Investigation of free amino acids, bioactive and neuroactive compounds in different types of tea and effect of black tea processing

This study aimed to investigate free amino acids, bioactive amines, and tryptophan derivatives in kynurenine pathway in white, green, oolong, black and pu-erh tea samples. Effects of black tea processing stages (withering, rolling, fermentation and drying) on these compounds were also investigated. Analyses of free amino acids, bioactive and neuroactive compounds were performed by using tandem mass spectrometry. Total free amino acid content of tea increased in withering stage while decreased in drying stage. Although all tea samples were found to contain gamma-aminobutyric acid (GABA) and kynurenic acid, only pu-erh tea contained tyramine and histamine. Furthermore, dopamine was detected in some white (0.8 mg/kg) green (0.3–0.6 mg/kg) and oolong (0.3–0.6 mg/kg) teas. Serotonin, kynurenine and tryptamine were not found in any tea samples. Concentrations of kynurenic acid in white, oolong, green, black and pu-erh tea were found to be within the range of 1.1 ± 0.3 and 13.0 ± 0.9 mg/kg. GABA and kynurenic acid concentration of tea increased 33% and 53% after withering, respectively. Dried tea contained lower GABA and kynurenic acid than withered tea. Tea could have health-promoting effects due to containing neuroactive compounds such as GABA, kynurenic acid and dopamine and, processing stages of tea affects the level of these compounds.