蓝莓不“蓝”？

Blueberries have a particularly distinctive color —hence their name.

But blue is a rare color in nature and few naturally occurring organic compounds give living things this color. So why exactly are blueberries blue?

It turns out that scientists recently figured out this conundrum —and it's not the fruit skin.

In a study published Feb. 7 in the journal Science Advances researchers found that tiny, randomly-arranged crystals in the fruit's waxy coating scatter light, giving blueberries their signature indigo appearance.

Blue hues rarely turn up in living things. The majority of examples, such as bluebells, butterflies and tropical frogs, rely on clever trickery to produce this shade (mainly to deter predators). Even blue rocks and minerals, like sapphires and lapis lazuli, are hard to come by.

And blueberries were even more of a mystery.

"The blue of blueberries can't be 'extracted' by squishing —because it isn't located in the pigmented juice that can be squeezed from the fruit. That was why we knew that there must be something strange about the color," study lead researcher Rox Middleton, a researcher at the University of Bristol in the U.K., said in a statement.

While blueberries contain strong pigments called anthocyanins, these have a deep reddish-purple color, completely different from the indigo shade of the fruit skin. However, like most plants, blueberries are coated in a thin layer of protective wax which acts as a waterproof coating and barrier against infection.

Middleton's team suspected the blue hue must come from outside of the fruit. So they removed a sample of this wax and recrystallized it on a piece of cardboard. To their delight, this created an ultra-thin crystalline coating with blueberry's trademark indigo color. When they looked closely at this layer, they found a random distribution of crystal structures within the wax which scatter blue and UV light to produce the fruit's signature color.

"It shows that nature has evolved to use a really neat trick, an ultrathin layer for an important colorant," Middleton said. "It was even more exciting to be able to reproduce that color by harvesting the wax to make a new blue coating that no-one's seen before."

The discovery opens up exciting opportunities for sustainable and biocompatible blue coatings and colorants.

The team is now seeing if there are simpler ways to prepare and apply the wax.

"Building all that functionality of this natural wax into artificially engineered materials is the dream!" Middleton said.

蓝莓有着极为独特的颜色——其名字也正由此而来。

但蓝色在自然界中十分罕见，极少有天然存在的有机化合物能赋予生物这种颜色。那么蓝莓究竟为什么是蓝色的呢？

原来，科学家们最近解开了这个谜题——答案并不在果皮上。

在2月7日发表于《科学进展》杂志的一项研究中，研究人员发现，蓝莓蜡质表皮中微小且随机排列的晶体能够散射光线，从而使蓝莓呈现出其标志性的靛蓝色外观。

蓝色在生物界中很少出现。大多数例子，比如蓝铃花、蝴蝶和热带蛙等，都是依靠巧妙的手段来呈现出这种颜色（主要是为了吓退捕食者）。即使是蓝色的岩石和矿物，如蓝宝石和青金石，也很罕见。

而蓝莓更是一个谜。

该研究的首席研究员、英国布里斯托大学的研究员罗克斯·米德尔顿在一份声明中说：“蓝莓的蓝色无法通过挤压‘提取’出来——因为它并不存在于能从果实中挤出的有色汁液中。这就是为什么我们知道这种颜色肯定有蹊跷。”

虽然蓝莓含有一种叫做花青素的强色素，但这种色素呈深紫红色，与蓝莓果皮的靛蓝色完全不同。然而，和大多数植物一样，蓝莓表面覆盖着一层薄薄的保护蜡，它起到防水和抵御感染的作用。

米德尔顿的团队怀疑蓝莓的蓝色一定来自果实外部。于是他们取下了一部分这种蜡的样本，并在一张纸板上使其重新结晶。令他们高兴的是，这形成了一层超薄的晶体涂层，呈现出蓝莓标志性的靛蓝色。当他们仔细观察这一层时，发现蜡质内晶体结构呈随机分布，这些晶体能够散射蓝光和紫外线，从而产生蓝莓的标志性颜色。

米德尔顿说：“这表明大自然已经进化出了一种非常巧妙的手段，即利用超薄的一层来作为重要的着色剂。更令人兴奋的是，通过采集这种蜡来制造出一种以前从未有人见过的新蓝色涂层，从而重现了那种颜色。”

这一发现为可持续且生物相容性良好的蓝色涂层和着色剂带来了令人振奋的机会。

该团队现在正在研究是否有更简便的方法来制备和涂抹这种蜡。

米德尔顿说：“将这种天然蜡的所有功能融入到人工合成材料中，这才是梦想啊！”

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