Coffee, oils and Clapham Common

A pour over at Röst Stätte coffee in Berlin. How does the preparation method affect the oils visible on the surface of a coffee?

Have you been making more coffee at home through the Covid-Lockdown times? Each morning, I have taken time to brew a coffee (or several depending on the way I feel that day) and then sit down and notice what is going on in the mug. The way the steam swirls upwards in turbulent patterns, the white mists on the surface of the coffee and the peculiar effects they have on reflected (and refracted) light from the coffee’s surface. And, the oils that appear on the surface of black coffee.

The appearance of these oils is very dependent on the way that you make your coffee. A cafetiere/French press is an immersion method of brewing coffee with no subsequent fine filtration of the grounds. It is therefore quite likely that the oils present in the roasted coffee bean will make their way to the surface of your coffee. If you brew by a pour over method on the other hand, it is thought that the paper filter should take out the oils as you brew. However, even when using a paper filter on a V60, a thin layer of oil can sometimes be seen on the surface of my coffee, visible in the sunlight on those mornings. How much oil is making it through the filter?

Regardless of whether you view the oils as important for the flavour or detrimental to it, there is something quite remarkable about oil patches on the surface of water: they can be a single molecule thick. Of course, you can pile more oil on the surface of the water and then you see the interference effects with light as you see on the surface of polluted rain puddles next to roads, but if there is a large enough area of water, the oil will spread out until it is one molecular layer thick.

How can we know that it is just one molecular layer thick? In one of those experiments that it is probably better to know about rather than to rush out to repeat, a clue came in the 1760s when Benjamin Franklin put a teaspoon of oleic acid (found in olive oil) on the surface of Mount Pond on Clapham Common. As he watched, the surface of the pond, which had been active with many capillary waves blowing over it, was calmed as the oil dispersed across the surface. First the oil remained in a small patch but it then grew, and grew until it reached the other side of the pond.

Mount Pond on Clapham Common. Probable site of Franklin’s 1760s experiment stilling the waves with a teaspoon of oil.

Franklin had been expecting the calming effect of the oil on the water waves, in fact he had been looking for it. On his journey to the UK from the USA he had been watching the wakes behind the ships in the fleet that were accompanying his ship on the journey. Two of the ships showed remarkably calm wakes, a fact that he had remarked upon to the ship’s captain. The captain had responded quite flippantly that it was probable that the cooks had emptied their greasy water over the sides of the ship. Mariners knew that oil and greasy cooking water, calmed the waves around the ships. We can learn a lot by talking to each other and listening to their experience.

The mariners knew that oil calmed the water but why? How? If we think about the oil as a surface layer over the water, it becomes possible to imagine an answer to this. Without the oil, when the wind blows over the water it will act to exaggerate the small perturbations on the surface of the water (caused by water flow, falling raindrops etc) which can then grow into waves. With a layer of oil on the surface, when the wind blows, if the oil is thick, it will act to blow the oil into a thinner layer covering the water surface. If the oil is thin already, it would take a lot of energy to stretch the oil surface to accommodate a growing wave. Either way, rather than exaggerate an existing perturbation on the surface of the water, the wind over an oily surface will tend to drag out the oil film, which will have the effect of calming any perturbations rather than encouraging them.

But how realistic was it that Franklin’s teaspoon of oil could have covered Clapham Common pond? About one hundred years after Franklin, Agnes Pockels and Lord Rayleigh were studying the effect of oil on the surface tension of water. As they did so, they calculated the thickness of thinnest oil layer that they could disperse over the surface of the water bath they were studying. Pockels calculated this thickness as 1.3 nm, Rayleigh at 1.6 nm, either way, a layer that is 10 000 times thinner than a grain in the smallest espresso grind coffee.

And one molecular layer thick.

It was while experimenting with surface tension that Agnes Pockels and Lord Rayleigh (separately) calculated that a layer of oil on water was just over 1 nm thick.

