Comparative Studies on Physicochemical and Rheological Properties of Concentrated Xanthan Gum in Milk Protein Isolate-Based Aqueous and Emulsion Systems

In this study, we investigated the physicochemical and rheological properties of xanthan gum (XG) in milk protein isolate (MPI)-based aqueous and emulsion systems at varying MPI concentrations (0–2.0%). MPI-based aqueous systems exhibited relatively higher ζ-potential values (-12.1 to -14.1 mV) than emulsion systems (-21.2 to -27.8 mV), suggesting a conformational transition of MPI during emulsification. The ζ-potential of the emulsion system decreased with increasing MPI concentration, leading to enhanced stability. The addition of XG further improved stability by forming a dense viscoelastic 3D network that restricted particle mobility. The intrinsic viscosity of XG in both systems decreased with increasing MPI concentration, indicating the formation of a more compact conformation due to electrostatic repulsion with MPI or emulsion particles. Consequently, the apparent viscosity and viscoelastic moduli values for XG increased, reflecting stronger intermolecular interactions among closely packed molecular chains. Notably, a more pronounced increase was observed in the emulsion system. The enhanced rheological properties are likely attributable to the formation of more negatively charged particles upon emulsification, which promoted XG molecule contraction in the emulsion system. This conformational transition facilitated interactions between negatively charged XG and positively charged peptide domains protruding on the emulsion particle surfaces. These findings highlight that emulsification alters MPI conformation, thereby changing its interaction with XG and leading to distinct rheological behaviors of XG in aqueous and emulsion systems. With this in mind, the design of gum-based thickeners for dysphagia should account for beverages containing emulsified phases to achieve optimal texture, stability, and swallowing safety.