Phenolic compounds, functional and antioxidant properties of flours from microwave pre-treated Bambara groundnut seeds

Abstract

Bambara groundnut (Vigna subterranea L.) is an underutilised and under-researched legume crop which is an important source of protein and other nutrients, mainly in countries in sub-Saharan Africa. It is consumed in a wide variety of forms such as soups, stews and increasingly as a flour with various food applications. Bambara groundnut is also susceptible to the hard-to-cook (HTC) defect as in many other legumes. Microwave processing is increasingly being used as a pre-treatment method for legumes to alleviate the HTC defect and inactivate anti-nutritional factors in the production of legume flours. This presents an opportunity for application of microwave processing to Bambara groundnut seeds for preparation of flours. Bambara groundnut also contain bioactive phenolic compounds with potential health-promoting properties in terms of combating non-communicable diseases (NCDs) associated with oxidative stress. However, phenolic compounds can be affected by thermal processing treatments such as microwave processing. In this study, the effect of microwave pre-treatment of Bambara groundnut seeds on the HTC defect as well as functional and antioxidant properties and phenolic content of its flours were determined. Seeds of two Bambara groundnut types (brown and red) were pre-soaked to a final moisture content of 40% and microwave treated at 900 W and 1200 W for 5 and 8 minutes with an air temperature of 130ºC. The effect of microwave pre-treatment on the cooking time (determined using a Mattson bean cooker) of the seeds were determined. The water solubility index (determined at 50ºC and 95ºC), nitrogen solubility index, pasting properties (determined using a rheometer) and thermal properties (determined using differential scanning calorimetry) of the resultant flours were determined. The total phenolic content (Folin-Ciocalteu method), antioxidant activity [2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) radical scavenging], phenolic profile and concentration [Ultra Performance Liquid Chromatography-Quadropole Time-of-Flight Mass Spectrometry (UPLC-QToF-MS)] and inhibitory effects against peroxyl radical-induced DNA damage of the resultant flours were determined. Microwave pre-treatment decreased cooking time of Bambara groundnut seeds possibly due to partial gelatinisation of starch, protein denaturation and depolymerisation of pectic substances in the middle lamella of parenchyma cells. The water solubility index, nitrogen solubility index and pasting viscosity of flours from microwave pre-treated Bambara groundnut seeds were significantly lower compared to the untreated flour. This could be due to the microwave-induced denaturation of proteins and exposure of hydrophobic sites as well as the formation of a hydrophobic protein-starch complex which decreased water uptake by starch and thus leading to a decrease in the afore-mentioned functional properties. The thermal properties (enthalpy) of the flours decreased with increase in microwave power and time, suggesting the melting of some crystalline regions of starch which was attributed to the partial gelatinisation of starch. Gallic, protocatechuic and vanillic acid were the main hydroxybenzoic acids while coumaric acid isomer, caffeoyl glycerol and ferulic acid hexoside were the main hydroxycinnamic acid derivatives identified in the Bambara groundnut seeds. Flavonoids identified in Bambara groundnut seeds were flavan-3-ols (procyanidin B2 dimer, procyanidin C2 trimer, catechin and catehin glucoside), flavonols (kaempferol, quercetin-3-O-glucoside and myricetin), flavones (apigenin), flavanones (hesperidin, hesperetin, naringenin, eriodictyol and eriodictyol-7-O-β-D-glucoside) and flavononols (taxifolin). The predominant flavonoids identified in brown Bambara groundnut samples were catechin and hesperidin while quercetin-3-O-glucoside and hesperidin were the predominant flavonoids in the red variety. Microwave pre-treatment of Bambara groundnut seeds led to both increases (greater extractability) and decreases (reduced extractability) in phenolic content (total phenolic content and concentration of phenolic compounds) and radical scavenging antioxidant properties of Bambara groundnut. Increases in phenolic content and antioxidant properties could be due to microwave-induced release of bound phenolic compounds while decreases in phenolic content and antioxidant properties may be due to microwave-induced oxidative degradation of phenolic compounds. Flavonoids were generally more thermally stable in the red variety than the brown during microwave treatments, suggesting that the effect of microwave pre-treatment on flavonoids in Bambara groundnut seeds could be variety-dependent. Microwave pre-treated Bambara groundnut seeds extracts showed protective effects against 2,2'-Azobis(2-amidinopropane) dihydrochloride (AAPH) radical-induced DNA damage which was attributable to their content of antioxidant phenolics. Overall, these results show the potential of microwave processing as a technique not only for reducing the cooking time of Bambara groundnut seeds but also for producing functional Bambara groundnut flours for different food applications. The retention of appreciable antioxidant properties supported by protective effects against AAPH radical-induced DNA damage by the flours indicates their potential to promote health in terms of offering protection from oxidative stress related non-communicable diseases. The outcomes and findings of this research open up further avenues and opportunities for increased utilisation of Bambara groundnuts which is an important crop for food security in sub-Saharan African countries.