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Forget the so-called morning banana diet, blue is the new yellow now that researchers in Europe and the US have explained why ripened bananas glow blue under ultraviolet light. Bernhard Kräutler, Simone Moser, Thömas Mueller, Marc-Olivier Ebert of the University of Innsbruck, Austria and Steffen Jockusch and Nick Turro of Columbia University, New York, USA, report in the journal Angewandte Chemie, how the intense blue hue is due to the breakdown of the high concentration of chlorophyll in the peel, which takes place during the ripening process of the curvy fruit. Fruit lovers everywhere are fully aware that bananas (Musa Cavendish) turn from a gum-curling green to yellow (their "memory" colour) and then on to dark brown as they ripen. Most of us have a favourite stage at which to indulge, but there are few people who would be so keen to eat a bright blue banana. The normal colour of a banana is mainly produced by carotenoid compounds and in daylight, these natural pigments appear yellow. In contrast, under UV light bananas ripened naturally or with added ethylene ripening hormone, glow a wonderful blue. Green, unripe bananas do not fluoresce in this way. "Surprisingly, this blue luminescence has been entirely overlooked," says Kräutler. The researchers used a crate of spectroscopic and other techniques, to analyse the structure of the main chlorophyll breakdown products that they suspected of giving rise to the blue glow. Multidimensional NMR spectroscopy provided the structure of the most intriguing of the fluorescent catabolites of chlorophyll (FCCs), labelled Mc-FCC-56. "Unpeeled yellow bananas showed blue fluorescence with a maximum near 450 nm, as did extracts from fresh peelings of yellow bananas," the researchers say, "The luminescence of such extracts was also similar to that of a solution of the main banana FCCs in methanol." They point out that fresh extracts from banana peelings analysed by high-performance liquid chromatography (HPLC) reveals about a dozen luminescent components with absorbance spectra typically with maxima near 317 and 358 nm. Intriguingly, one of the main compounds, Mc-FCC-56, revealed itself as containing a propionate ester group. This chemical modification has not previously been seen in a chlorophyll breakdown product. The team suspects that it has a stabilizing effect and might explain why the fluorescing intermediates in the banana skin are so long-lived. Earlier studies with other plants have only found short-lived fluorescing chlorophyll catabolites. This beggars the question: Why does the breakdown of chlorophyll occur differently in bananas than in other higher plants? Indeed, why does the breakdown process follow this path in the fruit when it is not the route taken by chlorophyll degradation even in banana leaves? There could be two answers to that question, Kräutler suggests. "In contrast to humans, many of the animals that eat bananas can see light in the UV range," he says, "The blue luminescence of the banana fruit could give them a distinct signal that the fruit is ripe." An alternative explanation may simply be that the chlorophyll degradation products serve a biological function for the banana itself. They are amazingly stable catabolites, the researchers say, and could help to prolong the viability of the ripening fruit. Either way, the thought of noshing on bright blue fruit is enough to make this banana fan switch to the more staid rosy-red apple. Reference:
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