Cheers! The chemistry of wine

How does acidity make a wine taste better?
28 August 2018

Interview with 

Alex Thom, University of Cambridge, Max Thompson, Rutherford's


The Naked Scientists and Alex Thom enjoy a glass of Reisling


Would a punting trip be complete without a tipple or two? Georgia Mills and Chris Smith are joined by theoretical chemist Alex Thom from the University of Cambridge, who is a keen onoelogist. But before the wine drinking starts, they learn a little about the scenery from Rutherford's tour guide Max Thompson. 

Alex - Theoretical chemistry tries to do chemistry mostly in a computer. That is quite hard because there’s a lot of atoms involved in chemistry usually. And so I work on making that better basically by designing algorithms to enable us to solve the problems more easily and on today’s computers.

Georgia - Why does it help to do chemistry on a computer instead of a dish?

Alex - Sometimes you just can’t do the experiments. Let’s say you want to look at the chemistry in the atmosphere of Jupiter or in the Sun, or even let’s say you’ve got a lot of different materials you want to screen for some exciting property like a solar cell. It’s just not economical to do these experiments or not possible, so if you can get answers from a computer that can at least help.

Georgia - And why is it difficult?

Alex - It’s all to the blame of physicists really. We need to solve the quantum mechanics underneath chemistry and, basically, every electron in a molecule is entangled with every other electron and you need to take into account all of those correlations and that gets really horrible very quickly.

Georgia - It sounds like tricky work which might bring us on to your other interest which is also chemistry related. So tell me about that.

Alex - Yes. I’ve had an interest for some time in wine and oenology which is the sort of study of winemaking and tasting wine.

Georgia - So does that mean that inside this basket there might be some wine?

Alex - There are a few bottles of wine in here, yes.

Georgia - Ohh! Hello!

Alex - And other jolities. I’ve brought a few bottles of different wines and it’ll be nice to try them and possibly even do some science on them.

Georgia - Possibly, maybe. Mainly drink them. How does chemistry come into wine tasting them?

Alex - Certainly in winemaking there’s a great deal of chemistry. A lot of vineyards will have their own oenologist, who’s basically a chemist.When you start making the wine from the grape juice a lot of things can go wrong and you need to be able to study it quite quickly and fix them. And to make products that are nice to drink isn’t easy so the more you know about what’s going on in your wine the better.

The flavour profile is made up of hundreds of different molecules. The main components are alcohol and some acidity and water. But the interesting bits are the flavour molecules which are very difficult to isolate and put together and they vary from year to year, and grape to grape, from wine to wine.

Georgia - And these are things when someone will say oh yes, it’s got hints of oak or something, that’s the flavour molecule doing that?

Alex - Exactly. You know it’s taste of guava, or pineapple, or even smell cut grass or that sort of thing. All of those are volatile molecules that you get when you smell it and also when you taste it.

Georgia - This has all got my mouth watering so perhaps - what’s the first one we’re going to taste?

Alex - I’ve brought an interesting wine here called a Reisling - that’s the grape, and I chose it because one of the important constituents of wine is the acidity. That’s what often makes the juicy feeling in the mouth for wine and I wanted to play with the idea of changing the acidity of wine and to see what the flavour changes.

Georgia - Oh, so we can actually change the acidity even though it’s already been made, put in the bottle, we can tinker?

Alex - The joy of chemistry is that we can play with some of these elements in a controlled fashion. Let’s try it as it is and we can comment on it, and then I can tell you what the official tasting notes say.

Chris - I love this. You can tell you’re a chemist, Alex, because you’ve got a pyrex beaker that you’re going to drink this out of.

Alex - Actually I got this at the International Chemistry Olympiad this year, so that was a gift there. It’s great.

Chris - It’s fantastic. It’s literally a pyrex like you would put on a retort stand on a gauze and boil away in a laboratory but it’s got a handle on the side.

Alex - I’m going to put a little of the Riesling in the glasses first. This is the unadulterated wine, as such, and we’ll taste that. Give it a swill in the glass maybe, get some of the flavour out.

Georgia - I think it’s a bit smokey. It’s a bit flowery and maybe a bit of citrus?

Alex - Yes, there’s citrus in there. The flowery is good. I can’t get the smoke myself but.

Georgia - Maybe someone went past smoking.

Alex - Yes, exactly. All manner of things in the atmosphere. But again, everything’s very original.

Chris - It is quite sharp. I’d say that’s quite an acid wine.

Alex - It is, yes. I picked it because it’s a Riesling which is known for one of the most acidic wines. That can be good and bad. People like acidity because it means they keep longer, they age longer, but if it’s too acidic it becomes sharp and horrible. So this is wine you might have on a hot summer’s day - a bit like today and the acidity makes it feel more refreshing.

Georgia - Very very nice. Can we see what happens when we change that acidity then.

