Gordon Shephard, South African Medical Research Council, Cape Town, highlights the analytical methods used to ensure our food is free from the natural toxins produced by fungi  

Food is the essence of life, yet the majority of people give little thought to the role analytical chemistry plays in ensuring our food is safe to eat. When food safety issues are raised, it is normally the perceived problems of pesticide - or other man-made chemical - residues in our foods causing concern. Yet natural toxins, produced by a range of microbiological organisms, are actually more potent toxins and carcinogens and therefore a greater threat to food safety. 

Amongst these natural toxins are the mycotoxins, secondary metabolites of filamentous fungi - more commonly known as mould.  These can infect agricultural products both in the field, during plant growth, as well as in poorly stored produce. Many of the mycotoxins are extremely stable and can survive in the agricultural raw material through to the finished product on the supermarket shelf. Similarly, their presence in animal feed can result in the carry-over of the toxin or its metabolites into animal products for human consumption. 

 

Maize kernels infected with mould can enter the food system

 

Although practically unknown in the developed world due to the vigilance of food safety authorities, human morbidity and mortality due to mycotoxin exposure is widespread in developing countries - especially in communities that are self-sufficient.

The mycotoxins have a diverse range of chemical structures and therefore biological effects. Although many hundreds of these toxins are known, researchers and food safety authorities concentrate on those produced by fungal pathogens of major crops - aflatoxins, fumonisins, trichothecenes (especially deoxynivalenol and T-2 toxin), zearalenone, ochratoxin A and patulin. And many countries now have legislated maximum levels of these toxins allowed in food. 

A characteristic of mycotoxin contamination is that it is not uniformly spread throughout an agricultural product. This means that specific sampling methods for each different product and toxin combination are needed.

A wide range of analytical methods have been developed for detecting mycotoxins in food. All these methods - apart from near-infrared - require extraction of the toxin from the food, using polar solvent mixtures, prior to analysis. These extracts, which still contain many soluble food compounds, can be analysed directly in enzyme-linked immunosorbent assays (ELISAs) or applied to a variety of screening methods such as lateral flow devices, dipsticks and biosensors. These methodologies all rely on the use of mycotoxin-specific antibodies to discriminate the mycotoxin from the coextracted food components and generally give semi-quantitative results. 

For more accurate determination of mycotoxins, the extracts require purifying.  The clean-up method of choice is solid phase extraction, where the mycotoxin binds to the sorbent, the impurities are washed through the column and finally the mycotoxin is released. Also popular are multifunctional columns - packed with adsorbent mixtures such as alumina and charcoal - that absorb the impurites as the mycotoxin extract passes through.   

After extract clean-up, mycotoxins can be analysed by thin-layer, gas or high-performance liquid chromatography. This last method coupled with ultraviolet, fluorescence or mass spectrometric detection is the most frequently used analysis technique. The use of tandem mass spectrometric detection here can provide multitoxin analysis combined with confirmatory evidence within the same experiment. Multitoxin analysis is useful for foods that can be contaminated by a number of different mycotoxins, produced by the same or different fungal species.

Interpretation of ancient writings suggests that mycotoxins have caused health problems since the earliest times of recorded history. And unless we can inhibit the toxin synthetic capability of the mycotoxin-producing fungi, we shall need to continue to tap into the advances in analytical chemistry to monitor these potent natural toxins.

Read Gordon Seymour Shephard's tutorial review 'Determination of mycotoxins in human foods' in issue 11, 2008 of Chemical Society Reviews