So to return to Clapham Common pond. 1 teaspoon is 5 cubic cm. If the oil formed a layer 1.5 nm thick over the surface of the pond, it would disperse over an area just slightly over 3000 square meters. It is perfectly possible for one teaspoon of oil to disperse over the surface of Mount Pond in Clapham Common. But what is possible is not necessarily advisable so let’s reverse the question and ask how much oil is on the surface of the coffee? Assuming that what is on our coffee is genuinely one molecular layer thick, or about 1.5 nm*. My cup has a radius of 4cm, meaning that the volume of oil on the surface is 0.0075 cubic millimetres. One metric teaspoon of olive oil is 5 cubic centimetres or 4.55 g. If we use the ratio of the volumes to calculate the ratio of the mass, we find that the oil we can see on the surface has a mass of about 7 micro grammes. A tiny amount, but a value consistent with studies suggesting that a small amount of cafestol (associated with the lipids in the coffee) gets through to the brew even in pour overs.

There is plenty to notice in a coffee, what do you see in yours?

*It is of course possible that the oils are actually thicker than this, but the paper filter does result in an oil film that is far from continuous across the coffee surface, suggesting that the oil is already stretched as far as it could be.

From fried eggs to coffee boules via milk rings

Egg no pales, coffee, garden centre
We can often see the Leidenfrost effect when we cook a fried egg. But could we see it while simultaneously preparing a coffee cocktail?

We have probably all come across the Leidenfrost effect, the splash of water into a hot frying pan causing drops of water to skirt across the hot surface before evaporating. We may even be familiar with it in frying pans and cooking surfaces. But what would happen if you swapped the frying pan surface for a (very hot) liquid surface. What happens to the Leidenfrost effect then?

One of the first differences between a frying pan and a bath of hot liquid (we’re not quite yet to the coffee bit) is that the frying pan based Leidenfrost effect requires a lot of heat: the frying pan has to be many degrees hotter than the boiling point of the liquid being levitated. But for the Leidenfrost effect to happen on liquid surfaces requires nowhere near so much heat. In some cases levitation can even occur if the liquid bath is just one degree higher than the boiling point of the levitating liquid. What makes a hot liquid so much different from a hot solid?

The first explanation could be that a liquid surface is absolutely flat at the molecular level. Frying pans and other surfaces have scratches and dents and all sorts of bumps that mean that bubbles can form at the interface and disrupt the levitation of the drop. Could this be it? Probably not as a complete explanation because people can study the Leidenfrost effect over semiconductor wafers which are also atomically flat and even there, many more degrees are needed between the temperature of the surface and the boiling point of the drop than are observed in liquid substrates.

A second explanation is that a liquid surface is able to deform a bit to support the weight of the drop above it, this means that the drop has more of a chance of remaining levitating above the liquid surface. And yet, it turns out that there is more than that happening in liquids as a recent study in a prominent physics journal showed.

If you look carefully at the surface of the coffee in the V60 jar, you will see it is bent underneath the drop on top of it. While the drop on the coffee here is not ‘floating’ because of the Leidenfrost effect (it is stable due to other effects described here), the fact that liquids may be able to bend under drops has been thought to make the Leidenfrost effect more stable on some liquid surfaces.

That study used a bath of silicone oil as the heated surface. The drops that levitated were either of two different liquids: ethanol (ordinary alcohol) or HFE-7100 (an engineered fluid designed to replace ozone depleting chemicals in certain industrial applications). What made the study so interesting was that tiny fluorescent particles were mixed with the silicone oil that allowed the researchers to see how the liquid underneath the drop was moving.

A toroidal vortex formed in the silicone oil under both the ethanol and HFE-7100 drops. We can see similar toroidal vortices in our V60 or by dripping milk into a glass of water; they are doughnut shaped regions of moving fluid, like smoke rings, they could be considered ‘milk rings’. But in this case, there was no drop entering into the bath of liquid as with the milk rings. The drop and the bath were not mixing at all. And, perhaps more puzzling, the direction of the rotation of the vortex was different for the two types of drop. For the alcohol drops, the liquid directly underneath the drop plummeted into the silicone oil before moving under and then back up to the surface to be pulled down at the centre again. Under the HFE-7100 it was different. There, the liquid at the centre of the doughnut surged up, dragged along the surface before going under and returning back once more to be pulled up at the centre of the torus.

