Two Fermentations, Two Clocks: What Really Happens When Pu-erh Ages

Tea Science｜Author：祥興茶行｜Published：2026-07-13

Pu-erh's transformation is two different processes on two very different timelines: a few weeks of microbial pile fermentation and years to decades of slow warehouse aging. Separating them explains why "older" is not automatically "better," and what the evidence says about health claims and food safety.

Walk into an old tea shop and you will often hear: "Pu-erh gets better with age, and the older it is, the more it is worth." That line mixes up two things: what happens inside a tea cake as it ages, and whether aging always moves in a good direction.

Pu-erh's transformation is two different things, happening on two different timelines. Once you separate the two, it becomes clear why "aged" does not equal "good," and what to watch out for.

First, Pu-erh Has Two Kinds of Fermentation

Many people think of pu-erh as a tea that ferments on its own if you leave it long enough. Under China's national standard GB/T 22111-2008, pu-erh is made from sun-dried leaves of Yunnan's large-leaf varietal and divides by process into two categories: raw pu-erh (sheng) and ripe pu-erh (shou), which undergoes "post-fermentation" [1]. The key step in post-fermentation is wo dui, or pile fermentation. Only ripe pu-erh goes through it, and it is the first timeline in this article.

The distinction in one line Ripe pu-erh's pile fermentation = weeks of controlled microbial solid-state fermentation in a factory (with pile heat and dominant microbes). Warehouse aging of finished raw or ripe pu-erh = years to decades of slow oxidation and enzymatic reactions in storage (with almost no vigorous microbial activity). The timescales, temperatures, and microbial ecologies all differ, so the two should be looked at separately.

The diagram below puts the two timelines side by side.

〔圖〕Side-by-side comparison of pile fermentation and warehouse aging timelines

Timeline One: Pile Fermentation, Weeks of High-Heat Fermentation

Pile fermentation is the key step in making ripe pu-erh. Sun-dried maocha is heaped into large piles, moistened, and covered, and microbes and heat work through it for several weeks. Heat, microbes, and enzymes all act at once.

Start with temperature. A study combining microbiome and metabolome analysis tracked one fermentation batch: the pile went from about 28°C on day 0 to nearly 50°C within two days, then stayed at roughly 51 to 63°C from day 3 to day 26 [3]. Thermotolerant microbes and high-temperature oxidation do their work during this hot stretch.

Next, the microbes. The same study saw the bacterial community change in stages: _Bacillus_ dominated early on, then _Staphylococcus_ and _Kocuria_ took over in the middle and late phases. Among the fungi, the genus _Aspergillus_ was present steadily throughout [3]. A review pooling many studies counts at least 93 fungal species (in 36 genera) and 53 bacterial species (in 22 genera) isolated from pile-fermented ripe pu-erh, with _Aspergillus niger_ detected most often and regarded as the dominant microbe of pile fermentation [4].

These microbes carry enzymes that break down large molecules. Among representative strains isolated from pile samples, _Bacillus subtilis_ had the strongest protein-degrading activity and _Aspergillus tubingensis_ the strongest pectin-degrading activity [3]. The tannase of _Aspergillus niger_ splits hydrolyzable tannins into gallic acid and turns the liquor deep red [12].

Scope of the research: the strain proportions and enzyme-activity figures above come from an experiment on a single batch at a single factory [3]. Another study inoculated _Aspergillus niger_ for "enhanced fermentation," a deliberate add-the-microbe design rather than traditional spontaneous pile fermentation [2]. Microbial profiles change with the leaf, the facility, and the season; these figures are one concrete case, not a description of all ripe pu-erh.

Timeline Two: Warehouse Aging, Slow Change Measured in Years

When ripe pu-erh leaves the factory, or raw pu-erh goes into a warehouse, warehouse aging begins. There is no pile heat and no teeming microbial population. What happens is mainly slow auto-oxidation and residual enzymatic reactions at room temperature, with change accumulating over years.

