Mycotoxins in the grain market

by Teresa Acklin
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A proactive stance will benefit handlers and processors dealing with trace toxins in grain.

   BY CHARLES R. HURBURGH JR.

   Mycotoxins are poisonous trace organic residues of mold deterioration. Mycotoxins are not alive themselves, but instead are very potent compounds causing a variety of human and animal health problems at very low dosages — parts per million (ppm) or parts per billion (ppb).

   Individual mycotoxins are produced by specific mold strains under more restrictive growth conditions than for the molds themselves. Mycotoxins are not automatically produced whenever grain becomes moldy. However, from a risk viewpoint, the likelihood of toxins is greater in damaged kernels than in sound kernels.

   Although aflatoxin has received the majority of public attention, advances in chemical detection methods have identified several others that can create problems for grain users. All mycotoxins are present in non-uniformly distributed trace concentrations. Normal bulk sample quality detection methods are hard to use for trace levels. Trace toxics in general will present increasing difficulty for bulk grain handlers.

   Table 1 on page 28 summarizes the major mycotoxin-producing molds, the grains usually affected and utilization limits. The Fusarium and Aspergillus strains account for the majority of toxin problems.

   The health impacts of mycotoxins are much less precise than regulatory limits or guidelines would suggest. The concept of mycotoxin poisoning was first discovered in England, when 100,000 turkeys mysteriously died of what was ultimately identified as aflatoxin.

   Most regulatory limits for trace toxics are set as a large safety factor (100 or more) applied to toxicological data from laboratory animals. In the U.S., if there are no limits established and the compound is a known carcinogen, then the so-called Delaney Clause sets the limit at zero until more definitive data is available. Table1 also gives the known effects of the mycotoxins listed. Not all are cancer causing. Aflatoxin is indeed a potent carcinogen, but the immediate effects of the Fusarium toxins are often more striking. Most of the economic impacts are on animal health.

      FUSARIUM AND ASPERGILLUS TOXINS.

   Cool, wet conditions favor the growth of Fusarium species molds, which can produce several mycotoxins detrimental to livestock. Fusarium strains can produce vomitoxin (also known as DON, deoxynivalenol or “refusal factor”), zearalenone (known as “giberella toxin”) and fumonisins. Swine are the food animal species most at risk from these mycotoxins, which usually are found in maize. Horses are extremely sensitive to fumonisins. Vomitoxin causes a dose-related reduced feed intake in swine at levels above 1 ppm in the ration. Higher levels (5 to 10 ppm) can cause vomiting and nearly complete feed refusal. If clean feed is offered, feed consumption resumes within 24 hours or less.

   There are no documented reproductive effects of vomitoxin in swine. Cattle have consumed up to 10 ppm vomitoxin with no adverse effects. The common absorbents used for aflatoxin, for example aluminosilicate or bentonite, are of little value against vomitoxin.

   Zearalenone is an estrogenic mycotoxin that occurs in maize and wheat. Sometimes zearalenone and vomitoxin occur in the same sample, although one usually predominates.

   Prepubertal gilts show signs of estrus, straining and prolapse at feed levels as low as 1 ppm. In cycling sows, zearalenone above 3 ppm has caused anestrus and pseudopregnancy, delaying the estrus cycle for up to 60 days. Very high levels of greater than 30 ppm can cause early embryonic death in sows. However, in late gestation, zearalenone is not likely to abort sows. Cattle given more than 10 ppm zearalenone may have infertility and interference with the estrus cycle.

   Fumonisins, a group of toxins produced predominately in maize, are believed to be most prevalent when cool, wet weather at crop maturity follows early season drought stress. Hepatic damaged and elevated serum liver enzymes occur in all livestock. Horses are highly susceptible to liver damage and also to the brain disease leukoencephalomalacia (moldy maize poisoning) at fumonisin levels above 10 ppm.

   Swine may have liver damage at feed fumonisin concentrations above 25 to 50 ppm, and cattle develop mild liver lesions at concentrations above 100 ppm. Liver damage in these species is transient, and liver function returns to normal when exposure stops. Feed concentrations above 100 ppm in swine can cause acute pulmonary edema. Toxin concentration is highest in broken grain, such as maize screenings.

   The Aspergillus species molds, generally characterized by greenish yellow coverage of kernels or ears, can produce aflatoxin and ochratoxin under stress conditions. Aflatoxin is a known carcinogen. Both cause animal health problems.

   Aflatoxin can be produced when maturing maize is under drought and insect stress with prolonged periods of hot weather (daytime highs above 32°C, nighttime lows above 24°C). Aflatoxin is a cancer promoter and an immunosuppressant.

