Food Supply Worries of an Agricultural Scientist Part 4: Aflatoxin

Field corn colonized by Aspergillus flavus



This post is going to be another struggle for balance.  The threat from this particular mycotoxin in the food supply is a so large that it makes the risks that worry most people look tame.  It makes the subject of one of my previous posts about another mycotoxin, vomitoxin, look like a virtual non-issue. Aflatoxin is one of the most potent acute toxins known and one of the most carcinogenic.  Because of this the average international tolerance for aflatoxin B1 in food is 4 parts per billion (PPB).  The average tolerance for food for children is 0.2 PPB and for milk 0.05 PPB (USDA ERS publication source for this data).  These are seriously low numbers.  I want to accurately represent the seriousness of this risk.  

At the same time I also want to accurately represent the extent to which the commercial food supply is now protected from that risk.  The same ERS document above reported US crop losses in 2003 from mycotoxins in corn, wheat and peanuts of $932 million and another $466 million for testing.  That is all for preventing this toxin from getting to us. There is a lot going on in the background that few people recognize.

Folks in the food industry may well ask “why even bring it up!?”  First of all, this is no secret.  My Google Alert for “Aflatoxin” sends me articles nearly every day.  Also I raise this issue to try to “calibrate risk.”  I saw an entry in a comment string on another blog the other day where someone wrote, “I hope this is a move towards chemical-free food.”  I’ll give that person the benefit of the doubt that they know that all food is made of chemicals (proteins, fats, carbs…).  Their concern was about synthetic pesticide residues.  I doubt that they know about “chemicals” like aflatoxin.  They should.  It is thousands of times more toxic than a typical pesticide residue.

As a 54 year-old, I grew up in a time when this toxin was barely even recognized or managed.  If you are younger the good news is that the food industry has been working hard to protect you from this risk for most or all of your life.  The bad news is that this is still one seriously nasty toxin and it is impossible to completely exclude from our food supply (it is also “equal-opportunity” for Organic or conventional, local or distant).  

Aflatoxin is a fully “natural” chemical that is an “intermediary metabolite” produced by a fungus called Aspergillus flavus (and some related species).  These are organisms that we plant pathologists call  “opportunistic pathogens”.  They can’t really do all that much damage to a healthy plant, but in certain plant tissues that are compromised by drought stress and/or insect damage, this fungus can grow and make it’s deadly toxin.  Aspergillus infection (which is most often an issue in field corn, peanuts, cotton seed, pistachios, Brazil nuts, and almonds) isn’t mostly a problem for yield losses. It is mainly a chemical contamination issue which hurts the grower because of “dockage.” (It really isn’t an issue for sweet corn because it is harvested before the stage where this occurs.  There are some other mycotoxins (deoxynivalenol, zearalenone and zearalenol, and occasionally fumonisin) that can occur in sweet corn, but by avoiding ears with worm damage those can be avoided.

For centuries, Aflatoxin was just an unrecognized part of the food supply.  In the late 20th century it began to be recognized as an issue.  Since then there have been tests developed to detect the toxin and FDA food regulations to make sure that our exposure to this toxin is limited as much as is possible.  It doesn’t mean that there are never issues. In 2006, 76 dogs died because of aflatoxin contaminated dog food.  Remember Peanut Corporation of America that knowingly shipped Salmonella contaminated peanuts and disrupted a huge part of the food supply?  As could be anticipated, that disreputable firm also shipped known aflatoxin contaminated peanuts.  These are relatively rare events, but they are real.  The system mostly protects us from exposure to this toxin, but it isn’t perfect.

For the Third World the scenario isn’t nearly as positive.  People in Africa that depend on peanut (ground nuts) for their protein are often poisoned (chronically and even acutely) by this toxin.  Peanut imports from India to Europe have recently dropped because of more rigorous Aflatoxin standards.  Do a Google News search for aflatoxin and you will see that this is a live issue.

There is a cool technology that might help contain this threat that I want to describe but not over-promote.  Back in 1992 or 1993 I was working at the biocontrol company, Mycogen.  I heard about an aflatoxin control method that was pioneered by Dr. Michael Cotty of the USDA.  I invited him to make a presentation to our company.  His idea was to overwhelm the natural, toxin-forming population of Aspergillus with a naturally occuring strain of the same fungus that still infects plants but which does not make the toxin (an “atoxigenic strain”).  This was done by growing the special fungal strain on cooked rice or wheat grains and then scattering them in the field so they would make spores that would colonize most or all of the plant tissues where the toxin-producing strains could otherwise grow.  The idea was to out-compete the toxin-forming strains.  As a biologist I was highly skeptical that he could overwhelm the natural population.  It turned out that Dr. Cotty’s data was positive and it has been confirmed and improved over the years.  As someone who worked on biological controls for many years I can appreciate this all-to-rare achievement.  This approach has now been commercialized and has even been picked up by a major ag technology company.  Its a little tricky from the marketing point of view because the grower who would need to apply it isn’t always the party that has the economic damage from the toxin.  We’ll see how it plays out. 

You might think that sounds creepy to intentionally spread a fungus, but a another, very closely related, similarly “atoxigenic” Aspergillus, (A. niger) has been used for centuries to make soy sauce and more recently citric acid.  There are other fungi we use to make cheese and of course the yeast that makes our bread and wine and beer is a fungus.  

Does this new biocontrol mean that aflatoxin is off of my “worry list?”  Unfortunately not. I’m encouraged that this biological control  technology might begin to further reduce the risk, but it can’t eliminate it.   So what do I as a scientist do about this “worry?.”  I avoid peanuts or tree nuts “in the shell”. There are good methods to detect aflatoxin contamination in shelled or processed nuts and that is good.   Unshelled nuts are on my very limited “do not eat” list along with trans-fats which I heard about and started avoiding 30 years ago.  Brazil nuts which are picked up from the jungle ground after unknown periods of residency there and opportunity for fungal infection, are absolutely on my “do not eat” list (If I’m wrong about that for the current supply I hope someone will correct me).

So, bottom line, I know that the commercial food system is doing all it can to protect us from aflatoxin and doing a far better job than when I was a kid. It is still an area that needs innovation, and I’m glad there are scientists still working on it.

You can certainly comment on this site or you can email me at [email protected].

Contaminated corn image by Pat Lipps, Ohio State University




  1. Mike

    Just interested in what you would suggest for those who enjoy growing as much of their own food as possible on their own land, as far as aflatoxin is concerned. Would sight-checking harvested foods for mold be enough to prevent real risk? I personally don’t have much interest in growing peanuts or corn, but I do plan to plant wheat, almonds, and walnuts.
    Also, would you mind sharing your complete “do not eat/drink/do” list? Thank you for your time.

  2. Mike

    Sorry, but before I forget – is there a list of food crops that are at risk for aflatoxin while others are not? You mentioned corn, peanuts, and tree nuts, I believe. Thank you again.

  3. Steve Savage

    The crops with the biggest aflatoxin risk are peanuts, almonds and other tree nuts, and field corn (not really sweet corn, it is too immature). There are many other mycotoxins, but most are less scary. As a gardener or small scale farmer I’d just avoid anything moldy or decaying. Sometimes you can cut away the effected part and eat the rest, but give it a good margin.
    I’ve always avoided peanuts or tree nuts in the shell (the others get inspected individually!). I avoided transfats for decades (back then you could only tell by the “partially hydrogenated” statement about the oil). I’m very careful about cross contamination from any surface where I have had raw meat in the kitchen. Most everything else is just about diversity and moderation.

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