On composition, a metabolomics study of 18 raw pu-erh samples of different ages found that aging consists essentially of the reduction, transformation, and condensation of flavanols (catechins, flavonol glycosides, and related compounds) and amino acids, together with the accumulation of organic acids and gallic acid [5]. A separate tracking study spanning about 10 years, quantified by nuclear magnetic resonance (qNMR), identified 3,384 metabolites in raw pu-erh, 619 of which were stably present at every age point, and confirmed that γ-linolenic acid and gallic acid keep accumulating with age [6]. The steady rise in gallic acid is considered one of the most robust markers of age, and this direction has been observed independently in several studies of both raw and ripe pu-erh [5][6][12].

On taste, the 18-sample study used quantitative descriptive sensory analysis and found that with longer aging, umami, bitterness, and astringency fell significantly, while sourness and "thickness" (kokumi) rose significantly [5]. That is what experienced drinkers mean when they say the bitterness and astringency fade and the tea turns mellow. On aroma, another study used high-resolution gas chromatography to follow raw pu-erh aged 7 to 21 years; the aroma profile moved roughly from fruity/floral, through smoky, toward an aged character, and several methoxybenzene compounds are commonly linked to the formation of that aged aroma [7].

Figures such as "509 signature metabolites" or "3,384 metabolites" are results from specific sample sets [5][6]; they do not mean every cake contains that many compounds. Chinese flavor terms for aged tea, such as camphor or jujube notes, also do not necessarily map one-to-one onto single molecules.

The chart below shows the direction of change of several major components during aging. It indicates only rising or falling, not actual values.

The overall pattern: ester catechins and other flavanols decline through oxidative polymerization, theabrownins and gallic acid accumulate, and caffeine stays relatively stable [5][8][12]. Health research on theabrownins is often overstated, so the next section deals with it separately.

Where Theabrownins Come From, and the Limits of the Health Research

The deep red color and smooth mouthfeel of the liquor come mainly from theabrownins. Reviews describe how they form: phenolics such as catechins are first oxidized by polyphenol oxidase (PPO) into quinones, which polymerize into theaflavins and thearubigins and finally couple with proteins, lipids, or caffeine to form theabrownins [9]. In dark teas, microbes such as _Aspergillus niger_ also take part in their formation through hydrolysis and oxidative polymerization [9].

A 2019 study in _Nature Communications_ found that theabrownins alter the gut microbiota in both mice and humans, suppressing bacteria with bile salt hydrolase (BSH) activity, and lower hepatic cholesterol through the chain "higher ileal bile acids → suppressed FXR-FGF15 signaling → the liver making more cholesterol into bile and excreting it in feces" [8]. The human evidence in this study is very limited:

🩺 Evidence level: mechanism established, human efficacy unproven The main lipid-lowering and weight-loss evidence comes from mice on a high-fat diet. The human part consisted of just 13 healthy young men aged 24–32, over 4 weeks, with no placebo control, a small before-and-after exploration [8], and the paper states that the subjects showed no significant weight loss [8]. What can be said for now is "the mechanism holds, with a preliminary human signal." It cannot be said that drinking pu-erh lowers cholesterol, blood lipids, or body weight. A related review also notes that the lipid-lowering activity of theabrownins has so far been shown only at the cell, animal, and mechanistic levels, with human clinical trials still to be done [9].

Ripe pu-erh does contain measurable natural lovastatin (a statin-class lipid-lowering compound). One study found that unstored raw pu-erh contained none at all (below the detection limit of 0.01 ng/g), while nearly all ripe pu-erh samples tested positive, at about 20.61 to 226.38 nanograms (ng) per gram of dry tea [13]. A pharmaceutical dose of lovastatin is about 20 to 80 milligrams a day, so the amount in tea is roughly ten thousand to a hundred thousand times lower (a comparison calculated from the reported contents, not one made by the original study). Reaching a drug dose would mean eating tens of kilograms of dry leaf, and brewing extracts only part of it anyway [13]. It is present only in trace amounts, has no pharmacological significance, and cannot replace any medication.

Precise-sounding claims circulate online now and then, such as "a top-journal human trial proved pu-erh lowers bad cholesterol by several percent." When checked, they usually trace back to no peer-reviewed original source, only commercial blogs. That is misinformation, and this article does not use it. A health figure precise to the decimal point but impossible to source deserves extra suspicion.

