Micro-Raman and fluorescence spectroscopy for the assessment of the effects of the exposure to light on films of egg white and egg yolk

In this work, non-destructive techniques were employed for the spectrofluorimetric and micro-Raman differentiation of solid films of egg yolk and egg white as a function of exposure to light. Using both techniques it is possible to discriminate between the samples on the basis of composition and follow some chemical changes involved during the light-ageing of egg proteins and fatty acid esters. In egg white, fluorescence emission spectra highlighted the presence of multiple fluorophores, with an effective broadening of the fluorescence emissions following light exposure, ascribed to the partial oxidation and depletion of tryptophan; in the Raman spectrum of dark-aged egg white a peak at 758 cm−1 is ascribed to the vibrational mode of tryptophan, which is completely absent in the spectrum of samples exposed to light. Changes in the fluorescence of egg yolk following exposure to light are ascribed to the degradation of amino acids, cross-linking between amino acids and the formation of oxygenated triacylglycerides. These results are corroborated with Raman spectra of the light-exposed samples, which are associated with wavenumber shifts with respect to the dark-aged sample for the Amide I and for the characteristic carbonyl vibration at 1744 cm−1 ascribed to the oxidation of fatty acid esters.