Thermal Gelation of Proteins from Cajanus cajan Influenced by pH and Ionic Strength.

Cajanus cajan [pigeon pea (PP)] is an important legume crop for subsistence agriculture and its seeds are an alternative plant-based protein source. PP protein isolates (PPI) are able to form heat-induced gels that could be used for food applications. The aim of this work was to study the influence of pH (2.1, 3.9, 6.3, and 8.3) and ionic strength (µ) (0.10 and 0.54) on thermal stability and thermal gelation of PPI obtained by alkaline extraction (pH 8.0) and isoelectric precipitation. Thermal stability of PPI changed with pH variation at low ionic strength (µ = 0.10), decreasing this dependence with the increase of ionic strength (µ = 0.54). At µ = 0.10, gelation capacity of PPI was lower at pH 2.1 and pH 3.9. These gels presented a coarse network, which entails low WHC. At pH 6.3 and pH 8.3, gels showed a solid-like character with a compact and homogeneous matrix, with better WHC. At µ = 0.54, gel formation was favoured at pH 2.1 and pH 3.9. G'20/G'95 ratio values and differential solubility results suggest that hydrogen bonds and electrostatic interactions could play an important role in gel formation at pH 6.3 and pH 8.3.

Comparative Study of Structural and Physicochemical Properties of Pigeon Pea (Cajanus cajan L.) Protein Isolates and its Major Protein Fractions.

Pigeon pea protein isolates (PPI) are an option to obtain a high yield of good quality proteins and represent a great potential for the food industry. In this work, physicochemical and structural properties of albumin (ALB), globulin (GLB), and PPI obtained at different pHs (8, 9, 10, and 11) were studied to deepen the knowledge of these proteins for future application. GLB presented protein aggregates and polypeptides characteristics of 7S vicilin subunits while ALB presented polypeptides with low molecular masses. GLB showed a more compact and less flexible structure than ALB fraction due to the distinct conformational characteristics found in DSC, fluorescence spectroscopy, Ho. These structural characteristics conferred GLB greater conformational stability (∆GH2O) than ALB fraction. The latter presented a higher proportion of ß-strand in aggregated structures. PPI11 showed the highest protein recovery, but the least So with more presence of protein aggregates with the least proportion of ß-strands in aggregated structures. A higher percentage of protein unfolding and exposure of hydrophobic residues to solvent was observed as the extraction pH of the isolates increased. Enthalpy change of transition decreased, and the maximum emission wavelength shifted to red in fluorescence spectroscopy. However, PPI11 showed only a slight increase in Ho (10%) with respect to PPI8. The variation in pH for protein extraction constitutes a simple, rapid, and low-cost method to obtain PPI with physicochemical and structural properties that will determine its functional properties and their use as food ingredients.