Mycotoxins are chemical compounds produced by fungi growing on organic substances such as corn, cottonseed or peanuts which, when consumed, have some undesirable effect on the animal consuming them. These effects can range from vomiting, feed refusal, weight loss, various types of tumors, and in some cases death. More than 100 toxic compounds produced by fungi have been identified and about 45 of these occur in grain crops. Some mycotoxins are rather rare in occurrence; others such as aflatoxin are quite common in some years.
Mycotoxins have probably been present in man’s food supply since the beginning of civilization. However, increased use of mechanical harvesting may have resulted in increased harvest of badly molded grains that would have been discarded with hand harvesting.
The seriousness of the mycotoxin problem varies with the year, the crop being grown and the intended use of the crop product. The producer of agricultural commodities stands to lose most from contamination of his product. For this reason, producers should be constantly aware of the possibility of mycotoxin contamination.
Aflatoxins are a type of mycotoxin produced by the Aspergillus flavus group of fungi. Aspergillus flavus is a common fungus and can be found in soil, air, and decaying plant residues. Infection by A. flavus and subsequent aflatoxin production can occur in the field, in transit and in storage. Most reports indicate that infection occurs in the field while aflatoxin production can occur whenever the product is exposed to favorable conditions either in the field or in storage.
Conditions favoring the aflatoxin-producing fungus, A. flavus are as follows:
|Temperature oF||86oF plus||80oF to 200oF|
|Relative Humidity||85% plus||62% to 99%|
|Kernel Moisture||18%||13% to 20%|
The figures listed above were taken from several sources and in some cases represent compromises among findings. In many cases, development of the fungus usually stops when the temperature is below 55°F and grain moisture is 12 percent or below.
Drought is considered to be one of the major factors that predisposes corn and peanuts to infection by A. flavus. In 1977, drought stressed corn in the southeastern states was plagued with a high incidence of aflatoxin. In 1980, aflatoxin was reported in higher than normal levels in the Southeast as well as Texas, Missouri and even southern Illinois. Corn is especially subject to aflatoxin problems when drought occurs.
Management Practices to Minimize Aflatoxin at Corn Planting and During the Season
- Select a regionally adapted variety.
- Use a balanced fertilization program designed for optimum yields.
- Select a planting date that has traditionally resulted in the highest yield on your farm or in your area.
- Follow normal cultural controls to limit damage by ear feeding insects including early planting dates.
- Attempt to best utilize your irrigation practices to deliver optimum water from silking stage to late dough stage.
- Make adjustments in combine ground speed and cylinder speed to minimize trash and broken kernels from ending up in the hopper. Aflatoxin is often associated with broken or light weight kernels.
- If drought has occurred during the season, consider harvesting irrigated or high yielding fields separately from dryland or poor yielding fields. This will allow some loads to escape from being contaminated with high numbers of infected kernels.
- Aflatoxin can increase in corn standing in the field if the moisture content is above 18 percent. If corn has dried below 15 percent, do not allow late season rains to raise the moisture content back up to levels where more aflatoxin can be synthesized by the fungus growth already in kernels. Harvest corn when hurricanes or late season fronts threaten.
- Improper handling of high moisture grain can lead to continued A. flavus growth and aflatoxin production before storage. Do not hold high moisture corn in wagons or similar holding areas for more than 6 hours. Place high moisture corn awaiting drying in a “holding bin” or “wet bin” and force air through it to keep it cool.
- Corn which collects in auger wells and pits around dump stations frequently contains the fungus A. flavus. Thoroughly clean all such areas before and after use. Remove leftover grain from trucks, trailers, holding bins, drying facilities and storage bins before beginning a new lot of grain.
- Moisture content is by far the most important factor affecting the growth of microorganisms in stored grains. The long-term safe storage moisture content for corn in Texas is 13 percent. After harvest, cool grain as soon as possible and dry down to 15 percent. Do not dry corn too fast as rapid drying may cause cracking and unacceptable quality losses.
Methods of Detecting Aflatoxin in Corn
Blacklight: Corn contaminated with kernels infected with Aspergillus flavus will often produce a characteristic bright greenish-yellow flourescence (BGYF) when examined in a darkened room under long wave ultraviolet light or what is commonly called a blacklight. This fluorescence is the result of the properties of Kojic acid. Kojic acid is another compound produced by A. flavus; it is not related directly to aflatoxin. The blacklight test is a presumptive test and assumes aflatoxin is present if the presence of Kojic acid indicates that A. flavus is present. Blacklight positive samples usually contain some aflatoxin but this method cannot determine if the load exceeds the F.D.A. Guideline of 20 ppb. If a load of corn is found to be positive with a blacklight test, it is recommended that a representative sample of this lot be taken and a determinative test such as a minicolumn or other test be performed.
Minicolumn Test or Holiday Minicolumn Test: The minicolumn test is a determinative test for aflatoxins. It is rapid, relatively inexpensive and can be performed at the buying point. This test is commonly employed to determine if corn exceeds the FDA Guideline of 20 ppb. If the sample used for analysis is representative of the entire load, it is an acceptable method for determining whether to accept or reject loads.
