Dark tea is a unique type of tea in China, which was named after the black appearance of finished tea products. It is fermented by a symbiotic association of bacteria and fungi. In China, it is usually called ‘Border-Sale Tea, as tea is literally sold on borders after commonly compressed into forms of bricks in western minority areas of China. The original dark tea was formed "in the cabin, on the horse’s back” during transportation. During the Tang and Song Dynasties, steamed green tea was produced in large quantities. Owing to the exposure to wind, rain and heat along the road, the tea underwent chemical changes under elevated temperature and moisture, and gradually reached a quality that is completely different from that of green tea. The volume production and sales of dark tea in the world is ranked just below black tea and green tea.

Dark tea is mostly produced in Hunan, Hubei, Yunnan, Guangxi, and Sichuan provinces. The raw materials for making dark tea and drinking habits vary geographically, leading to unique product forms and quality characteristics among places. Dark tea requires tea flush that has high maturity and is older than green tea and black tea. During its processing, there is a special step called “wet-piling” after fixing and twisting. The stimulating and astringent substances such as flavonoids, polyphenols and alkaloids in tea undergo oxidative polymerization and hydrolysis, which makes the dark tea mellow, lack astringency, and become enriched with special aromas such as aged-aroma and fungus flower aroma. The infusion color is yellow orange or red orange, and the brewed leaves are soft.

 Dark tea processing is divided into fixing, twisting, wet-piling, and drying steps, among which wet-piling is the key process for the formation of dark tea characteristics. Because the fermentation process is performed after fixing, dark tea is also known as “post-fermentation tea”, and because the fermentation process requires microorganisms, it is referred to as “microbial fermentation of tea”.

Fixing

The fixing principle of dark tea is very similar to that of green tea, as both of them use high temperature to inhibit enzyme activity. However, because the plucked tea flush of dark tea is relatively older and features lower water content, it is necessary to spray some water on the fresh leaves before fixing, which is conducive to even and thorough fixing. There are two fixing methods: manual fixing and mechanical fixing. The mature fresh tea leaves lead to a high cellulose content and a thick cuticle, hence high temperature and quick stir-fry are used for fixing. For manual fixing, iron pans with a diameter of 80-90 cm are commonly used. The pan temperature is generally 280-320℃ (536-608°F), the killing time is 2 minutes, and 4-5 kg of fresh tea leaves are used each time. At present, the roller fixing machine is commonly used, and the operation method and fixing degree are similar to green tea.

 Rolling

Two procedures are involved in the twisting process: initial twisting and re-twisting. Initial twisting is carried out when tea leaves are still warm after fixing, which makes the leaves form into a rudimentary strip shape, and the tea sap is squeezed out of the cells. Re-twisting constitutes curling the tea strips tightly again after wet-piling.

Wet-piling

Wet-piling is a unique and key process in the primary processing of dark tea. The site of piling should be under a room temperature of above 25℃ (77°F) and a relative humidity of about 85%. The preliminary twisted tea dhool should be immediately piled up to a height of 70-100 cm and covered with a wet cloth to preserve moisture and heat. The tightness, water content, temperature, and oxygen supply condition are all factors affecting wetpiling. The key parameters for the appropriate degree of fermentation are a temperature of about 45℃ (113°F) and leaves that are yellowish brown in color with distiller’s grain scent. If the leaves are yellow green in color and have a rough smell, this means that the piling is insufficient. If the leaves are colored dark and have a strong sour smell, it indicates excessive piling. 

The enzyme activity has been inactivated after fixing, but under a specific level of humidity and heat, the oxidation of polyphenols still continues as non-enzymatic automatic oxidation. As a result of this process, dark tea flavor turns into pure and mild. At the same time, sugars decompose into alcohols and organic acids. The smell of vinasse is a sign of moderate piling. The pile fermentation of dark tea makes large-molecule compounds in tea decompose into small-molecule compounds, which is helpful for the tea quality through metabolic processes under the participation of useful microbes, and makes the tea taste mellow and produce deep color liquor.

Drying

The drying methods of dark tea mainly include sun drying and fire drying. Sun drying is a traditional method that applies sunlight to remove moisture, resulting in a water content of about 13%. Fire drying refers to the application of pine wood during primary processing, leading to a final water content of about 10% and the attribution of a characteristic pine odor to dark tea. Drying dark tea by smoke-baking with pine wood can further improve the quality of tea, and make tea products high in aromas, mellow in taste and deep red in liquor color. Different dark tea producing areas have different raw materials and processing technologies, which leads to differences in the quality characteristics.

 Compressed Tea

In China, compressed dark tea is divided into Tuocha, Chocolate dark tea, Brick dark tea, and Cake dark tea according to shape. Brick dark tea, also known as border-sale tea, is a kind of brick-form tea made from older, coarse and rough leaves and branches. Brick dark tea is produced in Shanxi, Hubei, Hunan, Sichuan, Yunnan provinces, and is considered as a highly valued product in border areas.