Why would the two liquid drops show such different behaviour in the silicone oil substrate? It comes down to a competition of three forces. The first thing that you will notice is that if the levitating drop is slowly evaporating and will eventually disappear (as is the case with the frying pan), this means that it is absorbing heat from its local atmosphere in order to gain the energy needed for evaporation to occur (think about your hand getting cold after sanitising it with an alcohol liquid as the alcohol evaporates off). This means that the silicone oil immediately under the drop gets colder. Cold liquids are generally more dense than warm liquids and so the cold liquid sinks pulling the surrounding liquid down with it.

Linked with this effect is that the surface tension of a liquid decreases as the temperature of the liquid increases. This results in a flow of liquid from regions of low surface tension to regions of higher surface tension called a “Marangoni flow”. This is again something that we may have seen during the Covid-19 lockdown restrictions as videos were circulated showing the effect of soap on a layer of pepper scattered on the surface of water. The pepper retreats away from the soap because of these Marangoni flows which can in fact be very fast.

Milk rings can be formed by dripping milk into a glass of water. But similar fluid rings can also form in liquids hot enough to support cold Leidenfrost drops levitating above them.

These two effects draw the liquid down at the centre of the torus and push the liquid up at the edges, this is what dominates when ethanol is levitating above the silicone oil. In contrast, a third effect dominates for the levitating drops of HFE-7100. Both ethanol and HFE-7100 drops are evaporating above the hot silicone oil surface and as they do so, the gas that evaporates out of them under the drop flows out from the centre of the levitating drop to the edge. Just as with a gentle breeze on a pond, this vapour flow leads to a shear force on the liquid underneath that pulls the liquid out from the centre of the torus towards the edges, down and then, to complete the circle, back up through the middle.

Remarkably, despite their different rotation directions, both types of vortex contributed to keeping the drop levitating. You can read more about the study in the summary here or in the journal here.

Given that water boils at 100C and that alcohol boils at 78C, it is feasible that by dripping vodka or another strong alcohol based drink onto our freshly prepared coffee we may see a similar effect. It may certainly be worth a try. I’ll leave this as an experiment that you can tell me about on Twitter, Facebook or in the comments section below, but it is an experiment with a positive result either way. Perhaps you will see levitating alcohol drops above your coffee. But even if you don’t, you can at least keep trying until you have made an interesting coffee based cocktail.

With this ring…

vortices in coffee
Vortices behind a spoon dragged through coffee.

How many vortices do you see in your coffee? We finally arrive at the last in this series about the contributions of Helmholtz to the physics of a cup of coffee and the one that was to be the link with the (postponed) Coffee & Science evening at Amoret Coffee: vortices. But, beyond those that form behind a spoon, where do you see vortices in coffee and how can we connect them to dolphins?

Each morning as I prepare a pour over, I wait as each drop of coffee falls into the coffee bath below it. Some bounce up, some stay on the surface for some moments, many more pass straight through and get absorbed into the brew. I will admit that on most mornings, I am not thinking about the fact that I am watching one of the most beautiful pieces of physics unfold in front of my eyes and yet, this is how the processes occurring in the V60 were described by Lord Kelvin:

“[Helmholtz’s] admirable theory of vortex rings is one of the most beautiful of all beautiful pieces of mathematical work hitherto done in the dynamics of incompressible fluids.”

One of the most beautiful of all beautiful pieces of mathematical work? In my morning V60? How can we see these vortices as they fall? Sadly, it is perhaps easier to swap the coffee for plain water and drop food colouring into into it if we actually want to see these vortex rings form. As each coloured drop hits and goes through the surface, it forms a ring that curls up on itself and, if you are lucky, splits into many smaller rings, cascading to the bottom of the pot. You can see a film of the effect here or try it for yourself.

Vortex ring cascade, food colouring into plain water, V60 vortex
Dripping food colouring into a V60 of plain water: visualising the vortex rings that form every morning as you brew your coffee.

Each drop of coffee dripping from the filter into your coffee pot in the morning does this even if you can’t usually see it.

