Why do wet piles need turning and air?

Start with the essentials

Three key points

1. Conditions vary within a pileHeat, water and oxygen distribution require monitoring and handling.
2. Microbes work with the materialCommunities, enzymes and products depend on actual conditions.
3. Manufacturing connects with storageDrying, packaging and preservation continue managing the product.

Connect the process with the tea

The dark-tea process

Biochemistry

The core flow of wodui can be summarised as “water → pile → turn → dry”. After sun-dried maocha has been kill-greened, rolled and dried, its own enzyme activity has already been sharply reduced by the high heat. Wodui deliberately adds water and maintains warmth and humidity so that a microbial community can establish itself. Microbial metabolism generates heat, raising the core temperature; oxygen at the pile surface supports aerobic microbes. Turning serves to cool, add oxygen, even out the fermentation and prevent local overheating (“pile-burning”) [1][2][3].

If black-tea fermentation is “the leaf’s own enzymes performing”, wodui is “inviting a whole microbial band” — Aspergillus niger is the lead singer, bacteria and yeasts are the accompaniment, and moist heat is the stage lighting.

Microbial succession during wodui

Wodui is not the work of a single species; it is a dynamic succession of a microbial community. Chen et al. (2024) tracked the first 45 days of piling and found a clear succession: bacteria and yeasts are active early, Aspergillus niger dominates the middle phase, and fungal diversity declines as the pile matures [1].

Early stage (about 0–7 days). The leaves have just been watered and piled. Water is abundant and oxygen relatively available, so bacteria and yeasts multiply first. They break down simple sugars, produce organic acids and lower local pH, creating conditions for fungal growth; their metabolism also begins to raise pile temperature. Common early bacteria such as Bacillus can use tea polyphenols and amino acids, while yeasts contribute alcohol and ester aroma precursors [1][3].

Middle stage (about 7–30 days). As temperature rises and oxygen is consumed, Aspergillus niger gradually becomes the dominant fungus, often accounting for 60–95% of the fungal community by abundance [1][2]. A. niger secretes large amounts of extracellular enzymes — polyphenol oxidase, laccase, tannase, pectinase, cellulase and others — driving catechin oxidation/polymerisation and cell-wall degradation.

Late stage (about 30–45 days or more). Easily degradable substrates dwindle, pile temperature falls back, and some heat-tolerant fungi and bacteria continue to act while overall diversity drops. By now theabrownins have accumulated strongly and the pile enters a “finishing” phase that is terminated by turning and drying [1][3].

In Fuzhuan brick tea the situation differs slightly: after wodui the tea enters a controlled “flowering” step that encourages Eurotium cristatum to grow, producing the visible yellow cleistothecia known as “golden flowers”. This fungus breaks down polyphenols and produces various volatile compounds, giving Fuzhuan its distinctive fungal-floral aroma [3].

Enzyme systems and biochemical reactions

The defining feature of wodui is microbial pile-fermentation, not simple oxidation [1][2]. After fixation, the leaf’s own enzymes are largely inactive; the real drivers are the extracellular enzymes secreted by colonising microbes together with moist heat inside the pile.

The main enzyme systems include:

• Polyphenol oxidase and laccase — oxidise and condense catechins, generating theaflavins, thearubigins and eventually theabrownins;
• Tannase — hydrolyses ester catechins such as EGCG and ECG, reducing astringency;
• Pectinase, cellulase and protease — degrade cell walls, pectin, fibre and protein, softening the leaf and increasing soluble material;
• Glycoside hydrolases — hydrolyse glycosidically bound aroma precursors, releasing free aromatic compounds;
• Microbial metabolism itself — produces organic acids, alcohols, esters and other metabolites that contribute aged aroma and mellowness.

These enzymatic reactions work together with high-temperature, high-humidity non-enzymatic reactions (Maillard reactions, caramelisation) to drive tea polyphenols down, theabrownins up, soluble sugars up and caffeine partly down, producing dark tea’s “red, thick, brown-bright and mellow-smooth” quality [1][2][3].

