Effects of malting and dehulling on the functional structural antioxidant properties of Bambara Groundnut (Vigna Subterranean) flour

Abstract

Bambara groundnut (Vigna subterranean (L.) Verdc) is an underutilised legume crop rich in protein, fibre, and phytochemicals. This research aimed to examine the influence of dehulling and malting (120 h) on the functional, structural, and antioxidant properties of flours from three Bambara groundnut (BGN) varieties (cream, brown, and red). The techno-functional, thermal, pasting, structural properties, nutritional quality, antinutritional factors, and protein digestibility of BGN flours were determined. Furthermore, the polyphenol, antioxidant, phenolic, and metabolomic profiles were evaluated using Gas Chromatography Tandem Mass Spectrometry and Ultra-High Performance Liquid Chromatography-Quadrupole Time-of-Flight Mass Spectrometry. Malting significantly (p < 0.05) enhanced the water absorption capacity of BGN flours, with cream and red varieties having higher values (115.36 and 115.59%, respectively), whereas dehulling reduced it, from 95.15 to 80.03%. Dehulling and malting significantly enhanced (p < 0.05) the oil absorption capacity, bulk density, swelling power, and solubility of flours in all BGN varieties. Nonetheless, the true density, Carr index, and Hausner ratio of the flours from all BGN varieties were significantly reduced (p < 0.05). Dehulling significantly enhanced (p < 0.05) the L* value with cream variety having a higher value (89.87), whereas the a* value of all BGN flours was significantly reduced (p < 0.05) with cream variety having a lower value (0.26). Malting significantly enhanced (p < 0.05) the a* value of the cream and red BGN flours, while the L* value significantly decreased (p < 0.05), with red variety having a lower value of 79.08. Both traditional methods significantly increased (p < 0.05) the yellowness, hue, and chroma of the BGN flours. Dehulling significantly increased (p < 0.05) the peak viscosity, with the brown variety having a higher value (3331.50 cP), while malting significantly reduced (p < 0.05) it, with the red variety having a lower value (2169.50 cP). The breakdown and setback viscosity of the dehulled and malted flours significantly increased (p < 0.05), whereas the trough, final viscosity, and peak time significantly decreased (p < 0.05). Dehulling and malting significantly increased (p < 0.05) the onset and peak temperatures of the BGN flours, whereas the end temperature and enthalpy significantly decreased (p < 0.05). Malting significantly decreased the relative crystallinity of the BGN flours, with the brown variety showing the lowest value at 15.22%. The X-ray diffraction (XRD) patterns of the BGN flours did not show significant variations, and similar peaks were observed in the Fourier transform infrared spectroscopy (FTIR) spectra. Both methods altered the morphology of the BGN flours. Dehulling and malting significantly enhanced (p < 0.05) the moisture and protein content of BGN flours, with values varying from 6.01 (control brown variety) to 8.71% (malted brown and red varieties), and from 18.63 (control red variety) to 21.87% (dehulled brown), respectively. Dehulling increased (p < 0.05) the fat content from 5.82 (control red variety) to 7.84% (dehulled cream), whereas malting significantly decreased (p < 0.05) the fat content. Nevertheless, malting significantly increased (p < 0.05) the fibre content from 4.78 (control cream) to 8.28% (malted brown variety), while dehulling significantly decreased (p < 0.05) the fibre content. Both processing methods significantly decreased (p < 0.05) the ash and carbohydrate contents of BGN flours. Dehulling and malting significantly enhanced (p < 0.05) all essential and non-essential amino acids of BGN flours except tryptophan and asparagine, respectively. Dehulling and malting notably increased (p < 0.05) the phosphorus, magnesium, potassium, and sulfur of BGN flours, while calcium was significantly reduced (p < 0.05). Both processing methods significantly enhanced (p < 0.05) palmitic, arachidic, and y-Linolenic acids; however, they significantly reduced (p < 0.05) phytic acid and oxalate, especially with dehulling. Dehulling and malting significantly enhanced (p < 0.05) protein digestibility of BGN flours from 69.38 (control red variety) to 83.29 g/100g (dehulled cream variety). Malting significantly enhanced (p < 0.05) the total phenolic and flavonoid contents as well as antioxidant activity, but not in the case of dehulling. The phenolic profile of the processed BGN flours was lower than that of the whole Bambara groundnut, with quercetin-3-O-glucoside, myricetin, and catechin the most common metabolites in processed BGN flours. Dehulling and malting are useful traditional methods for modifying BGN flours with different techno-functional, structural, nutritional, and protein-digestibility characteristics, as well as metabolites, and their food applications should be encouraged.