Shiny Colors Crafted from Natural Fruit Wax

Jim Crocker
8th February, 2024

Shiny Colors Crafted from Natural Fruit Wax

A blueberry's blue hue (A) is not from its pigment but from a structural effect of its waxy coating, a fact revealed when the wax is removed (B) to show the dark red skin cells underneath (C)—a phenomenon also seen in plums (D).

Image adapted from: Middleton et al. / CC BY (Source)
The vibrant colors of fruit play a crucial role in attracting animals that help plants spread their seeds. While red, orange, and yellow fruits are common, genuinely blue fruits are surprisingly rare. This is puzzling, as many animals – particularly birds – have excellent vision for detecting blue shades[2]. Traditionally, fruit color comes from pigments, chemical compounds that absorb certain wavelengths of light and reflect others. However, recent research from the University of Bristol[1] reveals that some fruits appear blue not because of blue pigments, but due to the physical structure of their surface. The study focused on dark-colored fruits covered in a waxy bloom, such as blueberries, plums, and juniper berries. These fruits don’t contain blue pigments; instead, their blue hue is created by how light interacts with microscopic structures on the fruit’s surface. This phenomenon is called structural coloration – the same principle that creates the iridescent colors of butterfly wings or peacock feathers. Researchers discovered that the wax bloom consists of tiny, randomly arranged particles that scatter light. Specifically, these particles interact with light waves to enhance the reflection of blue and ultraviolet light. This creates the perception of a blue color, even though no blue pigment is present. To confirm this, the team extracted wax from the fruits and allowed it to recrystallize in the lab. The resulting material exhibited the same structural blue coloration, proving that the wax structure itself was responsible for the effect. This finding challenges previous assumptions about fruit coloration. Earlier work showed that fruit colors, while diverse, are less varied than flower colors[3]. This difference was thought to be due to constraints related to the chemical properties of pigments. However, the discovery of structural coloration adds another layer of complexity. It suggests that the limitations in fruit color diversity aren’t solely due to pigment chemistry, but also to the biological mechanisms required to create and maintain these surface structures. Interestingly, the researchers found that this structural blue coloration had evolved independently in different plant species. Despite variations in fruit shape and wax composition, the optical properties – how light interacts with the surface – were remarkably consistent across a wide range of plants. This suggests that there’s a strong selective advantage to having a blue appearance, and that structural coloration is an effective way to achieve it. The implications of this research extend beyond understanding fruit coloration. The epicuticular waxes responsible for this effect possess unique properties: they are self-assembling, self-cleaning, and capable of self-repair. These characteristics make them promising candidates for bioengineering applications, potentially leading to the development of sustainable and biocompatible optical materials. Furthermore, the study highlights the power of combining detailed physical analysis with biological observation to uncover previously unknown mechanisms in nature.

BiotechBiochemPlant Science

References

Main Study

1) Self-assembled, disordered structural color from fruit wax bloom.

Published 7th February, 2024

https://doi.org/10.1126/sciadv.adk4219

Related Studies

2) Animal colour vision--behavioural tests and physiological concepts.

Journal: Biological reviews of the Cambridge Philosophical Society, Issue: Vol 78, Issue 1, Feb 2003

3) How colorful are fruits? Limited color diversity in fleshy fruits on local and global scales.

https://doi.org/10.1111/nph.12157


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