LAISHA Explains
Plant pathology

Mycotoxins in grain and feed

The toxins some moulds leave in crops: aflatoxins, fumonisins, DON and others, how they get into food and milk, their limits, and how to prevent them.

Exam essential6 min readUpdated
On this page
  1. What mycotoxins are
  2. When and how contamination happens
  3. Limits and testing
  4. Preventing and managing mycotoxins
  5. Key points
  6. What the exams ask
  7. Questions people ask
  8. Sources
  9. Related on Laisha
  10. Discussion

Some moulds that grow on crops produce poisons. These mycotoxins are invisible, mostly tasteless, resistant to cooking, and can be present in grain that looks almost clean. They harm people and livestock, from acute poisoning to cancer and stunted growth in children, and they cause large economic losses when contaminated harvests are rejected.

Mycotoxins sit where plant pathology, food safety, animal nutrition and trade meet, which is why they appear in agriculture, food science and veterinary examinations alike.

What mycotoxins are

Mycotoxins are secondary metabolites of fungi, mainly of the genera Aspergillus, Fusarium, Penicillium and Claviceps. A crop can be infected by a fungus without containing toxin, and toxin can remain after the fungus has died. Several hundred mycotoxins are known; a handful matter most in agriculture.

The main mycotoxins in food and feed: aflatoxins from Aspergillus in maize, groundnuts, nuts and spices, which damage the liver and cause cancer; fumonisins from Fusarium in maize; deoxynivalenol and zearalenone from Fusarium in wheat, barley and maize; ochratoxin A from Aspergillus and Penicillium in stored cereals, coffee and wine; patulin in apples; and ergot alkaloids from Claviceps in rye and grasses MycotoxinMain fungiTypical cropsMain effects AflatoxinsAspergillus flavus,A. parasiticusmaize, groundnut, treenuts, spices, cottonseedliver damage, liver cancer;M1 passes into milk FumonisinsFusarium verticillioides,F. proliferatummaizedisease in horses and pigs;suspected human effects Deoxynivalenol(DON, vomitoxin)Fusarium graminearum,F. culmorumwheat, barley, maize(head blight, ear rot)vomiting, feed refusal,immune effects ZearalenoneFusarium speciesmaize, wheatoestrogenic: fertilityproblems, especially pigs Ochratoxin AAspergillus ochraceus,Penicillium verrucosumstored cereals, coffee,cocoa, dried fruit, winekidney damage;possible carcinogen Patulin · ErgotPenicillium expansum ·Claviceps purpureaapples and juice ·rye, grassesgut effects · ergotism(gangrene, convulsions) Field fungi (Fusarium, Claviceps) infect before harvest; storage fungi (Aspergillus, Penicillium) after, though Aspergillus also infects drought-stressed crops.
The main mycotoxins in food and feed, the fungi that produce them, the crops most often affected and their main effects on health.Diagram: Laisha, original.

Aflatoxins

Produced mainly by Aspergillus flavus and A. parasiticus in maize, groundnuts, tree nuts, spices and cottonseed, especially in warm climates. Aflatoxin B1 is the most potent natural liver carcinogen known and is classified by the International Agency for Research on Cancer as carcinogenic to humans (Group 1); the risk is multiplied in people with chronic hepatitis B. High doses cause acute poisoning (aflatoxicosis), as in outbreaks in East Africa in the 2000s that killed scores of people. Chronic exposure has been linked to stunting in children. When dairy animals eat aflatoxin B1, part of it is excreted in milk as aflatoxin M1.

Fusarium toxins

  • Fumonisins, mainly from Fusarium verticillioides in maize, cause fatal brain disease in horses and lung disease in pigs and are suspected of human health effects.
  • Deoxynivalenol (DON, "vomitoxin"), from F. graminearum and F. culmorum in wheat, barley and maize affected by Fusarium head blight or ear rot, causes vomiting and feed refusal, especially in pigs.
  • Zearalenone, an oestrogen-like toxin, causes reproductive problems, particularly in pigs.
  • T-2 and HT-2 toxins in oats and other cereals in cooler regions.

Others

  • Ochratoxin A from Aspergillus and Penicillium species in stored cereals, coffee, cocoa, dried fruit and wine; damages the kidneys.
  • Patulin from Penicillium expansum in rotting apples and apple juice.
  • Ergot alkaloids from Claviceps purpurea, which replaces rye and grass grains with dark sclerotia. Medieval epidemics of ergotism ("St Anthony's fire") caused gangrene and convulsions.

When and how contamination happens

Fungi are often divided into field fungi, which infect before harvest (Fusarium, Claviceps), and storage fungi, which grow after harvest (Aspergillus, Penicillium). The division is not strict: Aspergillus flavus also infects maize and groundnuts in the field.

Before harvest

  • Drought and heat stress at flowering and grain fill strongly increase aflatoxin in maize and groundnuts.
  • Insect damage (ear borers, pod borers) opens entry points.
  • Wet weather at flowering favours Fusarium head blight in wheat and barley.
  • Late harvest, lodging and leaving crops in the field to dry.

After harvest

  • Grain stored too moist (above about 14 per cent for most cereals, lower for groundnuts and oilseeds) and warm.
  • Damaged kernels, insect infestation and condensation in stores.

