Interaction between whey proteins and lipids during light-induced oxidation

Dairy products are a suitable group for polyunsaturated fatty acid (PUFA) fortification, but are very susceptible to light-induced oxidation. In this study, the combined effect of light and PUFA fortification on the stability of milk proteins and on the formation of protein-lipid complexes was studied. Therefore, emulsions were prepared containing whey proteins and lipids with a different degree of unsaturation (olive, soybean, fish and algae oil). PUFA stimulated the protein oxidation process, even after complete degradation of the photosensitizer. First order reaction kinetics were established for riboflavin and tryptophan degradation, as well as the formation of N-formylkynurenine, a tryptophan degradation product. Oxidation was further characterised by degradation of the essential amino acids lysine, histidine, methionine and tyrosine. Protein-lipid adduct formation, measured fluorometrically and by the protein-bound carbonyl groups, could be linked with a decreased extractability of the unsaturated lipids from the emulsion, but also caused the formation of protein aggregates of high molecular mass.