Sensitive Enzymatic Method for the Quantification of Theanine, a Principal Umami Component of Commercial Tea Beverages

Theanine (γ-glutamylethylamide) is the major “umami” component in tea and exhibits various favorable physiological effects in mammals. To quantify theanine contained in commercial tea beverages, a coupled reaction of γ-glutamyltranspeptidase from Pseudomonas nitroreducens NBRC 12694 and amine dehydrogenase from Paracoccus denitrificans NBRC 12442 was investigated. This method measures changes in the absorbance of 2, 6-dichloroindophenol at 600 nm and is fast (less than 10 min, including the time required for sample pretreatment). The calibration curve was linear between theanine concentrations of 0 − 200 μM, with R2 = 0.998. Theanine concentrations in samples of commercial tea beverages (black tea, green tea, and oolong tea) obtained using this method were similar to those obtained using the pre-column derivatization HPLC method. The proposed colorimetric assay for theanine determination is a reliable and timesaving method and can be used in food quality control and biological research.