What are aflatoxins?
The aflatoxins are a group of chemically similar toxic fungal metabolites (mycotoxins) produced by certain moulds of the genus Aspergillus growing on a number of raw food commodities. Aflatoxins are highly toxic compounds and can cause both acute and chronic toxicity in humans and many other animals. Their importance was first established in 1960 when 100,000 turkeys and other poultry in the UK died in a single event. The cause of this was eventually traced to a toxic contaminant in groundnut meal used in the bird’s feed. The contaminant was later named aflatoxin.
The aflatoxins consist of about 20 similar compounds belonging to a group called the difuranocoumarins, but only four are naturally found in foods. These are aflatoxins B1, B2, G1 and G2. Aflatoxin B1 is the most commonly found in food and also the most toxic. When lactating cattle and other animals ingest aflatoxins in contaminated feed, toxic metabolites can be formed and may be present in milk. These metabolites, aflatoxin M1 and M2, are potentially important contaminants in dairy products.
What foods can be contaminated?
Aflatoxins may be present in a wide range of food commodities, particularly cereals, oilseeds, spices and tree nuts. Maize, groundnuts (peanuts), pistachios, brazils, chillies, black pepper, dried fruit and figs are all known to be high risk foods for aflatoxin contamination, but the toxins have also been detected in many other commodities. Milk, cheese and other dairy products are at risk of contamination by aflatoxin M. The highest levels are usually found in commodities from warmer regions of the world where there is a great deal of climatic variation.
It is important to recognise that, although it is primary food commodities that usually become contaminated with aflatoxins by mould growth, these toxins are very stable and may pass through quite severe processes. For this reason they can be a problem in processed foods, such as peanut butter.
How do they affect human health?
At high enough exposure levels, aflatoxins can cause acute toxicity, and potentially death, in mammals, birds and fish, as well as in humans. The liver is the principal organ affected, but high levels of aflatoxin have also been found in the lungs, kidneys, brains and hearts of individuals dying of acute aflatoxicosis. Acute necrosis and cirrhosis of the liver is typical, along with haemorrhaging and oedema. LD50 (lethal dose) values for animals vary between 0.5 and 10 mg/kg body weight.
Chronic toxicity is probably more important from a food safety point of view, certainly in more developed regions of the world. Aflatoxin B1 is a very potent carcinogen and a mutagen in many animals, and therefore potentially in humans, and the liver is again the main target organ. Ingestion of low levels over a long period has been implicated in primary liver cancer, chronic hepatitis, jaundice, cirrhosis and impaired nutrient conversion. Aflatoxins may also play a role in other conditions, such as Reye’s syndrome and kwashiorkor (a childhood condition linked to malnutrition). Less is known about the chronic toxicity of aflatoxin G1 and M1, but these are also thought to be carcinogens, though probably a little less potent than B1.
Little is known about the level of dietary exposure to aflatoxins necessary to affect health, especially in humans, and diagnosis of chronic toxicity is very difficult. It is generally agreed that the best approach is to minimise the levels in all foods as far as is technically possible and to assume that any dietary exposure is undesirable.
How common is illness?
The incidence of chronic aflatoxicosis in humans is unknown and is almost impossible to estimate because the symptoms are so difficult to recognise. However, human liver cancer is quite common in parts of the world where aflatoxin contamination of food is likely and there may be a link, although this remains unproven.
Acute human aflatoxicosis is rare, especially in developed countries, where contamination levels in food and monitored and controlled. However, there have been outbreaks in some developing countries, notably in sub-Saharan Africa, where maize and groundnuts can be an important part of the diet and where the climate is suitable for rapid mould growth on crops in the field and in storage.
A notable outbreak occurred in India in 1974 when almost 400 people became ill with fever and jaundice after eating maize contaminated with between 0.25 and 15 mg/kg aflatoxin and more than 100 died. Major outbreaks have also occurred in Kenya, the largest in 2004 when 317 people were affected and 125 died, probably as a result of eating contaminated maize.
Where do they come from?
Aflatoxins are produced by at least three Aspergillus species. These are A. flavus, A. parasiticus and the much more rare A. nomius. These moulds are able to colonise a wide range of crops both in the field as non-destructive plant pathogens and in storage, and can grow and produce aflatoxins at quite low moisture levels (approximate minimum Aw 0.82) and over a broad temperature range (13-37oC).
Their growth is strongly influenced by climate and, although they are found all over the world, they are more common in tropical regions with extreme variations in temperature, rainfall and humidity. A. flavus invasion of groundnut crops in the field is known to be favoured by drought stress and maize crops are vulnerable if damaged by insect pests.
