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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.

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.

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