Every summer I go blueberry picking and I notice the many colors of blueberries, from the luminous indigo of unpicked berries on a bush, which turn nearly black after handling, to the deep red-purple stain they leave on fingers and fabric. What makes these berries so colorful?
This little fruit owes its multi-colored identity to two separate ways of generating color: a pigment and a structural effect. Like many eye-catching berries, blueberries contain pigment chemicals that selectively absorb some light wavelengths, reflecting the other colors back to our eyes for us to see. They also have a waxy layer, or bloom, that scatters and reflects blue light.
Dr. Rox Middleton, a researcher who studies the optical properties of plant-based materials, has investigated the blue of many fruits that have a wax bloom, including blueberries. Blue is an excellent advertising color for attracting fruit-eating animals, many of whom have vision that is sensitive to this hue. However, blue pigments are rarely seen in any fruit. The kinds of molecules that reflect blue light tend to be metabolically expensive, making the color impractical for many plants to produce.
Instead of blue pigment, blueberries contain pigments from the anthocyanin family, a group of hundreds of plant-based chemicals that contribute to red, purple, or black colors in fruits. Anthocyanins can also be blue. Anthocyanins change with pH, so they can shift from red to purple to blue if the liquid they are immersed in changes from acidic to basic. These anthocyanins give blueberries their underlying dark color that is then covered by a wax bloom.
This wax layer is created when “the [wax] molecules turn up and move to where they’re energetically relaxed,” Middleton said, which means they naturally come together in a stable, organized structure, forming crystals. The wax on blueberries gives the berry its distinctive velvety blue surface, but under a microscope the crystals that compose the wax are a hodge podge of tiny tubes. “They look like rolled up cinnamon sticks – curled up but not completely,” Middleton said.
These tubular crystals create blue through a “structural color” effect. Structural colors are created by physical shapes that are tiny enough to interact with light waves, causing effects such as constructive or destructive interference or selective scattering. These types of interactions essentially sort the light waves that reach our eyes, so we see some colors but not others. They can create the impressive effects from the brilliance of green peacock feathers to the rainbow swirls on a bubble.
The blue of blueberries is the result of those tiny tubes in their wax bloom scattering light in the blue range of the visible spectrum, reflecting it back for us to see. We often see blue from a very specific set of blue wavelengths, but the waxy bloom of blueberries reflects wavelengths from blue into ultraviolet light, actually peaking in the wavelengths of “near ultraviolet,” below the range perceived by humans. It’s possible that birds, many of whom can see ultraviolet light, use the mix of ultraviolet and blue light to recognize blueberries.
In blueberries, the colors that aren’t scattered pass through the wax layer and are absorbed by the dark anthocyanin pigment in the berries’ skin, so we can’t see them. Without that dark backdrop, the blue might not be visible: unripe blueberries look pale green because the anthocyanin pigment hasn’t accumulated, but the wax bloom is already present and scattering blue light. The blue of a blueberry wax bloom is a delicate effect: the light scattering is translucent and therefore much less visible without their pigment layer, and the wax can easily be rubbed off, so they lose their blueness entirely. Both the wax bloom and the pigment work together to create the distinctive blue of ripe blueberries.
Fully understanding the structural color mechanism is complicated and Middleton has many future questions to pursue. “These wax shapes are beautiful, but much more confusing. It’s an invitation to ask: what’s really going on?” they said.
I know that the next time I pick blueberries off our bushes, I’ll appreciate their variety of hues even more, knowing the complexity that makes them possible. But I’ll still pop them into my mouth!