Effects of green-keeping treatment on the functional constituents in flower tea of Eucommia ulmoides

Browning of the male flower of Eucommia ulmoides, which occurs easily and frequently after its collection from the living tree, has deterious effects on the quality of the flower tea since the functional constitutes are mostly impaired, which nowadays has become a bottleneck issue in the manufacture of Eucommia flower tea. Therefore the green-keeping procedure becomes one of the key steps in the production of the flower tea. The essence of green-keeping treatment is to protect functional constituents such as chlorophylls and phenols in the flower from degradation, in order to improve the sensory quality of the flower tea. In this study, different green-keeping treatments, including addition of salts (metal ions), ascorbic acid, citric acid, as well as steaming and quick boiling were compared to examine their effects on the functional constituents and sensory quality of the flower tea. The results showed that functional constituents were well maintained and the sensory quality was satisfactory after the flower was treated by steaming for 30-60 s, and spraying with 0.05% zinc sulfate or 0.1% citric acid. Processing procedures of making Eucommia flower tea were summarized systematically based on our previous study: the freshly picked flowers were air-dried, followed by green-keeping treatments that were selected from the current study, enzyme-deactivation, and microwave drying. This study would provide useful information for the tea industry in the aspect of maintaining the qualities of the flower tea both externally and internally.