To deal with mycotoxin problems, there must first be an understanding of the fungi which produce them, their growth parameters and interactions with crops. Mycotoxin control is both fungus specific and crop specific. Control of mycotoxins during growing seasons is a crop management problem. Control during storage is a food technology consideration.
Toxic fungal metabolites, known as mycotoxins, are chemically diverse and occur in a wide variety of substrates, including foodstuffs. Food safety is rising to a top priority in modulating commodity composition such as control of molds and yeasts. When ingested, mycotoxins have the potential to impair human and animal health, as well as predisposition to infectious diseases and reducing production efficiency, thereby resulting in economic losses in livestock.
Improvements in cereal grain variety and biotechnology, as well as advances in production management, have resulted in reduced insect damage, more timely harvest and a decrease in moldy cereal grains. However, these problems continue to occur since it is not possible to control the weather and other environmental conditions. Appropriate disposition of mycotoxin laden cereal grains will be a necessity. Utilization of contaminated cereal grains as animal foodstuffs is one alternative; therefore, identification and accurate determinations of mycotoxin levels are essential to channel products to appropriate end users.
The extent of raw corn contamination with mycotoxins varies with geographic location, normal annual climatic fluctuations, agronomic and storage practices, and the vulnerability of the plants to fungal invasion during all phases of growth, storage and processing. Levels of mycotoxins are influenced by environmental factors such as temperature, humidity and rainfall during preharvest and harvest periods. Often high levels of mycotoxins are associated with hot, dry weather followed by periods of high humidity. Insect damage also may be a factor.
At field and storage sites, mycotoxin presence may be associated with visual and/or aromatic evidence of mold growth; however, mold infection, with mycotoxin contamination, can be so subtle as to escape casual inspection. Of the mycotoxins found in corn, ie, aflatoxins, deoxynivalenol (DON, vomitoxin), fumonisins, ochratoxins and zearalenone, the most research has been reported for aflatoxins because of their carcinogenic potential; however, fumonisins and deoxynivalenol are more ubiquitous. Also, aflatoxins often are found in cotton, peanuts and rice. Specific mycotoxins appear to be limited to certain environmental loci and to specific crops (CAST 2003); however, one fungal species can produce multiple mycotoxins.
Because requirements for mold growth and mycotoxin development are specific, mycotoxin occurrences in grain masses are inconsistent due to differing microenvironments with mycotoxins generally occurring in hot spots. When assaying for mycotoxin presence and concentrations, there are three sources for variance: sample collection, subsampling and analysis. Sample collection is of greatest concern and shown to be the greatest source of error. Characteristically, individual kernels contain high levels of mycotoxins. As kernels are milled and particulate size decreases, variance decreases. Also, movement involves blending which contributes to sample homogenization. Therefore, inadequate sampling may result in erroneous data.
While controlling the occurrence of mycotoxins in foods may be possible, economic feasibility is questionable. If food borne mycotoxin regulations were based solely on direct health effects, questions of economic feasibility for meeting strict standards could be ignored, but with potentially disastrous consequences for less developed, food exporting countries (Wu 2005, Wu et al 2004). An important consideration for control of mycotoxins will be improvement of the fundamental knowledge behind the ecology and epidemiology of fungi which produce mycotoxins.
Common misconceptions throughout the cereal grain industry must be clarified. 1) Wherever fungus is observed, there is not necessarily a mycotoxin problem. 2) When mycotoxins are found in a specific locale in a given crop one year, they will not always be present at that site. 3) When a mycotoxin is detected in a crop at a specific site, the mycotoxin will not be present wherever the crop is grown. 4) Mycotoxins are not always produced in susceptible crops. 5) Not all mycotoxins are carcinogens. 6) Mycotoxins can be produced in the field as well as during harvest and storage.
Some mycotoxins can be removed prior to processing. Fusarium verticillioides, one of the fungi which can produce fumonisin, is found in the tip cap of the corn kernel; therefore, screening prior to fractionation of the kernel will eliminate most of the fumonisin in a load of grain. Aspergillus flavus, the fungus which can produce aflatoxin, is found throughout the corn kernel; therefore, screening often is ineffective.
There has been a commonly held belief that mycotoxins stress yeast during fermentation (Kelsall and Lyons 2003) resulting in lower ethanol yields. However, under controlled conditions (Murthy et al 2005), it was determined that aflatoxin B1 added at levels of 100, 200, 350 or 775 ppb did not affect fermentation rate nor final ethanol concentrations. Fungal metabolism results in conversion of oxygen and starch to monosaccharides and ultimately to water, carbon dioxide and heat:
C6H12O6 + 6 O2 Æ 6 H2O + 6 CO2 + 677 calories
Consequently, cereal grains laden with fungal growth may have less carbohydrate available for conversion to useful end products.
When using cereal grains, eg, barley, corn, wheat, as sources of starch for fermentation to ethanol, relative market value of coproducts resulting from the dry grind process must be considered. For producers to realize more income, they must measure and manage those items of interest to the end users. Technologies for improved fractionation of the grain kernel will result in distillers dried grains with solubles (DDGS) which have the oil and fiber removed. Therefore, resulting in animal foodstuffs that can be fed to nonruminants as well as ruminants (Rausch and Belyea 2006). Animal nutritionists must be provided precise and accurate data with respect to compositional characteristics of the DDGS.
