This chapter puts the “why” of tea making into six published core processes: green-tea fixation, white-tea withering, yellow-tea yellowing, oolong zuoqing, black-tea oxidation and dark-tea wodui. Each moves through leaf condition, key operations, possible transformations and quality observations, so that process names become more than a sequence of steps[1][2].
Reading note: the six categories can be compared by their transformation paths, but there is no single “oxidation percentage” that can be compared directly across cultivars, origins and factories[1][2].
The chapter is held together not by a single molecule or metric, but by four recurring questions: when enzymes and substrates meet, how heat and moisture redirect reactions, which compounds and volatiles change, and how those changes can be observed carefully in colour, aroma, taste and leaf appearance[1][2][3][4].
In this chapter
The Science of Green-Tea Fixation
How heat deactivates the leaf's enzyme in minutes, halting oxidation and locking in green tea's colour and freshness.
The Science of White-Tea Withering
White tea is the tea of "subtraction" — no kill-green, no rolling, just long withering and drying. It looks like nothing is done, yet slow water loss and gentle oxidation quietly unfold.
The Science of Yellow-Tea Yellowing (Men Huang)
Yellow tea is like "green tea that takes one more step." After kill-green, the warm leaves are sealed and sweltered so moist-heat turns them from green to yellow — losing grassiness, gaining mellow sweetness.
The Science of Oolong "Bruising" (Zuoqing)
Partial oxidation, sitting between green and black tea, achieved through repeated cycles of shaking and resting. Zuoqing shapes the "green leaf, red edge" and coaxes out oolong's celebrated floral-fruity aroma.
The Science of Black-Tea "Fermentation"
Black tea's "fermentation" isn't fermentation at all — it's an enzyme-driven oxidation that turns colourless catechins into golden theaflavins and red-brown thearubigins.
The Science of Dark-Tea Pile-Fermentation (Wodui)
The first three crafts rely on the leaf's own enzymes; dark-tea wodui hands the lead role to microbes — a true "post-fermentation" that turns polyphenols into theabrownins and makes ripe pu-erh deep, red and mellow.
Related chapters
- Processing — where these scientific principles appear in practice
- Botany — the fresh-leaf compounds that feed these reactions
- Flavor — how chemical reactions finally become sensory flavour
Comparison method: a shared transformation-path map can help organise the six categories conceptually; it does not replace judgment about specific conditions, raw material and sensory results[1][2].
Editorial note
Later work will deepen quality formation, process sampling and sensory verification, and multi-factor mechanisms of aroma and taste, with selective coverage of research topics such as yan yun and zuoqing. Each addition will distinguish established knowledge, findings made under defined experimental conditions and hypotheses still being tested; a “molecular mechanism” will not stand in for a complete craft and sensory judgment.
References
- Wan, X.-C. (2003). Tea Biochemistry (3rd ed.). China Agriculture Press. (Authoritative textbook on tea enzymology and the chemical differentiation of the six tea categories.)
- Shi, Z.-P. (Ed.). Tea Processing Science. China Agriculture Press. (Systematic treatise on the principles of tea processing and quality formation across the six categories.)
- Xu, Y.-Q., et al. (2018). Chemical compositions and biological activities of different types of tea. Comprehensive Reviews in Food Science and Food Safety, 17(2), 432–452. https://doi.org/10.1111/1541-4337.12339 (Review of aroma and compositional differences among tea types.)
- Wang, K., et al. (2010). Comparison of catechins and amino acids in white, green, oolong, black, and pu-erh teas. Food Chemistry, 121(2), 338–344. (Comparative analysis of amino acids and catechins across tea categories.)