Enzymatically Cross-Linked Sodium Caseinate as Techno-Functional Ingredient in Acid-Induced Milk Gels

Impact of pH on sodium caseinate binding and structural changes on montmorillonite surface

The immobilization of proteins onto clay surfaces has proven beneficial for pharmaceutical and environmental applications. This study examines the adsorption of sodium caseinate (Cas), an amphiphilic protein widely used in pharmaceutical formulations, onto sodium montmorillonite (Mt). Adsorption isotherms and kinetics were examined at two pHs, above and below Cas isoelectric point (IEP). Dynamic light scattering (DLS), zeta potential, Fourier Transform Infrared Spectroscopy (FTIR), and X-ray Diffraction (XRD) were used to characterize the complexes formed. Results showed that at pH 3, Cas reversed Mt surface charge from negative to positive due to electrostatic attraction, intercalating into the basal spacing. At pH 7, despite repulsion expected, different mechanisms occurred, including protein orientation on the external surface and Cas self-assembly on Mt, increasing the negative zeta potential. Fluorescence quenching and isothermal titration calorimetry (ITC) revealed binding parameters and thermodynamic interactions, identifying electrostatic, hydrogen, and hydrophobic forces at both pH levels. This study contributes to understanding protein immobilization mechanisms and key factors affecting protein-clay adsorption.

Size Modulation of Enzymatically Cross-Linked Sodium Caseinate Nanoparticles via Ionic Strength Variation Affects the Properties of Acid-Induced Gels

Enzymatic cross-linking by microbial transglutaminase is a prominent approach to modify the structure and techno-functional properties of food proteins such as casein. However, some of the factors that influence structure-function-interrelations are still unknown. In this study, the size of cross-linked sodium caseinate nanoparticles was modulated by varying the ionic milieu during incubation with the enzyme. As was revealed by size exclusion chromatography, cross-linking at higher ionic strength resulted in larger casein particles. These formed acid-induced gels with higher stiffness and lower susceptibility to forced syneresis compared to those where the same number of ions was added after the cross-linking process. The results show that variations of the ionic milieu during enzymatic cross-linking of casein can be helpful to obtain specific modifications of its molecular structure and certain techno-functional properties. Such knowledge is crucial for the design of protein ingredients with targeted structure and techno-functionality.