With cereal grain marketing based only on deductions rather than on premiums, producers do not have incentive to provide management which increases costs of production. Grain buyers procure characteristics, not attributes. If producers were paid on the basis of grain quality, management will be enhanced. At present, producers are paid by the ton (not bushels) delivered. Thus, if a producer delivers grain at 10% moisture, there is a decrease in gross income/acre; therefore, grain will be delivered at >15% moisture. To address this problem, grain must be sold on a dry matter basis.
Microbial Bacillus thuringiensis (Bt) based products have been used commercially for 40 years. The safety and advantages of Bt protected plants to control insects have been reviewed with the intent to enable a more science based discussion of the risks, safety and usefulness to producers, the environment and society (Betz et al 2000). Mean fumonisin levels were reported to be less in Bt corn than in control hybrids; the lower fumonisin levels in United States Bt corn hybrids were consistent with findings from France, Spain, Italy, Turkey and Argentina (Hammond et al 2004).
Modified grain plants may provide fewer broken stalks, less stalk required for standability, minimal volunteer corn the next growing season and decreased insect damage; therefore, reducing fungal and bacterial penetration that result in lower mycotoxin levels. Reducing the amount of stalk and husk required will decrease amount of fertilizer/bushel, result in fewer problems at picker head and gathering chains, and decrease material traversing the combine; therefore, reduce energy needs and diminish flow of four letter and hyphenated words. The concept of using carbon-carbon linkages in optimal ways is fundamental to providing food and fuel for life on this planet. It is essential to develop and support research directed to address the issues discussed above that impede optimal agricultural production and adversely affect human and animal health. Multidisciplinary and interinstitutional cooperation is essential to succeed in these research endeavours. Preharvest and postharvest methodologies and technologies must be developed to optimize food safety and economic return. The corn kernel consists of four main constituents: fiber, germ, starch and protein (Fig 1). Fiber in corn kernels is composed of pericarp fiber (outer covering of the kernel, mainly composed of dead cell wall material) and endosperm fiber (inner cellular fiber in the kernel).
Structurally, the kernel consists of 83% endosperm, 12% germ, 5% pericarp and 1% tip cap. Endosperm is comprised of starch (amylose and amylopectin) (83%), protein (8%) and endosperm fiber (3%) which has 70% hemicellulose (arabinose and xylose) and 30% cellulose. Fifty per cent of kernel fiber is pericarp fiber, which is 70% hemicellulose and 30% cellulose. Germ contains 33% oil (11% palmitoleic, 24% oleic and 62% linoleic acid) and 18% protein. Starch is the only fermentable component of the kernel. Germ, fiber and protein do not ferment and are recovered, as a single coproduct, DDGS, at the end of dry grind corn process.
In the dry grind process (Fig. 2), ground corn is mixed with process water (recovered from different sources in the plant) and subjected to liquefaction (conversion of starch to a range of 25 to 55 dextrose equivalents (DE) which is liquefied corn starch), saccharification (conversion of liquefied starch to maltose and glucose) and fermentation (conversion of monosaccharides and disaccharides to ethanol). The beer (a mixture of water, ethanol and nonfermentable solids) is stripped off for ethanol.
Remaining material (water and nonfermentable solids), also called whole stillage, is centrifuged to separate semisolid product called wet grains (30 to 35% solids) and a watery product called thin stillage (5 to 7% solids). Part of the thin stillage stream is recycled (backset) to the front end of the dry grind process (as process water) to mix with ground corn. Thin stillage is concentrated into a 25% solids syrup (condensed distillers solubles) by evaporating water. Syrup and wet grains are mixed and dried to 90% dry matter to produced DDGS. Therefore, there is no ‘back pipe’ (waste stream).
New technologies are being developed to fractionate the kernel into germ, pericarp fiber and endosperm fiber prior to fermentation. Fractionation technologies can be divided into wet and dry fractionation technologies. Wet technologies, such as quick germ (Singh and Eckhoff 1996), quick fiber (Singh et al 1999, Wahjudi et al 2001) and enzymatic dry grind (Singh et al 2005), involve a soaking step prior to fractionation. Depending upon the fractionation process used, DDGS with high protein (35 to 58%) and low crude fiber (2 to 8%) can be produced. Rausch and Belyea (2006) determined the value of DDGS produced from various technologies.
Adoption of those processes for producing marketable coproducts, the need for ascertaining locations of mycotoxins in the various process streams will be exacerbated. While the dry grind process will produce the same four products, ethanol, DDGS, carbon dioxide and water, as well as corn oil and fiber, relative amounts will change. With the enzymatic dry grind process, there will be 4 pounds of DDGS (compared with 16 pounds in the conventional process), 3 pounds of germ and 8 pounds of fiber. From the bushel of corn, we should continue to have 2.7 gallons of ethanol from starch. Multidisciplinary projects among engineers and scientists must be designed to elucidate basic information from which we will determine appropriate processing systems.
Citation
Tumbleson, M. E, V. Singh, K. D. Rausch, D. B. Johnston, D. F. Kendra, G. L. Meerdink, and W. M . Haschek. (2006). “Mycotoxin control during grain processing.” ASABE Paper No. 066040. St. Joseph, MI: ASABE.
Presented at the 2006 Annual International Meeting of the American Society of Agricultural and Biological Engineers.
References
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