Alex - Yes. I’m going to do a little modification to the acidity here so I’ve poured some of the wine into my beaker and we’re going to add to it some bicarbonate of soda. Now this is an alkaline salt basically. It’s just a standard kitchen chemical but… Because we’re on a punt it’s quite difficult to judge quantities and do this correctly, so into this wine I’m going to cheat and add a little bit of a homemade indicator. I’ve made this out of red cabbage last night. Hopefully it won’t change the flavour too much. But this wine is currently a nice sort of yellow colour and if I add a bit of this it should change to a startling pink colour, hopefully. And I can use this indicator to tell me how acidic the wine is, so this very pink colour means it‘s pretty acidic at the moment.

Georgia - it goes bluer when it’s more alkaline?

Chris - It currently looks like rosé doesn’t it.

Alex - Yes, exactly. I’m going to now add to it some bicarbonate of soda and hopefully that should fizz quite happily there, and it’s gone a bit darker purple. So we’re looking for a purplish colour rather than a green colour. If it’s gone too green it’s gone too alkaline.

Georgia - If feels like the acidity might have changed but we’ve also put in cabbage and a lot of soda. Is this really going to tell us anything?

Alex - I chose this partly because I had a red cabbage in the fridge last night. Yes, it’s certainly changed colour now. It’s gone a sort of salmony, maybe very light rose colour rather than the bright pink. It should now smell a lot less as I…

Chris - Yes. It certainly doesn’t smell the same.

Alex - Feel free to spit this out if you don’t like it.

Chris - Have we got a spitoon?

Alex - There’s a jug her we can use as a spitoon.

Chris - Oh god. That was grim. Ah man.

Alex - What's happened to it.

Georgia - It’s ruined! That’s what happened. Ugh.

Chris - That was rank.

Alex - It’s taken all that acidity away. What we’ve now got is a…

Chris - It hasn’t just taken the acidity away. It’s taken any semblance of wine away.

Alex - Yeah. And it basically tastes horrible.

Georgia - It’s like drinking washing up liquid or something.

Alex - Yes. I may have added a bit too much of this. The flavours I can still get - you can still taste the alcohol in this. So it tastes like a shot of alcohol but without anything much else.

The next experiment is to see if we can put the acidity back with a different acid. And so acidity is really really prized in wines because it gives them flavour, so most of the flavours disappeared as well. And if you’re in a really hot climate the grapes tend to turn all that acidity into sugar and they lose a lot of the flavour. This is an acid which I’m going to add which is a different acid from the one we had before. Most of the acid in the wine would have been an acid called “tartaric acid” which I’ve got somewhere in here and malic acid.

Chris - You’re making what lysergic acid? It’s a bit different kind of acid isn’t it?

Alex - Lactic! So this is stuff that builds up in your muscles when you exercise too quickly and it tastes a bit like yogurt.

Chris - Well, it’s gone the right colour again hasn’t it?

Alex - It’s gone a nice pink colour again.

Chris -  It’s back to pink so we know it’s more acidic again. I’ll give it a go. Go on then.

Alex - It’s pretty tart.

Chris - The smell is back.

Alex - The smell’s back.

Chris - Yeah. The smell, it’s wine again.

Georgia - Yeah, I can smell the wine again.

Chris - It’s back to Riesling that we had before. I tell you what, it’s a lot less disagreeable than what we did with the first effort with the bicarb.

Georgia - It’s like one of those sweeties that's super sour. So the flavours weren't destroyed by that red cabbage goop we put in, they were there?

Alex - Just hidden by changing the ph. As you can see, having a more acidic wine gives the flavours more of a chance.

Chris - You’re really enjoying that Georgia.

Alex - That was a very fine face.

Chris - Do you want some more? A top up?

Georgia - I’m quite alright.

Alex - So that’s what you can do and, of course, winemakers do this on a much more careful scale with acidity rather than just pouring a few things together in a punt.

Georgia - That's a really interesting example of how acidity is really important. I have new appreciation for acidity because it did taste absolutely awful and then the flavours did just sort of come back to life.

Alex - Exactly yes. I’ve got one more. We don’t need to finish all the bottles immediately.

Chris - There’s one each.

Georgia - Oh, I spy a word - champagne.

Alex - Yes, yes.

Georgia - Oh my goodness. You’re treating us.

Alex - Yes. It’s a good name in wine. I might just have a glass to catch if any champagne comes out... Oh no, not quite.

Georgia - Ahh!

Alex - Sorry, I put my finger over the bottle to stop it all from squirting out. And I managed to get….

Chris - This is Lewis Hamilton experience there for Georgia.

Alex - Cheers.

Georgia - Cheers.

Chris - Cheers Alex. Now what’s interesting about this is it’s got very tiny bubbles.

Georgia - Do the bubbles change the chemistry of the flavour? Does that make sense? Do they change how we perceive that chemistry?

Alex - Yes. The bubbles when they’re on your tongue and in your mouth they act as little pockets of air and it’s like swirling you glass. They produce these pressure gradients as they’re forming and they make the wine a lot more flavoursome effectively, so you taste the flavour molecules. If you leave champagne out for a day, you can get a nice wine but it doesn’t taste quite as nice after you’ve left it for a day.


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