Phil Kenkel and Kim Anderson
Aflatoxins, metabolites of the fungus aspergilus flavus, A.parasiticus and A.nomis are potent liver toxins and carcinogens in animals. Aspergilus flavus is common in corn and cotton seed mill. A.parasiticus is more common in peanuts. Growing season conditions associated with drought and high temperature during grain fill are associated with aflatoxin contamination. In order to detect aflatoxin, proper sampling and testing is essential. Postharvest aflatoxin contamination can develop when grain in improperly managed or stored.
Sampling for Aflatoxin
Sampling is the most important step in determining aflatoxin levels. Contaminated kernels are usually not uniformly distributed in a load. This makes it possible for one sample to contain a probe-full from a hot spot while the next contains no contaminated kernels. Aflatoxin is also measured at extremely low levels: parts per billion. One part per billion is equivalent to one second in 32 years. A single kernel of contaminated corn can have an aflatoxin level of 50,000 ppb. Forty kernels of this level of contamination is enough to put the bushel above the 20 ppb FDA action level. Basing an aflatoxin test on one or two probe samples is like taking two handfuls from the contaminated bushel and assuming you have one of the forty contaminated kernels in each handful.
The recommended sampling procedure for detecting aflatoxin in farm trucks or semi-loads is to take at least ten probe fulls and collect at least 10 lbs. of corn. The corn must be below 16% moisture unless the test will be conducted immediately (on-site). Delayed test results on high moisture corn may not be accurate because the fungus can continue to grow and produce aflatoxins.
The recommended procedure is to grind the sample fine enough to pass through a number 14 sieve (about the consistency of sand) and then throughly mix it. A 2 to 4 lb. sub-sample should then be ground so that it can pass through a No. 20 sieve (about the consistency of instant coffee) before testing for aflatoxin. Grinding can be done with a blender or coffee grinder. Grinders designed specially for grain will provide the best results and durability. The sample should be stored below 13% moisture to prevent the continued development of aflatoxin.
FDA Action Levels
Aflatoxins are currently regulated by the U.S. Food and Drug Administration (FDA). Aflatoxin contaminated commodities are considered by the FDA to be adulterated and thus prohibited from interstate commerce under section 402(a)(1) of the Food, Drug and Cosmetic Act. Action levels represent the regulatory standard as to the level of contamination at which the agency is prepared to initiate regulatory action. The FDA action levels are provided in Table 1. Corn having an unknown use or destination (across state lines) is subject to seizure if it exceeds 20 ppb. aflatoxin. Interstate shipments of corn containing between 20 and 300 ppb. aflatoxin must be accompanied by labeling specifying the aflatoxin concentration and the species for which it is intended.
|Action Level (ppb)*||Commodity||Species|
|20||Any food except milk||Humans|
|20||Feed||All species except specified below|
|100||Corn||Breeding Cattle, breeding swine and mature poultry|
|300||Corn||Finishing beef cattle|
|300||Cotton seed mill||All species|
* Action levels represent the regulatory standard as to the level of contamination at which the agency is prepared to initiate regulatory action.
Testing for Aflatoxin
There are several methods for determining the presence, or presumed presence of aflatoxin in grain. Examining kernels for yellow-green fluorescence under a blacklight is a commonly used presumptive test. The fluorescence only indicates the presence of a mold and a potential for aflatoxin. Ideally the sample should be cracked or coarse ground for blacklight examination. While there is no exact guideline, the presence of one kernel or two glowing particles is a good rule of thumb for defining a positive blacklight level. The blacklight test can best be used as a first check. A large percentage of samples testing positive by the blacklight test will test negative for aflatoxin under chemical analysis. Tips of corn, corn bees wings, soybean hulls and weed seeds can produce a similar glow under blacklight. It is also possible to get a “false negative”, that is, contaminated samples can fail to fluorescence.
Several chemical test kits are also available for use in detecting aflatoxin. Commercially available rapid test kits provide test results within minutes. Qualitative test kits indicate whether the sample is above or below 20 ppb total aflatoxins. These kits must be kept refrigerated and used properly. More involved (and more expensive) qualitative rapid test kits are also available which provide an actual measure of aflatoxin concentration. Testing by an independent analytical lab provides the most accurate results. Analytical testing is available at the Oklahoma Animal Disease Diagnostic Laboratory, 405-744-6623, Kansas State Grain Inspection, Topeka KS, 913-296-3451 and Enid Grain Inspection, Enid Oklahoma 580-233-1121also offer aflatoxin testing. Sources for aflatoxin test kits include:
Romer Labs Inc. (800-769-1380) and Neogen Corporation (800-234-5333).
