Variations in free amino acid fingerprints during quality formation of fresh scent-flavor Oolong tea

To investigate the metabolic alteration of free amino acids (FAA) in growing tea shoots and the production process impact on the chemical fingerprint patterns of FAA, spring shoots from the buds of Tieguanyin, Huangdan, Jinguanyin and Huangguanyin (Camellia sinensis) were harvested and processed into fresh scent-flavor Oolong tea. After pre-column derivation with 1-(quinolin-6-ylcarbamoyl)oxypyrrolidine-2,5- dione (AQC), FAA in samples of tea shoots and from the whole processing stages were determined with high-performance liquid chromatography. When the peak areas of the defined 18 FAAs were standardized or preferably transformed into natural logarithm, consistent visualized pattern recognitions would be obtained by either cosine similarity analysis, or nearest-neighbor cluster analysis based on Euclidean distance or principal component analysis. The samples could be divided into two groups according to the analysis. One was made up of Tieguanyin and Huangdan, and the other was Jinguanyin and Huangguanyin. Spring shoots could be distinguished well from the WIP (work in process) of fresh scent-flavor Oolong tea through their chemical patterns. The contents of 8 FAAs including theanine, glycine, glutamate and others altered obviously during shoot growth and remained rather low in banjhi buds with two to four leaves. Contents of the main FAAs did not change greatly during the process of fresh scent-flavor Oolong tea, and slightly higher content could be obtained in samples of semi-made tea than fresh leaves. These facts indicate that the contents and composition of FAAs changed more significantly during shoot growth than those in the process of fresh scent-flavor Oolong tea, and the results can provide effective references for raw material selection of fresh scent-flavor Oolong tea and its in-process quality control.