Changes in volatile compounds upon aging and drying in oolong tea production

Background: Long-term storage (aging) with periodic drying of fresh oolong tea gives rise to so-called old oolong tea. Alteration of aroma compounds is expected when a fresh oolong tea is converted into an old one, as the two teas smell drastically different. The aim of this study was to compare the volatile compounds in fresh and old oolong teas. Results: Significant differences were observed between the volatile compounds in fresh and old oolong teas. This observation suggested that long straight chains of alcohols and acids were putatively decomposed while shorter-chain acids, their amide derivatives and many nitrogen-containing compounds were generated during the tea conversion processes. The overall patterns of volatile compounds observed in five different preparations of old oolong tea were fundamentally identical. This consensus pattern was different from that observed in oolong tea either stored for more than 10 years without drying or prepared at relatively low temperatures and short baking time. Conclusion: Characteristic aroma nitrogen-containing compounds, including N-ethylsuccinimide, 2-acetylpyrrole, 2-formylpyrrole and 3-pyridinol, were consistently found in the examined old oolong teas. These compounds might be regarded as typical constituents at least for a certain kind of old oolong tea.