Effect of repeat unit structure and molecular mass of lactic acid bacteria hetero-exopolysaccharides on binding to milk proteins

Interactions of exopolysaccharides and proteins are of great importance in food science, but complicated to analyze and quantify at the molecular level. A surface plasmon resonance procedure was established to characterize binding of seven structure-determined, branched hetero-exopolysaccharides (HePSs) of 0.14-4.9 MDa from lactic acid bacteria to different milk proteins (β-casein, κ-casein, native and heat-treated β-lactoglobulin) at pH 4.0-5.0. Maximum binding capacity (RUmax) and apparent affinity (KA,app) were HePS- and protein-dependent and varied for example 10- and 600-fold, respectively, in the complexation with native β-lactoglobulin at pH 4.0. Highest RUmax and KA,app were obtained with heat-treated β-lactoglobulin and β-casein, respectively. Overall, RUmax and KA,app decreased 6- and 20-fold, respectively, with increasing pH from 4.0 to 5.0. KA,app was influenced by ionic strength and temperature, indicating that polar interactions stabilize HePS-protein complexes. HePS size as well as oligosaccharide repeat structure, conferring chain flexibility and hydrogen bonding potential, influence the KA,app.