Aflatoxin in peanut: why it starts in the soil — and what it costs at the border
Aflatoxin is the peanut industry’s most expensive contaminant. Understanding the fungus, the crop’s unusual biology, and where the risk is really set is the key to controlling it.
Aflatoxins are toxins produced by the soil fungi Aspergillus flavus and Aspergillus parasiticus. The most common form, aflatoxin B1, is one of the most potent naturally occurring carcinogens known — the naturally occurring mixtures are classified by the WHO’s cancer agency (IARC) as a Group 1 human carcinogen, mainly linked to liver cancer. That severity is why the tolerances are so low, and why aflatoxin, more than yield, often decides whether a peanut crop can be sold at all.
Why peanut, of all crops
Peanut is unusually exposed because of a quirk of its biology called geocarpy: after the flower is fertilised above ground, a stalk (the "peg") bends down and pushes the developing pod into the soil, where it matures underground. A. flavus is a common soil fungus in warm regions, so the peanut’s edible kernels effectively develop for weeks inside the fungus’s home. In most crops the harvested part hangs in the air; in peanut it ripens in direct, prolonged contact with the reservoir of the pathogen.
The toxin, and why limits are measured in parts per billion
There are several aflatoxins (B1, B2, G1, G2), and a related form (M1) that carries over into the milk of animals fed contaminated feed. Because the hazard is serious and you cannot reliably "process the toxin away" once it has formed, food-safety authorities set maximum levels in parts per billion (ppb) — vanishingly small amounts. Peanut kernels for direct human consumption face some of the strictest limits in the entire food trade, and a single lot over the limit can be downgraded or rejected at the border.
Maximum aflatoxin in peanut kernels for direct human consumption, by market
| Market | Limit | Basis |
|---|---|---|
| European Union | 4 ppb total (2 ppb B1) | Among the strictest in world trade |
| United States (FDA) | 20 ppb total (action level) | Industry often applies 15 ppb |
| China (GB 2761) | 20 ppb B1 | Set on B1, the most potent form |
The gap is the point: the EU tolerance is several times stricter than the US or China, so a lot that clears one market can be rejected at another border. Limits also shift by product, intended use, and over time — always verify the current rule for your destination.
The risk is set in the field, under stress
Contamination is often blamed on poor storage, and storage does matter — but a great deal of the risk is decided before the crop is ever lifted. Infection is driven by drought and heat stress late in the season: a water-stressed plant cannot defend its developing pods, and the fungus colonises them in the weeks before harvest. Anything that wounds the pod — insects, nematodes, mechanical damage, or simply over-mature pods — opens the door further. This is why a hot, dry finish to the season is a red flag, and why managing aflatoxin only after harvest is already too late for much of the damage.
Where and when aflatoxin risk builds
| Stage | What raises the risk | What helps |
|---|---|---|
| Field (pre-harvest) | Drought and heat stress at pod fill; pod damage; heavy A. flavus in the soil | Irrigation where possible; timely harvest; source suppression |
| Harvest & drying | Slow or uneven drying; leaving the crop damp | Dry quickly to safe moisture; avoid mechanical damage |
| Storage | High moisture and warmth; poor ventilation; insects | Keep dry and cool; ventilate; control pests |
The second window is drying and storage. Even a clean field crop will spoil if it is dried too slowly or stored damp and warm, because the fungus keeps producing toxin above a certain moisture level. Rapid drying to a safe moisture content, and cool, dry, well-ventilated storage, are the essential second line of defence. But note the asymmetry: good storage can stop more toxin forming — it cannot remove toxin that has already been made.
Attacking it at the source
Because so much of the risk is set in the soil and pod zone before harvest, one of the most powerful levers is to reduce the toxin-producing fungus at its source. Beneficial microbes can occupy the same niche as A. flavus and suppress it — a mechanism called competitive exclusion — lowering the toxigenic load the crop is exposed to. This is the logic behind biocontrol, and behind Huasheng’s Orvan™ technology, which couples aflatoxin suppression with improved nitrogen fixation in a single application. It complements good agronomy and drying rather than replacing them.
In short: aflatoxin in peanut is a field problem as much as a storage problem — much of the risk is written into the crop under late-season stress, before harvest. The most robust protection is layered: sound agronomy, fast drying and clean storage, and suppression of the toxigenic fungus at its source.