Handling and Storage Considerations
Corn should be dried to 15.5% moisture or lower within 24 to 48 hours of harvest to minimize the risk of mold growth and aflatoxin development. Natural air and low temperature drying do not achieve this level when corn moisture exceeds 18%. If in-bin drying systems are used, grain depth must be limited to a level which allows for over two cubic feet per minute of airflow for each wet bushel of corn. The probability for post-harvest contamination with aflatoxin is lower with high temperature drying systems because the corn is normally dried within 20 hours. A potential problem is the wet holding bin where mold growth can start and accelerate rapidly. It is also important to cool hot dried corn within 48 hours.
When loading corn storage bins opening the roof vents and running the aeration fans (push systems only) will help remove light trash and dust which can block air flow during aeration. The center core should be pulled out of each storage bin as soon as possible to remove the trash and fines which accumulate under the spout line. Inspect stored corn every 2 to 4 weeks. Monitor temperature throughout the bin and pull samples from below the grain surface at representative locations. Examine individual kernels from the samples for mold growth and insect activity. If the grain temperature changes by five degrees or more or the moisture content changes by ½% or more, the bin should be monitored closely and/or aerated. It may also be advisable to move the grain to another bin, and collect more samples while turning. Development of the aflatoxin producing fungus usually stops when the grain temperature is brought below 55°F and the grain moisture is 12% or lower.
Using Aflatoxin-Contaminated Grain
Broken and cracked kernels often have the highest concentration of aflatoxins. Cleaning grain with a gravity table or rotary screen can reduce aflatoxin concentration. Cleaning has the best chance of reducing aflatoxin for lots of grain with broken corn-foreign material content (BCFM) of 10% or more and moderate levels of aflatoxin (below 100 ppb.) Screenings should not be fed to any livestock because of the potentially high concentration of aflatoxins. The Oklahoma State Department of agriculture permits cleaning corn with an aflatoxin concentration up to 500 ppb. However the cleaned corn must have an aflatoxin level below, 300 ppb, be specially labeled and the corn screenings must be buried in an approved landfill or other designated tract of land.
When contaminated corn is used for ethanol production, aflatoxins do not appear in the distilled alcohol. However, the toxins are not degraded during fermentation but are simply concentrated in the spent grain by-products. Thus, most ethanol plants prefer not to use aflatoxin contaminated corn because of a desire to use the grain by-products for livestock feed.
One method to reduce moderate concentrations of aflatoxin contaminated corn is to blend the contaminated lot with clean grain. Blending may be a good alternative for an end-user who is blending feed for a less susceptible livestock species. If not done properly, blending poses the risk of contaminating clean corn with unacceptable levels of aflatoxin. Accurate sampling and testing is essential. If the concentration level is higher than anticipated, the entire blended lot may be above FDA action levels. Technically, the FDA does not permit the blending of contaminated and uncontaminated commodities, but does allow mixing different levels of “contaminated” commodities. However, when weather conditions have adversely affected crop conditions in wide geographic areas, FDA has relaxed its “no-blending policy” when the blending is conducted under specific guidelines from state authorities and the resulting mixture meets the aflatoxin guidelines for the particular species for which it is intended to be fed.
The Oklahoma Department of Agriculture will permit a commercial grain handler to blend corn with up to 500 ppb level of aflatoxin under certain restrictions. The blended lot must contain less than 300 ppb and the final product must be used only to formulate finished rations for beef cattle. The blended corn must also be sampled and tested for aflatoxin level. The blended corn must be labeled and the label must indicate that the corn has been blended, state the aflatoxin concentration and the date of the laboratory analysis. The label must also specify that the feed can only be used for finishing beef cattle.
Contaminated corn can also be detoxified with anhydrous ammonia. Ammoniation is not an FDA approved practice for corn in interstate commerce. Thus ammoniated corn must be used on-farm or sold for use within the state. Ammonia can be applied either as a gas (anhydrous ammonia) or liquid (aqua-ammonia). The ammonia reacts with the aflatoxin molecule to destroy its toxicity. Proper treatment can reduce aflatoxin concentrations by 95% or more. Swine and poultry may reject treated grain because of the ammonia smell.
Roasting corn at 290-300 oF can reduce aflatoxin content by 40-50%. However these temperatures are higher than those used in normal corn roasting or steam flaking process. Loss of feed value can be expected when using these high temperatures for treatment. Ensiling contaminated corn does not adequately degrade aflatoxins.
For more information:
Proceedings of 1996 Mycotoxin Workshop, Oklahoma Cooperative Extension Service, Division of Agricultural Sciences and Natural Resources.
Aflatoxins in Corn, Fact Sheet ID-59, University of Kentucky, Cooperative Extension Service, 1984.
Aflatoxins and other Mycotoxins, National Corn Handbook, Current Report 2105, Oklahoma State University, 1989.