The Scientific Limits of "Older Is Better": Storage and Food Safety

All the changes described above rest on one precondition: proper storage. In the wrong environment, the result goes the other way.

Because pu-erh is post-fermented and often stored for long periods, its processing makes it more susceptible to fungal contamination than other tea types, and mycotoxins in individual samples have reached several hundred ppb [11]. Detection rates vary widely between studies. Measured by confirmatory instrumental methods (such as LC-MS/MS), aflatoxin B1 detection rates in pu-erh range from 0% (one sample set in Fu et al., 2009, with nothing detected) to about 22.6% (Liu et al., 2017: 7 of 31 samples positive, mostly at low levels of 0.1 to 16.1 μg/kg). An official compilation of 452 samples from 6 institutions averages about 9.3% detection [14][18]. Ochratoxin A detection rates also range across studies from none detected to about one in ten; in one set, 4 of 36 samples (about 11.1%) were positive [10][14]. Some individual samples have exceeded legal limits [11][14]. The spread from zero to over twenty percent reflects differences in how each batch was processed, stored, and tested; "pu-erh = aflatoxin = cancer" is an exaggeration [14].

Why Properly Made Pu-erh Rarely Tests Positive for Aflatoxin

The toxin-producing fungi are not the main actors in pile fermentation. Aflatoxin is made by _Aspergillus flavus_ and _Aspergillus parasiticus_, and _Aspergillus niger_, which dominates pile fermentation, does not produce aflatoxin [14]. Pu-erh is low in fat and protein, and during pile fermentation dominant microbes such as _Aspergillus niger_ fill the available niches, leaving _Aspergillus flavus_ little room to establish itself. Even where it survives, most strains of _Aspergillus flavus_ do not necessarily produce toxin (research estimates that only about one in ten does, and only under suitable conditions) [4]. In one set of experiments, 3 strains of _Aspergillus flavus_ confirmed to produce aflatoxin were deliberately inoculated into a pile fermentation; tested by confirmatory methods including HPLC and LC-MS, none of the four aflatoxins was detected in any tea sample [4].

Testing method also matters. Many early reports used ELISA immunoassays, which the complex matrix of tea can easily push into false positives. In one study, the same batch of pu-erh tested positive across the board by ELISA but was entirely negative under confirmatory methods including HPLC and LC-MS/MS [14]. Researchers therefore consider ELISA unsuitable for testing aflatoxin in pu-erh, and confirmatory methods such as LC-MS/MS have "essentially resolved the false-positive error in pu-erh aflatoxin testing" [4]. When you see a report of aflatoxin found in pu-erh, check first which testing method was used.

Market testing agrees. In 2017, the Shenzhen Consumer Council sampled 10 commercial pu-erhs (half raw, half ripe, covering teas from 1995 to 2014) and tested them by confirmatory methods: aflatoxin was not detected in any of them [19]. A science explainer by academician Chen Zongmao, published by China's Ministry of Agriculture, answered the "drinking pu-erh causes cancer" rumor by stating that the carcinogenic risk from aflatoxin in pu-erh is extremely low [18]; the figures that set off the original scare came from artificially hot and humid "wet-stored" pu-erh, which does not represent normally produced tea [18].

The scattered high readings come from storage. The same review notes that pu-erh made in a normal production environment contains little toxin, but aflatoxin readily develops when it is piled or stored in high humidity [14]. A controlled pile has dominant microbes, high heat, and a short duration, so toxin risk is relatively low; molding in a wet warehouse is an uncontrolled process running in the opposite direction [4][14]. The negatives and low values therefore come mostly from legitimate dry storage, while the scattered high values point almost always to improper wet storage or poor-quality individual cases.

