What makes oil-in-water emulsions with pea protein stable? The role of excess protein in network formation and yield stress development

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.