Effect of Air Circulation in Low-temperature Vacuum Drying on Quality of Tieguanyin Tea

To improve the oolong tea quality and conserve energy in processing, the low-temperature vacuum dehydration was supplemented with forced air circulation for evaluation. Various means of air circulation were implemented to study their effect on the biochemistry and quality of the dried Tieguanyin tea. It was found that the external air circulation (A₂) produced higher amounts of water extracts (37.11%), tea polyphenols (25.22%), amino acids (1.83%) and flavonoids (6.05 mg·g^-1) in the tea than the close-loop air circulation (A₁) or without a forced air circulation (CK). The differences on the contents of polyphenols, amino acids and caffeine were not significant among the samples steeped for 3 times. The dried tea leaves were green, and the brewed tea light yellowish green in color. Sensory evaluation on the tea samples made by the various methods gave A₂ the highest score at 89.27±0.21, followed by A₁ at 86.27±0.26, and CK the lowest at 85.31±0.18.It appeared that, when the low-temperature vacuum drying was carried out with an external air circulation, the quality of Tieguanyin tea could be improved.