No blue in blueberries, it’s a trick of the light, says study

BLUEBERRIES are not actually blue and only appear to have a navy hue thanks to a trick of the light, a study has found.

The skin of the fruit is black, the berry has red juice and there is no blue pigment in either flesh or skin.

Analysis found that the colour comes from a colourless two micron-thick wax layer that covers the fruit. An intricate microscopic structure interferes with light itself and makes it seem blue to the naked eye.

White light from a bulb or the sun hits the wax and when it is reflected after bouncing around the wax’s structure it looks blue.

Scientists at the University of Bristol removed the wax, recrystallised it on a piece of card and analysed it in a lab.

Experiments revealed that it reflected both blue and ultraviolet light, but because the human eye cannot see ultraviolet light, the berries appear just as blue. However, animals with a wider range of optics frequencies will see both blue and ultraviolet, making blueberries look two-tone blue.

It is thought the fruit evolved to stand out more to entice frugivores, such as birds.

“The underlying pigment is more reddish; however the blueberry waxes do not contain blue pigment,” Dr Heather Whitney, study author told The Telegraph.

The wax is thought to help keep the fruit fresh and impervious to water but the new findings indicate it also plays a role in colouration and signalling.

The study, published in the journal Science Advances, reveals fruit can have colour without having pigment, known as “structural colour”.

A similar form of this in nature is the iridescence of hummingbird feathers, the scientists say.

Dr Rox Middleton, first author, said: “Those are all different structures to the ones in blueberries, but the idea is similar – colours can be reflected without pigment if there are tiny structures that light interacts with.

“It’s an effect that we see throughout nature – that’s how many shiny/shimmery feathers and insects get their colour – here we f ound t he effe ct widespread in fruits.”

The study also found that plums, sloes, grapes, juniper and barberries also have similar properties.

“From what we know about what fruit-eating birds can see, we know this must act as a visual signal for them,” Dr Middleton added. “It seems likely this is a feature that has evolved to attract fruit-eating birds and other animals.”