It is worth noting that enzyme sources are not single: fungi such as A. niger secrete extracellular enzymes into the leaf tissue; intracellular enzymes released when microbes die and lyse continue to act; and the high temperature and humidity themselves can activate some heat-tolerant enzymes or promote non-enzymatic browning. Thus wodui’s “transformative power” is a systems engineering outcome of microbial metabolism, enzymatic reactions and moist-heat chemistry, not a one-enzyme show [1][3].

Pathways of theabrownin and other key products

The transformation of polyphenols during wodui follows roughly this path [1][2]:

1. Catechin oxidation. PPO/laccase oxidise catechins to o-quinones.
2. Condensation to theaflavins and thearubigins. Similar to black-tea fermentation, but in wodui microbial action is more sustained and deeper.
3. Further polymerisation to theabrownins. Thearubigins and catechin derivatives continue to oxidise and polymerise into theabrownins — large, dark-brown molecules that dominate dark-tea colour and mellowness.
4. Direct microbial transformation. Some microbes can metabolise catechins directly, contributing to theabrownin and other metabolites.

Other components also change significantly:

• Protein / cellulose — partly degraded into amino acids and soluble sugars, improving mellowness and smoothness;
• Starch and pectin — hydrolysed into soluble sugars, adding sweetness;
• Caffeine — partly degraded or transformed by microbes, lowering stimulation;
• Aroma compounds — glycoside release, microbial metabolism and thermal reactions together produce aged, woody, fungal-floral and other notes.

Key compound dynamics

Zheng et al. (2024) reinforced the role of Aspergillus niger by inoculating it into the pile, which increased theabrownin production, improved microbial-community structure and lowered caffeine content — confirming that A. niger plays a central role in ripe pu-erh quality formation [2].

Parameters and quality defects

Parameters vary widely with leaf grade, season and batch size, and small laboratory piles do not simply scale to factory piles — a simulated pile-fermentation study in the food-science literature tested 30–45% watering levels and its sensory panel preferred the 45% treatment, above the watering range commonly used in production [4]. The table above gives typical ranges from textbooks and studies, not a single “standard recipe.”

In practice, tea makers judge temperature and humidity by hand, inserting a hand into the pile and adjusting turning frequency according to air temperature and raw-leaf moisture. Spring leaf is tender, lower in fibre, heats quickly but may peak lower; summer leaf is coarser, richer in carbohydrates, heats more aggressively and reaches higher peaks, needing closer monitoring. Above 65 °C microbial activity is inhibited or even killed, and scorched notes may appear; turning not only cools the pile but also moves surface-oxidised leaves to the centre and centre-heated leaves to the surface, evening out the whole batch [3].

Under-fermentation means microbial enzyme activity and moist-heat action remain incomplete. Theabrownin production is low, the leaf base stays greenish-brown, the liquor is pale, and grassy notes and astringency linger — lacking the aged aroma and mellow body expected of a finished dark tea [1].

Over-fermentation or poor temperature control creates quality defects. Excessive pile heat causes “pile-burning”: the leaf base scorches black and the liquor turns muddy. Too much moisture with poor ventilation encourages unwanted microbes, producing musty, sour or off odours [1][3]. Careful turning and water management keep the pile temperature in a safe range; a well-controlled post-fermentation actually reduces microbial risk [1].

Ripe pu-erh safety: the aflatoxin evidence

In 2017, claims that “pu-erh causes cancer” circulated widely, pointing at aflatoxins. The evidence-based conclusion: properly made ripe pu-erh carries very low risk — the real risk lies in improper storage.

• The toxin producers are not the pile’s dominant microbes. Aflatoxins are produced by a few species such as Aspergillus flavus and A. parasiticus. The dominant fungus in wodui, Aspergillus niger, does not produce aflatoxins, and the pile’s hot, moist 50–65 °C interior is unfavourable to aflatoxin production [3].
• Properly sampled surveys find very low detection rates. The Guangdong CDC tested 148 market pu-erh teas (aged 1–30 years): aflatoxin B1 was detected in only 2.03% (3 samples, at most 0.56 μg/kg); among 432 samples collected by the Guangzhou CDC in 2013–2015, none contained detectable AFB1 [6].
• Aged stored teas measure even lower. Chau et al. (2023) analysed 31 stored pu-erh teas from 1900–2021 by mass spectrometry: in teas aged over 30 years, aflatoxins were all below 1 μg/kg, far under the 5 μg/kg reference limit used in the study; deoxynivalenol (DON) peaked below 70 μg/kg (reference limit 1000 μg/kg) [7].
• Early “widespread detection” was largely an artefact. Older surveys mostly used ELISA, which tea polyphenols interfere with; with the more reliable mass-spectrometry methods, detection rates dropped sharply [6][7].

