Evaluation of bioactive components, antioxidant potentials, and mineral composition of aqueous extracts of watermelon pulp, seed, & rind (Citrallus Lanatus)

Abstract

Watermelon is a sweet juicy fruit, containing mainly three parts (pulp, seed and rind), which is consumed due to its thirst-quenching ability and low in calorie nutritional status. However, many consumers only consume the pulp, neglecting other parts (seed and rind), turning them to waste products, and these neglected parts have been reported to possess some nutritional potentials. Hence, this study sought to investigate the nutritional (mineral) compositions, bioactive compounds, and antioxidant abilities of the aqueous extracts of watermelon fruit (pulp, seed and rind). In the mineral composition studies, Ca, Mg, K, Na, Mn, Fe, Cu, P, and Zn were analysed, the studies revealed that potassium (K) was the most abundant mineral in the fruit with watermelon rind having the highest value (32625mg/l), watermelon pulp was higher with the value (13757mg/l) than watermelon seed having the least value (6053mg/l). Copper (Cu) was the least abundant mineral in the fruit, watermelon seed had the highest value (17.1mg/l), watermelon pulp had higher value (6.20mg/l) than watermelon rind with least value of 6.00mg/l. Furthermore, the antioxidant potentials (2,2- azinobis 3-ethylbenzo-thiazoline -6-sulfonate (ABTS), 1,1-diphenyl-2-picrylhydrazyl (DPPH), Total phenol, and Total flavonoid) of the aqueous extracts of watermelon fruit (pulp, seed and rind) were carried out and the results showed that watermelon seed was the richest in total phenolic content having (4.50 ± 1.17mg GAE/100g), watermelon rind was richer (0.56 ± 0.13mg GAE/100g) than watermelon pulp (0.30 ± 0.07mg GAE/100g). The total flavonoid assay showed the seed extracts with highest value (8.29 ± 2.06mg QE/100g), the pulp extract was higher (5.07 ± 0.71mg QE/100g) than the rind extract (4.71 ± 1.36mg QE/100g). The ABTS assay also revealed that watermelon pulp, seed, and rind had (32.14 ± 2.16mmol TEAC/100g, 11.4 ± 4.63mmol TEAC/100g, and 5. 36 ± 2.12mmol TEAC/100g) respectively. The scavenging abilities of the extracts showed that watermelon rind had highest scavenging ability (49. 77 ± 3.04 mgMAE/ml), while watermelon seed had higher scavenging ability (51.59 ± 7.86mgMAE/ml) than watermelon pulp having (59.37 ± 4.77 mgMAE/ml). Bioactive compounds such as flavonoid, saponin, alkaloids, cardiac glycosides & cardinocides and terpenoids were present in the whole watermelon fruit (pulp, seed and rind) while tannin and chalcone were not present in the whole fruit. The studies revealed that the neglected parts (seed and rind) exhibited significant antioxidant potentials which may inhibit overproduction of oxidants involved in the pathogenesis of many chronic diseases.