Researchers at the University of Bristol have discovered that tiny external structures within the wax coating of blueberries are responsible for their blue color.
This applies to lots of fruits that are the same color including damsons, sloes, and juniper berries.
In the study, published today in Science Advances, researchers show why blueberries are blue despite the dark red color of the pigments in the fruit skin. Their blue color is instead provided by a layer of wax that surrounds the fruit which is made up of miniature structures that scatter blue and UV light. This gives blueberries their blue appearance to humans and blue-UV to birds. The chromatic blue-UV reflectance arises from the interaction of the randomly arranged crystal structures of the epicuticular wax with light.
Unveiling Nature’s Color Tricks
Rox Middleton, Research Fellow at Bristol’s School of Biological Sciences, explained: “The blue of blueberries can’t be ‘extracted’ by squishing – because it isn’t located in the pigmented juice that can be squeezed from the fruit. That was why we knew that there must be something strange about the color.
“So we removed the wax and re-crystallized it on a card and in doing so we were able to create a brand new blue-UV coating.”
The ultra-thin colorant is around two microns thick, and although less reflective, it’s visibly blue and reflects UV well, possibly paving the way for new colorant methods.
“It shows that nature has evolved to use a really neat trick, an ultrathin layer for an important colorant,” added Rox.
Most plants are coated in a thin layer of wax which has multiple functions, many of which scientists still don’t understand. They know that it can be very effective as a hydrophobic, self-cleaning coating, but it’s only now they realize how important the structure is for visible coloration.
Potential Applications and Future Research
Now the team plans to look at easier ways of recreating the coating and applying it. This could lead to a more sustainable, biocompatible, and even edible UV and blue-reflective paint. Furthermore, these coatings could have the same multiple functions as natural biological ones that protect plants.
Rox added: “It was really interesting to find that there was an unknown coloration mechanism right under our noses, on popular fruits that we grow and eat all the time.
“It was even more exciting to be able to reproduce that color by harvesting the wax to make a new blue coating that no one’s seen before.
“Building all that functionality of this natural wax into artificially engineered materials is the dream!”
Reference: “Self-assembled, disordered structural color from fruit wax bloom” by Rox Middleton, Sverre Aarseth Tunstad, Andre Knapp, Sandra Winters, Susan McCallum and Heather Whitney, 7 February 2024, Science Advances.
DOI: 10.1126/sciadv.adk4219
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