listening to coffee

Listening to coffee

coffee tasting notes
Do we pay attention long enough to discern tasting notes such as those in the cup profile here? My current coffee, from Amoret – where you can currently buy this coffee and see if you can ‘hear’ these tasting notes.

Do we taste and appreciate coffee in a similar way to the manner in which we would appreciate a complex piece of music?

Perhaps the idea seems fanciful, maybe even non-sensical. How could it be that the way that we appreciate flavour is similar to how we listen (and how is this related to physics)? Coming from someone who is a clear amateur in both appreciating coffee and appreciating music, you would be forgiven for being a little dismissive (though I’d hope that you would trust me on the fact that there will be a link with physics). But, by being an amateur in taste, I think it is possible to see a first connection: it is in how much attention and learning (training or practise) we give to our perception of our sensation.

A great nineteenth century physicist, Hermann von Helmholtz, was also a medical doctor (and a keen amateur musician). In thinking about how we listen to sounds, Helmholtz suggested that “sensation” was physiological – the effect of the note on our ear or the chemical on our taste buds – but “perception” was psychological – how we hear the notes together or discern the flavour notes of a particular coffee.

Think about how you recognise a type of coffee that you love, or distinguish between a washed and a natural? Or how you know that the instrument that you can hear through the speakers is a violin. With the latter, it is because the fundamental note played is accompanied by a set of harmonics that are distinctive to that instrument. A flute or a piano will have a different set of harmonics and so a different sound. It has been through listening to different instruments that we have learned to identify them, but it is through training and practise (or experimental physics) that we can start to discern the various harmonics.

The way that we hear the different harmonics concerns the way that their waveforms add together. This is underpinned mathematically by Fourier analysis, which describes how any wave form can be made up of a summation of sinusoidal waves. Incidentally Joseph Fourier was also the scientist who proposed the idea of a greenhouse effect back in 1824 (which you can read more about here, or in relation to coffee here). Where you may have experienced these wave combinations is in tuning a guitar (or similar instrument). When you play two notes that are nearly exactly the same, but not quite, the waves of each will add together as they make their way from the plucked string to your ear. As they travel, at some points the two waves will combine to form a large amplitude wave and at other points the two waves will exactly cancel out. We would hear it as a type of “beating” (on-off-on-off) that you can hear as you attempt to tune the two strings together to play the same note. When the two plucked strings play the same note, the two waves will only add together to be louder, they will not cancel each other out and you should hear one, continuous and smooth tone.

Guitar, coffee
From resonances to the way we sense the world around us, there are a number of connections between coffee and music.

You can be an amateur musician and still appreciate the physics that is underlying this aspect of your ability to play (tuneful) music. But Helmholtz had noted a bit more than this. Owing to the way that waves combine, and which in the simplest case gives the ‘beats’ that you notice as you tune the guitar, when you play two notes together, if you listen carefully you will not only hear the two notes, but a third, a so-called combination tone. Discovered by the organist Georg Andreas Sorge in 1740 (you can hear one of his compositions here), this third note has the frequency of the first minus the second note. So, for example, if you were listening to C4 and G4 (at 264 and 396 Hz respectively), you would additionally hear a note at 132 Hz (C3). It is incredibly difficult to be able to discern such a combination tone which maybe part of the reason that it took so long to discover them. To learn to hear the note would take a lot of practise and no less attention when listening to a piece of music. How often do we truly listen to a piece of music to be able to do this?

Where Helmholtz came into this was that, not only did he explain the origin of this combination tone (in terms of the way the waves combined), he invented a device that allowed us mere amateurs to be able to hear it. One end of a tube was designed to fit snugly into the listener’s ear, with the other end open to the sound. The size of the tube determined which frequency of sound would reach the ear. Using these devices Helmholtz showed that, not only was the combination tone a real phenomenon, it had a mathematical basis in physics. And of course there was more. If you could hear the note of the subtraction of the two sound frequencies, you should be able to hear the note of the sum of these two frequencies too. In the example above, you should hear a note at 660 Hz. This combination tone had never been heard before, it came as a prediction of Helmholtz’s theory of how sounds added, itself sparked by a profound attention that he paid to listening to music.

Using a similar resonator to that used for distinguishing the combination tone based on difference, Helmholtz showed that this note too was audible. It was a prediction of what we should be able to hear based on the physics of what was going on. It extended our ability to perceive music.

The beat of a drum or the resonance on our coffee – the links between music and coffee go further than this.

In what way is this linked to tasting coffee? It is in how we learn to distinguish our taste. Just as a musician can, with time and attention, learn to discern at least a difference combination tone so, with practise, we can train our palette to discern intensities of sweet, of sour and subtleties of acids. We amateurs can hone our skills using the SCAA coffee flavour wheel, tasting each coffee we prepare to detect the sweet, roasted or floral notes that we read about on the packs of coffee we buy. To actually describe these coffees requires skill and a large amount of practise in cupping coffee. But to develop those skills to the point of being Q-grader requires an attention to detail that is quite incredible (you can read about the training needed to become a Q-grader here). Just as with music, for some of us, even a lot of practise will only ever allow us to appreciate the work of others rather than produce it ourselves.

