The kinetics of the reaction between epicatechin and various carbonyl compounds typically formed in cooked and stored foods were evaluated in model systems at pH 7.4 and 37 degrees C, and the corresponding reaction products in stored ultrahigh temperature (UHT) milk-added epicatechin were identified by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). The rate constants for the reactions of carbonyl compounds with epicatechin decreased in the following the order: methylglyoxal; 1.6 +/- 0.2 M-1 s(-1) > glyoxal; (5.9 +/- 0.3) X 10(-2) M-1 s(-1) > 5-(hydroxymethyl)furfural; (4.0 +/- 0.2) X 10(-2) M-1 s(-1) > acetaldehyde; (2.6 +/- 0.3) x 10(-2) M-1 s(-1) >= phenylacetaldehyde; (2.1 +/- 0.2) x 10(-2) M-1 s(-1) >= furfural; (4.3 +/- 0.1) x 10(-3) M-1 s(-1) > 2-methylbutanal and 3-methylbutanal; similar to 0 M-1 s(-1). Reaction products generated by epicatechin and methylglyoxal, glyoxal, S-(hydroxymethyl)furfural, and acetaldehyde were detected in UHT milk samples by incubating milk samples with epicatechin at 37 degrees C for 24 h. The lack of reaction between epicatechin and phenylacetaldehyde, furfural, 2-methylbutanal, and 3-methylbutanal in stored UHT milk may be due to their slow reaction rates or low concentration in stored UHT milk. It is demonstrated that epicatechin traps 5-(hydroxymethyl)furfural, acetaldehyde, glyoxal, and methylglyoxal and may thereby reduce off-flavor formation in UHT milk during storage both by trapping of precursors (methylglyoxal and glyoxal) for off-flavor formation and by direct trapping of off-flavors.