The processing of tea has many stages depending of the sought after finished tea type. For most teas, the process of preparing tea leaves for the cup starts with picking. However, for some Japanese Green teas, the plants are shaded for different lengths of time in order to increase the amino acid and chlorophyll levels in the leaves before they are picked. This is a significant factor in tea processing, but it is only employed for specific teas.
Picking and Withering
Once leaves are ready to be picked, once the new growth has a bud and few new leaves, those leaves and buds are ready to be picked. Immediately after picking, the leaves start to wither. This is the act of losing water content and passive oxidation occurs during this process. However, different withering techniques are employed in order to extract the water from the leaves to prepare them for further processing. Withering techniques include sunning the leaves on bamboo mats or with temperature and moisture controlled air.
Once enough of the moisture in the leaves has been extracted, the leaves are ready for either shaping or rolling–depending on the desired finished tea type. During these processes is where oxidation is either encourage or prevented. For Green teas and White teas, the oxidation of the leave leaves will be prevented or limited. For Black teas and Oolongs, oxidation will be encouraged until the desired levels are reached. Oxidation is encouraged by rolling and bruising the leaves so that the chemicals throughout the tea are exposed to the oxygen in the air. Oxidation is responsible for converting Catechins, which are Polyphenols in tea, into Theaflavins and Thearubigins, other types of Polyphenol. Theaflavins give tea sharp and bright flavors with a yellow color, while Thearubigins give it depth and its red color. Chlorophylls are also subjects of change in the oxidation process. Here, Chlorophylls break down contribute to the black or brown colors of dry tea leaves that have been oxidized.
All of these reactions affect the overall taste and appearance of the finished tea product, showing us just how important the Oxidation process is for finished tea. This is why Oxidation is started, stopped, or prevented for different types of desired tea. When oxidation is prevented, the Catechins in the tea do not convert to the Polyphenols that offer the darker colors and robust flavors. Instead, the leaves keep their green color and, and the flavors of the tea have more fresh, vegetal characteristics. Oxidation is stopped or prevented by the method of Fixing the leaves. This method can be understood as heating the leaves in order to dry them. This is commonly done by Pan Firing, Baking, Heated Tumbling, or by Steaming.
Leaf drying is the last step before the tea is sifted and ready for brewing. Most drying methods do not change the flavor of the tea, but charcoal roasting is a method of drying that does. This offers the tea a more roasted, burnt flavor. This roasting is typical in darker teas, such as Wulongs, and it is particularly desirable on cold days!
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