So-called plant epicuticular waxes are well known for their self-ordering, self-repairing and self-cleaning properties, their hydrophobicity, and for resisting insect and microbial pathogens. But the optical characteristics of these surface coatings have not been studied in great detail. This, and the observation that blueberry juice is dark red rather than blue, led Middleton and colleagues to speculate that the outer skin of a blueberry harbours a structural colour mechanism. Structural colour refers to the phenomenon in which particular nanostructures interfere with visible light in a way that gives rise to a perceivable colour.

The researchers first performed experiments with highbush blueberry (Vaccinium corymbosum, pictured) fruits. Basic checks showed that the blue colour disappears after mechanical removal of the surface, when rinsed with chloroform, or when applying immersion oil. Optical microscopy confirmed the dark red pigmentation of the epidermal cells underneath. The epicuticular wax was found to be transparent over the visible range. For colour to be perceived, an underlying absorptive background — in this case, provided by the fruit flesh — is required.

所谓的植物表皮蜡以其自排序、自修复和自清洁特性、疏水性以及抵抗昆虫和微生物病原体而闻名。但这些表面涂层的光学特性尚未得到详细研究。加上观察到蓝莓汁是深红色而不是蓝色，米德尔顿和同事推测蓝莓的外皮具有结构颜色机制。结构颜色是指特定纳米结构以产生可感知颜色的方式干扰可见光的现象。

研究人员首先对高丛蓝莓（Vaccinium corymbosum，如图）果实进行了实验。基本检查表明，机械去除表面、用氯仿冲洗或涂浸油后，蓝色消失。光学显微镜证实了下面表皮细胞的深红色色素沉着。发现表皮蜡在可见光范围内是透明的。为了感知颜色，需要一个潜在的吸收背景——在这种情况下，由果肉提供——。