Molecular characterisation and heat-induced gelation of pea vicilin and legumin

The most important globular pea proteins are legumin and vicilin, and a minor protein is convicilin. The first two have extensive molecular heterogeneity that is well documented in literature, and the latter possesses a distinctive highly charged N-terminal extension region. Characterisation of two vicilin fractions (one contaminated by convicilin) via column chromatography, gel electrophoresis, differential scanning calorimetry (DSC), circular dichroism and solubility experiments lead to the conclusion that convicilin is not a separate protein. It was denoted as the a -subunit of vicilin, and is another heterogeneous factor of this protein. Further experiments showed that when present in large amounts these a -subunits increase the minimum gelling concentration of purified pea proteins at near-neutral pH, and cause transparent heat-induced gels to be formed. This behaviour was attributed to the repulsive forces on the N-terminal extension region at near-neutral pH, and was supported by the fact that no difference in the gelation behaviour of the two vicilin fractions was observed a low pH values where the repulsive charges would have been neutralised. These a -subunits also appeared to have an impact of the gelation of the pea protein isolates when present in sufficient quantity. Heat-induced gelation of legumin was compared with its analogous protein in soybean, namely glycinin. Overall the results of DSC and small deformation rheology showed that both the proteins have the same physical and chemical driving forces acting during gelation, but soybean glycinin, unlike legumin, was consistently able to form reheatable gels. Comparison of the amino acid profiles of the two proteins gave no indication as to why these homologous proteins form gels with different gel network stabilities. When comparing protein isolates and legumin from different pea cultivarsit was shown that the contribution of legumin to pea protein isolate gelation was cultivar specific and that The contribution of legumin to pea protein isolate gelation was no easier to characterise, and although disulphide bonds were seen to played a role in gelation, but they did not demonstrate the gel strengthening ability that they are often reported to posses.