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.
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.