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If you’re British or UK-resident, then you’ve probably come across Ribena. If not: well, it’s a blackcurrant cordial – you add water to it to make a blackcurrant-flavoured drink. It has a distinctly purple colour, and, according to market research, in 2018 3.3 million people in the UK drank it.

Figure 1: Some blackcurrants.

1) Ribena was actually invented in Bristol. In the 1930s there was an oversupply of milk, and Dr Vernon Charley, a scientist at Bristol University’s Long Ashton Agricultural Research Station, was looking at ways of making a blackcurrant flavouring that could be used to prepare milkshakes. Along the way, he came up with a cordial that could be added to water to make a nice drink. A local drink manufacturer named Frank Armstrong heard about this and started to produce it commercially, naming it Ribena after the Latin names for Blackcurrants, Ribes Negrum. (The Bristol Post, 17 September 2013).

Figure 2: The Long Ashton Research Station, on 29 May 2020. It’s now a day nursery.

During World War II, traditional sources of Vitamin C such as oranges and lemons were in short supply; handily, blackcurrants contain a lot of this essential nutrient. Their production was thus increased significantly, and pretty much the nation’s entire supply came to Bristol to be turned into cordial. The Government distributed this free to children in the UK, thereby creating a generation with a taste for blackcurrant squash; to this day, over 90% of the national blackcurrant harvest is turned into Ribena (https://www.ribena.co.uk/about/).

Figure 3: The basic core of anthocyanins

2) Blackcurrant’s distinctive purple colour is due to the presence of chemical compounds called anthocyanins. These are a class of molecules that are found in many different red or purple plants – we’ve mentioned them in the context of Rhubarb and leaves on this blog, and they’re in blueberries, purple grapes and aubergines as well. In addition, the well-known chemical indicator that can be made from red cabbage works because anthocyanines change colour with pH (https://www.compoundchem.com/2017/05/18/red-cabbage/).

Anthocyanins all have the same basic structure, shown in Figure 3. When drawing chemical structures we tend to use R as a shorthand for ‘some other stuff attached here’, and in the case of anthocyanins that other stuff attached to atom 3 is a sugar molecule. The process of attaching the sugar to the core requires light, and you can see this in nature, when parts of an apple that are exposed to the sun turn red before those parts that are not in the sun. The colour is due to the two 6-membered rings (the atoms labelled 1-8) absorbing light, and the presence of the oxygen atom and the positive charge is important for the colour you observe. The corresponding all-carbon compound with a similar core is called naphthalene (Figure 4), and although this also absorbs light, it does so in the ultra-violet part of the spectrum which the human eye cannot detect, so it doesn’t look coloured. Red, green and blue light are all reflected making napthalene crystals appear white. By contrast, in the anthocyanins the presence of the oxygen atom and the positive charge mean that light of lower energy, from the green region of the spectrum, is absorbed, leaving red and blue behind to generate the purple colour we see.

Figure 4: The structure of naphthalene.

3) Normally at this point, we tell you some more advanced chemistry, but today we’ve got an experiment for you to try at home. The question we’re going to try and answer is related to those anthocyanin molecules, and is: how many of them are there in a glass of Ribena? What do you think? 10? 200? A thousand? A million? A trillion?

A thought to bear in mind if you’re hazarding a guess now is that people generally struggle to understand just how small a molecule is. To ponder this further, consider that there are as many molecules in a teaspoon of water as there are teaspoons of water in all the oceans on Earth.

Full disclosure: this is a bit complicated. You’re going to need a smartphone and a computer monitor and some understanding of science. But, we’d be delighted to hear from you if you’ve given it a go – leave us a comment. We’ve only just written this, and your feedback will help us improve it.

The instructions are here: Instruction Document.

Credits: Chris Adams

Picture credits: Figures 1, 3 &4, Public Domain from Wikimedia Commons.