Why ripe bananas get the blues

If you have ever taken a ripe banana to a rave — and let’s face it, who hasn’t? — you will have discovered that it glows an intense bright blue under the ultraviolet (“black”) light beloved of party organisers.

This curious phenomenon has recently been investigated by an international team of scientists from the University of Innsbruck in Austria and Columbia University in New York. In a report in the journal Angewandte Chemie, they explain that the blue glow is connected to the degradation of chlorophyll during the ripening process.

The fluorescent breakdown products are concentrated in the banana peel and are colourless under normal light, so that all one sees is the usual yellow colour, for which carotenoids must take most of the blame. The fluorescence under black light occurs both in fruit that matures naturally and in fruit that is artificially ripened by exposure to ethylene gas.

The intensity of the luminescence progressively decreases as the bananas reach the end of the ripening process. Unripe bananas do not glow at all.

Setting out to solve the fruity enigma before it drove them bananas (sorry), the research team used a range of spectroscopic techniques to analyse the structures of the main breakdown products.

They identified a propionate ester group that had never been seen before in a chlorophyll breakdown product and found that this modification has a stabilising effect on the molecule.

But why should bananas catabolise chlorophyll differently from other higher plants? Two possible explanations are offered by Bernhard Kräutler, the research team’s top banana (sorry again). One is that, unlike humans, many banana-chomping animals can see light in the ultraviolet range and the blue luminescence may be a signal to them that the fruit is ripe.

The other suggestion is that the great stability of the banana’s chlorophyll degradation products helps to prolong the viability of the ripening fruit.

Take your pick.