And though these rings must have been seen before Helmholtz’s paper in 1858, and even dolphins play with them in the sea, no one had attempted a mathematical model until Helmholtz. Helmholtz founded his mathematics on several theorems including the fact that a vortex cannot terminate within the fluid. It either has to terminate at the boundary of the fluid (like the vortex formed behind a spoon being dragged through coffee) or it has to close on itself (it forms a vortex ring) (more info here, opens as pdf).

Helmholtz seems to have come to vortices via an interest in organ pipes. He noticed that vortex sheets form at the inner surface of the pipe that can contribute significantly to the internal friction of the air flow through the pipe*. This means that, at the boundary between the moving air and the stationary air at the pipe edge there is a region of turbulent flow which leads to the formation of vortices. For Helmholtz, this had immediate consequences for measuring the speed of sound using pipes. Because where as previously the length of the organ pipe had been taken to be the distance between the maximum vibration (anti-node) and minimum vibration (node) of the sound wave, Helmholtz noticed that the presence of vortex sheets at the surface of the pipe would lead to an apparent lengthening of the resonator. If you used the length of the pipe to calculate the speed of sound, you would be very slightly wrong*.

As he investigated further, he found that these same surface-vortex effects explained a feature of organ design that had been known empirically but never explained. Why is it that in order for the character of the sound to be similar for each note, notes played through short, fat pipes must be accompanied by notes played through tall thin ones? Again it is to do with the air flow past the surfaces of the organ pipe.

vortices, turbulence, coffee cup physics, coffee cup science
Another cool consequence of boundary layers: Vortices created at the walls of a mug when the whole cup of coffee is placed on a rotating object (such as a record player).

In fact, these vortex sheets that appear at the boundaries between fluids appear so often, you can start to see them everywhere! They are in a cup of coffee if you put it on a record player (as with the picture of ink in a takeaway cup here) and they are in clouds that show a Kelvin-Helmholtz instability. Appearing like a series of waves on a cloud in the sky, Kelvin-Helmholtz instabilities occur when a layer of cold dry air flows fast past a layer of hot and humid air. At the boundary of the two layers, a vortex structure forms and because the hot humid air encounters the cold dry air within that vortex, clouds can form at the boundary which reveal the vortices driving them. Although the conditions to create them must occur quite frequently, they last only a very short amount of time (less than a minute is typical) and so are considered quite rare. Look out for them next time you can see that the weather is changing and the clouds are fairly high in the sky.

Of course, it is not just on Earth and in coffee that we see these vortex structures. We see them in the weather patterns of other planets, in the solar wind and in jets leaving supernovae. And it is not just in fluids that Helmholtz’s mathematics of vortices proved useful. In Helmholtz’s equations the fluid velocity associated with a vorticity described (exactly analogously) the magnetic force produced by an electric current distribution*.

Kelvin Helmholtz instability in clouds over the M3 in January 2020
A Kelvin-Helmholtz instability in clouds over the M3 in January 2020.

Far more could be said about Helmholtz’s work on vortices and its links to both coffee and the weather on Saturn, but that will have to wait until the next Coffee & Science evening at Amoret. Until then, enjoy watching these astonishing structures in your coffee and let me know if you observe anything interesting with them.

This is the last in a series of articles on the contributions of Helmholtz to our understanding of coffee. You can read an introduction here, his work on vision and colour here, the sounds of coffee here and the energy of coffee here. Next time, we’ll be back to experimenting with coffee, please do let me know (on Twitter, FB or in the comments) of any experiments you have been doing at this time, what have you seen in your brew?

*”Worlds of Flow”, Olivier Darrigol, Oxford University Press, 2005

Making coffee work

Cogs, Wimbledon Common, Windmill, Contact S2b, instant coffee and washing soda developer
Cogs on Wimbledon Common, taken using a film based camera and developed using (instant) coffee. A way to make coffee ‘work’ but not the one linked to Helmholtz.

Search online and you can find videos of machines that lift cogs or turn wheels that are powered by the steam rising off a coffee. You can see an example here and find instructions for how to build your own here (a “stay at home” hobby project perhaps?).

Although such machines are very much for the hobbyist, the principle of the steam engine drove the industrial revolution. And, even now, much of our power network relies on somehow heating water which then drives a turbine that generates electricity. Underlying this is the principle that energy can be transformed from one type to another but is, ultimately, always conserved.

