Tiny, yet impactful: Occurrence and role of micelles and very small oil droplets on lipid oxidation in surfactant-stabilized emulsions

Lipid oxidation is a major factor limiting the shelf-life of food emulsions, and is also a notoriously complex phenomenon. This complexity arises from the involvement of many physical structures present at several scales (1–10,000 nm), of which the smallest ones are often overlooked during emulsion characterization.
In this work, we explored the role of the colloidal structures at the nanoscale (10–200 nm) in rapeseed oil-based emulsions stabilized by different concentrations of a nonionic surfactant. Our hypothesis was that such structures include surfactant micelles, swollen micelles and very small oil droplets, which may have marked implications for lipid oxidation by being important yet underestimated targets of the involved reactions, and/or by transporting reactive molecules. By deploying cryogenic transmission electron microscopy (cryo-TEM), we analyzed the size of very small oil droplets and of surfactant micelles, which are typically overlooked by dynamic light scattering (DLS) analysis when larger droplets are concomitantly present. Their size and oil content were shown to be stable over incubation, but lipid oxidation products were overrepresented in these very small droplets [1]. In a separate set of experiments, we showed that when mixing two emulsions containing ‘clean’ oil droplets and pre-oxidized ones, respectively, relatively hydrophilic and reactive lipid oxidation products, such as 4-hydroperoxy-2-nonenal, equilibrated between oil droplets within 30 minutes. However, such a transfer did not occur for larger molecules such as triglyceride-bound hydroperoxides [2]. This work thus provides important insights regarding the exchange of molecules between oil droplets, or limitations thereof, in relation to lipid oxidation in model O/W emulsions. It also highlights the importance of the fraction of “tiny droplets” for the oxidative stability of such systems.

[1] ten Klooster, S. et al. J. Colloid Int. Sci., 2023, 652, 1994.
[2] ten Klooster, S. et al. 2023, Food Chem., 405, 134992.