The moment a tea leaf is plucked from the tea plant, it begins to wither or wilt. The amount of this unavoidable, uncontrolled wither the leaves experience depends on how much time elapses from the time the leaves are plucked in the field until the leaves reach their destination and they are further processed and the quality of care that is given to them during this time. Withering is also a controlled process used in tea production. Tea producers use a balance of moisture and air-flow during a controlled wither to moderate the reduction of moisture in tea leaves until it reaches a desired level and to achieve other physical and chemical goals.
The physical goal of withering is to reduce the moisture content in the leaf, making the leaf flaccid and pliable, which prepares the leaf for shaping and rolling. The chemical goal of withering is to allow the aroma and flavor volatiles to develop in the leaves and for grassy aromas to be released. The longer the wither, the more aroma and flavor compounds develop in the leaves as many of the chemical compounds in tea degrade into volatile compounds. Since the leaves are cut off from their supply of energy, they also begin to break down their stored carbohydrates for use as energy. Loss of moisture causes cell walls to break down, initiating polyphenol oxidase and peroxidase activity – the reaction known as oxidation. Chlorophylls also begin to degrade at this time.
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 in the shade, or indoors in troughs with forced air. The air may be heated to speed up the process. The withering process is complete once the tea has achieved its desired percentage of water-loss. Great care is also given to the density of the withering leaves to ensure that they wither evenly.
Photo source: http://www.flickr.com/photos/litrate/