. 1984) but the species cultivated in the Philippines are Dioscorea alata or ubi and Dioscorea esculenta or tugui (BAS 2006). Annual production of ubi in the Philippines is 26,464 metric tons for the period 2000-2005, while