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FORGET squeezing fruit to see if it鈥檚 ripe鈥攋ust look at the way it
glows in the dark. That鈥檚 the message from Portugal, where scientists have
discovered that, when ripe, fruits such as apples and kiwi fruit fluoresce
brightly if they are zapped with a laser. They hope the effect will provide a
way to tell the age and ripeness of fruit on an industrial scale, without anyone
having to touch it.

Alexander Kharmalov, a physicist at the University of Coimbra in Portugal,
discovered the effect by accident. For a laugh, he placed an apple in a device
for measuring fluorescence鈥攍ight emitted by an object when struck by light
of another wavelength. To his surprise, the pungent gases given off by the apple
produced light when excited by the machine鈥檚 laser. How they might do this is
not known. 鈥淚n theory, these volatile organic gases should not fluoresce, so I
was very surprised,鈥� he says.

In further tests, Kharmalov and his colleague Hugh Burrows discovered that
it鈥檚 not just the gases that fluoresce. The apple鈥檚 skin also seems to have
something to do with it. 鈥淭he membrane is clearly important,鈥� says Burrows. When
the gas molecules come up against the porous membrane of the skin, only the most
energetic make it through, the researchers speculate. They believe these
molecules are in a more excited state of vibration than usual. When the laser
light hits the molecules, they drop into a lower vibration state, releasing
energy in the form of light. 鈥淚nitially, I was very sceptical,鈥� says Burrows.
鈥淏ut the results seem genuine.鈥�

Kharmalov has also found that the fluorescence spectrum varies with the
fruit鈥檚 age, becoming more intense as the fruit matures and gives off more gas.
鈥淎nyone who keeps apples at home knows they smell more strongly as they get
older,鈥� he says. The fruits also fluoresce more strongly at the blue end of the
spectrum as they age and give off more methanol.

This fluorescence 鈥渟ignature鈥� should make it much easier for producers to
determine the age and ripeness of fruit. At the moment this must be done
subjectively by squeezing the fruit and inspecting it.

Kharmalov believes the technique will be useful for fruits such as apples,
lemons and kiwi fruits, which are often stored for long periods, but may not be
well suited to fruits such as strawberries, which ripen very rapidly. Kharmalov
is also able to tell which part of the world coffee beans come from just by
their fluorescence spectra.

Producers certainly need fast, cheap ways of determining ripeness without
damaging fruit. Douglas Henderson, chief executive of Britain鈥檚 Fresh Produce
Consortium in Peter borough, says: 鈥淣obody has come up with a reliable
non-invasive system, so this could be of considerable benefit.鈥�

So Kharmalov is now beginning work on a commercial device that exploits the
effect. He says it may even be small and cheap enough for people to use while
shopping: 鈥淏efore you buy, how do you know whether fruit has been stored in a
bad atmosphere for three months?鈥� asks Kharmalov. 鈥淭his could tell you.鈥