Press Release
Angewandte Chemie International Edition 2008, 47, doi: 10.1002/anie.200803189 Nr. 43/2008 Blue BananasRipening bananas glow an intense blue under black lightContact: Bernhard Kräutler, University of Innsbruck (Austria) Registered journalists may download the original article here: Blue Luminescence of Ripening Bananas Ripe bananas are of course yellow. However, under black
light, the yellow bananas are bright blue, as discovered by scientists at
the University of Innsbruck (Austria) and Columbia University (New York,
USA). The team, headed by Bernhard Kräutler, reports in the journal Angewandte
Chemie that the blue glow is connected to the degradation of
chlorophyll that occurs during ripening. In this process, colorless but
fluorescing breakdown products of chlorophyll are concentrated in the
banana peel.
 © Wiley-VCH
The usual appearance of bananas is mainly the result of carotenoids. Under normal
light, these natural pigments appear yellow. Under UV light, known to
partygoers as black light, ripening bananas appear blue instead. There
is no difference between naturally ripened bananas and those ripened
with the use of ethylene gas. Green, unripe bananas do not fluoresce.
The intensity of the luminescence correlates with the breakdown of the
green pigment chlorophyll. As the ripening continues to progress, the
blue glow decreases. “Surprisingly, this blue luminescence apparently
has been entirely overlooked,” says Kräutler.
By means of various spectroscopic techniques, the team analyzed the
structure of the main breakdown products. In doing this, they identified
a propionate ester group, a modification never seen before in a
chlorophyll breakdown product. This group has a stabilizing effect and
could explain the unusually long duration of the fluorescing
intermediates in bananas. Fluorescing chlorophyll catabolytes have
otherwise only been found as short-lived intermediate products in higher
plants.
Why does the breakdown of chlorophyll occur differently in bananas than
in other higher plants, including even banana leaves? Kräutler suggests
two different explanations: “In contrast to humans, many of the animals
that eat bananas can see light in the UV range. The blue luminescence of
the banana fruit could give them a distinct signal that the fruit is
ripe.” Or perhaps the chlorophyll degradation products also serve a
biological function for the banana. The amazingly stable catabolytes
could help to prolong the viability of the ripening fruit.
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