Physics

Here’s why blueberries aren’t blue — but appear to be

Tiny structures in the berries’ coat seem blue, despite dark red pigments in fruit’s skin

Nanostructures in the waxy coating on these berries reflect blue and ultraviolet light. That gives blueberries their characteristic false color, a study shows.

Daniel Hurst Photography/Moment/Getty Images

By Erin Garcia de Jesús

March 5, 2024 at 6:30 am

Roses are red and blueberries look blue. But the berry’s color is not really true. The fruit’s waxy coat just masquerades as blue.

A waxy covering coats some blue-colored fruits — such as blueberries, dark grapes and certain plums. This wax contains a host of tiny structures, each less than a thousandth the thickness of a piece of paper. Such nanostructures scatter blue and ultraviolet (UV) light. To our eyes, that makes these fruits look blue. Birds — which can see UV light — probably see such delicious snacks as bluey-UV.

Blue is not a common color in nature. While some fruits do appear blue, few contain pigments in that color. Blueberries, for instance, contain a heaping amount of anthocyanin (An-thoh-SY-uh-nin). That skin pigment should leave each sphere a dark red.

But if you rub off the outer layer of wax, a blueberry no longer looks blue — or red. Instead, it’s completely dark, Rox Middleton says. Middleton is a physicist who works at the University of Bristol in England and at Dresden University of Technology in Germany. Structures in the fruits’ waxy outer layers create blue hues that are faux, her team now shows.

Scanning electron micrograph images of miniature structures seen on a blueberry, an Oregon grape and a plum. — Researchers viewed the skins of blue-colored fruits under a high-magnification microscope. The resulting images revealed that the fruit skins have texture on their surface made of tiny features called nanostructures. The nanostructures in blueberries (left) reflect blue and ultraviolet light. That covers up dark red anthocyanin pigments found underneath the waxy coat on the fruit’s skin. Similar structures found in Oregon grapes (center) and plums (right) also make those fruits appear blue.R. Middleton *et al*/*Science Advances* 2024

The researchers zoomed in on the skin of a variety of fruits using a high-resolution microscope. This turned up an assortment of tiny molecular structures. Additional experiments revealed that these nanostructures scatter blue and UV light.

Middleton’s team shared its findings February 7 in Science Advances.

The team also managed to re-create this effect in the lab. They placed wax from Oregon grapes (Mahonia aquifolium) in a solution of carbon-based molecules. The wax dissolved and turned transparent. After being spread on a card, the wax dried into a crystalline material. That’s a solid in which molecules form a repeating pattern. As a crystal, the waxy layer again appeared blue.

Creating materials that mimic a blueberry’s coating might prove useful. “This kind of coloring is cool because it doesn’t stain,” Middleton says. It could provide a new way to tint plastics or makeup blue.