Of course, training our palettes requires drinking a lot of coffee, but it also means making mixtures of salty or sweet liquids and thinking about how they taste. Cupping hundreds, thousands, of coffees and paying attention to the complete flavour profile of them. Is there a flavour equivalent to Helmholtz’s summation combination tone that is waiting to be discovered? It will need someone skilled in matters of coffee appreciation and experimental science. Someone who has demonstrated the attention required to carefully listen to the taste of our coffee but who can also work on the theory of how those flavours are perceived. There are many people working on the physics, chemistry and physiology of taste and smell. Could you be one of them?

This is the third in a series of the contributions of Hermann von Helmholtz to our appreciation of the physics in coffee – it goes far beyond the vortices he may be famous for. The introduction is here while the contribution of Helmholtz to our understanding of colour and vision is here. Future posts will consider hot coffee and of course, what happens as we stir it. Much of the material for this post has been found as a result of reading Michel Meulder’s excellent biography of Helmholtz: “Helmholtz: from enlightenment to neuroscience” (2001).

Coffee whispering

coffee and cassette tape in Batch and Co
What does listening to your coffee tell you? Would a long black sound different to an Americano?

How does your coffee sound? Does an espresso sound different to a latte? Could you deduce how the milk had been frothed, or what milk had been used, by listening to your coffee before you drank it?

To see why there may be an effect, it’s worth thinking about your coffee for a moment. The tiny bubbles in the crema of an espresso are different from the larger bubbles of a milk froth made of semi-skimmed milk in a cappuccino. Bubbles of non-dairy milks may be different too, particularly if the initial small bubbles have combined to form larger bubbles as the froth ages. Indeed, sound is used as a characteristic of coffee: think about the sounds made by a steaming wand in milk. Somehow the environment of a cafĂ© would not be the same without the constant hiss and whistle of a cappuccino being made. But can we use it to experience our coffee more fully? Not just the aroma, taste, sight and feel but also, can we start to listen to our coffee?

Take the example of the sound of a dripping tap: each drop of water falling into a bowl of water left under the tap ready for washing up later. Each “plink” is telling you something about the size of the drop coming from the tap. Intuitively, or perhaps from experience, we know that small drops produce a higher pitch, a higher frequency, than large drops: small drops ‘plink’, large drops ‘plonk’. But there is something wrong with this example, because, despite what we may think, we are not hearing the drops at all, only a consequence of the drops.

Drops on a coffee can reveal a lot, but this time we’re interested in the sound that they make.

As the drop falls, it creates a hole in the surface of the water, a dent that grows and then closes in on itself, so that the drop of water has formed a bubble of air under the water surface. As this bubble is unstable, it pulsates under the water just before it collapses and it is this pulsation that we hear. As the frequency of the pulsation will depend on the radius of the bubble, air cavities of different sizes will produce different sounds. And because a larger drop will generally produce a larger hole under the water, the larger drop will generate deeper sounds: plonks rather thank plinks.

How does this relate to the sounds made by your coffee? Well, it turns out that the sound of a bubble bursting reveals a lot about the surface tension and the size of the bubble. A recent study published in Physical Review Letters measured the sounds made by bursting soap bubbles through 24 microphones placed around each bubble. Analysing the sounds, the group found that not only could they ‘hear’ how the air escaped the bubble, by analysing the sounds recorded in the microphones they could determine, quantitatively, the movement and forces of the bubble ‘skin’ as it retreated back and the bubble burst. They suggested that listening to bubbles and liquid surfaces could be a complementary tool to high speed photography for understanding the forces on a liquid. This may prove useful for example when thinking about how a pond skater moves on the surface of the water.

To think about what this may mean for coffee, take the Aeropresses I’ve been making recently. First, I wet the grounds and allow a first stage of de-gassing to start. The sound here is of an almost continuous hiss, not entirely dissimilar to the sound you hear when you put an empty seashell to your ear.

latte art, hot chocolate art, soya art
Could we detect a difference between a semi-skimmed milk latte and an oat milk hot chocolate by the sound that they make? Some people listen to their bread in order to know when it is cooked. What does listening to coffee reveal?

At this point it was hard to know whether what I was hearing were the grinds or the ‘sound’ of the Aeropress ‘shell’. Topping up the chamber with water, the bubbles on the surface of the coffee became larger, and of a different form. And they sounded different too! A few pops, and a hiss.

Did I learn anything (apart from that putting one’s ear to the top of an Aeropress does get quite hot and a steamed ear is a strange experience)? I learned that there was much more to my coffee than I had appreciated, that there is always more to discover. It was almost as the author of the 1933 paper about determining the size of bubbles in water by the sound said:

“As a matter of fact we know very little about the murmur of the brook, the roar of the cataract, or the humming of the sea.”

What will you hear in your coffee? Do let me know, in the comments below, on Twitter or over on Facebook.