RADIO waves are showing promise as an alternative to using ozone-depleting chemicals to destroy insect infestations in stored fruit and nuts.
These pests are currently dealt with using the cheap and effective chemical methyl bromide. But because methyl bromide damages the stratospheric ozone layer, its use in industrialised countries will be banned from the end of 2005 under the Montreal Protocol.
Insect pests such as the navel orange worm and Indian meal moth, which infest walnuts, almonds and pistachios, are killed if their body temperature rises to around 50 掳C for as little as a couple of minutes. A team led by Judy Johnson, an entomologist at the US Department of Agriculture鈥檚 San Joaquin Valley Agricultural Sciences Center in California, has found that exposing them to radio waves will do this. 鈥淥nce insects lose moisture, they鈥檙e just toast,鈥� she says.
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To generate the radio waves, the USDA team used a standard industrial food drying system that operates at 27 megahertz, which corresponds to a wavelength of 10 metres. Crucially, this did not damage the nuts. Insects absorb more energy than the nuts because of their high concentrations of body fluids, says Juming Tang, a bioengineer at Washington State University in Pullman, who is also working on the project.
But while the process works well with dry products, it is less successful with fruit because their higher liquid content means they absorb more of the radio energy. Moreover, liquid is not distributed evenly in fruits, leading to patchy heating.
To make the heating effect more even, the researchers put fruit in a tank of water. Using this technique, they have effectively eliminated codling moth larvae from cherries by delivering the heat energy to the centre of the fruit in less than a minute, Tang says.
The radio drying process should also work with tropical tree fruits that can tolerate short periods at high temperatures. But temperate fruits such as apples and pears are a problem because of their low heat tolerance.