3-chlorotyrosine formation versus other molecular changes induced by hypochlorous acid in proteins: A study using dairy proteins as a model

• The carbonyl content significantly increased upon HOCl oxidation.
• Lys, Tyr, Trp and Met were the most sensitive sites of oxidation within proteins.
• HOCl induced molecular changed proved to be pH dependent.
• Protein carbonyls for both proteins and SH groups (of caseins) were pH independent.
• 3-chlorotyrosine is a potential marker for HOCl induced damage in food proteins.

General markers of HOCl induced changes in food proteins were evaluated and compared with the specific indicator 3-chlorotyrosine using dairy proteins at various oxidant/protein ratios and at different pHs. Protein aggregation was more pronounced at alkaline pH and already observed at a ratio of 0.3 mmol HOCl/g protein. Tryptophan, methionine, tyrosine and lysine in whey proteins showed more degradation at pH 8.0, whereas methionine and histidine in caseins were more vulnerable for degradation at pH 5.8. Total thiol content was strongly decreased, up to 75% at 4.8 mmol HOCl/g whey protein with more degradation at acidic pH while in caseins it remained constant. The available lysine content notably decreased upon HOCl treatment and was more pronounced at pH 8.0. The levels of 3-chlorotyrosine increased as function of the oxidant/protein ratio and reached a maximum at 2.8 mmol HOCl/g whey and casein proteins. The 3-chlorotyrosine concentration was observed the least at pH 8.0, while the increase in protein carbonyls depended only on the HOCl/protein ratio, but not on the pH. It is concluded that 3-chlorotyrosine provides a more accurate assessment of the impact of HOCl damage on proteins in foods.