Nutritional Potentials, Antioxidant Activities and Phenolic Composition of White and Purple Sweet Potato Flesh and Peels

Abstract

Although sweet potato peels, which may contain some nutrients and phytochemicals are usually discarded before consumption yet little research has been carried out on its nutraceutical value. Therefore, this present investigation seeks to compare the nutraceutical potentials of peels and flesh from white and purple fleshed sweet potato. The evaluated antioxidant indices were higher in peels of the studied potato. The results of the total phenolic content, total flavonoid content, reducing property, lipid peroxidation, ABTS and DPPH radical scavenging activities revealed higher levels in the peels of the evaluated sweet potato extract compared with the flesh for both white and purple varieties of sweet potato. The HPLC-DAD analysis(mg/g) revealed presence of some phenolic acids and flavonoids, with higher level of the identified phenolic compounds in the peels of the evaluated sweet potato (Purple Peel Sweet Potato: gallic acid; 0.65, chlorogenic acid ; 2.25, caffeic acid ; 2.97, ellagic acid; 0.18, epicatechin; 0.70, rutin ; 0.68, quercitrin ; 1.94, kaempferol; 1.16; White Peel Sweet Potato: gallic acid ; 0.61, chlorogenic acid; 1.46, caffeic acid ; 3.08, ellagic acid; 0.75, epicatechin ;0.30, rutin; 0.17, quercitrin ;1.85, kaempferol ;1.51). In addition, caffeic acid was shown to be the most abundant phenolic compound in the peels of white and purple sweet potato (3.08mg/g, 2.97mg/g) while isoquercitrin was the most abundant phenolic compound in the flesh of the purple sweet potato (1.86mg/g) and quercitrin was the most abundant phenolic compound in the flesh of white sweet potato (1.79mg/g). The result of the proximate indices revealed a higher value of carbohydrate in the flesh (White Fleshed Sweet Potato: 65.10±0.28%, Purple Fleshed Sweet Potato: 65.17±0.93%) and also higher value of crude fibre, fat, protein, and ash contents in peels of purple (8.92±0.14%, 23.17±1.11%, 7.94±0.10% and 5.40±0.09%) and white sweet potato (7.67±0.29%, 21.85±1.91%, 8.40±0.12% and 3.16±0.28%). Higher amount of most evaluated minerals (mg/kg) were reported in both peels of purple and white sweet potato compared with the evaluated flesh of purple and white sweet potato (Purple Peel Sweet Potato: zinc; 75.14±0.08, iron; 410±0.13, copper; 9.38±0.06, calcium; 1343.30±0.41, magnesium; 497.04±0.41 and manganese; 151.04±0.30; White Peel Sweet Potato: zinc; 28.38±0.42, iron; 486±4.47, copper; 29.31±0.17, calcium; 1064.90±4.86, magnesium; 485.61±0.72 and manganese; 16.16±0.07). Similarly, amylose-amylopectin ratio was also reported to be higher in peel of purple sweet potato (13.67±0.23%) and peel of white sweet potato (11.94±0.09%) than the corresponding flesh of the evaluated sweet potato. The result of this investigation showed that leaving the sweet potato peel intact prior to cooking will contribute to the nutraceutical potentials and could be explored in the management of free radical mediated disorders.