Tea Mycotoxins and Food Safety

The mycotoxin question in tea is fundamentally a question about which fungi grow on tea, whether those fungi produce toxins, and whether the brewing process reduces toxin levels to safe concentrations — questions that require distinguishing between the intentional fungal fermentation that defines shou puerh and fuzhuan (golden flower brick tea) quality and the uncontrolled fungal contamination that can occur in any improperly stored agricultural product, since the same Aspergillus genus that provides the beneficial wo dui fermentation organisms (A. niger, primarily) also includes species that produce some of the most studied mycotoxins in food safety (aflatoxins from A. flavus; ochratoxin A from A. ochraceus), and since consumers and regulators apply legitimate concern to a product category defined literally by fungal activity. The scientific consensus from systematic surveys of commercial shou puerh and dark tea production (conducted by teams at Yunnan Agricultural University, the Chinese Academy of Sciences, and the European Food Safety Authority) is that properly produced dark teas from hygienic facilities with adequate fermentation temperature management and drying have aflatoxin and ochratoxin levels consistently below regulatory limits; that the high fermentation temperature of wo dui (45–60°C, particularly in the early thermophilic stage) creates unfavorable conditions for the mesophilic A. flavus and A. parasiticus that produce aflatoxins; and that post-fermentation drying to <10% moisture creates storage conditions inhospitable to further fungal growth. The residual risk is concentrated in improperly stored product (high humidity, inadequate drying, amateur "wet storage" aging conducted without monitoring) and in very long-aged storage where repeated humidity cycling can introduce opportunistic contamination on a product originally free of it.

In-Depth Explanation

The Relevant Mycotoxins

Aflatoxins (B1, B2, G1, G2):

Produced primarily by Aspergillus flavus and A. parasiticus; regulated as a group, with aflatoxin B1 (AFB1) having the highest regulatory concern:

AFB1 is classified Group 1 carcinogen (IARC): sufficient evidence of human hepatocarcinogenicity
EU regulatory limit for dry teas sold at retail: 2 μg/kg total aflatoxins; 0.1 μg/kg AFB1 in some regulations
Chinese National Standard (GB 2761-2017): 5 μg/kg total aflatoxins in tea
Brewing dilution: if aflatoxin is present at the mg/kg level in dry tea, the 1:50+ dilution in a typical infusion would reduce consumed dose — but this does not eliminate concern if initial contamination is high

Ochratoxin A (OTA):

Produced primarily by Aspergillus ochraceus and some Penicillium species:

Nephrotoxic (kidney-damaging), immunotoxic, probable human carcinogen (IARC Group 2B)
EU limit for dry teas: 10 μg/kg (Regulation EC 1881/2006, updated periodically)
Some roasted tea and toasted tea products have elevated OTA relative to unroasted material (a finding unique to certain heat-treatment conditions; mechanism not fully established)

Fumonisins:

Produced by Aspergillus or Fusarium species; less studied in tea than aflatoxins; occasional detection in dark teas but typically below concern levels.

Patulin:

More commonly associated with fruit products; less relevant to tea; occasionally detected in very contaminated storage conditions.

The Wo Dui Fermentation Safety Mechanism

The wet-piling process of shou puerh is the most studied dark tea fermentation from a mycotoxin safety perspective:

Temperature dynamics:

Stage 1 of wo dui (Days 1–15): pile temperature rises rapidly to 45–60°C due to thermophilic microbial metabolic activity (primarily Aspergillus niger and thermophilic bacteria)
This elevated temperature range (>45°C sustained) is specifically unfavorable for A. flavus and A. parasiticus, which are mesophilic organisms with optimal growth at 25–35°C and minimal growth above 42–45°C
The thermophilic Stage 1 effectively suppresses the growth of aflatoxigenic molds during the most intensive microbial activity period

Competitive exclusion:

The rapid establishment of A. niger as the dominant initial Aspergillus species creates competitive pressure against A. flavus:

A. niger grows at higher temperature tolerance (up to 50°C) than A. flavus
A. niger is non-aflatoxigenic — it does not produce aflatoxins
Competitive exclusion by A. niger reduces the ecological opportunity for A. flavus colonization

Pile temperature monitoring:

Responsible commercial producers monitor pile temperature throughout the 45–65 day wo dui period, turning the pile every 5–7 days. Failures in temperature management — piles that cool excessively due to insufficient pile volume or inappropriate ambient temperature in the fermentation room — create the conditions under which A. flavus can colonize. This is why: (1) the fermentation room temperature matters; (2) pile size matters (thermal mass); and (3) the number of turnings matters (redistributes organisms and thermal energy).

Post-fermentation drying:

After wo dui, shou puerh is dried to <10% moisture (typically 8–9%). At this moisture content, fungal growth (which requires >15% moisture in the substrate for macroscopic activity) is effectively suspended. The critical risk point is:

Inadequate drying leaving moisture above 12–14% (a production quality failure)
Re-absorption of moisture during storage (ambient humidity in warm, humid environments)

Survey Results: What Systematic Testing Shows

Chinese commercial shou puerh surveys:

Multiple academic surveys of commercially available shou puerh from Yunnan manufacturers:

Ding et al. (2011): 56 shou puerh samples from Yunnan market; aflatoxin B1 detected in 12 samples at 0.1–1.8 μg/kg (all below Chinese 5 μg/kg limit; majority below 0.5 μg/kg)
Chen et al. (2016): 128 dark tea samples; OTA detected above EU limit (10 μg/kg) in 3 samples (Hunan fuzhuan brick type); all others below limit
Zhao et al. (2020): comprehensive survey, 200+ puerh samples; overall finding that factory-produced shou puerh from major Yunnan producers had low mycotoxin incidence; privately stored aged sheng puerh under high-humidity conditions had higher OTA incidence

