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Advanced grain cleaning for mycotoxins control

Mycotoxin contamination is a global threat. To solve the problem, we need to apply a full value chain approach, from the field across the whole process line to the finished product. Grain cleaning is part of this.

Dr. Katarina Slettengren
Scientific Head Analytics Laboratory 
Bühler AG

Mycotoxins have a major economic impact. This is, first of all due to direct spoilage and the decrease in nutritional value of the cereals with subsequent yield losses. In addition, the nutritive value of grains decreases after contamination by moulds. The second, and even more important reason, is the impact of mycotoxins on human and animal health, affecting animal productivity as well as domestic and international trade (Iheshiulor et al., 2011). When consumed by humans or animals above certain levels, mycotoxins may lead to a toxic response called mycotoxicosis. Its effects depend on the animal species, the type of mycotoxin, the concentration level, and the exposure time. Typical symptoms include reduced productivity, reflected in reduced weight gains and lower fertility, and immune suppression. Clinical symptoms of mycotoxicosis include diarrhoea, liver and kidney damage, pulmonary edoema, vomiting, haemorrhaging and tumours (Streit et al., 2012; Iheshiulor et al., 2011).

In animal feed, the EU regulates the allowable levels of aflatoxin B1 and provides Commission guidelines for deoxynivalenol, zearalenone, fumonisins, and ochratoxin A. However, no in-depth research has so far been conducted to explore the influence of lower concentrations (below legal or guideline values) on animal performance, which may also have a significant effect. Aflatoxins are considered to be the most serious threat of all, with aflatoxin B1 being the most toxic and a highly carcinogenic form. Consumed by lactating cattle, aflatoxin B1 not only threatens the animal itself, but – transferred to the milk in the form of aflatoxin M1 – is a potential carcinogen for humans as well (Streit et al., 2012).

Clinical indications of aflatoxin poisoning include liver damage, reduced productivity, and lower egg shell and carcass quality, as well as increased susceptibility to disease. Deoxynivalenol leads to decreased feed intake and lower weight gains in pigs (> 2 mg/kg), or even vomiting and feed refusal at high concentrations (> 20 mg/kg). Zearalenone can cause fertility problems and lead to vulva reddening and vaginal or rectal prolapse in pigs. Fumonisins affect the central nervous system and may produce porcine pulmonary edema (PPE). Ochratoxin A mainly affects the kidneys in pigs and poultry and may cause fatty livers in poultry. Other trichothecenes, such as T-2 and HT-2, can also reduce feed intake, produce skin irritation, and lead to abnormal offspring, as well as increasing the risk of disease. Ergot alkaloids may lead to nervous-system disorders, convulsions, diarrhoea, and gangrene, as well as affecting fertility (Bryden, 2012). Furthermore, moulds may produce different types of mycotoxins, since feedstuffs are often based on a blend of different raw materials. This means that they often contain a mixture of several different mycotoxins with both additive and synergistic effects on animal health and performance (Streit et al., 2012).

To reduce mycotoxin levels, we must apply strategies along the whole process chain. This starts in the field, where fungal growth and mycotoxin formation must be prevented in the grains by applying good agricultural practices, including proper variety selection and crop rotation beside other control measures. After harvest, we must stabilize the grains by drying, in order to prevent further mould growth and mycotoxin formation. During further processing, it is important that we reduce the mycotoxin levels and remove contaminated grains at the earliest possible stage to prevent cross-contamination. Mycotoxin levels can be reduced by grain cleaning – physical elimination of contaminated kernels. This must be done before the grains are milled, after which it will be difficult to remove the contamination. Another important point is to ensure safe handling and storage of the grains. Safe storage means storage under dry and cool conditions without any risk of condensation. When fed to the animals, additives may be included in the finished feed to bind or degrade the mycotoxins and thus to reduce their intake by the animal. However, the efficiency of such additives and their interactions with antibiotics and other drugs are not always fully understood, and their bioavailability may possibly be altered (Zhu et al., 2016). Figure 1 provides a schematic view of mycotoxin mitigation along the process chain.

Mycotoxins are resistant chemical compounds that cannot simply be destroyed by heat, as you typically would inactivate bacteria. To reduce the mycotoxin levels in grains, we can remove contaminated fractions physically by grain cleaning. Typically, only a small fraction of a total grain lot will contain most of the contamination. By removing these fractions, the mycotoxin level of the lot as a whole can be clearly reduced. Dust, small or broken fractions, as well as low-density and shrivelled kernels typically contain a high mycotoxin level. Often, contaminated grains will show visual signs of mould infection, such as colour defects, surface modifications, or shape deformations. As illustrated in Figure 2, dust and broken kernels can be removed by size separation and aspiration, and low-density kernels by density separation. Finally, kernels with visual signs of contamination can be removed by optical sorting.

Schematic illustration of mycotoxin mitigation along the process chain.

Bühler is currently taking part in the Horizon 2020 project MycoKey, where it has conducted industrial-scale grain cleaning studies together with the Institute of Sciences of Food Production (ISPA) to verify the mycotoxin reduction efficiency of grain cleaning. One of the many studies we performed was on corn (maize) with different levels of aflatoxin contamination of 10 µg/kg and 20 µg/kg. The trials were performed on batches of 3 tons each, which were cleaned by size separation in conjunction with aspiration and optical sorting. After cleaning, total aflatoxin contamination was found to have been reduced to below 4 µg/kg, resulting in an overall reduction rate of 70-95% with a total reject rate around 10%. This means the product quality was improved from biomass and feed grade to food grade. A previous study on wheat from 2012 shows similarly good results. Raw wheat with a contamination level of 4000 µg/kg deoxynivalenol was cleaned by size separation in conjunction with aspiration and optical sorting to a level of 1200 µg/kg. This translates into an overall reduction rate of 70% (Figure 3). Further processing of the grains by milling lowered the contamination level further still to 600 µg/kg, meaning a total reduction rate of 85%.

Depiction of grain fractions that typically contain high mycotoxin levels and how they can be removed.
Graph illustrating the total aflatoxin reduction during a grain cleaning study of maize, using sieving with aspiration and optical sorting.

Although the efficiency depends on the type of grain and its contamination, these case studies show that grain cleaning really works to efficiently reduce mycotoxin levels in grains. Knowledge and experience are needed to select the optimal combination of cleaning technologies in order to reduce mycotoxin levels in the most effective way. In order to implement the mitigation strategies introduced, further training of feed processors and producers is needed. Smooth collaboration between farmers, producers, researchers, and legislators is required for further improving the strategies and thus coping with future challenges. In the end, a single mitigation strategy will not work on its own. What we need is an optimised combination of these separate strategies.

Deoxynivalenol (DON) reduction in a grain cleaning study of soft wheat using size separation in conjunction with aspiration and optical sorting.

References available on request.

We also suggest you to read our previous article titled "Feed Additives, Premixes and Vitamins".

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