The reactivity of hexanal, (E)-hex-2-enal, 4-hydroxyhex-2-enal, and 4-hydroxynon-2-enal in oil-in-water emulsions and their respective compartments, in the presence and absence of protein, was studied at 40 °C. In aqueous buffer, hexanal oxidized to hexanoic acid. In the presence of protein, an additional loss occurred, presumably as a result of adduct formation with cysteine. Similarly, (E)-hex-2-enal oxidized to (E)-hex-2-enoic acid in aqueous buffer, and the results suggested that this acid is also able to form adducts with proteins. 4-Hydroxyalk-2-enals showed the highest reactivity in all models evaluated. Especially in protein-containing systems, they were not detectable anymore or their initial concentration was seriously reduced. 4-Hydroxynon-2-enal was the most reactive of the substances studied. The reactivity of the aldehydes was influenced by their partition within emulsions, which was remarkably not correlated with their hydrophobicity. These findings need to be considered when using these aldehydes as lipid oxidation markers in foods.
Keywords: aldehydes; lipid oxidation; oil-in-water emulsions; whey protein.