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December 2002

Feature

 


Food for thought

Izelle Theunissen, MRC News

Contaminated maizeMention the words 'food safety' and you're guaranteed to stir up some emotions around organic ("organic is natural and good") and genetically modified or GM foods ('Frankenstein foods'). But people are less knowledgeable about mycotoxins - and this can shed a different light on the food safety debate.

Mycotoxins are naturally produced food-borne metabolites of fungi that are natural contaminants of agricultural crops. Their toxic effects in animals have been known for a long time, and therefore health authorities worldwide have regulated mycotoxin levels in human food and animal feed.

One such example is the mycotoxin aflatoxin B1 (AFB1), which is produced by the fungus Aspergillus flavus that grows on peanuts before or after harvesting and under poor storage conditions. In 1993 the International Agency for Research on Cancer (IARC) classified AFB1 as a carcinogen - a substance that can cause cancer in humans.

The tolerance levels of aflatoxins in human foodstuffs are regulated internationally and in South Africa by Government Notice No. R 313 of 16 February 1990. 

Another prime example is patulin - this is a toxic secondary metabolite that is produced by a number of fungi, most important of which is Penicillium expansum. This fungus is a well-known post-harvest pathogen that causes 'blue mould rot' or 'soft' rot' in apples.

Patulin has been shown to possess mutagenic properties (can cause damage to the genetic material of cells), to have adverse effects on developing rat fetuses, and to cause immunotoxic, neurotoxic and gastro-intestinal effects in rodents. Recommended specifications are that patulin levels should not exceed 50 parts per billion in products intended for human consumption (United Kingdom Committees of Toxicity, 1992).

This begs the big question: what role do these and other mycotoxins play in organic and GM foods?

Mycotoxins and organic foods

Organic agriculture is generally defined as cultivation of crops without the use of fungicides or pesticides, implying that such produce are 'naturally good and healthy'. This has resulted in an increasing demand for organic foods in developed countries - and these foods cost more to produce commercially than 'conventional' crops.

Rural communities in developing countries who depend on subsistence farming have always made use of organic farming methods, but without using the variety of 'natural compounds' and/or biocontrol agents to try and control fungal infection and insect manifestations as is done in commercial organic crops in developed countries.

But either way, the fact remains that organic foods that are grown without the use of insecticides and fungicides may be expected to be infested by insects and infected by fungi to a larger extent than conventionally grown foods. Infestation by insects can in many cases lead to fungal infections that produce mycotoxins - and therefore the food is concomitantly contaminated with higher levels of mycotoxins.

Research by the MRC's PROMEC Unit indicates that patulin levels in apple juice made from conventionally grown apples ranged from 250-4000g/l. But in organically produced apple cider, a study done by other researchers has found levels of up to 45 000g/l.

Mycotoxins and GM foods

One example of a GM food is transgenic maize (Bt-maize). These hybrids are genetically modified with genetic material from a naturally occurring soil-borne bacterium, Bacillus thuringiensis, that produces a protein that is toxic to certain insect pests, for example maize stalk borers.

These stalk borers cause damage to maize ears or kernels and this damage is often associated with Fusarium ear rot. The fungus Fusarium verticillioides is one of the most prevalent seed-borne fungi associated with maize intended for human and animal consumption throughout the world. The fumonisins, a family of food-borne carcinogenic mycotoxins, were first isolated in 1988 from cultures of F. verticillioides at the PROMEC Unit.

Fumonisin B1 (FB1) was shown by researchers to cause equine leuko-encephalomalacia, pulmonary oedema syndrome in pigs, and primary hepatocellular carcinoma in rats. In addition, detailed mycological analyses of home-grown maize samples intended for human consumption from different oesophageal cancer (OC) rate areas in Transkei during six seasons over the period 1976-1989 revealed a statistically highly significant correlation between the incidence of F. verticillioides in maize and the OC rate.

The Bt-maize strains, which are resistant to insects such as maize stalk borers, have been shown to significantly reduce the levels of insect damage - therefore yielding maize crops with significantly lower fumonisin concentrations compared to their non-Bt counterparts. Another plus is that the maize yields of the Bt-strains are larger than their non-Bt counterparts. So despite the current discussions surrounding GM foods, it appears that Bt-maize hybrids could play a major role in lowering fumonisin levels in maize products, which should ultimately enhance the quality and safety of maize for animal and human consumption, particularly in an African context.

Following extensive media coverage of high levels of aflatoxins that were allegedly found in peanut butter given to school children in the Eastern Cape Province under the Primary Schools Nutrition Programme (PSNP), the PROMEC Unit published a Policy Brief (No 3, June 2001) ( http://www.mrc.ac.za/policybriefs/polbrief3.htm) to emphasise the serious health implications of supplementing the diet of school children with peanut butter containing aflatoxin. It is further of great concern that the liver cancer risk increases significantly if a child suffering from hepatitis B virus (HBV) infection consumes aflatoxin-containing foodstuffs, due to the synergism beteen AFB1 and HBV in causing liver cancer. Therefore, an increase in liver cancer incidence in South Africa within 20-30 years from now is possible.


For more information about mycotoxins in food, contact Prof. Walter Marasas (tel.: (021) 938-0244 or e-mail: wally.marasas@mrc.ac.za or Dr Hester Vismer (tel.: (021) 938-0287 or e-mail: hester.vismer@mrc.ac.za).

Useful links:

LINK Maize our Staple Food - Should Toxins Concern us?

LINK Your hair - 'a dietary time-clock'

LINK Aflatoxin in peanut butter

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