Changes in the Physicochemical Properties of One Bud and Two Leaves at Different Leaf Positions during Congou Black Tea Processing

Fresh leaves of Fuding Dabai tea (Camellia sinensis (L.)) consisting of one bud and two leaves were used to manufacture Congou black tea, and the changes in the physicochemical properties and microstructure of the bud and the two leaves were explored during the manufacturing process. It was found that during the rolling process, the color of the bud and the leaves gradually turn from green to red, but the degree of color change of the lower leaf was lower than that of the bud and the upper leaf. Besides, optical microscopic (OM) and transmission electron microscopic (TEM) analysis showed the structures of mesophyll cells and chloroplasts in both leaves were destroyed in the rolling process, indicating the beginning of fermentation. The chemical composition analysis showed that during the Congou black tea manufacturing process, the contents of tea polyphenols and catechins decreased, the contents of theaflavins and thearubigins increased at first, then decreased and finally leveled off, and the content of theabrownins kept increasing; the contents of all these compounds changed most during rolling. There was no significant difference in the oxidation degree of total catechins, epigallocatechin, epigallocatechin gallate, epicatechin gallate and theaflavins between the two leaves (P > 0.05), indicating that their fermentation degrees were similar. During the entire process, the contents of chlorophyll a, chlorophyll b and total chlorophyll decreased and the contents of pheophytin and pheophorbide increased for the bud and the leaves. The content of chlorophylls in the lower leaf was always much higher than that in the upper leaf (P < 0.05). This study provides a theoretical basis for processing high-quality congou black tea by improving the utilization rate of fresh tea leaves.