glowing-bananas

Before last year no one knew ripe bananas glow blue under ultraviolet light. Now the group which made the discovery have found when bananas ripen, little blue halos appear around dark spots on the peel. However, the exact reason for the blue glow is still a mystery.

Sadly the chemists did not chance across the luminescence while eating a banana under a sun bed. Instead, it came while they were looking for yellow compounds, as reported here, last year. Researchers from the University of Innsbruck, Austria and Columbia University, US, found the blue luminescence originates from fluorescent chlorophyll catabolites (FCCs), particularly one called Mc-FCC-53, which they indentified through a combination of high performance liquid chromatography, mass spectrometry and nuclear magnetic resonance. This latest banana breakthrough came after the team used fluorescence analysis to reveal that under UV light, the dead spots that appear on a banana’s skin are surrounded by intense bright blue halos.

glowing-banana-halos

FCCs are short-lived intermediates of chlorophyll degradation in many plants. However, FCCs in bananas differ from those in other plants by the presence of a propionate ester group, which serves to stabilise the compounds. Since FCCs seen in bananas seem to be specially modified for increased stability, they are assumed to have some beneficial function. The authors suggest the blue luminescence is a sign of cell death, since the halos materialise as the fruit changes from ripe to rotten.

The blue glow may serve to warn fruit-eating animals about the ripeness of the bananas. Many animals, including parrots, can observe a wider range of UV wavelengths than humans. Another possibility is that the FCCs delay the onset of rotting, or serve some other, as yet unknown, function.

Tom Bond

Reference:
Moser et al., PNAS, 2009, doi 10.1073/pnas.0908060106

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