Mycotoxins and their control post-harvest

Mycotoxins are a common hazard in the food industry, with the rate of incidents of contamination increasing over time. Agricultural products – specifically cereal grains, and fresh and dried fruits and vegetables – may be contaminated by fungi that produce these mycotoxins as secondary metabolites. This has a negative effect on food safety, and consequently on the health of consumers. This begs the question: what measures should be put in place to mitigate – or preferably, prevent – contamination? And how does one verify that products are safe?

The effect of mycotoxins on humans

Over 100 fungi species have been found to contain more than 400 toxic secondary metabolites, also known as mycotoxins. Exposure to mycotoxins via ingestion of contaminated foodstuffs may cause health issues in the consumer, including carcinogenicity. The extent of the effect of mycotoxin exposure on the consumer depends on the type of mycotoxin consumed, as well as the demographic and current health status of the consumer.

Studies have shown that exposing humans to mycotoxins results in the dysregulation of cellular processes such as DNA replication and protein synthesis, and should be avoided altogether. For this reason, there are maximum limits of these toxins permitted in food and animal foodstuffs, as determined by regulatory bodies.

Techniques to limit mycotoxin exposure

The food industry can limit consumer exposure to mycotoxins through a combination of techniques to limit and prevent fungal contamination of foodstuffs and feed. Note that fungal contamination of foodstuffs and feed presents as a dual hazard: a chemical hazard, in the form of mycotoxin production, as a direct consequence of fungal contamination, which is a microbiological hazard. Therefore, when assessing the contamination risk that these hazards present, the risk control strategy is to address the limiting of the biological and chemical hazards together and synergistically.

Fungal and (by extension) mycotoxin contamination of agricultural products usually occurs during the post-harvest period of the supply chain – the time after harvesting that includes cooling, cleaning, sorting, storage and packing, but before further processing into secondary products. Post-harvest contamination control is critical; however, contamination can occur during any stage of the food supply chain, even the growing stages; and if contamination cannot be prevented, processing techniques must be effective enough to remove the resulting toxins.

These are some of the most practical post-harvest techniques for reducing fungal contamination and the production of mycotoxins :

Temperature and humidity control

All microorganisms have an optimum moisture content and temperature for growth; to reduce their growth, the temperature and humidity of foodstuffs is controlled during post-harvest storage. When the temperature and equilibrium relative humidity are high, there is more microbial growth. Thus, a dryer storage area (below 85% relative humidity) and low temperatures (below 21⁰C) are preferred to hinder microbial growth.

Depending on the foodstuffs in question, keeping storage areas within these parameters can be a challenge. For example, this is easier to implement for cereal grains than fresh produce, as produce needs higher humidity to maintain its ‘fresh’ quality for longer, and a much lower temperature than 21⁰C. Controlling these factors is even more of a challenge in hot and humid countries, where refrigeration and ventilation may not be available.

Irradiation

Although often seen as a controversial method of hazard elimination, ‘irradiation’ as used on foods is based on the principle of using ionising energy to inactivate microorganisms by altering their cellular structure or mechanisms, thereby hindering microbial growth.

The effectiveness of irradiation depends on many factors, such as dosage, the attributes of the microorganism in question, and the environmental conditions in which the foodstuffs are kept. Irradiation does have a negative impact on organoleptic features and the micronutrient value of fruit and vegetables, and so should only be used on foodstuffs for which a colour, texture or nutritional change is not seen as devastating.

Use of electrolysed oxidising water

This is obtained from an electrolysed sodium chloride solution which transforms water molecules and chloride ions into chlorine oxidants. These have antimicrobial effects, due to increased levels of hydroxyl groups in the water which act as free radicals that cause a breakdown of the structure of spores, thereby decreasing the production of mycotoxins.

A summary of the advantages and disadvantages of each technique is given in Table 1 below.

Many other techniques can also be employed during processing, such as modified atmosphere packing (MAP) of packed foodstuffs, photodynamic disinfection methods (UV-C light, during a disinfection step in the process), antifungal agents as ingredients in edible coatings, and even biological control during the growing stage of the foodstuffs, to limit contamination by fungi. An example of this would be the direct use of certain antagonist microorganisms as biocontrol agents (BCAs), or the use of microbial metabolites such as antifungal proteins (AFPs) produced by competitor microorganisms.

Most food manufacturers use a combination of techniques throughout the supply chain to ensure maximum reduction of both the biological and the chemical hazards presented by fungal contamination, thereby reducing the number of mycotoxins present in a foodstuff.

However; how does one know whether these techniques that are implemented are effective or not?

How, when and what to test

When a method or a combination of methods is used to reduce fungal contamination, it is important to validate and verify the effectiveness of the methods used on microbial reduction.

Validation and verification are two very distinct and different processes. Validation entails proving that a control programme is effective – for example, temperature and humidity control. It often involves submitting samples to a reputable and accredited laboratory. The methods used (generally HPLC and UPLC) can detect and quantify the level of mycotoxins present in a particular foodstuff.

Laboratory-based methods are the first choice for validation, because they are sensitive; also, it’s possible to compensate for known sample matrix effects, by employing additional sample-preparation and analyte-extraction techniques.

Verification means showing that the validated control was performed correctly – for example, verifying that a (validated) photodynamic disinfection step has been performed correctly. Ideally, it should be done on site.

Due to the associated financial and time constraints, laboratory-based methods are not ideal for verification. But there are rapid systems available for this type of testing to verify that foodstuffs are not contaminated by mycotoxins.

There is no silver bullet, when it comes to controlling hazards; however, a well-thought-out approach combining the different techniques described above will help to ensure that your products are safe from fungal contamination, thus looking after both the consumer and your brand.

The FACTS multi-disciplinary team includes food scientists with industry experience in validating and verifying control measures throughout the food supply chain. They will give guidance on which controls are most applicable for your product matrix, and assist with risk assessments and with setting up a robust testing schedule to eliminate hazards. FACTS facilitates mycotoxin testing, with the added value of result interpretation.