Mould growth and aflatoxin production during storage of crops is also important, especially if drying is inadequate, or storage conditions allow access for insect or animal pests.
Are they stable in foods?
Aflatoxins are quite stable compounds and survive relatively high temperatures with little degradation. Their heat stability is influenced by other factors, such as moisture level and pH, but heating or cooking processes cannot be relied upon to destroy aflatoxins. For example, roasting green coffee at 180oC for 10 minutes gave only a 50% reduction in aflatoxin B1 level.
The stability of aflatoxin M1 in milk fermentation processes has also been studied and although appreciable losses do occur, significant quantities of the toxin were found to remain in both cheese and yoghurt.
How can they be controlled?
The ability of aflatoxin-producing fungi to grow on a wide range of food commodities and the stability of aflatoxins in foods mean that control is best achieved by measures designed to prevent the contamination of crops in the field and during storage, or detection and removal of contaminated material from the food supply chain.
For primary producers
Pre-harvest control of aflatoxins is best achieved through general Good Agricultural Practice (GAP) to include such measures as:
- Land preparation, crop waste removal, fertiliser application and crop rotation
- Use of fungus- and pest-resistant crop varieties
- Control of insect pests
- Control of fungal infection
- Prevention of drought stress by irrigation
- Harvesting at the correct moisture level and stage of maturity
The most important and effective control measure in post-harvest handling and storage is the control of moisture content and hence, the water activity of the crop. Ensuring that susceptible crops are harvested at a safe moisture level, or are dried to a safe level immediately after harvest is vital to prevent mould growth and aflatoxin production during storage. The safe moisture level varies between crops – for maize it is approximately 14% at 20oC, but for groundnuts it is much lower, about 7%. These moisture levels must be maintained during storage and transport.
It is also important to ensure that the moisture content does not vary too much in a bulk-stored crop. Small localised ‘wet spots’ can develop mould growth and these can extend to neighbouring areas as the fungus produces metabolic water during respiration. Insect and animal pest damage can also act as focal points for fungal growth.
For food processors
Physical separation of contaminated material can be an effective means of reducing aflatoxin levels in contaminated commodities. For example, colour sorting is often used to remove mouldy peanuts from bulk shipments. Density segregation, mechanical separation and the removal of fines and screenings from grain and nut shipments can also be effective measures.
Chemical decontamination methods have been investigated, especially for material used in animal feed, but most of the methods investigated are impractical, or produce toxic by-products. Biological decontamination has also been considered, and a single bacterial species, Flavobacterium aurantiacum, has been shown to remove aflatoxin B1 from peanuts and corn.
Although decontamination methods for aflatoxin M1 in milk and dairy products have also been investigated, most of these are not practical for the dairy industry. The only really effective control is to minimise the contamination of materials used in animal feed for dairy cows.
For enforcement agencies
Many countries monitor imported commodities that are susceptible to aflatoxin contamination, such as pistachios and Brazil nuts, by sampling and analysis. A number of analytical methods have been developed based on TLC, HPLC and ELISA and there are also rapid screening kits available. However, moulds and aflatoxins in bulk food shipments tend to be highly heterogeneous in their distribution and it is essential to ensure that an adequate sampling plan is used to monitor imported materials.
In some commodities, such as figs, aflatoxins fluoresce strongly under UV-light and this can be used as a rapid screening test for high concentrations.
Are there rules and regulations?
Around 100 countries around the world have regulations governing aflatoxins in food and most include maximum permitted, or recommended levels for specific commodities.
The EU sets limits for aflatoxin B1 and for total aflatoxins (B1, B2, G1 and G2) in nuts, dried fruits, cereals and spices. Limits vary according to the commodity, but range from 2-12 μg/kg for B1 and from 4-15 μg/kg for total aflatoxins. There is also a limit of 0.050 μg/kg for aflatoxin M1 in milk and milk products. Sampling and analytical methods are also specified.
Limits of 0.10 μg/kg for B1 and 0.025 μg/kg for M1 have been set for infant foods.
US food safety regulations include a limit of 20 μg/kg for total aflatoxins (B1, B2, G1 and G2) in all foods except milk and a limit of 0.5 μg/kg for M1 in milk. Higher limits apply in animal feeds.
Both Australia and Canada set limits of 15 μg/kg for total aflatoxins (B1, B2, G1 and G2) in nuts. This is the same as the international limit recommended for raw peanuts by the Codex Alimentarius Commission.