If you open up a ripe blueberry, the blue skin on its outside does not match the dark, reddish purple color inside of the fruit. However, their skin does not actually contain blue pigments, which would normally be creating this color. Instead, the random arrangement of microscopic structures on their natural wax coating gives these berries a blue appearance. The findings are described in a study published February 7 in the journal Science Advances and can be applied to other fruits including slides, damsons, and juniper berries.
[Related: Meet the first electric blue tarantula known to science.]
The color blue is one very rare in nature, with fewer than one in 10 plants sporting the common human favorite. It’s partially because there is not a true blue pigment in the natural world. To appear this color, the molecules in both plants and animals have to perform tricks to make them appear blue to the human eye. In some flowers like bluebells, this primarily occurs when naturally occurring pigments or colorants are mixed the way you can mix different paints to change the color. Red pigments called anthocyanins are the most common in nature and changes to the acidity in the pigment changes the color. Combined with reflected light, dayflower, hydrangeas, and cornflowers can all have colorful azure flowers. It’s not entirely clear why plants may 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 on the naturally produced thin layer of wax on their skin. The wax serves multiple functions in the plant kingdom, including as a self-cleaning coating or for added protection, but scientists still don’t understand a lot of what the wax does and why.
“We realized that there are loads of blue pigments in nature, as well as ‘tricks,’ or ways that nature makes blues without pigments,” study co-author and University of Bristol physicist Rox Middleton tells PopSci. “We just didn’t know–couldn’t find anywhere written–why blueberries and fruits like them are blue.”
The blue in the blueberries couldn’t be extracted simply by squishing it since the blue is not located in the pigmented juice that comes from squishing the fruit. This led the team to believe that there was something strange about the color.
In the study, Middleton and the team examined the wax of a blueberry using an electron microscope. They found that the layer of wax that surrounds blueberries is composed of tiny structures that work by scattering blue and ultraviolet (UV) light, while absorbing other colors of light. The arrangement makes the berries appear blue to humans and blue/UV to birds and other species that can see UV light, despite not having blue pigments in the waxy skin itself.
To look closer, they removed the outer wax from the very and recrystallized it on a card in the lab. They created a new blue-UV coating and removed a very thin substance that creates color called a colorant from the skin. It was only about two microns wide, which is less than a strand of human hair.
Diagram showing how the wax structure reflects light. CREDIT: Rox Middleton.
“They [the colorants] scatter blue and UV and they let the other colors pass through without absorption,” says Middleton “That’s why it’s so important that there are dark pigments underneath to ‘mop up’ the rest of the light. If there was a bright pigment or white scattering material underneath, that light would come through, and the color would look mixed or washed out.”
It’s not yet fully clear if the UV light in blueberry’s skin attracts birds. The study does show that nature has evolved a “really neat trick” in the form of an ultrathin 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 coating.
[Related: Indigo, vermillion, and other ancient colors that have decorated the world for millennia.]
“There are hints at a similar effect in a particular caterpillar, but I would say not a great deal [is] actually published with spectra,” says Middleton.
Reproducing this colorant in the lab could pave the way for new methods of creating pigments. The team plans to look into easier ways of recreating this coating and using it to create more sustainable and even edible UV and blue-reflective paint.
“It was a really nice reminder that there is so much to discover about the world around us,” says Middleton. “Plants are amazing!”
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