Harvest and post-harvest control
The appropriate harvest time is essential for reducing the risk of a mycotoxin contamination. In general, it can be stated that early harvest leads to lower concentrations of mycotoxins. Additionally, special attention should be paid to careful harvesting procedures.
Another important point is the usage of appropriate harvest equipments that is correctly adjusted to avoid damage to the kernel, since damaged kernels could be predisposed to infection during storage. Additionally the equipment should be free from previous harvest residues to avoid cross contamination. Fusarium spores in the soil are a ubiquitous issue and it is therefore important to avoid allowing the soil to contact the harvester machine to reduce the risk of mycotoxin contamination of healthy grain.
Before storage the grain should be cleaned of shrunken or damaged kernels to reduce the infection rate.
Hygiene/Storage facilities and conditions
Firstly storage of the grain without delay as well as good storage facilities are necessary to decrease the mycotoxin hazard.
Some important points to achieve good storage conditions:
- Clean silo from former harvest residues
- Avoid dead volumes
- Beware of “wet spots”
- Adapt silo size to herd size
Most important factors to control during storage:
- moisture content
- chemical preservatives
- antifungal agents
During storage drying the crop by aeration is a very important procedure. The moisture content should be 15% or below to reduce fungi and toxin exposure. Mycotoxins are chemically very stable, therefore there is no significant reduction during the drying step.
The main limiting factors for fungal growth are water activity (available water content to mold in a substrate) and temperature. The minimum required water activity for fungi metabolism is higher than 0.65.
Grains are very good insulators. Due to this fact the grains in the center of the silo will have the same temperature as during harvest, a relatively high temperature. The grains next to the silo walls cool faster because of the decreasing environment temperatures. The convention currents of warm air from the center meet cool, damp atmospheric air at the top and sides of the storage bins causing condensation and leading to the formation of “wet spots” which are the ideal places for fungal growth and subsequent mycotoxins’ production.
For this reason homogenous temperature during storage should be kept in mind as well as rotation of the grains as these procedures can reduce the risks of wet spots. Moreover the temperature in the storage facilities can increase as a result of microbiological activity, increasing the risk of fungal growth and mycotoxins’ production.
Furthermore the silo size must be adapted to the herd size in order to avoid storing the grain for too long.
Insect and rodent control is very important because their activity in stored products creates favourable microclimates for fungal growth. They damage kernels and therefore predispose these parts to fungal manifestation.
Chemical preservatives and antifungal low toxicity like propionic acid agents should be used to supplement good management practice rather than as a substitute for safe, clean handling practices of mixed feed. These mold inhibitors cannot eliminate mycotoxins in the grain but they will inhibit further fungal growth during storage.
It is very important to keep in mind that with bad storage conditions and without antifungal agents the mycotoxin load can increase with different mycotoxins!
Stability of fungi and mycotoxins
Due to their low molecular weight and their chemical structure mycotoxins are very stable. They are resistant to storage and high temperature. Therefore the mycotoxins content cannot be reduced significantly after drying the grains or during several manufacturing processes.
Trichothecenes like deoxynivalenol and T-2 toxin are stable at 120°C and moderately or partially stable at 180°C or 210°C. Fumonisins are quite heat stable and for this reason the toxin content can not be reduced significantly during processes below 150°C. Additionally, the effectiveness of such heating treatment often depends on time and pH-value. During storage, milling and cooking zearalenone is very stable and moreover the content of this toxin does not decrease at high temperatures. Ochratoxins are fairly stable and can only partially degrade during cooking, fermentation process and roasting.