Rheological and microstructural characteristics of lentil starch–lentil protein composite pastes and gels

Abstract

The rheological and structural changes in heat induced pastes and gels of lentil starch and lentil protein composites were investigated at various starch to protein ratios. The starch fraction (φs) in the mixtures was varied from 0 to 1 and the total solid content was maintained at 25% (w/w). Results showed that the gel strength of the composite gels increased exponentially with the increase in lentil starch fraction. The pasting temperature increased and the paste viscosity decreased with the increase in the lentil protein fraction. The high starch composite gels (φs > 0.5) showed higher elastic (G′) and loss (G″) moduli at higher temperature (60 °C) than at lower temperature (10 °C). The high protein composite gels (φs ≤ 0.5) showed higher G′ and G″ values at lower temperature (10 °C) than at higher temperature (60 °C). Segregation of protein-rich domains was observed in the high starch gels (φs > 0.7) whereas low starch composite gels (φs ≤ 0.5) appeared more homogeneous. The microstructure of composite gels appeared to be more fragile with larger pore size and thinner wall compared to the microstructure of starch gel. Both non-covalent interactions (hydrophobic and hydrogen bonding) and covalent bonding were found to contribute to the gel structure and firmness of these composite gels. The NaCl concentration increased the paste viscosity and gel firmness of the composite gels up to 0.25 mM above which the magnitudes of these parameters were decreased. Both the paste viscosity and the gel firmness of the composite gels were found to be higher above the isoelectric point of lentil protein and vice versa. From this study, textural properties of the composite gel/paste were found to be strongly affected by the proportion of the starch and protein as well as the extrinsic factors (pH, ionic strength, presence of reducing agents). Therefore, understanding of gelling behaviour of lentil starch and protein in composite gel would be helpful for structure formation of these two biopolymers in mixtures and would help their application in new product development.