Formation of Phenylacetaldehyde from l-Phenylalanine Mediated by Enzymatic Oxidation of (−)-Epigallocatechin and (−)-Epigallocatechin Gallate during Black Tea Fermentation

Enzymatic oxidation during black tea processing participates in the formation of aromatic aldehydes from amino acids. The present study investigated the effects of (−)-epigallocatechin (EGC) and (−)-epigallocatechin gallate (EGCG) enzymatic oxidation on phenylacetaldehyde formation from l-phenylalanine. Results showed that the phenylacetaldehyde yield increased over 4-fold with the addition of polyphenol oxidase and EGC/EGCG, while excessive addition of EGC and EGCG inhibited further production. Through liquid chromatography tandem mass spectrometry, six EGC/EGCG–l-phenylalanine adducts, including aminated EGC/EGCG, amino-linked condensation products, and their decarboxylated derivatives, were identified and kinetically characterized. Their structures and temporal variations supported a plausible novel pathway involving reduction, decarboxylation, oxidation, and hydrolysis of Schiff base intermediates to form aminated EGC/EGCG and phenylacetaldehyde. Model fermentation involving tea leaves showed that exogenous addition of l-phenylalanine increased the phenylacetaldehyde content dose-dependently and promoted adduct formation. This research offered new insight into tea aroma formation driven by catechin oxidation.