Influence of natural rheological modifiers on O/W emulsions: rheological and low-field NMR study

In this study, the mechanisms by which natural rheological modifiers influence the emulsification of oil-in-water (O/W) emulsions were investigated. Under different zero-shear viscosity gradients, the rheological properties, aqueous phase microstructure, and emulsification performance were examined. Rheological tests revealed that the shear-thinning behavior of xanthan gum and sodium hyaluronate reduced the emulsion viscosity during homogenization, facilitating droplet dispersion. Both xanthan gum and sodium hyaluronate maintained a stable viscosity during heating, demonstrating excellent thermal stability and supporting stable network formation during emulsification. Owing to its weak thickening capacity, λ-carrageenan required high addition concentrations at equivalent viscosity gradients, which hindered effective droplet dispersion. LF-NMR analysis indicated that xanthan gum had the highest bound water content, suggesting strong hydrogen bonding. Zeta potential measurements revealed that xanthan gum possessed the greatest negative charge, which promoted molecular chain extension. Emulsification analysis demonstrated that both xanthan gum and sodium hyaluronate improved the emulsification performance. The emulsification effect increased with increasing xanthan gum concentration, whereas it decreased with increasing sodium hyaluronate concentration. λ-carrageenan reduced emulsification effectiveness. This study revealed that xanthan gum improves emulsion stability through synergistic mechanisms involving hydrogen bonding, hydrophobic association, and electrostatic repulsion, which strengthen interfacial films and promote three-dimensional network formation. The molecular structure of natural rheological modifiers determines their network conformation, which mediates intermolecular interactions with emulsifiers and governs the arrangement of both components at the oil-water interface, thereby influencing emulsification performance. This thickening behavior, dictated by molecular conformation, also regulates the migration rate of emulsifiers to the interface. Furthermore, this study utilized rheological simulations to replicate the thermodynamic and kinetic conditions inherent in the emulsification process. Under a consistent set of simulation parameters, the influence of variations in modifier network conformation on the resulting droplet size distribution was systematically investigated.