Fungi in the Grain: How Storage Transforms Coix Seed from Medicine to Mycotoxin Risk

New Study Reveals Dynamic Fungal Shifts and Toxin Production in Herbal Grain Under Heat and Humidity

When Functional Food Meets Fungal Risk

Coix seed—better known as Job’s tears or 薏苡仁 (Yìyǐrén)—holds a special place at the crossroads of traditional medicine, global wellness, and modern agriculture. It is celebrated as both food and healing grain, and its story stretches across centuries and continents. Yet, as a recent study by Guo et al. (2025) makes clear, the journey from field to table is increasingly fraught with a modern, invisible threat: fungal contamination and the build-up of dangerous mycotoxins. For all the progress in food processing and transport, the lesson remains the same: the safety of a harvest is shaped by every step along its path.

The Experiment: One Grain, Many Risks

To unravel the risks, the researchers studied Coix seed in three different forms: unshelled, shelled, and polished. Each form was stored under two sets of conditions: one mimicking high temperature and high humidity typical of tropical storage or transport, and the other representing ambient room conditions. Using DNA metabarcoding, they tracked shifts in fungal communities, while LC–MS/MS (liquid chromatography–tandem mass spectrometry) quantified mycotoxin accumulation over time. Samples were examined at four weeks and three months, reflecting realistic storage intervals encountered in global supply chains.

Results: How Fungi and Toxins Take Hold

The results reveal how easily Coix seed can transition from medicinal food to mycotoxin risk. Under warm, humid conditions, Aspergillus species dominated, accompanied by rising levels of aflatoxins, among the most potent naturally occurring liver carcinogens. Under more moderate, ambient conditions, risk did not disappear—it shifted. Fusariumspecies increased in prevalence, producing zearalenone (ZEN), an estrogenic mycotoxin known to disrupt endocrine function.

Rather than eliminating danger, storage conditions merely exchanged one fungal threat for another. Across all treatments, mycotoxin concentrations rose steadily from four weeks to three months, underscoring that time itself is a risk multiplier—even when grains appear visually intact.

The Protective Role of the Hull

One of the study’s most practical insights concerns the outer hull. Unshelled Coix seeds consistently harbored lower fungal loads and reduced mycotoxin levels compared with shelled or polished seeds. The hull functions as a natural protective barrier, limiting moisture ingress, oxygen exposure, and fungal penetration. While polishing improves appearance and convenience, it also strips away this biological defense, increasing susceptibility during storage and transport.

This finding challenges modern processing priorities that favor refinement over resilience.

Industry Implications: A Wake-Up Call for Modern Food Systems

For producers, exporters, and regulators, the implications are significant. Coix seed is often shipped long distances and stored in bulk, conditions that magnify fungal risk. Current drying and storage standards—especially in tropical and subtropical regions—may be insufficient to control mycotoxin accumulation over time.

Equally important is the study’s warning against visual safety bias. Polished grains may look cleaner and command higher prices, yet they can carry higher invisible risks. This trade-off is not unique to Coix seed and likely applies to other herbal grains, spices, and functional foods.

A promising development highlighted in the research is the use of DNA metabarcoding as an early-warning system. By detecting shifts in fungal communities before toxin levels spike, producers could intervene proactively rather than react after contamination is established.

Practical Recommendations

The study supports several actionable strategies:

• Maintain unshelled storage whenever feasible to preserve natural protection.
• Minimize exposure to heat and humidity during storage and transport.
• Adopt DNA-based fungal monitoring, especially for high-value or medicinal grains.
• Educate stakeholders that visual cleanliness does not equal safety.
• Use moisture-proof packaging for shelled or polished products intended for long storage.

These measures shift food safety from reactive testing toward preventive risk management.

Old Wisdom Meets New Challenges

This research sits at the intersection of traditional knowledge and modern science. Coix seed has long been valued for its health benefits, yet today’s globalized supply chains introduce slow-building, invisible hazards that historical practices never faced at this scale. Protecting functional foods now requires both respect for natural plant defenses and advanced molecular surveillance.

Sometimes, what appears to be “just a shell” is the difference between nourishment and risk.

