Why blueberries are blue

The blueberry has been a part of our diet for nearly 13,000 years, and for good reason. They’re high in vitamin C, high in antioxidants, and contain a good amount of fibre. But for all this time, the source of their iconic colour hasn’t been so clear. Until now. Researchers at the University of Bristol have discovered the secret behind this berry’s brilliant blue. The lead author on the paper is Rox Middleton…

Rox - I've been looking at some other blue fruits, which aren't blue because of pigment, but because of structures inside them. But then I suddenly realised that I didn't actually know how blueberries made their colour. I'm used to the idea that there might be really, really tiny structures that are causing the colour. And so initially when we started looking at the fruits, it was a case of rubbing them and realising that the colour was really carried by the waxy coating on the outside.

Chris - Is this the same science that explains why, for instance, butterfly wings look the colours that they are? It's the shapes of tiny structures on the surface of the wings that give them the colour, it's not because they've got pigments in them.

Rox - That's exactly right. This time the structures are really quite different to the way that butterflies do it. But it's exactly the same idea that there are small structures which interact directly with the light waves but don't absorb any of the light like a coloured pigment would do.

Chris - How did you do this then? How did you actually test that and prove that is what is going on? It's the stuff on the surface of the blueberry that's making it look blue.

Rox - Well, we did make a computer model and look at how that happened, but the other way of doing it was by removing that wax from fruits and then it's completely clear when it's a inner solution then letting that dry out and then re evaporating that wax onto the surface of a piece of black card and we got the blue colour back in that coating.

Chris - Why don't blueberries, if they want to be blue, why don't they just make a chemical that's blue rather than going to all the trouble of making some waxy stuff on their surface that makes them look blue?

Rox - The crazy thing is they do have pigments in them, which could be blue. So they are filled with anthocyanins and the anthocyanins are really important because they make the surface of the fruit look really dark. And without them being dark, you wouldn't be able to see the blue at all. Anthocyanins make fruits red and black, but they can also be blue. But it depends on having certain conditions inside the cell to make them blue. So it might be that it has to have a metal complex with the anthocyanins or you might have to have another really big molecule. And both those things are energetically expensive is what we say.

Chris - And so the fruit just made the choice to go down the route of adding another layer on the outside to make itself look blue, even though the stuff inside could be used for that purpose. It's easier to make a new chemical that it puts on the outside.

Rox - It seems like it, but in fact actually these coatings are on most surfaces of most land plants and they just normally don't make the plant look blue. They have loads of other functionalities, which might be another reason why it's good for the fruit to have that coating. And then a slight adaptation can make that into an also blue coating.

Chris - And what's the purpose of it? Does it have a role beyond changing the colour of the fruit? Is that its primary role or is it there to do other things and it just happens to have this bluing effect in the context of something that is dark?

Rox - Well, we don't know about the other effects. There’s still loads to discover about waxy coatings. What was kind of exciting was to find the same blue colour or you know, similar range of blue colours happening across loads and loads of different fruits. And we do know that it's important for fruits that they are highly visible to animals and birds that eat them. And we were able to look at how birds see things. So what the visual system of a bird does and see that this is really colourful to birds.

Chris - Yeah, because birds have got the ability to see a wider repertoire of colours than we can haven't they?

Rox - Certainly some do. There are lots of birds that don't see UV, but there are loads of birds that do see UV and then also red, green and blue.

Chris - So does this waxy coating work into the UV as well? It's not just the visual visible blues that we can see. Birds looking in the UV, are they going to see something different then in the UV with this wax coating?

Rox - So we know that they see UV and we know that they see blue, so they would see the two together.

Chris - There are multiple plants that have dark fruits that seem to deploy this strategy. Have they all arrived at this independently because it works or are they all in some way related distantly and they're getting it from one common ancestor?

Rox - I think it's a really interesting question. The thing is that they're very, very distantly related. Some of them aren't even angiosperms. They aren't even the flowering plants. So like pine trees. But then basically all of the land plants have this wax, but normally the wax is not blue.

Chris - Is there anything we can do - apart from it's academically very satisfying to be able to explain this, but is there anything we can do with it? Now we know they do this and we know how they do it and we know how it's working. Can we use it?

Rox - Yeah, potentially We could make coatings which have the same effect, which use the same structures and we could make them out of a different material or we could even use this material. Since it's a biological material, it's sustainable and it's often also a waste product. So maybe we could use that and it just self assembles to make these coatings itself. So we could perhaps use that as a colourant or a protective coating. Something that protects against UV perhaps.