Co-adsorption and displacement behavior of phenolic acid derivatives at a β-lactoglobulin-stabilized oil-water interface and the interactions of both solutes

Interfacial proteins interact non-covalently or covalently with small molecule food ingredients present in emulsions such as partly interfacially-active phenolic acid derivatives (PCD). However, these interactions might influence the interfacial behavior of the proteins depending on pH value and chemical-structural properties of both interaction partners. The aim of this study was to analyze the impact of pH 6.0 or 9.0 and the chemical-structural properties of selected, closely related PCD on their co-adsorption and displacement behavior at beta-lactoglobulin (beta-Lg)-stabilized oil-water interfaces. Additionally, their interactions with the protein were characterized. At pH 6, hydrophilic monomeric PCD adsorbed predominantly at the interface and were dissolved primarily in the aqueous phase as being localized by fluorescence microscopy. Hydrophobic monomeric PCD were predominantly adsorbed at the interface or migrated into the oil phase, resulting in partial displacement of the interfacial protein due to conformational changes. For more polar PCD, a formation of hydrogen bonds with the interfacial protein was detected. Selectively consecutive washing steps enabled stepwise breakdown and quantification of the different non-covalent interactions. Electrostatic repulsion and steric hindrance inhibited polymeric PCD from adsorbing at the interface at pH 9, which was shown with pendant drop analysis. These PCD mainly interacted hydrophobically with the interfacial protein film. Less covalent adducts with the protein were found at pH 6 compared with pH 9. Additionally, PCD preferentially polymerized under alkaline conditions instead of reacting covalently with the protein, which was analyzed with mass spectrometry.