Cowpea seed coats and their extracts : phenolic composition and use as antioxidants in sunflower oil

Abstract

Seed coats of cream-coloured Bechuana white and purple-coloured Agriblue cowpea varieties and the freeze-dried forms of their acetone extracts (CPE) were analysed for total phenol content using the Folin-Ciocalteu and Ferric Ammonium Citrate methods. The seed coats and CPE of both cowpea varieties were analysed for antioxidant activity using the Trolox Equivalent Antioxidant Capacity (TEAC) assay. HPLC (High Performance Liquid Chromatography) was done to determine the free phenolic acid composition of the CPE. The effect of the seed coats and CPE on the peroxide value (PV) of sunflower oil was determined during storage at 65oC over a 16-day period. The seed coats and CPE were added to the sunflower oil at concentrations of 1800 ppm and 900 ppm and their effect compared to that of the synthetic antioxidant tertiary butyl hydroquinone (TBHQ) added to the oil at 200 ppm. Seed coats and CPE fromBechuana white cowpeas contained higher levels of total phenols than the corresponding samples of Agriblue. This differed from the generally observed trend that cowpeas with darker-coloured seed coat generally contain higher levels of total phenols than light-coloured cowpeas. More cowpea varieties need to be tested to confirm this observation. The ABTS•+ radical scavenging activity of the seed coats and CPE of Bechuana white cowpeas were higher than those of Agriblue. According to the HPLC chromatograms, both cowpea varieties had similar types of phenolic acids namely, the cinnamic acid derivatives p-coumaric, ferulic and sinapic acids and the benzoic acid derivatives p</i<-hydroxybenzoic acid, 3,4 dihydroxybenzoic, syringic, gallic and vanillic acid. These were present in higher amounts in Bechuana white than Agriblue. Phenolic acids derived from cinnamic acid tend to have higher antioxidant activity than benzoic acid derivatives. The higher antioxidant activity of Bechuana white seed coat acetone extracts and CPE was attributed to the presence of phenolic acids, especially the cinnamic acid derivatives in higher amounts in Bechuana white than Agriblue. CPE and seed coats of both cowpea varieties reduced the formation of hydroperoxides in sunflower oil as shown by their lower peroxide values compared to oil without added antioxidant during storage. This antioxidant effect of the additives was attributed to the presence of phenolic compounds. The ability of the CPE and seed coats of both cowpea varieties to reduce formation of hydroperoxides in sunflower oil was dependent on their concentration in the oil. However, the seed coat and CPE of both cowpea varieties were not as effective as TBHQ in reducing the formation of hydroperoxides in the oil. Bechuana white CPE and seed coats were more effective than Agriblue in reducing the formation of hydroperoxides. This was because Bechuana white had higher levels of total phenols and higher radical scavenging activity than Agriblue. For both cowpea varieties, the seed coats were less efficient in retarding the formation of hydroperoxides than the CPE. This was because the extracted phenolic compounds in the CPE were in a freer form to diffuse in the oil to exert antioxidant effects compared to the unextracted seed coat.