Fluorescence spectroscopy as a novel approach for the assessment of myofibrillar protein oxidation in oil-in-water emulsions

The oxidation of oil-in-water emulsions (37 °C/10 days) containing increasing levels of myofibrillar proteins (MP) (0.5%, 1% and 2% on the basis of lipid content) was investigated. Protein oxidation was assessed by measuring the loss of natural tryptophan fluorescence and the increase in fluorescent protein oxidation products (FP) using fluorescence spectroscopy. Lipid oxidation was simultaneously analysed by measuring the increase of conjugated dienes (CD) and hexanal. The oxidative degradation of tryptophan residues occurred as an early event in MP oxidation whereas FP formed later as secondary protein oxidation products. The shift of the tryptophan maximum fluorescence emission wavelength during oxidation provided information about the location of oxidising tryptophan residues. Emulsions with higher concentrations of MP displayed higher tryptophan fluorescence and yielded a higher amount of FP. MP acted as inhibitors of lipid oxidation because emulsions with higher MP contents contained lower levels of CD and hexanal. Significant negative correlations were found between tryptophan fluorescence and CD, reflecting the timely interaction between primary lipid oxidation products and protein oxidation. Both spectrophotometric techniques were useful although the loss of tryptophan fluorescence is more reliable since it is a specific measurement which is not affected by the presence of other fluorescent protein oxidation products.