Formation of Electrostatic Complexes within Admixtures of Lentil Protein Isolates and Anionic Polysaccharides (κ-Carrageenan, De-acyl Gellan Gum and Gum Arabic)

Abstract

Proteins from plant sources are generally less soluble and have poorer functionality compared to animal proteins. The overall goal of this thesis was to better understand mechanisms associated with the formation of electrostatic complexes involving mixed systems of lentil protein isolates (LPI) and three different anionic polysaccharides (gum Arabic (GA), κ-carrageenan (κ-CG) and de-acyl gellan gum (GG)). A better understanding of mixed systems should lead to the development of formulated ingredients for targeted applications. Findings also may lead to enhanced utilization of lentil proteins as food and/or biomaterial ingredients with improved functionality over the protein alone. Maximum complexation occurred in the 1:1 LPI:GA mixed system (total biopolymer concentration (Cp) = 0.05%, w/w) at pH 3.50 with complexation following two pH-dependent structure forming events associated with the formation of soluble (pHc) and insoluble (pH1) complexes at pH 5.87 and 3.62, respectively. The addition of GA resulted in a shift of the LPI isoelectric point (pH 4.70) to a lower pH (3.17). The addition of sodium chloride (NaCl) disrupted coacervation, whereas the addition of urea caused a drop in the magnitude of the observed maximum optical density (O.D.). Increasing the temperature to 60°C resulted in a shift in turbidity curves towards more acidic pH and a decrease in maximum O.D. relative to the control (21-23°C). The addition of GG or κ-CG to LPI resulted in a suppression of LPI aggregation by electrostatic repulsion with a shift in net neutrality of the formed complexes to a lower pH (4.36) compared to LPI alone (pH 4.70) as measured by electrophoretic mobility of a 15:1 LPI:GG/κ-CG mixed system (Cp = 0.05%, w/w). The addition of salts resulted in disruption of formed LPI:GG/κ-CG complexes, and no polysaccharide-ion specific sensitivities were evident (i.e., Ca2+ to GG or K+ to κ-CG). Complexation was primarily driven by electrostatic attractive forces with secondary stabilization by hydrogen bonding. Hydrophobic interactions were thought to play a role in the stabilization of LPI-LPI aggregates. Removal of the lentil hull had a minor effect on complexation. Initial interactions occurred slightly above the pI of the LPI where biopolymers carried net negative charges with polysaccharide chains interacting with positive patches on the protein’s surface.