Characterization and modification of Bambara groundnut globulin fractions for the enhancement of functional properties

Abstract

There is a growing interest in the utilization of leguminous grain proteins for food and industrial applications. Bambara groundnut is a xerophyte pulse grain and a potential source of protein that can replace soybean protein, a trusted and widely used food ingredient in the food industry. However, the use of Bambara groundnut proteins including the subunits (legumin and vicilin) is limited in food applications. The understanding of the composition of Bambara groundnut proteins at the subunit level is vital to unlocking their potential and facilitating utilization. In this study, Bambara groundnut globulin is characterized in terms of the structures, composition, and physicochemical properties at the subunit level, and then modified using atmospheric plasma and enzymatic hydrolysis. Bambara globulin consisted of about 70% vicilin, whilst legumin protein was found in relatively low quantity. Gel electrophoresis revealed three major protein bands in globulin similar to vicilin with predominant β-sheet structures. The presence of a disulfide bond was also revealed in legumin. Bambara globulin showed major vicilin (7S, Mw: 120 kDa) and minor legumin (11S, Mw: 410 kDa) components. Fluorescence and hydrophobicity data suggested a folded structure for the legumin fraction dominated by the helical secondary structure compared to the vicilin fraction. Bambara proteins contain an appreciable amount of methionine that is even higher than the FAO/WHO recommended value. Bambara vicilin had the highest amount of negatively and positively charged amino acids compared to globulin and legumin. This coincides with its high solubility profile (approximately 82% at pH 3.5). The least gelation concentration (LGC) significantly increased in the order of globulin (8%) < legumin (18%) < vicilin (20%) at pH 7. Bambara groundnut proteins formed weakly structured gels as indicated by the frequencydependent behaviours of both the storage (Gʹ) and loss (Gʺ) moduli with a difference of lesser than 1 log cycle. The highest Gʹ of vicilin gel indicated more firmness of the gel compared to the gel formed by globulin and legumin. The sol-gel transition temperatures increased in the order of globulin (40℃) < legumin (50℃) < vicilin (80℃). The Gʹ and Gʺ of globulin showed relatively low dependency on heating time beyond the gel point compared to legumin and vicilin subfractions, suggesting a more rapid establishment of its gel network during gelation. Vicilin gel consisted of a microporous structure with a small lath sheet-like structure compared to globulin and legumin. Emulsifying stability of the proteins significantly differed (p < 0.05) at pH 7. The foaming capacity of the vicilin fraction was significantly (p < 0.05) higher than that of the storage protein at pH 3, 7, and 9. Atmospheric cold plasma-activated water (PAW) and enzymatic modification of Bambara groundnut globulin were further assessed. The cold plasma treatment resulted in the loss of the helical structure of Bambara globulin. The plasma treatment increased the hydrophobicity of Bambara globulin indicating an unfolded structure that was also reflected in the observed redshift in fluorescence intensity. No major changes were observed in gel electrophoresis, protein surface charge, and solubility profiles, except for about a 20% reduction in the glutamic acid content of the amino acid profile. Bambara globulin had reduced emulsifying capacity after treatment with PAW. However, foaming capacities were significantly better and stable at up to 15 mg protein/mL. Hydrolysates produced from Bambara groundnut globulin and vicilin, respectively using a combination of pepsin and pancreatin were investigated for ACE and renin inhibitory activities. The hydrophobic amino acid residues in both globulin and vicilin hydrolysates are high, improving the entry of their peptides into target organs via hydrophobic associations. Surface hydrophobicity increased significantly (p<0.05) with an increase in peptide size from <1 to <3 kDa with that of vicilin hydrolysate and membrane fractions having the highest values. The low molecular weight peptide (<1 kDa) membrane fractions from globulin at 1 mg/mL exhibited significantly higher (p<0.05) invitro ACE inhibitory activities compared to vicilin hydrolysate and its fractions. However, higher molecular peptide fraction (<3 kDa) favoured renin inhibitory activity at the same concentration. Vicilin is the major protein fraction of Bambara groundnut globulin. Bambara groundnut globulin was stabilized by disulfide linkages from the legumin, a minor fraction of the storage protein. Bambara globulin and its subfractions formed a weakly structured gel with the dominance of an elastic structure. The dominancy of the β-sheet structure in vicilin protein and the high crosslink density of the vicilin gel could be related to the firmness of the vicilin gel. The variations in the gel points of Bambara globulin and the subfractions were linked to the differences in their amino acid and subunit composition, the thermal unfolding properties of the protein fractions, and the presence of disulfide linkages. Modification of Bambara groundnut globulin using cold plasma-activated water treatment and enzymatic hydrolysis, respectively increased the hydrophobicity of the protein and influenced the emulsifying and foaming properties and the invitro angiotensin-converting enzyme (ACE) and renin inhibitory activities. Therefore, Bambara groundnut globulin could be a potential functional ingredient in the food system. The low molecular weight peptide (<1 kDa and <3 kDa) membrane fractions from globulin have the potential to serve as functional bioactive peptides against hypertension.