Different wet processing methods, combining alkaline or acidic protein extraction with isoelectric precipitation or ultrafiltration, were employed to obtain a number of protein isolates from a defatted chickpea seed (var. “Thiva”) flour. Depending on the isolation method applied, the resulting protein isolates were enriched either in protein constituents belonging to the chickpea seed globulin (TpI, TUFG) or to the albumin fraction (TUFA) or contained a mixture of proteins belonging to both fractions (TUF). The differences in protein composition between the isolates, as well as the impact of the extraction conditions, were reflected in their protein solubility, surface hydrophobicity, sulphydryl group content and thermal properties. The interfacial activity and film-forming ability of the isolate protein constituents were evaluated, indicating that, in general, the composition of the isolates in terms of globulins or albumins influenced to an appreciable extent their surface properties. The method applied for isolate preparation also strongly influenced the emulsifying and emulsion stabilizing-properties, during long time ageing, of the chickpea proteins. Emulsion storage at freezing conditions resulted in extensive droplet aggregation and coalescence. Following heat treatment, on the other hand, flocculation phenomena mainly occurred, with the isolate obtained by isoelectric precipitation from an alkaline protein extract (TpI) producing emulsions that exhibited very high stability against heat coagulation. These results are attributed to compositional and possibly structural differences existing between the protein constituents of the chickpea isolates and, hence, to the number and the extent of exposure of reactive hydrophobic and sulphydryl side groups of the protein molecules. The differences in protein composition between the isolates and the impact of extraction conditions were also reflected in the onset temperature of gelation. On the other hand, the protein isolate gelling behaviour, minimum gelling concentration and water loss during gel compression, depended mainly on the method applied for isolate preparation rather than on the protein composition. In fact, as was observed from oscillatory rheometry measurements, the isolates obtained by ultrafiltration exhibited a lower gelling concentration and produced gel networks of higher elasticity, at a relatively low protein content, compared with the isolate obtained by isoelectric precipitation. On the other hand, large deformation tests revealed that the isolates obtained by ultrafiltration produced gels of a weaker network structure when compared with the isolate obtained by isoelectric precipitation, probably due to differences in the thickness of the strands that constitute the protein network. The gel solubility tests revealed a balanced interplay between the covalent and the physical interactions in the development of the gel network structure by the heat-denatured chickpea proteins.
National Documentation Centre (EKT)
