Investigating roles of oil pre-oxidation on emulsion stability: Dispersed-phase and interfacial-film both matter

The inadvertent flow of mildly/moderately oxidized oil into production critically causes quality uncontrollability in food emulsions. In this paper, we explored the effects of oil pre-oxidation (oxidized before emulsification) on the emulsion stability and interfacial properties of two protein emulsifiers (myofibrillar protein, MP; sodium caseinate, SC). These effects were further found to be different dependent on the proteins native structural and physicochemical properties. For MP with a highly structured fibrous conformation, the adsorption capacity, diffusion and rearrangement rate, and dilatational elastic modulus were decreased (by at least 5.74%, 16.25%, 11.21%, 10.04% respectively) at the oxidized-oil/water interface, which were mainly explained by the increased oil polarity and the generated interfacial-active oxidation products that impeded the interfacial unfolding and protein-protein interactions. With an appreciably high adsorbed amount (adsorption ratio >90%), the MP's oxidized-oil emulsion stability was largely determined by the interfacial properties, and thus significantly decreased with oil oxidation (even at the mild oxidation level). In contrast, for SC with a disordered structure, the rearrangement rate and dilatational elastic modulus were similarly decreased (by at least 14.61%, 12.56% respectively) at the oxidized-oil/water interface, while the adsorption capacity and diffusion rate were increased (by at least 5.40%, 13.16% respectively) possibly due to the enhanced hydrogen bonding between the more hydrophilic SC and the higher polarity oil. With a significantly lower adsorbed amount (adsorption ratio <10%), the SC's oxidized-oil emulsion stability was mainly decided by the oil inherent properties but not interfacial performance, which thus significantly increased with oil oxidation (even at the mild level) due to the growing oil polarity and density. These findings highlight that mild oil pre-oxidation could have critical impacts on emulsion stability, and the mechanism exploration provides scientific guidelines for the quality control and rational formulation of labile unsaturated oil-based emulsion foods (e.g. fat substituted emulsified meat products).