   Effects are possible on poultry and small swine at 20 ppb. Aflatoxin-contaminated grain should not be fed to lactating dairy cattle, as it will carry to the milk at 1/200 to 1/400 dilution rates.

   Ochratoxin is a relatively uncommon mycotoxin of maize, but has been reported several times in the U.S. Midwest. Swine are the most likely animals affected by feed levels of 1 to 3 ppm. Ochratoxin causes increased water consumption and urination with renal tubular damage which may result in permanent scarring of the kidneys.

   Mycotoxins create great concern, even panic, among consumers. As the understanding of growth conditions increases, there is an increasing tendency to anticipate outbreaks and to search for the first reports of toxins in new harvest. Most mycotoxin production occurs in the field before harvest, rather than in storage. Poor storage practice can increase already existing mycotoxin levels.

   Table 2 summarizes the weather conditions required for toxin production, as well as is known. Clearly, the Aspergillus and Fusarium toxins are unlikely in the same area in the same year. However, the real problems occur when abnormal weather spreads the risk to larger areas than the market is accustomed to handling.

   The crucial point is that, even in situations where mycotoxins have been detected, there are usually as many or more clean lots as contaminated ones. Crops and growing regions must not be unilaterally condemned by rumor. The key element in preventing unjustified extrapolation of problems is an effective screening and follow-up program.

   Weather conditions are not a guarantee of mycotoxins. Our knowledge of growth conditions is not precise enough to predict incidence with absolute certainty. Mycotoxin production is highly weather-sensitive, and conditions can change rapidly. An effective scouting-preharvest check program in your trade area is the only way you can assess your local situation.

      SAMPLING, TESTING AND HANDLING.

   Mycotoxins are a kernel-by-kernel situation. A few kernels have very high levels (several thousand times the average), and most have none. Measurement methods require small ground samples (2 to 50 grams), so sampling and sample handling are obvious problems.

   A representative mycotoxin analysis cannot be made with original samples less than 2.25 kilograms, and most laboratories recommend 4.5-kg samples. Samples should be either diverter-based or multiple probings. If other tests are run on the sample, no material should be removed before preparing for mycotoxin analysis.

   The entire original sample should be ground before subsampling for analysis portions. The reason for this goes back to the kernel-to-kernel nature of mycotoxins. Dividing whole grain samples (for example with a Boerner divider) carries a high probability of not detecting the toxin.

   There are several test methods for mycotoxin analysis. They are generally summarized in Table 3 on page 30. All the chemical methods require ground samples and the handling of liquids.

   Mycotoxin tests should be done in a separate area apart from physical factor grading. Recognize that mycotoxin testing will take time, labor and money. This is why an advance survey/scouting program is valuable.

   The immunoassay-based tests are the most common. Operators need training and practice to be accurate. Mycotoxin tests are not as forgiving of operator error as the common bulk sample quality tests.

   The net result of typical sampling and analysis errors is that mycotoxin data have an uncertainty of plus or minus 25% to 40% of the reported value. This is the reason contradictory results can be obtained from the same load or storage. Obviously, repeat analyses on the same small ground sample will be much closer, but the sampling and sample preparation contributes error not associated with the test itself.

   Mycotoxin policy is sensitive and often unclear. Deliberate blending to dilute toxin levels is illegal in many countries if the level of regulated toxin is known prior to the blending.

   In the United States, for instance, the Food and Drug Administration receives data from grain inspection and state laboratory officials, but it generally lacks resources to act on bulk grain data before the grain has been moved, sold or used. In practice, user requirements and specifications for the delivered grain control mycotoxin problems more effectively than government agencies can.

   The higher the percentage of over-tolerance lots, the more likely it is that some resold grain will exceed limits. In the case of aflatoxin, U.S. export grain must be tested. Other mycotoxins are often limited by contract. In the years when aflatoxin was over tolerance in 15% to 20% or more of the inbound lots, random blending with no segregation of inbound grain failed to protect grain handlers.

   In the U.S., “reconditioning” is now allowed for aflatoxin-contaminated maize. Since January 1993, reconditioning by mechanical cleaning at less than 50% of the rated capacity of the cleaner is acceptable. Only one attempt and resample is permitted.

   There are mixed reports on the effectiveness of cleaning at reducing mycotoxin levels. Generally, in farm grain, the breakage levels are low, and cleaning is not as effective. Moldy kernels are weaker, and therefore will break in handling. Cleaning at a later point seems more effective against mycotoxins than cleaning at the farm/country elevator.

