Antioxidant Capacity of Indigenous Root and Tuber Crops of Sri Lanka as Affected by Simple Processing

Abstract

The indigenous root and tuber crops in Sri Lanka have not been exploited by the food industry. With a view to establish their nutritional importance and promote consumption and industry usage, this study determined the contents of selected bio active compounds and antioxidant properties in 15 root and tuber crops varieties namely, Amorphophallus paeoniifolius, Canna indica (2 Selections), Dioscorea alata (6 varieties), D. bulbifera, D. esculenta, Maranta arundinacea and Xanthosoma sagittifolium (3 varieties), both in their raw and processed (boiled) forms. Antioxidant capacity (AOC) of yams was determined by Ferric Reducing Antioxidant Power (FRAP) Assay and 2,2-diphenyl-1-picrylhydrazyl (DPPH) Radical Scavenging Assay. Total phenolics (TPC) and ascorbic acid contents (AAC) were determined by Folin-Ciocalteu method and 2,6-dichlorophenol indophenol visual titration method, respectively. Chemical analyses were performed on triplicate samples. The AOC in the raw form ranged from 56 mg TE/g dry weight (D. bulbifera) to 30 mg TE/g dry weight (M. arundinaceae) while it showed a 0.4% (D. alata ‘Raja ala’) to 45% (D. alata ‘Hingurala’) decrease when boiled. The TPC in the raw form was highest in D. bulbifera (190.5 GAE; mg/100 g) while it was lowest in D. esculenta (6 mg/100 g). Dioscorea alata ‘Jaffna Rasawalli’ showed the highest loss (95%) of phenolics when subjected to boiling. The Xanthosoma sagittifolium varieties ‘Kiri ala’ and ‘Isuru’ had the highest AAC (12 mg/100 mg) in raw form while it was lowest (2 mg/100 mg) in M. arundinaceae.   D. bulbifera and X. sagittifolium ‘Kaha kiri ala’ retained relatively higher levels of antioxidants in their boiled forms. The contents of bioactive compounds in yams tended to decrease when subject to boiling. However, the degree of loss depended on the yam variety.  Further research should focus on developing processing technologies to minimize the loss of bioactive compounds in root and tuber crops.