References

• Guo, X. et al. (2025). Dynamic changes in fungal communities and mycotoxin production in Coix seed under different storage conditions. Food Control, xx(x), xxx–xxx.
• World Health Organization (WHO). Mycotoxins.
  https://www.who.int/news-room/fact-sheets/detail/mycotoxins
• Food and Agriculture Organization (FAO). Mycotoxin prevention and control in foodgrains.
• Zearalenone — PubChem.
  https://pubchem.ncbi.nlm.nih.gov/compound/Zearalenone
• Aflatoxin B1 — PubChem.
  https://pubchem.ncbi.nlm.nih.gov/compound/Aflatoxin-B1

Frequently Asked Questions

What is coix seed and what are its traditional medicinal uses?

Coix seed (Coix lacryma-jobi var. ma-yuen, also called Job’s tears) is a grass grain grown across tropical and subtropical Asia, used for both food and medicine. In Traditional Chinese Medicine (TCM), coix seed is classified as having properties that strengthen the spleen, remove dampness, clear heat, and expel pus. It is used in formulations for conditions including arthritis, oedema, respiratory infections, and digestive disorders. Modern pharmacology has identified multiple bioactive compounds in coix seed including polysaccharides (coixan), fatty acids (coixenolide), and lactam compounds that show anti-inflammatory, immune-modulating, and potential anti-tumour properties in laboratory studies. The seeds are also widely eaten as a food grain in congees, teas, and desserts.

Which fungi are most commonly found contaminating coix seeds?

Coix seeds in Asian markets have been found contaminated by a range of fungal species. Primary storage moulds include Aspergillus flavus and A. parasiticus (aflatoxin producers), A. niger, A. ochraceus (ochratoxin A producer), and multiple Penicillium species. Field fungi including Fusarium species may also colonise seeds during harvest and initial drying. Studies of commercial coix seed products in China, Taiwan, South Korea, and Japan have found that aflatoxin contamination—primarily B1 and B2—is present in a significant proportion of samples, with some studies finding contamination rates of 15–30% of commercial samples at levels exceeding regulatory limits.

What mycotoxins are associated with coix seed contamination?

Aflatoxins (B1, B2, G1, G2), produced by Aspergillus flavus and A. parasiticus, are the primary mycotoxin concern for coix seeds. Aflatoxin B1 is the most potent natural carcinogen known and is classified as a Group 1 human carcinogen by IARC. Ochratoxin A (produced by A. ochraceus and Penicillium verrucosum) has also been detected in coix seed samples and is classified as a possible human carcinogen. For herbal medicinal products, where consumers often take daily doses over extended periods (unlike casual dietary exposure), mycotoxin accumulation may be particularly concerning even at low per-dose concentrations. Most Asian pharmacopoeias now set aflatoxin limits for coix seeds (typically 5–10 ppb total aflatoxins, 2 ppb aflatoxin B1).

How can consumers tell if coix seeds are safe from mold?

Visual inspection provides some guidance but is not reliable for mycotoxin detection, as aflatoxins are invisible to the naked eye and undetectable by smell at dangerous concentrations. Indicators that suggest higher mold risk in coix seeds include: visible discolouration, black spots, or white/green powdery surface growth; musty, earthy, or abnormal odour; seeds that appear shrunken, discoloured, or have damaged seed coats; and products stored in non-airtight packaging under warm conditions. For assurance of safety, purchase from reputable suppliers with documented quality control testing; look for products with mycotoxin test certificates; avoid bulk-purchased seeds with unclear provenance; and store purchased seeds in airtight containers in cool, dry conditions away from moisture.

How should coix seeds be stored to prevent mold contamination?

Optimal storage of coix seeds requires controlling the three primary mold growth factors: moisture, temperature, and oxygen availability. Store seeds at moisture content below 12–13% (achievable by purchasing well-dried product and maintaining dry storage conditions); keep storage temperature below 15°C where possible (refrigerator storage is appropriate for long-term storage of significant quantities); use airtight containers such as glass jars or vacuum-sealed bags rather than paper or cloth packaging that allows moisture absorption; and check stored seeds periodically for any signs of moisture accumulation (clumping, condensation inside containers). For bulk storage in hot climates, traditional methods of mixing seeds with dried ash or storing in clay vessels in cool locations have been validated as effective by modern testing.