Some fermentation microbes can also break down toxins that are already present. _Aspergillus niger_ carries enzymes that hydrolyze ochratoxin A (a carboxypeptidase and ochratoxinase), cleaving it into the far less toxic OTα plus an amino acid fragment, a known mechanism in food detoxification research [20][21]. An _Aspergillus niger_ strain isolated from tea can degrade aflatoxin B1 with its intracellular enzymes (one experiment measured about 88.59% degradation in 72 hours); this is enzymatic breakdown, which is a different thing from tea leaves physically adsorbing toxins and keeping them in the spent leaves rather than releasing them into the liquor [22]. Toxins have also been seen to fall during actual fermentation of dark teas, but that was Fu brick tea (one study measured about a 56.75% drop in ochratoxin, positively correlated with _Aspergillus_ counts), not direct evidence from pu-erh pile fermentation [23].

⚠️ Limits of breakdown during fermentation: this breakdown happens only during fermentation, depends on live microbes and active enzymes, and acts on toxins already present at that time. It does not guarantee zero residue in the finished tea, and it does nothing about ochratoxin that grows later when a cake sits in an improper wet warehouse. Most _Aspergillus niger_ strains do not produce toxin, but some strains can produce ochratoxin [10]. Residual ochratoxin is water-soluble and does dissolve into the liquor. Fermentation can at most reduce part of the toxin load; the real safeguard is tea from legitimate producers that passes inspection and has been dry-stored.

Brewing does not destroy the toxins. What gives some reassurance is how little gets into the cup:

🍵 From leaf to liquor 1. The concern is the toxin, not live microbes: the food-safety issue is the toxin that molds produce (a chemical), not live mold or bacteria on the leaf, so killing microbes is not the point. 2. The toxins are heat-stable and do not brew away: aflatoxin breaks down substantially only above 160°C, and ochratoxin A at around 180°C [17]. Brewing water at about 100°C is not hot enough to break down these toxins. 3. Aflatoxin transfers poorly into the liquor: experiments found that aged pu-erh leaves bind aflatoxin strongly; ten successive infusions released only about 2.94%, with most of it staying in the spent leaves [15][16]. 4. Risk once transfer rates are counted: a cumulative risk assessment that built the actual leaf-to-liquor transfer rate into its calculation found hazard quotients (HQ) below 1.0 for every consumer group, an acceptable non-carcinogenic risk [16]. Precondition: all of the above assumes tea from legitimate producers that passes inspection and has been dry-stored. For improperly wet-stored tea, the detections and exceedances described earlier remain a risk [14][16].

⚠️ Does not apply to ochratoxin A: low transfer holds only for aflatoxin. Ochratoxin A is highly water-soluble and does dissolve into the liquor, with transfer rates of about 34–64% [16], so aflatoxin's low transfer cannot be used to reason about ochratoxin.

Dark teas from legitimate manufacturers carry extremely low safety risk [14][16]. Pu-erh's safety and quality depend on controlled processing and good storage, not on being kept as long as possible.

The Takeaway, in One Cup

For how to choose a good cake, see our beginner's guide to buying pu-erh; for the general principles of tea fermentation, see our explainer on what tea fermentation is. The next time you hear "the older the better," ask first: which fermentation, and how was it stored?

