A study found that ripening bananas appear bright blue under UV light as a result of chlorophyll breakdown in banana peels.
Scientists from the University of Innsbruck in Austria and Columbia University in New York discovered the blue glow by looking at ripe Cavendish bananas under UV light. The bright yellow colour the peel takes on as the fruit ripen is caused by the loss of chlorophyll. What distinguishes these chlorophyll breakdown products from those observed in other higher plants is that they fluoresce blue under UV light. Such catabolytes had only been seen as short-lived intermediates of chlorophyll breakdown, whereas in the peel and leaves of bananas they are abundant and unusually long-lived.
Scientists are not sure why the degradation process is different in bananas than in other plants. They speculate that since many of the animals which eat bananas can see light in the UV range, the blue luminescence gives them the distinct signal that the fruit is ripe. Another possibility is that the antioxidant properties of these breakdown products extend the viability of ripening fruits and senescing leaf cells.
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