EFFECTS OF CALLOSOBRUCHUS MACULATUS INFESTATION ON ANTIOXIDANT ACTIVITIES, PHENOLIC PROFILE AND NUTRIENT COMPOSITION OF THREE COWPEA (VIGNA UNGUICULATA) VARIETIES

Abstract

There is a growing interest in preventive health care for the development of natural antioxidants from plant food . Cowpea (Vigna unguiculata L. Walp.) is an important legume crops in most part of the world. Bruchid Callosobrochus maculatus is a well known pest that deteriorates cowpea which may affect the functional property of the food if not properly managed. Therefore, the present study is to evaluate the effect of the pest infestation on the nutrient composition ( proximate and mineral composition), phenolic profile and antioxidant indices (total phenol, total flavonoid, hydroxyl radical scavenging, reducing power, ABTS radical scavenging assay, DPPH radical scavenging assay, inhibition of lipid oxidation) of three selected cowpeas commonly consumed in Nigeria. The result of the proximate composition (%) showed a decreasing value in the protein and carbohydrate content with respect to infestation. Although, there was an increasing value of moisture, ash and fat content after infestation, while there was no appreciable difference in the fiber content. Similarly, the mineral content (Ca, Mg, Na, K, P, Zn, Fe and Cu) showed a decreasing value (mg/kg) with respect to infestation. Phytochemical screening revealed the presence of alkaloids, flavonoids, steroids, terpenoids, saponnins, tannins and cardiac glycosides in both infested and non infested cowpea varieties. Quantification of phenolic compounds in the studied cowpeas revealed the presence of some phenolic acids and flavonoids with lower quantities of phenolic compounds in the infested cowpea varieties.The result also revealed that infestation causes an increase in the evaluated anti-nutrients (phytate, oxalate and alkaloids). The result of the antioxidant potential showed that ife brown has the highest antioxidant activity with a lower activity after infestation with Callosobruchus maculatus. The total phenolic content (mg/g) of the selected cowpea varieties were Ife brown: 1.059, oloyin brown: 0.481, Sokoto white: 0.772. The level of reduction (%) of total phenolic content after infestation was 73.2, 32.2, and 31.5 for Ife brown (IFB), oloyin brown (OB) and Sokoto white (SW) respectively. The highest ferric reducing power (mg/g) was recorded for oloyin brown (0.082) and the least value was recorded for sokoto white (0.032). Similarly, there was a decreasing reducing power after infestation (IFB: 45.1%, OB: 56.6%, SW: 40.6%). The highest total flavonoid content (mg/g) was recorded for ife brown (0.951) and the least was recorded for sokoto white (0.357) with a significant reduction after infestation (IFB: 40.3%, OB: 37.7%, SW: 47.9%). A higher level of reduction in hydroxyl radical scavenging activity was only recorded for ife brown (46.3%), with lower level of reduction in oloyin brown (8.9%) and Sokoto white (5.8%). Some level of inhibition against lipid oxidation was recorded for the studied cowpeas (IFB: 51.8%, OB: 53.9%, SW: 52.7%), However, a lower level of decrease in the activity against lipid oxidation was noticed after infestation (IFB: 7.1%, OB: 6.9%, SW: 11.6%). 2, 2’-azinobis-3-ethylbenzothiazoline-6-sulfonic acid ( ABTS) and 1, 1-diphenyly-2- picrylhydrazyl (DPPH) radical scavenging activity of the studied cowpeas showed a better antiradical action at the lowest selected concentration (0.02mg/ml) of the phenolic extracts with oloyin brown having the highest radical scavenging activity. However, no appreciable difference was recorded for the ABTS and DPPH radical scavenging assay after infestation. Overall, the results of the present investigation revealed that pest infestation do not only affect the nutrient composition of the studied cowpeas, but also have an adverse effect on the functionality of the plant food. Therefore, the result of this investigation showed that management of this pest is not only essential to retain the food value but also to maintain the functionality of the plant food so as to have the optimum effect in the preventive health care.