If you open a ripe blueberry, the blue skin on the outside doesn’t match the dark purple and reddish color inside the fruit. However, their skin does not actually contain the blue pigments that would normally create this color. Instead, the random arrangement of microscopic structures in their natural wax coating gives these fruits their blue appearance. The findings are described in a study published February 7 in the journal Advances in science and can be applied to other fruits, including slips, berries, and juniper berries.
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The color blue is a very rare color in nature, with less than one in 10 plants having the common human favorite. This is partly because there is no true blue pigment in the natural world. To display this color, molecules in both plants and animals must perform tricks to make them appear blue to the human eye. In some flowers such as bells, this mainly happens when natural pigments or colorants are mixed in the way you can mix different dyes to change the color. Red pigments called anthocyanins are the most common in nature, and changes in acidity in the pigment change the color. Combined with reflected light, sunflowers, hydrangeas and cornflowers can all have colorful blue flowers. It’s not entirely clear why plants might go to this trouble to be so blue, but a unique color may help them attract pollinators like bees.
For blueberries, the blue hue comes from the thin layer of wax naturally produced on their skin. Wax serves many functions in the plant kingdom, including as a self-cleaning coating or for added protection, but scientists still don’t understand much about what wax does and why.
“We realized that there are many blue pigments in nature, as well as ‘cheats’ or how nature makes blue without pigments,” says study co-author and University of Bristol physicist Rox Middleton. PopSci. “We just didn’t know—we couldn’t find it written down anywhere—why blueberries and fruits like them are blue.”
The blue in blueberries cannot be extracted by simply crushing them as the blue is not found in the pigmented juice that comes from crushing the fruit. This led the team to believe that there was something strange about the color.
In the study, Middleton and team examined the wax of a blueberry using an electron microscope. They discovered that the wax layer surrounding blueberries is made up of tiny structures that work by scattering blue and ultraviolet (UV) light, while absorbing other colors of light. The adjustment makes the berries appear blue to humans and blue/UV to birds and other species that can see UV light, despite the fact that there are no blue pigments in the waxy skin itself.
To get a closer look, they removed the outer wax and recrystallized it on a card in the lab. They created a new blue-UV layer and removed a very fine color-creating substance called pigment from the skin. It was only about two microns wide, which is less than a human hair.
“They [the colorants] they scatter blue and ultraviolet rays and they let other colors through without absorption,” says Middleton, “That’s why it’s so important to have dark pigments underneath to ‘sweep’ the rest of the light. If there was a bright pigment or white diffusing material underneath, that light would get through and the color would look washed out or washed out.
It is not yet entirely clear whether UV light on blueberry skin attracts birds. The study shows that nature has evolved a “really neat trick” in the form of an extremely thin layer for an important colorant. It was also surprising that this had not been documented before and that the team could recrystallize the wax and make a new blue layer.
[Related: Indigo, vermillion, and other ancient colors that have decorated the world for millennia.]
“There is evidence of a similar effect in a particular caterpillar, but I would say not much [is] actually published with spectra,” says Middleton.
Reproducing this colorant in the laboratory could pave the way for new methods of creating pigments. The team plans to look at easier ways to recreate this layer and use it to create more durable and even edible UV and blue-reflective paint.
“It was a very nice reminder that there is so much to discover about the world around us,” says Middleton. “Plants are amazing!”