Research by Bernhard Kräutler and colleagues of the University of Innsbruck and Columbia University, New York, shows that ripe bananas fluoresce blue under ultraviolet light. The glow is attributed to previously undiscovered chlorophyll-breakdown products, which occur as a green banana ripens and yellows. The colourless but fluorescing substances are concentrated in the banana peel and the effect disappears once the banana is too ripe.

It is not understood why these substances are produced. The breakdown products certainly help to give ripe bananas their distinctive bright-yellow appearance, and they could act as brighteners to make ripe bananas more visible. This is analogous to the brighteners in detergents that make white clothes appear brighter in sunlight and glow under ultraviolet light. The researchers point out that “most humans would consider the idea of a blue banana to be unappetizing”, but the fluorescence may make the ripe bananas more distinctive to animals that can see UV light. In addition, the chemicals have antioxidant properties that could provide protection against decay, keeping the fruit ripe for longer.

Further reading

S Moser et al. 2008 Angewandte Chemie International Edition 47 8954.