Overview of Dark Tea (Heicha)

Dark tea, known in Chinese as hei cha (黑茶), is a fully post‑fermented tea category that undergoes microbial transformation after the initial manufacture. Unlike green, black, or oolong teas, which rely primarily on enzymatic oxidation, dark tea’s characteristic flavor, color, and health‑promoting compounds arise from controlled fermentation driven by fungi, bacteria, and yeasts. The most famous representatives include Pu’er (Yunnan), Liu’an (Anhui), Fu Brick (Hunan), and Tibetan brick teas. The processing of dark tea is therefore a multi‑stage journey that balances enzymatic steps with prolonged microbial activity, and each step is deliberately designed to shape the substrate for the succeeding fermentation phase.

This article breaks down the dark‑tea manufacturing process into its core operations, explaining the purpose of each step, the typical methods employed, the key time‑and‑temperature parameters, and the common regional variations that arise from adjustments to these parameters. The information draws on classical sources such as Lu Yu’s Cha Jing (8th century), the Yuan‑dynasty Tea Manual (Cha Pu), and modern microbiological and food‑science studies published in journals like Food Chemistry and Journal of Agricultural and Food Chemistry.

Step 1: Raw Material Selection and Preparation

Purpose: To obtain leaves with the appropriate chemical composition (polyphenol, protein, and carbohydrate levels) and physical integrity that will support both the initial enzymatic steps and the later microbial fermentation.

Typical Practices:

  • Harvesting is usually done during the late spring to early summer flush when the shoots have one bud and two to three mature leaves. In Yunnan Pu’er production, the standard is one bud + two leaves (sometimes three leaves for higher‑grade bricks).
  • Leaves are transported quickly to the processing facility to minimize wilting and microbial spoilage. In traditional settings, bamboo baskets are used; modern factories employ ventilated plastic crates.
  • Upon arrival, the fresh leaf is spread thinly on bamboo mats or concrete floors for pre‑withering (also called sun‑withering or indoor withering) to reduce surface moisture from ~75 % to 55‑60 %. This step lasts 2‑4 hours at ambient temperature (20‑28 °C) and is crucial for making the leaves pliable for rolling without breaking cellular structures.

Key Parameters: Target moisture content after withering: 55‑60 %; ambient temperature: 20‑28 °C; relative humidity: 60‑7 % (to avoid excessive drying).

Variations: In some Fu Brick processes, the leaves undergo a longer indoor withering (up to 6 hours) at lower temperatures (18‑22 °C) to develop a higher soluble sugar content, which fuels the later fungal growth during piling.

Step 2: Initial Processing – Fixing (Sha Qing) and Rolling

Purpose: To deactivate oxidative enzymes (primarily polyphenol oxidase and peroxidase) that would otherwise cause undesired enzymatic browning, while simultaneously shaping the leaves to rupture cell walls and release intracellular substances that become substrates for microbes.

Fixing (Sha Qing):

  • Traditionally performed in large woks (guo) heated to 240‑260 °C. The leaves are tossed continuously for 2‑5 minutes until the leaf color turns a dull green and the grassy aroma diminishes.
  • Modern facilities may use drum‑type steam‑fixing machines where saturated steam at 100‑110 °C is applied for 30‑60 seconds, achieving enzyme inactivation with lower energy consumption and more uniform heat distribution.
  • The fixing step reduces polyphenol oxidase activity to less than 5 % of its original level, as verified by spectrophotometric assays.

Rolling:

  • After fixing, the leaves are still warm (≈80‑90 °C) and are fed into rolling machines (rou nian) that apply pressure and torsion. The typical rolling time is 15‑30 minutes, with pressure ranging from 0.5 to 1.5 MPa depending on leaf thickness.
  • Rolling serves two functions: (1) mechanical disruption of vacuoles, releasing polyphenols, sugars, and amino acids; (2) forming the characteristic strip or needle shape that influences later drying rates and the surface area available for microbial colonization.
  • In some dark‑tea traditions (e.g., Liu’an), a light rolling (rou qing) is performed to keep the leaves relatively intact, preserving a higher proportion of soluble sugars for the subsequent piling stage.

Key Parameters: Fixing temperature: 240‑260 °C (wok) or 100‑110 °C (steam); fixing time: 2‑5 min (wok) or 30‑60 s (steam