The concept is easiest to visualise with a pendulum or a swing. At its height, the pendulum has a maximum of potential energy but is not moving (so its kinetic energy is zero). As it passes through its minimum height point, the speed of the swing (or pendulum) is maximum and the potential energy at a minimum. Indeed, the amount of potential energy lost by the pendulum is equal* to the amount of kinetic energy gained. The same can be extended to “work” which, in the language of physics is always energy. If a certain amount of energy is put in, a certain amount of work can be produced. In a closed system and without loss, the amount of energy you put in is the amount of energy you get out. In any real system, some of that energy is lost to heat or other methods of loss.

Press Room coffee Twickenham
Whether you make your coffee as a pour over or an espresso, the principle of conservation of energy is always the same. The energy you put in will equal the energy you get out (with some lost as heat as the coffee cools). Pour over at the Press Room, Twickenham.

To use a more coffee related example, in an espresso machine, work is done to put the water under high pressure (and separately to heat it). This pressurised water is then allowed to escape through the coffee puck where the work originally done pressurising it gets transformed into the kinetic energy (speed) of the water going through the group head. Changing the pressure changes the speed at which the water goes through the puck in an analogous way to how changing the height of the pendulum drop affects the speed as it goes through its central point. Of course you won’t always see this because changing things like the grind size in the espresso puck will also affect the route that the water takes as it travels through the puck and so the actual speed at which you see the coffee-infused water leaving the espresso basket will be affected by that too. The real world is never quite so easy as the ideal.

A pour over works in a different way. Here, the energy is stored in the water ‘bath’ in the filter as gravitational potential energy. As the water falls, it gains kinetic energy at the expense of this gravitational energy (or height). As the espresso machine also works with gravity, the conclusion would be that the water will move much faster through the espresso puck than the pour over bed. That this often doesn’t seem to be so is again because of the effects of the resistance of the coffee bed or espresso puck, on the espresso and the pour over.

This concept of the conservation of energy has been engrained into us from an early age. And so it may be surprising that it is a fairly recent principle in physics. For although versions of this principle had been considered for many years, it had not been recognised until the 1840s (by James Joule and Robert Mayer in 1843 and 1842 respectively) that work and heat were interchangeable. And it wasn’t until 1847 that Helmholtz recognised that all energy was conserved. Although at that time he was using the word ‘force’ for what we now call ‘energy’, and what we now call kinetic energy was thought of as a ‘living’ energy. He wrote:

“… the loss in the quantity of potential force is always equal to the gain in living force, and the gain of the first is the loss of the second. Thus, the sum of the existing living and potential forces is always constant.”**

So, among the many contributions to physics that he made, Helmholtz also has a claim to being among those who developed the field of thermodynamics which remains crucial both for physics and for our industrial and technological progress.

Rag&Bone, Rag & bone, coffee Victoria, coffee Westminster
Rag & Bone Coffee in front of St Matthew’s Church. Much of our understanding is based on our assumptions about how the world works. The challenge for us is to identify those assumptions that underlie our thinking.

There is perhaps a cautionary note here for any who are tempted to think that science and religion are always somehow in opposition. For the British scientists who contributed to the development of the idea of conservation of energy (such as Joule and William Thomson (Lord Kelvin)), the concept was founded on the idea of a Creator God: as only God could create or destroy, so it followed that energy of itself, could never be either created nor destroyed, it could only be transformed from one form to another. The idea of a God was, for them, implicit in the idea of the conservation of energy**.

Helmholtz had a philosophical disagreement here. For him, the principle was founded on a Kantian understanding of philosophy***. Certainly certain things had to be assumed at the beginning (such as the principle of causality and the existence of matter outside of our perception of it). But once these assumptions had been made, the principle of conservation of energy followed in a deterministic manner.

Does this matter? In our everyday experience of engines and the way things work, conservation of energy certainly seems to be crucial. We no longer question the principle but assume that one form of energy is transformed into another and is continuously conserved even as it is dissipated into the universe as heat as our coffee cools. But nonetheless, Helmholtz’s understanding was founded on certain assumptions, beliefs, just as Joule and Kelvin’s. It helps to be aware of the philosophical underpinning of our science so as to ensure we don’t have over confidence in what we can, and cannot, know.

