Study: Tiny Structures in Wax Coating of Blue-Pigmented Fruits Give Them Their Blue Color

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November 30, 2024

Many visually guided fruit-eaters have eyes highly adapted for blue sensitivity, which makes it perhaps surprising that blue pigmented fruits are not more common. However, some fruits are blue even though they do not contain blue pigments. In new research, scientists from the University of Bristol and elsewhere investigated dark pigmented fruits with wax blooms, like blueberries, plums, and juniper cones, and found that a structural color mechanism is responsible for their appearance.

Structural color in wax bloom produces blue appearance on fruits across a wide phylogeny: (A) undamaged highbush blueberries growing on the plant; (B) blueberry with (i) unmodified wax, (ii) mechanical wax removal, (iii) chloroform wax removal, (iv) surface application of (almost) index-matching oil, (v) surface application of water, (vi) outer skin peeled to reveal flesh, and (vii) underside of peeled skin; (C) transmission optical microscopy of peeled blueberry skin from internal edge showing red pigmentation in epidermal cells. Scale bar – 200 μm; (D) (i and ii) a selection of plum (Prunus domestica) fruits with different cell pigmentation, (i) with wax intact and (ii) with wax removed. Image credit: Middleton et al., doi: 10.1126/sciadv.adk4219.

“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,” said lead author Dr. Rox Middleton, a researcher at the University of Bristol.

“That was why we knew that there must be something strange about the color.”

“So we removed the wax and re-crystallized it on 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,” Dr. Middleton said.

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.

Now Dr. Middleton and colleagues plan 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.

“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,” Dr. Middleton said.

“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.”

The results were published in the journal Science Advances.

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Rox Middleton et al. 2024. Self-assembled, disordered structural color from fruit wax bloom. Science Advances 10 (6); doi: 10.1126/sciadv.adk4219