Beyond a subsistence crop: Structure and functionality of Bambara groundnut proteins

Background and Aim

In recent years, the importance of shifting from a largely animal-based protein diet to one rich in plant proteins have become increasingly apparent.  This is attributed to many factors of which most can be linked to aspects of food security, sustainability and climate change.  To that end, legume crops and specifically the so-called “neglected and underutilised species” are increasingly recognised as sources with the potential to address several of these factors important to the protein transition.  Bambara groundnut (Vigna subterranea (L.) Verdc.) [BGN] is one such a crop, which with a reported protein content of up to 27%, is increasingly recognised for its potential as an alternative plant protein source.  Research efforts have therefore been geared towards highlighting the functionality of BGN proteins, which although being of great value, are still lacking in terms of the link between the observed macroscopic functionality and the molecular and mesoscopic structural properties of the proteins; i.e. the structure-function relationship.  This is especially true for the gelation functionality of BGN proteins, which up till now have only been investigated for the minimum concentration above which a solid-like material can be observed after heating.  The aim of this thesis was therefore to gain insights into the structural properties of BGN proteins obtained from different processing streams and their resulting gelation properties.

Methods

To identify the interplay between processing, structure and functionality, BGN proteins of varying purity as obtained via different extraction routes were characterised for their physicochemical and structural properties, followed by characterisation of their gelation behaviour.  Hereto, techniques such as size-exclusion chromatography coupled to multi-angle laser light scattering provided detailed information on the molecular weight and size characteristics of the proteins, whilst microstructures were characterised with microscopic techniques (i.e. cryo-scanning electron microscopy and confocal laser scanning microscopy).  The gelling behaviour of the proteins were characterised in terms of their rheological responses and linked to scaling models, as a means of describing the resultant gel networks.  These techniques were also applied to evaluate the effect of pre-treatments (soaking and roasting) on the seed microstructures and composition of the resultant fractions, as well as to investigate the synergistic enhancement of BGN proteins in admixture with whey protein isolates.

Results and Conclusions

Vicilin proteins which are trimeric proteins (molecular weight around 150–170 kDa) were confirmed as the major storage proteins in BGN seeds with a high purity (>90% protein content); whilst also being present as the major component in the less pure BGN protein isolates.  The role of vicilin in the BGN protein isolates was evident in the gelation behaviour, where the rheological responses in terms of the elastic modulus G’ of the protein isolates, revealed a concentration dependence which closely resembled that of vicilin in the presence of 200 mM NaCl (conditions for highest solubility of BGN vicilin).  Furthermore, both BGN vicilin and BGN protein isolates were characterised as weak-link fractal structures, whilst microstructural images revealed inhomogeneous aggregates for both fractions.  These findings thus show that high purity BGN proteins (i.e. vicilin proteins) are not necessary to achieve the same functionality as the protein isolates with a lower purity.  Hence, the applicability of dry fractionation as a mild processing technique was also explored, which resulted in protein-enriched fractions with evident gel formation.  Branching out further on the processing aspect, it was observed that soaking as a pre-treatment method is beneficial to address the hard-to-cook properties associated with the seeds, whilst retaining a similar composition as that of the raw seeds.  Finally, BGN protein isolates also showed potential as partial animal protein replacers, based on the change in the rheological responses when combined with whey protein isolates.  Overall, the findings presented in this thesis provided new knowledge and thereby a better understanding of the structure-function behaviour of BGN proteins, which can be used to strengthen the position and steer the application of BGN as an alternative plant protein source.