References

  1. 中華人民共和國國家標準（2008）。《GB/T 22111-2008 地理標誌產品 普洱茶》。國家標準化管理委員會。
  2. Zhang, 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. Hu, X. et al. (2025). Integrated Microbiome–Metabolome Analysis and Functional Strain Validation Reveal Key Biochemical Transformations During Pu-erh Tea Pile Fermentation. _Microorganisms_. PMC12388847.
  4. Ma, C. 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
  5. Ma, B., Ma, C., Zhou, B. et al. (2025). Quantitative descriptive analysis, non-targeted metabolomics and molecular docking reveal the dynamic aging and taste formation mechanism in raw Pu-erh tea during the storage. _Food Chemistry: X_. PubMed 39968040.
  6. Chen, L. et al. (2025). Non-volatile metabolite profiles and taste quality of Pu-erh raw tea on different storage years by metabolomics and 1H qNMR. _Food Research International_, 219, 116967. https://doi.org/10.1016/j.foodres.2025.116967
  7. Liu, Y. et al. (2025). Characterization of Volatile Substances in Pu-erh Tea (Raw Tea) at Different Storage Times. _Foods_, 14(5), 840. https://doi.org/10.3390/foods14050840
  8. Huang, F. et al. (2019). Theabrownin from Pu-erh tea attenuates hypercholesterolemia via modulation of gut microbiota and bile acid metabolism. _Nature Communications_, 10, 4971. https://doi.org/10.1038/s41467-019-12896-x
  9. Zhang, Y. et al. (2025). Formation, biotransformation and health benefits of theabrownins (review). _Food & Function_, 16(16), 6346. https://doi.org/10.1039/d5fo02046h
  10. Xu, W., Zhao, Y., Jia, W. et al. (2023). Reviews of fungi and mycotoxins in Chinese dark tea. _Frontiers in Microbiology_, 14, 1120659. https://doi.org/10.3389/fmicb.2023.1120659
  11. Mycotoxins in Tea: Occurrence, Methods of Determination and Risk Evaluation (2018). _Toxins_, 10(11), 444. https://doi.org/10.3390/toxins10110444
  12. 高曉余、嚴亮、趙艷等。微生物多樣性與普洱茶品質關係研究進展。《廣東農業科學》，廣東省農業科學院。
  13. Zhao, Z., Pan, C., Liu, S., Li, C. et al. (2013). Exposure assessment of lovastatin in Pu-erh tea. _International Journal of Food Microbiology_, 164(1), 26–31. https://doi.org/10.1016/j.ijfoodmicro.2013.03.018
  14. Xu, W. et al. (2022). Regulation of fungal community and the quality formation and safety control of Pu-erh tea. _Comprehensive Reviews in Food Science and Food Safety_, 21(6). https://doi.org/10.1111/1541-4337.13051
  15. Zhao, X. et al. (2022). Determination of four aflatoxins on dark tea infusions and aflatoxin transfers evaluation during tea brewing. _Food Chemistry_, 398, 133905. PubMed 36413842. https://pubmed.ncbi.nlm.nih.gov/36413842/
  16. Wang, X. et al. (2023). Comprehensive and cumulative risk evaluation of dietary exposure to aflatoxins and ochratoxin A on fermented teas worldwide by a new assessment model. _Food and Chemical Toxicology_ (PII S0278691523007238). https://www.sciencedirect.com/science/article/pii/S0278691523007238
  17. Thermal stability of aflatoxin B1 and ochratoxin A (2013). _Mycotoxin Research_, 29, 113–117. PubMed 23604747. https://pubmed.ncbi.nlm.nih.gov/23604747/
  18. 陳宗懋（2019）。〈院士科普解讀「喝普洱茶致癌」謠言〉。中華人民共和國農業農村部・農產品質量安全監管司。https://jgs.moa.gov.cn/kptd/201904/t20190418_6186244.htm
  19. 深圳市消費者委員會（2017）。市售普洱茶黃麴毒素檢測結果（10 款市售樣本以確認法檢測全數未檢出）。健康時報／新浪健康轉載。https://health.sina.com.cn/news/2017-09-22/doc-ifymfcih2331021.shtml
  20. Zhang et al. (2022). Degradation of Ochratoxin A by a UV-Mutated _Aspergillus niger_ Strain (FS-UV-21). _Toxins (Basel)_. PMC9146908. https://pmc.ncbi.nlm.nih.gov/articles/PMC9146908/
  21. (2024). _Aspergillus niger_ Ochratoxinase Is a Highly Specific, Metal-Dependent Amidohydrolase Suitable for OTA Biodetoxification in Food and Feed. _Journal of Agricultural and Food Chemistry_. https://doi.org/10.1021/acs.jafc.4c02944
  22. Wang et al. (2020). Degradation and Detoxification of Aflatoxin B1 by Tea-Derived _Aspergillus niger_ RAF106. _Toxins (Basel)_, 12(12), 777. PMC7762301. https://pmc.ncbi.nlm.nih.gov/articles/PMC7762301/
  23. (2025). Microbial communities and metabolite dynamics in the flowering fermentation of Fu brick Tea: Correlations with mycotoxin degradation. _Food Bioscience_（abstract 級）. https://www.sciencedirect.com/science/article/pii/S221242922500882X

Teas mentioned