How Can You Tell Whether Corn Is Fungus-Ridden?

April 14, 1999

TALKING to plants to keep them healthy is considered crazy enough. But listening to them? Well that's exactly what researchers in Illinois propose doing to weed out kernels of corn that could end up poisoning your breakfast. Grain stores often become infected with fungi that produce harmful toxins, and the food industry has to prevent such poisons ending up in cornflakes or animal feed.

Current tests use ultraviolet light, which makes certain species of fungus glow in the dark. Grains that fluoresce bright green-yellow are thrown away. The trouble is that this BGYF test, as it's known, misses around 15 per cent of infected kernels.

So chemists Sherald Gordon and Richard Greene of the National Center for Agricultural Utilization Research in Peoria, Illinois, have devised a more accurate way to spot contaminated corn. They recognise infected grains by listening for telltale noises made by mouldy kernels when they are bombarded with an infrared strobe light.

The strobe's short pulses make kernels heat up and cool down very quickly, says Greene. The dissipating heat produces sound waves which can be picked up by a microphone. "Clean kernels absorb light and give off sounds at specific wavelengths," Greene explains. "But fungi alter the kernels' chemical and physical structure." These changes mean infected kernels produce sounds at slightly different wavelengths.

The researchers used a technique known as Fourier Transform Infrared Photoacoustic Spectroscopy (FTIR-PAS) to record the kernels' moans and groans, which are analysed by neural network software written by computer scientist Bruce Wheeler at the University of Illinois. "The network achieves a 96 per cent accuracy in spotting clean and contaminated corn. It's definitely more sensitive than the BGYF test," Greene says.

Because the technique works on single grains, Greene believes it should be ideal for laboratory testing of suspicious samples-though not for an automated test in grain factories. For this the chemists are investigating a similar technique called Transient Infrared Emission Spectroscopy (TIRS), which is better for spotting infected kernels on a conveyor belt.

TIRS was devised by John McClelland of Iowa State University in Ames, who realised that heated kernels also emit small amounts of infrared light at specific wavelengths, and that these wavelengths vary depending on the health of the grain. Because the emissions are short-lived, kernels can be heated and tested with TIRS as they pass by on a factory production line.

Greene does not envisage either FTIR-PAS or TIRS replacing the fluorescence test. "We don't know why yet but the different approaches seem to miss different infected grains." A combination of the two could weed out even more of the infected kernels, he says.
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Author: Matt Walker
New Scientist issue 17 April 1999

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