So Helmholtz can teach us something else as we gaze into our coffee. Our world is multifaceted, and what we believe about what the world is, influences and informs our understanding of how the world works. Our challenge is to look into ourselves as we sip our coffee and to start to see what we believe we know and what we can actually know. And if we were to really do that, what conflicts would we find?

*With the usual caveats of no energy being lost to friction etc.

**”Helmholtz and the British Scientific Elite: from force conservation to energy conservation”, David Cahan, Notes Rec. R. Soc (2012), 66, 55-68

***”Helmholtz: from enlightenment to neuroscience”, M Meulders, MIT press, 2010

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).

Looking at coffee

coffee at Watch House
Observing the colours in our coffee can reveal much more than just the chemistry of the cup.

How do you see your coffee in the morning? Through blurry eyes, a red-ish/brown liquid that you may admit to noticing more for its aroma and taste than for how you look at it? But what is it about that lovely red colour of a fresh filter coffee viewed through sunlight? And what about the way that the glass jar curves towards you and then bends away, how do we perceive distance?

The colour question has historically been more problematic. For Aristotle, the rainbow was composed of a mix of three colours, which fitted with Pythagorean numerology*. Newton thought there were seven, which fitted with the harmonies in music theory. Goethe (who also developed a colour theory) liked to quote “If you show a red rag to a bull it becomes angry, but a philosopher begins to rage as soon as you merely speak of colour”**.

Today, in schools we are taught that there are three primary colours for light: red, green and blue. This is because all colours of light can be observed by careful weightings of these three colours and when the three are combined we see white light. But what does it mean that light has primary colours? Is not light just a vibration, why is it that we see colour at all? It comes down to our physiology and how we sense the world.

It was Thomas Young (who also showed the wave-like properties of light) who first proposed that these three ‘fundamental’ colours were associated with three types of ‘resonator’ in our eyes. The idea was significantly developed by Hermann Helmholtz during the 1850s. Each type of receptor responded to light at all frequencies but responded most sensitively in a smaller range. Generally humans have three types of frequency sensitive (and so colour sensitive) receptors, though those with colour blindness have fewer and there are even some of us with four. Most of us though, have three types of receptors sensitive in the red, blue and green regions of the spectrum. Hence we perceive the light as white if these three types of receptor are stimulated equally, that is, if we combine blue, red and green light. The red colour seen as you brew a fresh pour over of coffee in front of a window through which sun is streaming at dawn, is red because of these activated red-sensitive cone receptors in your eye.

Sun-dog, Sun dog
A ‘rainbow’ of colour as seen in a ‘sun dog’ observed in central London. But what is colour really?

But Helmholtz went further than this. Have you ever been staring at a bright object and then turned away towards a dark wall and had the experience of seeing the same bright object ‘projected’ on the wall but in a different colour? Both Goethe and Helmholtz observed themselves as they ‘saw’ these phantom images and watched the images as they changed colour before eventually fading. While Goethe incorporated his observations into his general colour theory, Helmholtz linked the phenomenon to these same cone receptors in the eye. He realised that if your red-sensitive colour receptors had become saturated by watching a bright red object (such as a red-hot piece of iron for example), they would not respond so quickly when you looked away at a blank bit of wall. So if you, for example, ordinarily perceived the wall as white, because the red-colour receptors had been taken out for a while, your blue and green receptors would dominate while the red would not respond and so cause you to observe a greener phantom image. Would we ever see a green phantom coffee?

Unlike the question of the colour, the question of depth perception has some thoroughly more modern elements. For while many had thought about how we realise that space has depth, the binocular effect of our two eyes was not realised until relatively recently. In fact, that two, 2D images taken from slightly different angles and viewed separately through each eye appeared as if they were a 3D image, was only discovered in 1838. Prior to that, it had been thought that perhaps we knew about depth because of our learned familiarity with the size of objects, much as Fr Ted explained the distant cows to Dougal (which is one of the clips here).