Limits and testing

Countries set maximum levels for food and feed, and Codex Alimentarius sets international standards used in trade. Examples of Codex levels are 15 µg/kg of total aflatoxins in groundnuts destined for further processing, 10 µg/kg in ready-to-eat almonds, hazelnuts and pistachios, and 0.5 µg/kg of aflatoxin M1 in milk. Some jurisdictions set much lower limits, which affects exporters.

Testing is complicated by uneven distribution: a few highly contaminated kernels can contain most of the toxin in a lot, so sampling plans (many small samples combined, ground and sub-sampled) matter as much as the laboratory method. Methods include rapid test strips and ELISA kits for screening, and chromatographic methods (HPLC, LC–MS/MS) for confirmation.

Worked example

From contaminated feed to milk

A dairy cow eats 10 kg of compound feed a day containing 20 µg/kg of aflatoxin B1, and gives 25 litres of milk. Assume about 2 per cent of the ingested B1 appears as M1 in milk (typical carry-over is about 1 to 3 per cent, higher in high-yielding cows).

  1. B1 eaten per day = 10 kg × 20 µg/kg = 200 µg.
  2. M1 excreted in milk = 200 × 0.02 = 4 µg per day.
  3. Concentration = 4 µg ÷ 25 L ≈ 0.16 µg/kg (taking 1 L ≈ 1 kg).

That is below the Codex level of 0.5 µg/kg, but three times a stricter limit of 0.05 µg/kg used in some markets. Feed quality controls milk safety, which is why dairies test both.

Preventing and managing mycotoxins

Prevention throughout the chain works far better than trying to remove toxins later.

In the field

  • Resistant or tolerant varieties, and varieties adapted to the season so that grain fill avoids drought.
  • Irrigation where possible during flowering and grain fill; timely sowing.
  • Controlling insects that damage ears and pods.
  • Biological control of aflatoxin: applying atoxigenic strains of Aspergillus flavus, which do not produce aflatoxin, so they outcompete toxin-producing strains in the field. This has reduced contamination in maize and groundnuts in parts of Africa, the Americas and Asia.
  • Crop rotation and residue management against Fusarium; fungicides at flowering where approved and justified.

At harvest and in storage

  • Harvest at maturity, avoid damaging grain, and dry quickly to safe moisture.
  • Sort out damaged, discoloured and mouldy kernels, by hand or machine; much of the toxin is in a small fraction of the grain.
  • Store dry, cool, clean and protected from insects; hermetic storage helps.

Downstream

  • Diverting contaminated lots to safer uses where permitted, blending only where regulations allow (blending to dilute is prohibited in many jurisdictions for food).
  • Binders (clays and other adsorbents) and enzyme products in animal feed to reduce absorption of some toxins.
  • Processing such as cleaning, dehulling and nixtamalisation of maize reduces some toxins; ordinary cooking does not destroy most of them.

Key points

  • Mycotoxins are toxic fungal metabolites, mainly from Aspergillus, Fusarium, Penicillium and Claviceps; they survive cooking.
  • Aflatoxin B1 is a potent liver carcinogen (IARC Group 1) in maize, groundnuts and nuts; aflatoxin M1 passes into milk.
  • Fusarium toxins include fumonisins (maize), DON and zearalenone (cereals); ochratoxin A affects stored products; ergot affects rye.
  • Drought, insect damage, wet flowering weather and moist, warm storage increase contamination.
  • Codex levels include 15 µg/kg total aflatoxins in groundnuts for processing and 0.5 µg/kg M1 in milk; sampling must account for uneven distribution.
  • Prevent through resistant varieties, irrigation, insect control, atoxigenic biocontrol, prompt drying, sorting and good storage.

What the exams ask

  • School-leaving and entrance papers: what mycotoxins are, examples such as aflatoxin and ergot, and the crops they affect.
  • Degree and research-fellowship entrances in plant pathology, food science and animal nutrition: producing fungi, conditions for contamination, health effects, field and storage fungi, and control strategies including atoxigenic strains.
  • Food safety and veterinary examinations: maximum levels, sampling and testing, carry-over into milk, and feed binders.

Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.

Questions people ask

Does cooking or roasting destroy aflatoxin?

Not reliably. Aflatoxins and most other mycotoxins are heat-stable. Roasting groundnuts reduces aflatoxin only partly; removing damaged kernels and preventing mould are what work.

Is mouldy grain safe for animals?

No. Livestock are affected by many mycotoxins, often more than people, and toxins can pass into milk, eggs and meat. Mouldy grain should not be fed without testing.

Why do mycotoxin tests on the same lot give different results?

Toxin is concentrated in a few kernels, so small samples vary a lot. Following official sampling plans, with many increments and proper grinding, gives reliable results.

Sources

  1. IARC. Some Traditional Herbal Medicines, Some Mycotoxins, Naphthalene and Styrene. IARC Monographs Volume 82, 2002; and Volume 100F, 2012.
  2. Codex Alimentarius. General Standard for Contaminants and Toxins in Food and Feed (CXS 193-1995), as amended.
  3. FAO and WHO. Safety Evaluation of Certain Contaminants in Food. JECFA reports, various.
  4. Wild, C. P. and Gong, Y. Y. "Mycotoxins and human disease: a largely ignored global health issue." Carcinogenesis 31, 71–82 (2010).
  5. Bandyopadhyay, R. et al. "Biological control of aflatoxins in Africa: current status and potential challenges in the face of climate change." World Mycotoxin Journal 9, 771–789 (2016).

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