   Mycotoxin-contaminated kernels are less dense than sound kernels. Density-separation is used for peanuts but has not as yet been practical for grain elevators.

   Controlled applications of vapor ammonia will break down aflatoxin (not other mycotoxins). This has been used for maize and cottonseed. The maize turns dark in the process, and its subsequent use is limited to feed.

   The best method for handling mycotoxin problems is to obtain good advance information as to the potential in your area, then use both visual and objective screening to sort out the worst lots from inbound grain. The emergence of a problem is not the time to plan your operating and testing strategy.

   The future will hold increasing concerns over trace toxics in general, not just mycotoxins. Analytical technology steadily increases the number of detectable compounds. Customer demands will increase in proportion to knowledge. Preplanning for the problems that are possible in your area, then periodic review of those plans, is a good investment.

   Experience has shown that all grain can find a legitimate, safe use if mycotoxin problems are not approached as kneejerk responses to panic. Be prepared.

   Dr. Charles R. Hurburgh Jr. is a professor of agricultural and biosystems engineering at Iowa State University, Ames, Iowa, U.S., specializing in grain quality, processing and handling. This article is based on his presentation to the 65th conference of the Grain Elevator and Processing Society. Animal health information in the article was provided by Dr. Jim McKean, extension veterinarian, lowa State University.

Strategy for testing and dealing with mycotoxins

   • Know which toxins are possible in your area.

   • Understand the weather conditions likely to cause toxin development.

   • Have a strategy for getting a pre-harvest estimate of toxin levels.

   • Have a facility-specific testing and handling procedure and train your personnel.

   • Know where toxin-contaminated grain could be used.

   • Have information and options available for producers, do not leave customers hanging and arrange for quantitative testing if needed.

   • Review the plan periodically and update as technology changes.

   • Hope you never need to use the plan, but know you must have one.

Table 1: Grain mycotoxins

ProducingGrains often
MycotoxinmoldaffectedUse limitsaAnimal effects (most sensitive)cHuman effectsd
AflatoxinAspergillusMaize, peanuts, Human: 0Depressed immune system, intestinalIn extreme, liver
cottonseed,Animals, excepthemorrhage, liver degenerationdeterioration;
sorghumdairy: 20 ppblow level intestinal
disorders; cancer
acceleration
OchratoxinAspergillus,Maize, wheat, rice,Poultry: 0.25 ppmKidney degeneration, decreased egg
Penicilliummixed feed Swine: 1 ppmproduction
VomitoxinFusariumMaize, wheat, rice,1 ppmFeed refusal, gastric disorders
hay, mixed feed,(monogastric animals)
screenings
ZearalenoneFusariumMaize, hay, mixed 1 ppmReproductive disorders
feed, screenings(monogastric animals)
FumonisinFusariumMaize, screeningsHorses: 10 ppmBrain damage, liver degeneration,
heart failure (horses, swine, poultry)
aLimits, in parts per million or parts per billion, suggested for most sensitive use; other uses may allow higher levels.
cDoes not mean other animals are insensitive. Consult veterinarian.
dAflatoxin documented. Others not determined conclusively.

Table 2: Favorable weather conditions for toxin production

MoldFavorable weather/storage conditions
AspergillusFieldHot, dry at silking (maize), or grain fill (all);
high temperatures 32°C to 35°C, nighttime lows
above 24°C; drought, insect stress.
StorageaMoisture greater than 17%;
temperature over 16°C.
FusariumFieldCool, wet at grain fill; excessive rainfall;
can be relieved by warm, dry weather
after maturity.
StorageWet ear maize (above 22% moisture);
unlikely in dry, bulk grain
aOther molds normally crowd out Aspergillus in wet grain in storage.

Table 3: Methods of mycotoxin analysis

Time
MethodTypeper sampleComments
Black lightScreening1 to 2 minutesAflatoxin only; 40% to 60%
(ultraviolet)false positives; more than
four glowing particles per
2.25-kg sample indicates
high likelihood of greater
than 20 ppb.
ImmunoassayScreening5 to 10 minutesSeveral toxin kits
test kitsavailable; some chemistry
involved; ground, 5-g to
20-g samples required;
gives yes or no answer.
ImmunoassayQuantitative5 to 20 minutesSeveral toxins available.
with reader
ChromatographyQuantitative2 to 4 hours overLaboratory confirmation;
2 daysimpractical for elevators;
all toxins can be tested
this way.
Screening types do not measure the level of contamination, only estimate presence or absence of mycotoxin.
Quantitative types estimate concentration of mycotoxin in ppm or ppb.

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