Shadows reveal a lot. From the position of the light source to information we interpret as informing us about how different objects relate to each other. And again, why is it that shadows appear blue on snow?

Apparently between 1855-59, 29% of scientific papers concerning the eyes were about this problem of stereoscopy or binocular vision. Helmholtz’s contribution to the debate was to show how much of our realisation of depth was a learned but unconscious process and also how much relied on the involuntary movement of our eyes to ‘calibrate’ the surroundings after fixing on something. That movement of your eye that is impossible to control but you can watch in others as they concentrate is there for us to check that what we think we are seeing is what we are seeing.

Just how much is revealed to us, about the coffee and ourselves, by our gazing at it? When Feynman discussed colour vision in volume I of his Lectures on Physics, he wrote “We make no apologies for making these excursions into other fields, because the separation of fields… is merely a human convenience, and an unnatural thing. Nature is not interested in our separations, and many of the interesting phenomena bridge the gaps between fields.”*** Our world is intricately connected, we only have to gaze at our coffee to have an intuition as to how much this is true.

This post is one of a series about the contributions of Hermann von Helmholtz to how we understand the world around us. The introduction is here and it will be followed by thinking about what we hear in our coffee, the heat of our coffee and, of course, what happens when we stir it.

*”The Rainbow Bridge: rainbows in art, myth and science” R.L. Lee Jr, A.B. Fraser, Penn State University Press (2001)

**”Helmholtz: from enlightenment to neuroscience” M. Meulders, MIT press, (2010)

***”The Feynman lectures on physics volume I”, Feynman, Leighton and Sands.

Connectivity

Shades of light and dark. How do we see shadow, colour, depth? How is it linked to the physics of coffee?
Superscript and subscript

The other morning, grinding coffee in order to prepare a V60 (the last of a fantastically complex Natural El Salvador from Amoret coffee), I was hit by the intense aroma of rich, freshly ground beans. It seems at the moment that we are surrounded by more vivid impressions of things that have, in reality, always been there, but that have previously been obscured by other features of our lives. Such things have been revealed by the changes to our lives that have come about as the result of the “lock-downs” needed to reduce the transmission of Covid-19. The birdsong that seems more dramatic and intense than before the traffic subsided. The colours of the trees as the spring light bounces off and filters through the leaves no longer surrounded by a misty haze of pollution (now suggested through its absence). And of course the smell of the coffee hitting our olfactory senses.

Superscript and subscript

Before this period of social distancing and self-isolation, I had been preparing for another in the series of Coffee & Science evenings at Amoret coffee in Notting Hill. The title of the evening had been “Space Coffee” and we were going to explore the connections between what happened in your coffee cup with features that you can see in the atmospheres of planets such as Saturn and Jupiter. Actually the connections are a lot wider than that and can be seen on the Earth too, but the atmospheres of Jupiter and Saturn have some very peculiar structures that you may not immediately think could possibly be linked to your coffee cup. One of the key people who worked on the science behind this was Hermann von Helmholtz (known as H2 to his friendsa). For the Coffee & Science evening, the important work of Helmholtz was on vortices and fluid rotations, but it turns out that he has more links with a coffee cup than that, connections that can even give us some food (drink?) for thought in this time of separation.

which will win, gravity or light
The world has not really been turned upside down, but certainly the way that we view it could be. An opportunity to re-assess our view points?
Superscript and subscript

Helmholtz made many contributions to the understanding of our world including how we see it. In addition to inventing the ophthalmoscope (in ~1850), Helmholtz was interested in the way in which we perceive colour and how we manage to see in 3D. Thinking about the way in which we see things like light and colour and developing on the idea that how we perceive our world is, ultimately, received in each of our own minds via our sense organs, Helmholtz compared the sensations of light and colour to symbols of language: ways in which we interpret the world around us. As Michel Meulders writes in his fascinating biography of Helmholtz (told from the view point of a medical doctor rather than a physicist)b, Helmholtz had

“…stated lyrically that we should thank our senses, which miraculously gave us light and colour as responses to particular vibrations and odour and taste from chemical stimuli. We should thank the symbols by which our senses informed us of the outside world for the spell-binding richness and the living freshness of the sensory world.”

