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This study investigates the stability and structure of oil-in-water emulsions stabilized by pea protein. Of the wide range of emulsion compositions explored, a region of stability at a minimum of 5% w/v pea protein and 30-50% v/v oil was determined. This pea protein concentration is more than what is needed to form a layer covering the interface. X-ray scattering revealed a thick, dense protein layer at the interface as well as hydrated protein dispersed in the continuous phase. Shear-thinning behavior was observed, and the high viscosity in combination with the thick protein layer at the interface creates a good stability against creaming and coalescence. Emulsions in a pH range from acidic to neutral were studied, and the overall stability was observed to be broadly similar independently of pH. Size measurements revealed polydisperse protein particles. The emulsion droplets are also very polydisperse. Apart from understanding pea protein-stabilized emulsions in particular, insights are gained about protein stabilization in general. Knowledge of the location and the role of the different components in the pea protein material suggests that properties such as viscosity and stability can be tailored for various applications, including food and nutraceutical products.

This study investigates the structural properties of oil-in-water emulsions stabilized with pea protein using small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) techniques particularly as regards the hydration of the protein. The high protein content needed for stability (>5% w/v) is identified as being present mainly as a dispersed component in the aqueous phase contributing to network formation and increased viscosity. The hydration of the protein was distinguished through measurements with various D2O/H2O ratios. The dispersed protein is highly hydrated (75–80% water content). Additionally, pH-dependent scattering studies revealed significant structural rearrangements and aggregation of pea protein at acidic pH and around the isoelectric point, without greatly impacting emulsion stability. A range of pea protein concentrations were studied with increased amount of dispersed material in the aqueous phase at higher protein concentrations that results in smaller oil droplets and enhanced emulsion stability. Overall, these findings highlight the complex interplay between protein hydration, pH-induced changes and aggregation, and concentration on emulsion stability. This provides insights for optimizing protein-based emulsions in food applications, paving the way for novel protein-based emulsifiers.

Emulsions stabilized with pea protein exhibit enhanced stability only if excess protein is present in the continuous aqueous phase. We hypothesize that the additional protein, beyond the interfacial layer surrounding the oil droplets, is important for the emergence of a yield stress as well as for the overall stability and properties. Stable emulsions with oil concentrations of 40-60% v/v were prepared and compared to layers from various separated emulsions. Confocal microscopy visualized both the oil droplets and the protein distribution. Rheological measurements were used to assess mechanical properties and network formation. Small angle X-ray scattering provided quantitative structural information. Results identified that stable emulsions have a protein layer encapsulating the oil droplets and that excess protein forms irregular aggregates in the aqueous phase. Rheological analysis indicated that the protein aggregates contribute to network formation and give rise to a yield stress which enhances stability. Only for sufficiently high protein concentrations were the emulsions stable. Other samples separated and the upper phases were always similar in emulsion composition regardless of the initial component fractions. This study highlights the dual role of pea protein in emulsions as a dispersed protein network and as interfacial material. Determination of the most favourable emulsion composition provides insight into design of stable emulsions for applications.

Hypothesis

Pea proteins can act not only as interfacial stabilizers of oil-in-water emulsions but also as gelling agents in the continuous phase. Protein gelation, rather than droplet jamming, may be the main mechanism of emulsion stability, providing a physical explanation for the creaminess of high-protein plant-based emulsions.

Experimental

Spin-echo small angle neutron scattering (SESANS) with D₂O/H₂O contrast variation was used to study 15% pea protein dispersions and emulsions with 40–60% rapeseed oil, 7.5% protein at pH 3 to 6.5. SESANS investigates length scales up to tens of micrometres, enabling simultaneous analysis of protein networks and oil droplets without dilution. Complementary small angle X-ray/neutron scattering were used to validate protein aggregate size, and hydration.

Findings

Protein dispersions at neutral pH formed mass fractal networks with small individual building blocks (radius ∼38 Å, hydration ∼70%). Emulsions consisted of oil droplets embedded in these networks, with droplet radii decreasing at higher oil fractions due to an effective higher protein concentration in the continuous phase, creating a denser network. Dispersions and emulsions at lower pH contained aggregated clusters of denatured proteins. These coarse and inhomogeneous networks gave increasing droplet radii at lower pH. Contrast variation enabled the separation of protein and oil droplet scattering, demonstrating that protein gelation rather than droplet jamming is the main mechanism of stability. This gives a physical explanation of the high viscosity of high-protein plant-based emulsions and is promising for these plant materials to be used as gelling agents in food applications.

Global interest in milk alternatives increases rapidly due to health awareness, their allergen-friendliness, and concerns about sustainability. While dairy product microstructure and rheology are widely studied, plant-based alternatives remain less understood, with limited comparative studies of different plant sources and brands. This study uses ultra-small, small and wide-angle X-ray scattering (USAXS, SAXS, WAXS) to analyse structural fingerprints of commercial plant-based milk, yoghurt and cream alternatives versus dairy products. These techniques allow characterization across multiple length scales from large oil droplets and aggregated structures to carbohydrate/protein networks and glyceride crystalline phases. Correlations between intensity and fat (USAXS) and carbohydrate content (SAXS) provide structural insights, while SAXS and WAXS data correlated with solid fat and crystal packing are important for melting behaviour and viscosity perception. Light scattering confirmed fat-content-related size trends and revealed larger structures of non-lipid materials in plant-based samples. The study provides a basis for understanding scattering data where structural fingerprint plots, using colour scales to compare intensity and intensity gradient, allow ready data interpretation that will be beneficial for analysis with artificial intelligence (AI) tools. This approach helps optimize plant-based formulations by connecting structure and functionality and demonstrates the potential of scattering techniques in food structure research and design.