Thin-Layer Chromatography (TLC): The thin layer chromatography method for determining aflatoxin is a more precise measure of determining aflatoxin concentrations in corn. If this method is coupled with AOAC (Association of Official Analytical Chemists) approved extraction methods, it is superior to other methods of quantitating aflatoxins. This method is commonly employed by testing laboratories.
Rapid Test Kits: Recently, several commercial firms have marketed rapid test kits for use in determining the aflatoxin concentration in corn samples. These test kits are self contained and provide all the necessary instructions to complete an analyses on-farm, at the elevator, or at the buying point. Several kits are available at varying prices (ranging from $5-15/test depending on quantity ordered). The following table includes sources for 3 such test kits.
|Product Name||Company||Price / Test (1996)||Other|
|$407.50 / 50||Fluorescence|
2990 Anthony Rd.
Burlington, NC 27215
|$15 / 2||5 or 20 ppb|
|Agri-Screen Field Test Kit||Neogen Corp.
620 Lesher Pl.
Lansing, MI 48912
|$94 / 12||20 ppb|
Other methods of determining and quantifying aflatoxins are available. As research discovers new techniques, they will likely be employed. It is important, however, to remember that aflatoxins are concentrated in a few kernels that contaminate an entire load. For this reason, a representative sample is essential to determine the degree of contamination. A multi-level probe sampling at several sites and depths will give the best results. AOAC approved methods generally agree that an initial sample weight of 10 pounds (5 kilograms) is desirable.
Reducing Aflatoxin in Contaminated Corn
Detoxification: Several compounds are under investigation for detoxification of aflatoxins from corn. One such method being examined in southeastern states is use of anhydrous ammonia. This method has shown some promise in feed corn, but problems with feed refusal have limited its use. Also, kernel discoloration results from this method, and this has prohibited its use in food grain corn.
Blending: The physical mixing of contaminated (greater than 20 ppb) corn with uncontaminated (less than 20 ppb) corn is not a recommended practice. While occasionally successful, too often the result is a larger lot of contaminated corn. Efforts to separate and work with contaminated lots through screening or alternative usage will give the most consistent and desirable results in marketing contaminated corn.
Screening: The use of small opening screens has resulted in successfully reducing the aflatoxin concentration in food grade or other high value corn. Screening removes broken kernels which often contain a high percentage of the aflatoxin contamination. Screening has been coupled with the use of vibrator or gravity tables which remove any light-weight, infected kernels and thus further reduce the aflatoxin concentration in food grade corn. Screening corn can significantly reduce the aflatoxin concentration in corn with only a minimal (5 to 10 percent) loss in grain weight or yield.
Management Practices to Minimize Aflatoxin in Peanuts
Planting and During the Season
- A. flavus is common soil fungus and may increase in population on certain crop residues including peanuts. Fungal populations increase on certain soils where peanuts follow peanuts. Using crops other than peanuts or corn on these problem fields helps reduce the population of the fungus in the soil.
- Peanut or corn residue on the soil surface favor the increase of the organism. Deep burial of crop residue is recommended.
- Control pod damaging insects such as lesser cornstalk borer. Damage by this insect may increase incidence and detection of this problem.
- In problem fields where peanuts follow peanuts, a late planting may be advisable. When peanuts mature during the cooler temperatures at the end of the growing season, it lessens the degree of Segregation III occurrence.
- Use an inverter digger to keep pods off the soil surface while curing within the windrow. Do not place more than two rows together in a windrow unless vines are very small. This increases ventilation around the pods and facilitates rapid and uniform drying. Leave peanuts in the windrow only long enough to achieve the desired combining moisture of 18 to 25 percent. Increased length of exposure heightens the chance of exposing dry peanuts to late season rains.
- Some mold damage and aflatoxin accumulation develop during harvesting and curing. Adjust combines to prevent pod damage and transport peanuts in vented trucks and trailers to prevent heating. Force air through the truck or trailer.
- Dry peanuts immediately following harvesting without undue delays in transportation equipment.
- Do not blend or mix damaged or lower quality peanuts with high quality ones in the same truck or trailer. Harvest good fields or good portions of a field into separate trucks or trailers. Remember that it only takes one kernel to condemn an entire load.
Methods for Detecting Aflatoxin in Peanuts
The visual method for detecting aflatoxin in peanuts is a presumptive test based on the observance of visible growth of Aspergillus flavus on a sample drawn from a peanut load. Any lot of farmer’s stock peanuts which contain any visible A. flavus mold on any kernels or portion of kernels will be classified as Segregation III regardless of the percentage of damage. These peanuts must be diverted to oil stock where the use of meal is restricted to non-feed uses. This determination should be made with the aid of a low power microscope. This procedure is indirect and does not give an indication of the actual quantity of aflatoxins present.