The claim

The “Guinness Coin Test” is the statement that you can tell whether the beer is ready to drink by the sound the glass makes when you hit it with a coin.

Our conclusion

The beer definitely sounds different, which is due to the air bubbles (which disappear after a short time).

We explain how this works in the fact check.

Yesterday was another one of those pleasant evenings. I sat in the Irish pub around the corner and had a beer. During the course of the evening, a nice gentleman from the next table started chatting. He explained to me a trick that would allow me to see when the freshly tapped Guinness had settled in the glass and reached its ideal drinking state. To do this we just need to have a coin. A coin and of course a freshly tapped Guinness.

The whole thing works like this: if you got a freshly tapped Guinness and tapped the glass with a coin, you would hear a certain sound. So a “clang”. Immediately after tapping, we can actually see that the beer is still a little foamy and bubbly. Over the next few minutes, however, this slightly foamy, effervescent state changes and the beer becomes darker. So if you hit the glass again with the coin after a few minutes, you will hear a different noise, i.e. a different “clang” or even a “clink”. When we hear this other “klong,” the Guinness is supposedly ready to drink.

That's the legend that the gentleman at the next table not only told me, but also demonstrated to me several times during the evening. Now the reality is that a field test in this form cannot be carried out on site in a busy Irish pub without disturbing influences. Ultimately, we have various factors and impairments that do not exactly lead to a provable result in this form. On the one hand, we have various environmental external noise factors that affect our sound tests. On the other hand, it is never guaranteed that the person checking the coin always acts on the glass with the same intensity. Another problem is not least the selective perception, as one also hopes for the desired result to some extent. Let's get to the point: there was of course no way we were able to carry out a proper fact check last night. Especially not when several test items have been tried out over time.

Guinness fact-checked

But what is behind this statement? We started a fact check this morning WITHOUT a pub or beer. The first question is whether Guinness even has the possibility of changing its physical properties so that they are also noticeable acoustically. I immediately rejected the first idea of ​​starting another self-test. This doesn't have to happen again (or maybe it does?). Instead, I preferred to interview experts and look for relevant literature.

My first clue came from Florian Aigner, a physicist, author and science explainer. Aigner noted to me that floating bubbles in a vessel can change the sound. In this respect, the original statement would even be plausible. And this could also be the difference between Guinness and other beers.

Guinness is a stout that is drafted with 70% nitrogen (or even more, depending on where you are). By the way, this is also the reason why there is a ball in the Guinness cans, this is the so-called floating widget . The nitrogen that is added during the tapping process gives the beer its character in this form and also creates an interesting bubble effect (see: “ Bubble cascade in Guinness beer is caused by gravity current instability ”).

As already mentioned at the beginning of this text, we can observe immediately after tapping that the beer is still slightly foamy and bubbling and is still a little light brown in color. This changes within a short period of time. The frothy fizz disappears, the foam on the surface becomes firmer and the beer becomes darker overall.

What if I hit the glass with a coin? Yes, the timbre may indeed have changed! Physicists refer to this as the “cappuccino effect” or “hot chocolate effect”. This effect means that air bubbles in a liquid change the speed at which sound travels through the liquid. And in the end you can really hear it (also: Hot Knowledge – The Cappuccino Effect ).

So basically it's not really about Guinness, but about any liquid that is "contaminated" by air bubbles or in which the bubbles disappear. Compare “The Sounds of Teacups and Glasses”:

Experiment
Air was introduced into the water using an ordinary aerator in a faucet. A cup with an inner diameter of 8 cm and a depth of 8 cm was filled to a depth of 7.5 cm. Approximately 0.25 seconds after filling the beaker, the bottom was tapped once with a metal spoon. After another 3.5 seconds the floor was tapped again.

http://drphysics.com/teacup/teacup.html

In fact, it was shown in this experiment that the resonance spectra change when different amounts of air bubbles are in a liquid. Long story short: There is also a wonderful video in which we can watch this exact experiment.

“Guinness is like a good guest who has to sit down first”

And in the end, that's the effect that we were able to observe and probably hear in our Guinness glass last night (it was loud, the coin wasn't always even, etc.). “Seeing” is certainly enough and you can also wait until the content visually calms down. This typical surf must “roll up from below”. You can already observe this with the eye.

Ergo: The glass also made a different sound after the beer had acquired its characteristic dark brown color and the foamy, bubbling streaks had disappeared. It has “settled” and has now acquired its soft and full taste, which is typical of a stout. So it is “ready to drink”. And the “Guinness Coin Test” should be an indicator of this.

At this point I would like to say a loud “Slainte!” to Manuel Michenthaler.

That could also be of interest

The voice in your head: WhatsApp and trust! “When I write to Tom on WhatsApp, I always hear the Styrian talking, even though I read standard German.” What does that mean for us? Read here!


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