For scale, the Chinese national standard GB 2761-2017 sets the aflatoxin B1 limit at 20 μg/kg for foods such as peanut oil and 0.5 μg/kg for infant formula — the pu-erh values above sit at very low levels [8]. So normal production and normal storage raise no aflatoxin concern for ripe pu-erh. What deserves attention instead is mouldy stored tea: if a tea has gone mouldy in damp conditions, stop drinking it. Keep stored tea dry and ventilated.

Representative styles

Different dark-tea categories emphasise different microbial communities and parameters. Ripe pu-erh highlights Aspergillus niger-dominated deep transformation; Fuzhuan foregrounds controlled “flowering” and Eurotium cristatum; Liubao is often combined with cellar ageing to develop its betel-nut aroma and cooling sensation.

Ripe pu-erh uses Yunnan large-leaf sun-dried maocha, high pile temperature and a long cycle, accumulating abundant theabrownins. The liquor is red and thick, with aged, woody and glutinous aromas. High-grade ripe pu-erh is prized for being slippery, mellow, sweet and clean, without pile odour or off-notes.

Fuzhuan brick tea goes through a separate “flowering” step after wodui, controlling temperature at 25–30 °C and relative humidity at 60–75% to encourage uniform golden flowers inside the brick. The amount and distribution of golden flowers are important quality markers; their metabolites give Fuzhuan its characteristic fungal-floral aroma [3].

Liubao tea from Wuzhou, Guangxi, traditionally follows “wodui → steaming → pressing → cellar ageing”. After wodui it is often stored in air-raid shelters or dedicated cellars for long-term ageing, where continued microbial action under relatively high humidity gradually develops its “red, thick, aged and mellow” character with betel-nut aroma.

Ya’an Tibetan tea / Kangzhuan represents Sichuan border tea. The raw material is relatively coarse and mature, the wodui cycle is long and the microbial community is complex; the finished tea is steamed and pressed into bricks or strip tea, mainly supplied to Tibetan areas. Its taste is mellow and full-bodied, resistant to boiling, with woody and aged notes.

Common misconceptions

1. “Wodui is just mould.” Proper wodui is dominated by beneficial fungi such as Aspergillus niger; the hot, moist pile environment suppresses most toxin-producing moulds. Mouldiness is the result of uncontrolled damp storage, not proper wodui.
2. “Ripe pu-erh always contains aflatoxins.” Multiple surveys and mass-spectrometry studies show that properly produced and normally stored ripe pu-erh has extremely low aflatoxin detection rates and levels far below food-safety limits.
3. “The longer the wodui, the better.” Too long leads to excessive theabrownin accumulation, dull aroma, flat taste and loss of vitality; quality depends on the balance of time, temperature and humidity.
4. “Raw pu-erh turns into ripe pu-erh with age / Raw pu-erh is dark tea.” Incorrect. Ripe pu-erh is a manufactured dark tea (hei cha) resulting from artificial moist-heat microbial post-fermentation. Raw pu-erh is fundamentally sun-dried green tea that matures via slow natural non-enzymatic oxidation in dry storage. Their biochemical pathways and constituents are fundamentally different; raw tea never transforms into ripe tea.
5. “Dark-tea post-fermentation can continue indefinitely.” After drying, moisture falls to a safe range and microbial activity largely stops; long-term ageing is very slow oxidation plus low-level microbial action, not unlimited fermentation.

See also

• Black-tea oxidation — enzymatic oxidation, for contrast with microbial fermentation
• Ripe pu-erh craft — the practical steps from maocha to ripe tea
• Yellow-tea yellowing — another non-microbial transformation after fixation

Both ripe and raw pu-erh evolve over time, but they take two completely different chemical routes:

Modern artificial wodui was standardized around 1973. It is crucial to understand that wodui is NOT merely an “accelerated version” of raw pu-erh aging; it is an entirely distinct biochemical remodeling driven by fungal extracellular enzymes and moist-heat reactions: catechins are deeply converted into theabrownins, creating a signature reddish-brown liquor and earthy mellow smoothness. Raw pu-erh in dry storage, conversely, retains high catechin vitality and mountain character through slow non-enzymatic auto-oxidation accompanied by dry-storage microflora. Their metabolic pathways and end-products diverge completely. National standard GB/T 22111 strictly distinguishes raw from ripe pu-erh based on whether post-fermentation wet-piling has occurred [5].

