Quantifying the distribution of proteins at the interface of oil-in-water food emulsions

Emulsifiers play an essential role in ensuring the physiochemical stability of food emulsions. In the case of mayonnaise, proteins contained in egg yolk act as emulsifiers. Here, we employed stochastic optical reconstruction microscopy (STORM) to localize proteins at the oil/water droplet interface using fluorescently labeled antibodies. To quantitatively analyze the distribution of proteins, we first simulated homogeneous and heterogeneous distributions. We then implemented the relative position distribution (RPD) analysis to extract the histogram of relative distances between all neighboring localizations. By analyzing the local maxima of the histogram, we could classify distributions at droplet interfaces as homogeneous, partially heterogeneous, and heterogeneous. The model fitting over the RPD histogram using a 2D probability function further provided a localization precision amplitude consistent with the analysis of the local maxima. As a model system for mayonnaise, we used emulsions prepared with combinations of phosvitin, phospholipids, apolipoprotein B (apoB), and sodium dodecyl sulfate (SDS) as emulsifiers. The binary phosvitin/SDS model emulsion showed a partially heterogeneous distribution of phosvitin around the droplets. The ternary phosvitin/phospholipid/SDS and apoB/phospholipid/SDS emulsions showed increased heterogeneity of phosvitin and apoB. Quantification of heterogeneity at droplet interfaces may provide insights in factors determining the physical and chemical stability of emulsions.