Aflatoxins are highly toxic and carcinogenic metabolites produced primarily by certain isolates of filamentous fungi viz., Aspergillus flavus and Aspergillus parasiticus. These toxigenic fungi invade the agricultural commodities in the field prior to harvest or during post-harvest operations such as drying or curing and in storage, and secrete aflatoxins. Aflatoxin contamination is a main problem in a wide range of agricultural commodities including peanut, corn, pistachio nuts, almonds, rice, wheat, and spices like chilli, black pepper and turmeric. Several recent studies have reported the occurrence of aflatoxins in dates, dried apricot and prunes as well. Aflatoxins were first discovered after the outbreak of “Turkey X disease” in turkey poults in 1960 in England, which caused mortality of about 1,00,000 turkey poults. The consumption of mold-contaminated peanut meal was attributed to the death of turkey poults. Analysis of the contaminated peanut meal revealed the presence of the fungus A. flavus and its toxic metabolites. The toxic metabolite produced by the fungus was named as “aflatoxin” representing the first letter of the Genus “Aspergillus” and the first three letters of the species “flavus”. So far 18 different types of aflatoxins have been described among which aflatoxin B1 (AFB1), aflatoxin B2 (AFB2), aflatoxin G1 (AFG1) and aflatoxin G2 (AFG2) are frequently occurring in food. The letters “B” and “G” in the name of the toxins refer to the colour (Blue, B; Green, G) of the fluorescent emissions under ultraviolet light and the numbers 1 and 2 indicate their relative mobility in Thin-layer chromatography on silica gel. The fungus A. parasiticus produces AFB1, AFB2, AFG1 and AFG2, whereas, A. flavus produces AFB1 and AFB2. Among the aflatoxins, AFB1 is the most prevalent and dangerous mycotoxin to humans and considered as one of the most potent hepatocarcinogens known. The International Agency for Research on Cancer classified the AFB1 as class I human carcinogens.
Consumption of aflatoxin-contaminated food products leads to the disease called “aflatoxicosis” in humans. Acute and chronic exposures to aflatoxins result in a diverse range of symptoms including liver cancer, immunosuppression, poor weight-gain, and rapid death. When cow, sheep, goat, camel or other lactating cattle consumes AFB1-contaminated feeds, AFM1 will be formed as a result of the metabolic process in the liver of animals and excreted in the milk. Several studies reported the presence of residual aflatoxins in the liver and meat of broilers when fed with aflatoxin-contaminated feeds. In the case of laying hens, aflatoxins and their metabolites can be found in the eggs. Humans are exposed to aflatoxins directly by ingestion of aflatoxin-contaminated foods, or indirectly by consumption of aflatoxin-contaminated eggs, meat, milk and dairy products.
The presence of aflatoxins in the food chain threatens people’s livelihood, food security and human health. Aflatoxins have been detected in the blood of pregnant women and in breast milk. Therefore many countries have established regulations regarding the permissible levels of aflatoxins in foods. The Food and Drug Administration (FDA) of USA has set an aflatoxin tolerance limit of 20 parts per billion (ppb) for foods. Because of the high cost of analysis and non-availability of dedicated research laboratories, only a few countries are effectively monitoring the levels of aflatoxins in their food products.
The presence of toxin-producing fungi on the agricultural products may not be obvious sometimes because, the molds can be removed during the processing of agricultural products. However, the aflatoxins, which are produced by the toxigenic fungi, are extremely stable under most conditions of storage, handling and processing. Storage of food products under inappropriate conditions (e.g., high temperature and moisture levels) may also favour the growth of toxigenic molds and subsequent production of aflatoxins. A high level of exposure of people to aflatoxins is a serious threat to public health. To ensure the food safety and quality, the susceptible agricultural products must be monitored regularly to minimize the risk of aflatoxin hazard. Mold-contaminated agricultural commodities should not be used for the production of human foods and animal feeds. The moisture content of the agricultural products should be reduced below 12% to prevent mold growth. Storage of agricultural products at low temperature (<10°C), relative humidity (<65%), proper sorting of damaged, mold-infected and aflatoxin-contaminated cereal grains and tree nuts can help reduce aflatoxin levels in the foods. Increasing awareness among families, farmers, consumers and traders about human health impacts associated with aflatoxins and imposing appropriate regulatory measures on import of high-risk agricultural commodities will help in reducing exposure of people to aflatoxins.
This article is written by Drs. Rethinasamy Velazhahan and Abdullah Mohammed Al-Sadi from the Department of Crop Sciences of the College of Agricultural and Marine Sciences at Sultan Qaboos University.