PROXIMATE COMPOSITION, ANTIOXIDANT AND ANTIMICROBIAL PROPERTIES OF THE SKIN OF SELECTED ROOT AND STEM TUBERS

Abstract

The crude extracts of the skins of five roots and stem tubers-Dioscorea rotundata (white yam), Dioscorea alata (water yam), Ipomea batatas (sweet potato), Manihot esculenta (cassava) and Xanthosoma sagittifolium (cocoyam) were analyzed for proximate composition. Antimicrobial activities of the extracts were assayed against Xanthomonas spp, Escherichia coli, Staphylococcus aureus, Pneudomonas syringae, Bacillus subtilis, Candida albicans. Quantitative antioxidant screening was carried out for total phenols, total flavonoids, tannins and free radical scavenging ability using 2,2-diphenyl-1-picryhydrazyl (DPPH). The result from proximate analyses showed that potato skin had the highest ash content of 25.07±0.09 % while white yam skin had the lowest ash content of 9.07±0.07 %. Cassava skin had the highest moisture content of 7.16±0.06 % and water yam skin had the lowest moisture content of 3.45±0.00 %. Cocoyam skin had the highest fat content of 3.56±0.02 % and white yam skin had the lowest fat content of 1.82±0.03 %. Cassava skin had the highest crude fibre content of 5.88±0.01 % and potato skin had the lowest crude fibre content of 3.29±0.02 %. White yam skin had the highest protein content of 10.78±0.06 % and water yam skin had the lowest protein content of 6.77±0.01 %. Water yam skin had the highest carbohydrate content of 68.55±0.00 % and potato skin had the lowest carbohydrate content of 56.29±0.08 %. For antimicrobial activity, the skin extract of I. batatas (sweet potato) showed antimicrobial efficacy against Xanthomonas spp and E. coli recording 4.65 mm and 3.38 mm inhibitory zones respectively, at a concentration of 0.5 mg/ml. There were no zones of inhibition observed in the skin extracts of D. rotundata (white yam), D. alata (water yam), M. esculenta (cassava) and X. sagittifolium (cocoyam) against any of the test microorganisms. For antioxidant screening, white yam skin had the highest phenol content of 14.96±0.07 % and cocoyam skin had the lowest phenol content of 7.47±0.09 %. Potato skin had the highest flavonoid content of 10.79±0.00 % and cocoyam skin had the lowest flavonoid content of 1.49±0.01 %. White yam skin had the highest tannin content of 1.41±0.01 % and cocoyam skin had the lowest tannin content of 0.72±0.01 %. Cassava skin extracts exhibited the highest free radical scavenging ability of 44.51±0.29 % while water yam skin had the lowest radical scavenging ability of 29.97±0.06 %. The skin of D. rotundata (white yam), D. alata (water yam), I. batatas (sweet potato), M. esculenta (cassava) and X. sagittifolium (cocoyam) are nutritionally and medicinally valuable. Sweet potato skin may provide new sources of protective chemicals in agriculture and medicine