Complexation of Whey Proteins with Carrageenan

Abstract

The formation of electrostatic complexes of whey protein (WP) and a nongelling carrageenan (CG) was investigated as a function of pH, ionic strength, temperature, and protein-to-polysaccharide (Pr:Ps) ratio. On lowering the pH, the formation of soluble WP/CG complexes was initiated at pH_c and insoluble complexes at pH_φ, below which precipitation occurred. The values of the transition pH varied as a function of the ionic strength. It was shown that at [NaCl] = 45 mM, the value of pH_φ was the highest, showing that the presence of monovalent ions was favorable to the formation of complexes by screening the residual negative charges of the CG. When CaCl₂ was added to the mixtures, complexes of WP/CG were formed up to pH 8 via calcium bridging. The electrostatic nature of the primary interaction was confirmed from the slight effect of temperature on the pH_φ. Increasing the Pr:Ps ratio led to an increase of the pH_φ until a ratio of 30:1 (w/w), at which saturation of the CG chain seemed to be reached. The behavior of WP/CG complexes was investigated at a low Pr:Ps ratio, when the biopolymers were mixed directly at low pH. It resulted in an increase of the pH of the mixture, as compared to the initial pH of the separate WP and CG solutions. The pH increase was accompanied by a decrease in conductivity. The trapping of protons inside the complex probably resulted from a residual negative charge on the CG. If NaCl was present in the mixture, the complex took up the Na⁺ ions instead of the H⁺ ions. Keywords: Whey proteins; carrageenan; electrostatic interactions; complexation