Dried flowers, shrunk plants and beauty long past – the concept of “withering” or “wilting” tends to generate a negative image in our minds. But withering plays a central role in tea production: instead of drying the leaves, it softens them. We’ll explain the chemical processes behind this, and reveal why this also increases the caffeine content, among other things.
After the leaves of the tea plant have been harvested, they are processed further. Traditionally, tea leaves go through five steps before they can produce a tasty infusion. The process starts with withering. In a similar manner to cut flowers or freshly mowed grass, tea leaves start withering as soon as they have been plucked from the tea plant. In professional tea production, however, withering is done on purpose and is controlled by influencing temperature and humidity.
In times gone by, withering was often done in the open air. The tea leaves were spread out on bamboo mats or sheets. The process has now been modernised and usually takes place in factories. The freshly picked leaves are spread out evenly on grids in withering troughs that are 25 to 30 metres in length, and are aerated from various directions using large ventilators. The process can take an entire day, depending on the ambient temperature and humidity.
The leaves are weighed twice to determine whether the withering process can be ended: first when the fresh leaves enter the factory and then again 10 to 20 hours later. While fresh tea leaves contain 73 to 80 percent water, the withering process causes them to lose about 30 percent of their moisture and thus also some weight. This makes the leaves go limp, but also more supple. Why is this the case? The withering process causes the membranes of the tea leaves to open, making them more permeable and at the same time softer. The advantages of this are primarily of a practical nature: During the production step that follows, the leaves can be more easily rolled and do not break. It also means that the machines remain clean and fully functional. If the leaves were still too moist they would stick to the machines and, in the worst case, bring them to a standstill.
The withering time can also be used to store the fresh leaves to bridge the time to subsequent processing. During so-called “chemical withering”, the leaves are not aerated on an ongoing basis but are subjected to cool air from time to time. No chemicals are added to the leaves in this process – even if the name suggests otherwise. Instead, the word “chemical” refers to the chemical changes that occur naturally in the leaves. Because: It’s not just the structure of the leaves that changes, it’s also their composition. The caffeine content also increases, particularly in the case of black tea. The more effective the withering process, the more invigorating the finished tea product.
New flavours also develop over the course of time. This is because many of the chemical compounds in the leaves disintegrate into volatile compounds and aromas during the withering process. So, if you were to take a walk through a tea factory, you would experience a pleasant fragrance. Skilled tea producers therefore know when the withering process is complete – not only because of the weight of the leaves but also because of their sense of smell.
Withering plays a key role, particularly in black tea, but also impacts on the unmistakeable aroma of other types of tea. Pu-Erh tea such as the bitter-sweet DARK CHOC is only withered for a short period of time, and green tea often not at all. This is reflected in the taste, since the chlorophyll content of the leaves is reduced during withering. So: the shorter the withering time, the greener the appearance and the more grassy the aroma. If the leaves are withered for a longer time, the colour in the cup becomes darker and the aromatic compounds more intense, giving rise to a sweet, floral infusion.
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