Theabrownins and metabolic regulation: landmark findings

Folk beliefs often tout ripe pu-erh as an agent to “melt away dietary fat”. In 2019, Prof. Wei Jia’s group at Shanghai Jiao Tong University published landmark research in Nature Communications, delineating the precise gut-liver axis mechanism [9]:

• High molecular weight remains in the gut: Theabrownins, the bulky polymerized pigments formed during wodui, are barely absorbed into the bloodstream. Instead, they act locally within the ileum and colon;
• Selective suppression of bile salt hydrolase microbes: Theabrownins selectively inhibit gut bacteria possessing bile salt hydrolase (BSH) activity (such as certain Lactobacillus and Bacteroides species);
• Hepatic cholesterol catabolism activation: Decreased BSH activity leads to the accumulation of conjugated bile acids (e.g., T-β-MCA) in the ileum. This suppresses intestinal farnesoid X receptor (FXR) and fibroblast growth factor 15 (FGF15) signaling, releasing feedback inhibition on the hepatic rate-limiting enzyme cholesterol 7α-hydroxylase (CYP7A1);
• Accelerated cholesterol conversion and fecal excretion: The alternative hepatic bile acid synthesis pathway is upregulated, converting excess endogenous cholesterol into bile acids for fecal elimination, thereby significantly ameliorating hypercholesterolemia [9].

Flavour sketch

Tea Science · core comparison

For the full six-tea-class comparison, see The Science of Tea: From Leaf to Six Tea Types.

References

1. R-008 — Zhang, W., et al. (2024). Pile-fermentation mechanism of ripened Pu-erh tea: Omics approach, chemical variation and microbial effect. Trends in Food Science & Technology, 146, 104379. https://doi.org/10.1016/j.tifs.2024.104379
2. R-009 — Wang, Y., et al. (2024). Enhanced Fermentation of Pu-Erh Tea with Aspergillus niger: Quality and Microbial Community Analysis. Molecules, 29(23), 5647. https://doi.org/10.3390/molecules29235647
3. R-031 — 施兆鹏, 夏涛 主编 (2016).《茶叶加工学》(第3版). 中国农业出版社. ISBN: 978-7109214347.
4. R-267 — 冯超浩, 刘通讯 (2013).《不同潮水量条件下普洱茶渥堆过程化学成分的变化》. 食品科学, 34(7), 135–139. https://doi.org/10.7506/spkx1002-6630-201307028
5. R-086 — GB/T 22111-2008.《地理标志产品 普洱茶》(Product of geographical indication—Pu-erh tea). 国家质量监督检验检疫总局、国家标准化管理委员会.
6. R-268 — 广东省疾病预防控制中心 (2018).《消费者报道》专访张永慧：“普洱茶致癌说”不成立. https://cdcp.gd.gov.cn/ywdt/mtbd/content/mpost_3444485.html
7. R-269 — Chau, S. L., Zhao, A., Jia, W., & Wang, L. (2023). Simultaneous determination of pesticide residues and mycotoxins in storage Pu-erh tea using ultra-high-performance liquid chromatography coupled with tandem mass spectrometry. Molecules, 28(19), 6883. https://doi.org/10.3390/molecules28196883
8. R-270 — GB 2761-2017.《食品安全国家标准 食品中真菌毒素限量》. 国家卫生和计划生育委员会、国家食品药品监督管理总局.
9. R-299 — Huang, F., Zheng, X., Ma, X., Xiang, H., Zhou, P., Zhao, A., … & Jia, W. (2019). Theabrownin from Pu-erh tea attenuates hypercholesterolemia via modulation of gut microbiota and bile acid metabolism. Nature Communications, 10(1), 4971. https://doi.org/10.1038/s41467-019-12896-x