The secret to a blueberry’s hue is in the structure of its wax coat.
Waxy coverings on blue-colored fruits such as blueberries (Vaccinium corymbosum), grapes (Vitis vinifera) and some plums contain nanostructures that scatter blue and ultraviolet light, researchers report February 7 in Science Advances. That makes these fruits look blue to people. Birds — capable of seeing UV light — probably see such delicious snacks as blue-UV (SN: 4/3/01).
Blue is not a common color in nature. And while there are some known blue fruits, few contain pigments in that shade. Blueberries, for instance, contain a heaping amount of anthocyanin, a skin pigment that should give each sphere a dark red color.
But structures in the fruits’ waxy outer layers can create their own blues. Devising ways to mimic a blueberry’s color-forming coating might one day provide a new way to give plastics or makeup a blue tint. “Using this kind of coloring is cool because it doesn’t stain,” says Rox Middleton, a physicist at the University of Bristol in England and Dresden University of Technology in Germany.
To better understand what is special about the berries’ waxy coverings, Middleton and colleagues looked at a variety of fruits under a scanning electron microscope. The resulting images showed an assortment of tiny molecular structures. Additional optical experiments revealed that all the structures scatter blue and UV light.
“When you rub the outside of a blueberry and take this outside layer of wax off,” Middleton says, “then you can see underneath is completely dark.”
The team also managed to re-create this effect in the lab. Wax from Oregon grapes (Mahonia aquifolium) became transparent when it was dissolved with chloroform. When the wax recrystallized after being spread on a black card, the layer regained its blue hue.