Cut open a blueberry, and you will find that its flesh is pale; crush the blueberry skin, and a deep maroon juice gushes out. This reveals to us a surprising fact: there is no blue pigment in blueberries. So, where does their iconic blue colour come from? Researchers at the University of Bristol in the UK have discovered that this colour is not hidden within pigments at all, but rather, arises from a physical mechanism known as structural colouration (1).
The eyes of many visually guided fruit-eating animals are highly sensitive to blue light, making blue a bold and distinct signal colour that effectively attracts animals to help disperse seeds. Compared to other fruits, blue fruits are quite rare. Most plant colours have dedicated pigment molecules: green comes from chlorophyll, yellow from lutein, and orange from beta-carotene. However, producing blue pigments through this chemical pathway requires plants to synthesise large pigment molecules that absorb lower-energy light (red light). Constructing such complex molecules, however, consumes a substantial amount of the plant’s energy. To ingeniously resolve this ecological paradox between high reward and high cost, blueberries did not stubbornly stick to this chemical route. Instead, they evolved a magical surface coating that "manufactures" blue on a physical level.
Blueberry skin is rich in anthocyanins, but these anthocyanins mainly appear dark red or almost black. If you observe a blueberry closely, you will find its surface is covered with a thin layer of "white frost" that cannot be washed off by water. This substance, botanically known as "bloom," is a microscopic layer of surface wax crystals, and it is the true maker of the blueberry's blue appearance. The blue-ultraviolet colour of these dark-pigmented fruits is produced by an optical mechanism known as "structural colour."
The essence of classic structural colouration phenomena, seen in peacock feathers or Morpho butterfly wings, lies in the highly regular, periodic arrangement of nanostructures within the material. These ordered arrays have specific lengthscales that cause light waves of certain wavelengths to undergo coherent interference, producing highly saturated colours and distinct reflectance peaks in the spectrum. At the same time, because the geometric effects of physical interference change depending on the incident angle of light, these biological surfaces often exhibit iridescent, shifting colours as they are moved. However, the colour of a blueberry does not change with the angle. Electron microscopy confirms that blueberry bloom has absolutely no regular structure: it is highly disordered, consisting of randomly arranged, hundred-nanometre-scale non-spherical scatterers. Therefore, the colouration mechanism of blueberries cannot be explained by the interference of classic periodic structures, but is instead driven by how these scatterers interact with light.
Spectral measurements further corroborate this mechanism: unlike classic structural colour, the spectral signature of blueberry bloom shows no distinct reflectance peak within the visible light range, but instead exhibits an inverse relationship between wavelength and reflectance, where shorter wavelengths show higher reflectance. As a result, this scattering property is biased toward shorter wavelengths. The wax crystals scatter the shorter-wavelength blue and ultraviolet light illuminating the blueberry's surface back into the external environment.
Colours produced by such microscopic physical structures are non-absorbing, meaning that the tiny epicuticular wax crystals creating this blue illusion are actually translucent. If the fruit relied solely on this wax layer, unscattered red and yellow light would be reflected back from the underlying layers and mix with the surface blue light, making the fruit look washed out or white. Therefore, all non-absorbing structural colours must rely on a dark "background" with strong absorption capabilities to completely eliminate the stray light that penetrates through. This is where the abundant anthocyanins in the epidermal cells of the blueberry skin come into play: these deep dark-red pigments, which look almost black upon observation, form an ideal light-absorbing base that completely absorbs all the medium- and long-wavelength light, penetrating the wax layer, and accentuating the blue scattered by the surface.
The research team confirmed the dominant role of epicuticular fruit wax through various methods. When mechanical wiping or the organic solvent chloroform is used to destroy this layer of bloom, the blue colour on the blueberry's surface immediately disappears, exposing the true dark anthocyanin appearance of the underlying skin. Furthermore, the researchers successfully reproduced the structural colour of this fruit wax in the laboratory: they extracted the plant's epicuticular wax using chloroform, and then recrystallised it using thermal evaporation and deposition techniques in a vacuum environment. These molecules underwent self-assembly after deposition, automatically forming nanocrystals similar to those on the natural fruit surface, successfully reproducing that blueberry blue on a black substrate.
The researchers also observed various plant fruits from different evolutionary lineages and found that this phenomenon of producing structural colour via bloom is extremely widespread in nature. Interestingly, the surface wax crystals of different blue plants vary vastly in microscopic morphology covering diverse geometric structures such as the ring, slab, rod, and tube shapes. Despite their different microstructures, they exhibit highly convergent optical properties in spectral reflectance, all producing similar blue-ultraviolet reflectance curves. From an ecological perspective, this creates a survival paradox: the fruit accumulates high concentrations of anthocyanins to provide nutrition and antioxidant protection, yet this makes it darker and harder to detect. This paradox is ingeniously resolved through the bloom, which covers the dark anthocyanins and produces a blue colour via structural colouration, sending a strong visual signal to seed dispersers like birds.
The wax layer on plant surfaces has diverse functions. Previously, researchers only knew that it could serve as a highly effective hydrophobic and self-cleaning coating, with multiple properties such as resistance to insect pests and pathogens, but it was unclear how important its structure was to colouration. This research broadens our understanding of non-absorbing photonic surfaces and is expected to be integrated into bioengineering in the future, being developed into novel, sustainable optical biomaterials with self-repairing functions or eco-friendly colourants.
References
Middleton R, Tunstad SA, Knapp A, Winters S, McCallum S, Whitney HM. Self-assembled, disordered structural color from fruit wax bloom. Sci Adv. 2024;10(6):eadk4219.
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