Characterisation of marama bean protein

Abstract

Marama bean, Tylosema species, is an underutilised indigenous oilseed legume from Southern Africa. It is a very good source of protein, similar to soya beans and peanuts. The marama plant is very hardy and drought-tolerant. Therefore, it has great potential as an alternative to these other oilseed legumes. To facilitate the use of marama protein as a functional ingredient in food systems, the knowledge of protein composition and structure as well as its functionality is indispensable. In this study, the physicochemical characteristics of marama protein were determined. Specifically, the microstructure of protein bodies in two species (T. esculentum and T. fassoglense) of marama beans was compared with that of soya beans. Furthermore, the composition and functionality (thermal and rheological properties) of marama bean protein were determined and compared with soya bean protein. Marama bean parenchyma cells showed clustered spherical protein bodies surrounded by lipid bodies similar to soya beans. T. esculentum seemed to contain smaller sized (4 ± 2 μm) protein bodies per parenchyma cell as compared with T. fassoglense (7 ± 4 μm). Marama protein bodies contained spherical globoid and druse crystal inclusions, which were absent in soya bean protein. P, K, Mg and Ca were the major minerals in marama, which probably originated mainly from storage protein sites. The high level of tyrosine in marama protein, almost 3 times that of soya protein, was confirmed. Marama protein was also slightly richer in proline compared to soya protein. By SDS-PAGE, marama protein exhibited fewer protein bands compared iii with soya. The patterns of these bands in marama under non-reducing and reducing conditions were similar, suggesting an absence of disulphide bonds. The vicilin (7S) and acidic 11S subunits seemed to be absent in marama. This is most unusual in legume proteins. Only a major basic legumin (11S) (20 kDa), medium (63 kDa) and high (148 kDa) molecular weight protein bands were separated for marama. The pI’s of most polypeptides in the marama proteome map were between 6-10, indicating that marama protein is a more basic protein compared to soya protein. Marama protein was characterised by one major endothermal transition (96oC) compared to soya which had two. Marama protein was highly extensible compared to soya protein and even wheat gluten. It was also very adhesive compared to gluten. However, dynamic oscillatory data indicated that marama protein has a less stable protein structure than gluten. With added peroxidase, the storage modulus (G’) of marama protein dough increased with time, suggesting the formation of new and strong protein networks. These new networks probably resulted from tyrosine oxidation and crosslinking. SDS-PAGE and HPLC/MS data from incubated doughs suggest that dityrosine crosslinks may be important in increasing the structural stability of marama protein. The protein body structure of marama is similar to soya in terms of spherical shape and localisation within the parenchyma cells. However, the protein composition of marama is very different from that of soya. Marama protein thus offers new opportunities of protein utilisation in food and non-food systems. Marama protein may be employed in baked goods, especially in the production of gluten-free products. The high extensibility of marama protein may be useful for gas retention in bread dough. The use of marama protein may be explored in the production of microspheres/nanoparticles. Potential applications for these particles may be in bioplastic films. Marama protein nanoparticles may also be used as a bioadhesive due to its high adhesiveness compared to soya protein and even gluten.