Formation of di-Tyrosine in pasteurized milk during shelf storage.

[Display omitted] • Di-Tyr is a protein crosslink produced by light-induced oxidation. • Formation of di-Tyr is detected in milk after 1-day light exposure. • Immune gold labelling evidenced presence of di-Tyr on casein micelles. • Formation of di-Tyr may change casein susceptibility to proteolysis. • No protein aggregation was observed in milk after 14-day light exposure. Formation of the protein crosslink di-Tyrosine was studied in PET-bottled pasteurized milk exposed to fluorescent light in a commercial display cabinet. An HPLC method with fluorescence detection was developed and intra-laboratory validated using pure di-Tyrosine synthesized on purpose. Di-Tyrosine was detected after 1-day lightening and increased up to 7 days, reaching around 250 and 320 µg/g protein in whole and partly skimmed milk, respectively. Afterward, a progressive decrease occurred. By transmission electron microscopy with specific immune gold labelling, presence of di-Tyrosine was observed for the first time on the surface of casein micelles of lightened milk. The crosslink formation, however, did not bring to protein aggregation phenomena detectable by laser light scattering measurements. Exposure to light also induced degradation of riboflavin and decrease of yellowness index. Di-Tyrosine proved to be a suitable indicator to evaluate the progress of protein oxidation in pasteurized milk during storage on the market. [ABSTRACT FROM AUTHOR]