Modified Peptides as Indicators for Thermal and Nonthermal
Reactions in Processed Milk
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Abstract
Site-specific
relative quantification of β-lactoglobulin
modifications in heated milk and dairy products was performed to determine
their thermal and nonthermal origins and to evaluate marker candidates
for milk processing. Therefore, formation kinetics of 19 different
structures at 26 binding sites were analyzed by ultrahigh-performance
liquid chromatography–tandem mass spectrometry with multiple
reaction monitoring (UHPLC-MS/MS/MRM) after specific protein hydrolysis.
The results indicate that (i) site-specific analysis of lactulosyllysine
may be a more sensitive marker for mild heat treatment than its overall
content; (ii) <i>N</i><sup>ε</sup>-carboxymethyllysine,
N-terminal ketoamide, and asparagine deamidation are of thermal origin
and may be good markers for rather intensive heat treatment, whereas <i>N</i><sup>ε</sup>-carboxyethyllysine reflects thermal
and nonthermal processes; (iii) the relevance of methylglyoxal-derived
arginine modifications is low compared to that of other modifications;
(iv) oxidation of methionine and cysteine is a rather weak indicator
of thermal impact; and (v) the tryptophan modifications formylkynurenine
and kynurenine are of nonthermal origin and further degraded during
processing