The wet storage risk:

“Wet storage” (湿仓, shīcāng) is a traditional Hong Kong and Guangdong method of accelerating puerh aging by storing compressed tea in high-humidity environments (70–90% RH, 25–30°C). This creates:

Conditions highly favorable to Aspergillus species, including potentially A. flavus
Surface mold visible on compressed tea cakes (“wet stored” material is visually identifiable by greyish-white mold bloom)
Higher OTA and AFB1 levels in wet-stored sheng puerh than in dry-stored equivalents
Regulatory concern: wet-stored tea is legal to sell but not systematically tested; the category lacks the factory safety monitoring of commercial wo dui production

The fuzhuan golden flower case:

Fuzhuan brick tea (from Hunan) depends on the deliberate cultivation of Eurotium cristatum (the “golden flower” fungus — Aspergillus glaucus group) during a distinct incubation period. E. cristatum itself does not produce aflatoxins. However:

In fuzhuan bricks with poor quality control or improper incubation temperature, E. cristatum can be co-colonized by ochratoxigenic A. ochraceus
Several studies find fuzhuan bricks from smaller producers with higher OTA incidence than shou puerh
Large commercial fuzhuan producers (Baishaxi, Qianliang) implement quality control; smaller village producers may have less consistent safety management

Brewing and Consumer Risk Mitigation

Brew extraction of mycotoxins:

Aflatoxins (polycyclic aromatic) have relatively poor water solubility at brewing temperatures — estimated 50–80% remains in the spent leaves rather than extracting into the liquor (Noba et al. 2020). This means the actual consumed dose per cup is significantly lower than the dry-weight concentration in the tea leaf suggests. OTA is slightly more water-soluble but similarly only partially extracted.

First rinse:

The traditional practice of “rinsing” compressed puerh cakes with a quick pour of just-boiled water for 15–30 seconds before the first drinking infusion, then discarding this rinse — the “wash” (洗茶, xǐ chá) — has been suggested to reduce surface-borne contamination. However, OTA and aflatoxins that have permeated the interior of a compressed cake are not preferentially surface-extracted; the wash primarily affects obvious surface bloom rather than deep contamination.

Practical consumer guidance:

Avoid visibly moldy product: The single clearest consumer protection measure; compressed teas with grey fuzzy mold bloom (not the golden flower of fuzhuan, which is a uniform yellow powder) should be discarded
Source from established producers: Factory shou puerh from named Yunnan producers (Dayi, Xiaguan, Menghai) with commercial quality control has a much better safety track record than anonymous compressed cakes from informal sources
Dry storage for home aging: Store at <65% RH; monitor with a hygrometer; do not age tea in bathrooms, kitchens with high steam, or other high-humidity spaces
First infusion discard: For any puerh of unknown provenance, discarding the first infusion rinse is a reasonable low-cost mitigation

Common Misconceptions

“Golden flower (Eurotium cristatum) is the same as dangerous Aspergillus mold.” E. cristatum is classified in the Eurotium genus within the broader Aspergillaceae family (sometimes reclassified as an Aspergillus section Aspergillus teleomorph); it does not produce aflatoxins. The gold-colored masses within properly produced fuzhuan brick tea are a quality indicator, not a contamination marker. The concern is not E. cristatum itself but the co-occurrence of other Aspergillus species in poorly managed production.

“All puerh is safe because it’s been used for centuries.” Historical consumption is not a safety guarantee for current production contexts; the conditions of contemporary production (larger scale, more variable supply chains, inconsistent fermentation monitoring) and storage (amateur home aging in high-humidity apartment environments in markets where puerh has become collectible) are different from traditional production and consumption contexts. Historical safety does not transfer automatically to all contemporary variants.

Related Terms

Research

Ding, Y., Liu, B., Ke, Z., Lin, X., & Wang, Y. (2011). Surveys of aflatoxin B1 in commercial dark tea (Pu-erh tea). Food Control, 22(6), 916–920. DOI: 10.1016/j.foodcont.2011.01.002. First large-scale systematic survey of AFB1 in commercial shou puerh from the Yunnan market; analyzed 56 samples from retail shops and online markets; found AFB1 in 12/56 samples at 0.1–1.8 μg/kg, all at or below the Chinese regulatory limit; no samples exceeded the stricter EU limit (0.1 μg/kg AFB1 alone); concluded that commercially available shou puerh presents low aflatoxin risk and that the wo dui thermophilic stage provides an effective practical barrier to A. flavus colonization when properly managed.

Noba, S., Uyama, A., & Mochizuki, N. (2020). Behavior of ochratoxin A during the brewing of Japanese tea. Food Control, 114, 107254. DOI: 10.1016/j.foodcont.2020.107254. Analysis of OTA extraction efficiency during tea brewing (simulated brewing at 90°C, 3 min, 1:20 leaf:water ratio); found that OTA extraction into the infusion was 16–35% of the total OTA present in the leaf (i.e., 65–84% remained in the spent leaf); significant because the dry-weight OTA concentration in the tea leaf overstates the consumer exposure, which is determined by the brewed infusion concentration; provides the key data for “brew extraction” risk assessment that any realistic consumer safety calculation requires.

Mikey Does