The moment a tea leaf is plucked from the tea plant, it begins to wilt naturally, a process we call withering. But once the tea leaves reach the processing facility, this process is controlled by the tea producer. The purpose of a controlled wither is to prepare the leaves for further processing by reducing their moisture content and to allow for the development of aroma and flavor compounds in the leaves. Controlling the withering process means closely monitoring humidity, temperature and air-flow over time. A controlled wither can occur outside with tea leaves laid out gently on bamboo mats or tarps, or indoors in troughs with forced air. The air may be heated to speed up the process if necessary. Great care is also given to the density of the withering leaves to ensure that they wither evenly. The withering process is complete once the tea leaves have achieved a desired percentage of water-loss. This is determined by the final weight of the tea leaves after withering or by the flaccidity and changes in the aroma of the leaves.
During withering, the moisture content in the leaf is reduced by about one-third to one-half, making the leaf flaccid and pliable. This prepares the leaf for further processing, usually shaping and rolling. On the chemical side of things during withering, chlorophyll in the leaf begins to degrade, caffeine levels slowly rise,, flavor and aroma volatiles develop in the leaves and grassy aromas dissipate. Since the leaves are cut off from their supply of energy, they also begin to break down their stored carbohydrates for use as energy. The loss of moisture also causes the cell walls to break down, initiating polyphenol oxidase and peroxidase activity – the reaction known as oxidation. The longer the wither, the more aroma and flavor compounds develop in the leaves. This is because during the withering process, many of the chemical compounds in the leaves degrade into volatile compounds. In fact, many tea makers use their sense of smell to tell when the withering process is complete. If the leaves are withered too long, polyphenol and peroxidase activity will cease due to dehydration. Once withering is deemed complete, processing continues.
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