Withering is the first mechanized stage of tea production once it reaches the factory. The picked leaves are laid out in long wooden or concrete troughs with a mesh screen at the base that allows air to pass though. A large fan is located at one end of the trough, which blows air from underneath the leaves through the mesh. Over a period of around 9-16 hours (depending on the humidity and temperature of the environment), the leaf will begin to reduce in moisture content from approximately 74 and 83% moisture content at the time of picking (the leaf at this stage is very stiff and waxy) to 68 to 70% moisture content when at the end of withering (Here the leaf will be softer and more pliable). For best quality, an air temperature of a steady 25-30°C is ideal, beyond 35°C the quality falls as leaf is overheated.
A study was conducted to see the effect of different moisture content of withered leaf on the activity of oxidative enzymes polyphenoloxidase (PPO) and peroxidase (PO) and on the formation of theaflavins (TFs) and thearubigins (TRs). Significant reduction in PPO and PO enzymes, TFs, TRs and theaflavin digallate equivalent was observed with the decrease in moisture content of withered leaf. Te best range appears to be 68-70% moisture content depending on the specific tea cultivar.
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.
At certain times, such in the advent of rain near the time of harvest, it might be necessary to remove the excess moisture form the leaves using hot air during withering. Accelerated withering in this way is generally not desirable, as this process evaporates some of the desirable volatile taste and flavor components in the leaf, which results in less character expressed in the tea.
As the moisture is removed, the leaf will continue to become softer and eventually become quite easy to bruise, so care is taken during this process not to handle the leaf too often. Generally the leaves are turned 2 to 3 times to ensure even withering of all of the leaf in the withering trough. The colour in the leaf begins to change as the green Chlorophyll begins to degrade. The softening of the cells that occurs during withering also supports a uniform distribution of polyphenol oxidases which will later be used to oxidise the leaf during the oxidization process.
It is possible to under-wither or over-wither the leaf; both will have a negative impact on the character and flavor of the made tea. If the tea is under withered then the leaf will remain staff and there will be insufficient friction to adequately roll the leaf that will in turn result in poor oxidization. If the tea is over-withered, there will be a large percentage of dust and flavor and character may be lost. Well-withered tea allows for effective rolling, with a lot of the leaf being ruptured or twisted allowing for the release of the tea liquor form the vascular bundles contained within the leaf.