Important factors in prevention
As already discussed, many factors can affect the production of mycotoxins. Some of the most important effects originate from biological factors, environmental factors, harvesting, storage and distribution or processing (Bryden W.L. 2012).
Biological factors include susceptibility of the crop, spore load and compatibility of the mycotoxin producing fungi (Brydan W.L. 2012). In general, the biological factors can be classified under factors based on the interaction between the crop and the invading fungi (Tola and Kebede, 2016). Some plants are known to be more resistant to the production of high levels of mycotoxins. Such crops possess certain genes that are involved in inhibiting mycotoxin synthesis. Genetic modification allows researchers to develop crops that are less susceptible for fungal invasion and mycotoxin production. The development of such crops could contribute to reducing the mycotoxin challenge (Nesic et al. 2021).
Environmental factors include for example temperature, humidity, mechanical injury and crop damage by insects (Bryden, W.L. 2012). Temperature is considered to be important in determining which fungi and mycotoxins can grow and infect the crop. Some fungi thrive better in low temperatures, while others grow ideally in higher temperatures. The temperature will therefore determine which mycotoxins will be produced. Aflatoxin risk for example increases with warmer temperatures and reduces when temperatures are lower, all because A. flavus thrives well under heat (Perrone et al. 2020). In general, it can be stated that fungi in the field need more moisture for growth than during storage. The suggested moisture levels needed for growth are about 22-25% of wet weight. This moisture will be used by the fungi for germination of the spores and also for growth (Hesseltine, C.W. 1979).
Mechanical injury of crops causes an increase in susceptibility to fungal invasion. Regardless of the environmental conditions, the growth of fungi is increased when the kernels are injured compared to uninjured kernel (Neme and Mohammed, 2017). The injuries provide an easier entrance for the fungi into the crop (Magan, 2003).
Insects are considered to be a major problem, since they can also damage the crops, making the crops more susceptible to invasion by toxigenic fungi and the associated production of mycotoxins (Chulze, S.N. 2010). They can form kernel wounds, that make it possible for the fungi to enter to the endosperm. Some larvae feed on the kernels and therefore cause damage. Besides, insects also function as vectors that transfer the fungal spores from the surface of a crop, to the inside of the kernel or stalk (Neme and Mohammed, 2017). It is therefore considered very important to reduce (insect caused) damage during the growing season of the crops, but also during drying and storage (Chulze, S.N. 2010).
When harvesting the crops, it often occurs that the crops are mechanically injured, allowing for high inoculation of fungal spores (Hesseltine, C.W. 1979). The moment of harvest can affect the amount of mycotoxins in the raw materials. In general, the sooner the harvest, the lower the mycotoxin levels (Jouany, J.P. 2007). For example, for fumonisins in corn, infection can occur in the beginning of ear development, but the mycotoxin production keeps increasing during further development and maturing of the corn ear. This suggests that early harvest can aid in preventing high mycotoxin levels (Warfield and Gilchrist, 1999).
Another important consideration during harvest is the cutting height of the used machinery, since this affects the interaction between the soil (possibly containing new fungi) and the crops. An initial rinsing step should be implemented to eliminate crops that are injured. Also, crops that are clearly infected, for example with foot rot, should be avoided during harvest (Jouany, J.P. 2007).
It was shown that the moisture level of the crops during harvest strongly affect the final production of mycotoxins (Ono et al. 2002). This indicates the importance of the time of harvest; whether the humidity is high (during morning dew or rainfall) or low (in the afternoon) (Jouany, J.P. 2007).
A distinction can be made between field fungi and storage fungi. In general, the fungi originate from the field, however they can further expand during storage. During storage, fungal production depends on the moisture levels, temperature, composition of the raw materials, and presence of insects. The importance of minimizing access of the ingredients by insects was already discussed, but humidity and temperature are also considered very important in reducing the production of storage mycotoxins. Fungi need humidity levels of more than 65% for growth during storage, and most growth occurs at temperatures between 10 and 40 °C (depending on the type of fungi). By keeping moisture levels and temperatures below these values, mycotoxin production during storage can be minimized (Atanda et al. 2011). In general, crops with moisture levels of over 130 g/kg are prone to the development of fungi and mycotoxins (Brydan, W.L. 2012). Sometimes, it can occur that there are leaks in storage packaging, floods or condensation, which cause the stored goods to become wet, allowing for increased production of mycotoxins. Such events should be avoided at all times (Hesseltine, C.W. 1979).
As already briefly mentioned in the section on the general information on mycotoxins, growth of fungi also depends on the bioavailability of nutrients (Milani, J.M. 2013). In general, it seems that fungi grow best when the substrate contains high levels of carbohydrates (Pitt and Hocking, 2022).
Besides insect damage occurring in the field, insects often attack stored ingredients and can cause an accumulation of moisture, serving as optimal circumstances for the growth of fungi and production of mycotoxins. Keeping insects away from the crops, in all stages of the production process, is an important factor in minimizing mycotoxin levels in the feed (Chulze, S.N. 2010).
The available oxygen and carbon dioxide can also influence the production of fungi during storage. Most fungi need oxygen but there are some efficient oxygen scavengers that can even grow at low levels of oxygen, for example Penicillium expansum. In general the oxygen level, amount of oxygen that is dissolved, is considered to be an important factor for mould growth. High levels of carbon dioxide can reduce fungal growth, however some species are still able to grow under high levels of carbon dioxide, such as Penicillium roqueforti (Pitt and Hocking, 2022).