Blue halos of doom

Under ultraviolet light, rings around the brown spots in aging bananas may signal the transition from ripe to rotten

Ghostly rings of light surround the dying brown spots in overly ripe bananas, researchers report. The halos, which glow intensely blue under ultraviolet light, may mark the beginnings of cell death and may signal to animals such as apes that the bananas are ready for eating.

AGING BLUES Bananas that are yellow in daylight (left) glow blue under ultraviolet light (right), and their age spots sport an intensely blue ring. Moser et al./PNAS 2009

Bernhard Kräutler of the University of Innsbruck in Austria and his colleagues had already found that yellow bananas have a blue hue under UV light. Green bananas, on the other hand, don’t. The blue, the researchers knew, comes from a class of fluorescent chemicals that are produced and build up when chlorophyll breaks down.

In the new study, reported online September 7 in Proceedings of the National Academy of Sciences, the team determined what happens under UV light when the bananas become overripe. Though the brown spots themselves are dull, the team found that a ring of flesh surrounding the spots glows with an intensity three times that of the yellow peel. By analyzing the light, the researchers found that the chlorophyll-breakdown chemicals were more abundant in the rings than elsewhere in the banana.

Scientists do not yet know if the halo serves any biological purpose for the bananas. But Kraeutler speculates that animals may use the light to see whether fruit is ripe or rotten.

For example, he says, scientists have postulated that apes use color vision to detect fruit in the woods and that they see short-wavelength light, such as blue light verging on ultraviolet, better than humans do. “There’s a very high correlation between the ability to see fruit and the development of color vision,” Kraeutler says.

The halos may also provide a way for researchers to study cell death in bananas, and possibly other plants, without resorting to invasive procedures.

So far these fluorescing by-products of chlorophyll breakdown have only been found to accumulate in one other plant, the peace lily. The fluorescing chemicals, though briefly present, also do not seem to build up a glow in apples and pears.

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