Raising a glass to the chemistry of wine

We took to the seas, well the river Cam, to try the chemistry of wine with Alex Thom
31 December 2018

Interview with 

Alex Thom, University of Cambridge


Wine glasses on a bar


It's time for Georgia Mills' favourite moment of 2018. And, for whatever reason, 2018 certainly featured a fair amount of wine tasting. There was the “taste” show in April, where we explored different flavours. Izzie tried beer fermented with saliva more recently (not the best!). But then there was our punting show back in summer… Where Chris Smith and Georgia punted down the Cam, picking up and dropping off scientists along the way. Including Chemist Alex Thom who created some experiements with wine...

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.

Chris - 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 here we can use as a spitoon.

[Chris spits our wine overboard]

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.


Add a comment