Heteroprotein complex formation of soy protein isolate and lactoferrin: Thermodynamic formation mechanism and morphologic structure

Abstract

Heteroprotein complex coacervate (HPCC) is one of the most promising electrostatically driven biopolymer materials. In this paper, the formation conditions, thermodynamic formation mechanism, and morphologic structure of soy protein isolate/lactoferrin (SPI/LF) complex coacervate were investigated. The SPI/LF complex coacervates prepared under optimal conditions were electrically neutral and followed the principle of charge compensation. Electrophoresis and isothermal titration calorimetry verified that all individual SPI fractions participated in HPCC and the stoichiometry of the SPI/LF complexes was the same as their optimal mixing ratio. Moreover, SPI/LF complex coacervation was thermodynamically favoured (ΔG < 0), which resulted from entropy gain (TΔS > 0) and negative enthalpy change (ΔH < 0). Not only electrostatic interactions but also hydrogen bonds participated in the complex coacervation. The SPI/LF interaction improved the heat-stability of heat-sensitive lobe in LF. Furthermore, the SPI/LF complex (pH 6.25, SPI/LF = 1:3) exhibited distinct granules, whereas the uniform crosslinking structure appeared in the other SPI/LF complex (pH 6.6, SPI/LF = 1:4). Atomic force microscope showed the sphere complex (pH 6.25-SPI/LF = 1:3) with a diameter of 50–150 nm and the chain-like complex (pH 6.6-SPI/LF = 1:4) with a length of 50–150 nm and a width of 20–80 nm.