Changes in Morphological and Functional Characteristics of Tea Leaves During Japanese Green Tea (Sencha) Manufacturing Process

Japanese green tea (Sencha) is mainly manufactured by six processing steps: steaming, primary rolling, rolling, secondary rolling, final rolling, and final drying. The morphology and microstructure of tea leaves were observed, and total polyphenol content (TPC) and total antioxidant activity indexed by FRAP and DPPH were evaluated in tea leaves and infusions during processing. Folds or wrinkle-like microstructures on the tea leaf surface increased during processing, particularly during the primary rolling (PR) step. Leaf shape also mainly collapsed during the PR step. In tea leaves, the values of TPC, DPPH, and FRAP during processing ranged from 189 to 283 mg GAE/g dry matter (DM), 2670–4363 μmol FeSO4/g DM, and 1519–2327 μmol TE/g DM, respectively, whereas in the hot water extraction as a tea infusion decreased and ranged from 14 to 135 mg GAE/g, 391–2961 μmol FeSO4/g DM, and 219–1049 μmol TE/g DM, respectively. These functional attributes increased mainly from steaming (ST) to rolling (RL) steps. The results indicated that the processing conditions at each step, especially at the steps for ST, PR, and RL, influenced the apparent quality of the final tea leaf products and the functional properties of tea leaves and infusions.