As mammals, we can't see in the UV region without technological aid, so most of us probably have never noticed that optimally ripe bananas actually fluoresce blue. Scientists also overlooked this phenomenon until Austrian and American chemists carefully studied the breakdown of chlorophylls in the peels of ripening bananas. In an advanced Angewandte Chemie article, they reported that the natural process of aging is what causes bananas to glow blue under UV light.



Image © Wiley.

As plants age, they lose their green color because the chlorophylls that are vital for photosynthesis get converted into colorless chemicals called NCCs (Nonfluorescent Chlorophyll Catabolites). NCCs are found in aging leaves and the peels of ripe fruit like bananas, apples, and pears. On the way to forming NCCs, chlorophylls do go through a brief intermediate state where they are Fluorescent Chlorophyll Catabolites, which are (surprise!) fluorescent. However, FCCs have only been detected in minute quantities in plants because they are normally very short-lived.

Thus, scientists were amazed to find that ripe bananas glowed blue under UV light (you have to wonder if someone accidently threw a banana peel under a UV light, or if they screened for this on purpose). They studied the strength of the blue luminescence relative to a banana's storage time and found that green bananas, which have plenty of chlorophylls, had virtually zero fluorescence. The fluorescence of the peel would steadily increase until it reached a maximum after 2 days of storage. The production of FCCs from the degradation of chlorophylls occurred when the bananas are the most yellow. Then, the fluorescence decreased rapidly until the peel had very little luminescence by day 5, as the FCCs continued on the path of aging and were converted to NCCs. In white light, the bananas would appear dark yellow with brown spots at this stage.

By studying the structure of the FCCs in bananas, the chemists found that a key chemical modification changed a free acid group typical of FCCs to an ester functional group. They propose that the ester group is responsible for the added stability of banana FCCs, allowing them to accumulate long enough to be seen under UV light before they undergo further chemical reactions.

As to why this occurs in bananas, the scientists speculate that animals with vision in the UV region may use the blue luminescence when looking for a suitable meal. Whatever the reason, you can still take advantage of this research. If you're ever in doubt of your banana's ripeness and have access to a black light, you know what to do.

Angewandte Chemie Int. Ed., 2008. DOI: 10.1002/anie.200803189