Yellow Bananas Fluoresce Blue

Ripe yellow bananas appear blue under ultraviolet light, reports a research team led by Bernhard Kräutler, a chemistry professor at the University of Innsbruck, in Austria (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200803189). The blue luminescence is due to previously unidentified chlorophyll breakdown products (catabolites) that are generated as bananas ripen and change color from green to yellow. These tetrapyrrolic catabolites contain a propionate ester group that may protect them from isomerization, in contrast to a free acid moiety found in previously identified chlorophyll catabolites that degrade more quickly in other plants. The researchers propose that fluorescent chlorophyll catabolites could be optical brighteners in vivo and therefore contribute to the bright yellow color of a ripe banana; the amount of the catabolites decreases as a banana overripens and the color dulls. The scientists further suggest that the blue fluorescence could be a ripening signal to fruit-eating animals, many of which have a window of vision that includes UV light. The chlorophyll catabolites may also function as antioxidants to prolong the life of the ripening fruit.