Superscript and subscript

What does it mean that I should thank my senses for the way in which I smell, see and hear the coffee beans as they are ground?

Superscript and subscript

The connections between Hermann von Helmholtz and coffee are more than just the vortices that form, and more than the fact that Michael Faraday once served him cups of it while he was preparing lectures for the Royal Institutionc. We’ll be exploring those links over the next few weeks, from how we see coffee, through how we hear it and eventually to what ties it all together. Please keep checking back but also, do let me know what new sensory symbols you have perceived in this time of opportunity to attention.

Superscript and subscript

a “Worlds of Flow”, Olivier Darrigol, Oxford University Press (2005)

Superscript and subscript

b “Helmholtz: from Englightenment to Neuroscience”, Michel Meulders, MIT press (2010)

Superscript and subscript

c “Helmholtz and the British Scientific Elite: from force conservation to energy conservation”, David Cahan, Notes & Records of the Royal Society, 66, 55-68 (2012) doi:10.1098/rsnr.2011.0044

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 café 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.

A key ingredient at Second Shot

Coffee, hot chocolate and cake at Second Shot in Marylebone,
Second Shot
Coffee, hot chocolate and cake at Second Shot in Marylebone

First impressions count, and the first impression we got of Second Shot‘s second branch north of Marylebone Station, was of a very friendly, local spot type of cafe. A small crowd were sitting around a table discussing a topic in an animated way. Various others were popping in or out, chatting with the barista, one person was sitting at a table with a laptop. And despite the fact that the weather was turning and the sky was becoming an ominous grey, the cafe itself seemed bright and open, with plenty of light coloured wood to complement the large windows.

We decided on a long black, hot chocolate and nut free brownie. Not wanting to spoil the initial taste of the coffee, I waited for the long black to arrive so that I could try that first before trying a bit of the brownie. The coffee was very well made, from Square Mile, and a perfect complement to the squidgy but moreish brownie.

On the walls, plants were hanging in pots with their leaves trailing down, while the light was reflected off a series of drawings sketched using coffee as an ink (they were for sale). It is surprisingly easy to make ink out of many different household (and not quite so household) items. Coffee is one base ingredient and, not surprisingly, makes a brown ink. But to bind the ink and to make it more viscous, gum arabic is frequently added to home-made ink recipes. The gum arabic is needed particularly if you are going to write with the ink with a fountain pen as it makes the ink viscous enough that it can flow through the nib.

coffee art, pictures using coffee ink, sketch, Second Shot
Hanging plants and coffee art, the walls at Second Shot, Marylebone.

Just as with the gum arabic, often ingredients are added to a product that are crucial to it, but that we do not realise they are there. Another example is the seaweed extract that is added to some plant based milks in order that they produce a better milk froth. But what if you don’t add all the ingredients at the same time, what happens if you add only one ingredient of the ink rather than all of those that are necessary?

One type of black ink that has been used for centuries is made from oak galls. It is even thought that ink based on galls was used to write the 4th century Codex Sinaiticus (the oldest complete copy of the Christian New Testament that we know of). The galls form on oak trees when the larvae of different types of wasp secrete a chemical that produces abnormal growth in the tree. The gall is the protective casing around the wasp larva, a type of home for the wasp larvae as they mature. Once the wasp has matured, it burrows through the growth which has turned from green to brown and it leaves a tiny little hole where it exited the gall. Galls can be seen on many oak trees during the summer and autumn and I’ve even seen them on the oak trees of various central London parks.

After the galls have been crushed and soaked, iron sulphate is added to the gall solution to produce a deep black ink*. Again, gum arabic is added to bind the ink to the paper and to increase its viscosity. But it is this ink that was also a popular ‘invisible ink’ first described over 2000 years ago. Philo of Byzantium** (who lived from 280-220 BCE) describes making an ink with the oak galls only. Forget about the iron sulphate for the moment. When it dries, the ink is nearly invisible on the paper, it can easily be missed and so can be sent to a collaborator as an invisible message. When the collaborator washes the paper with a solution of iron sulphate, the black ink appears on the paper and the message is revealed. Although the recipe has been known for two millennia, it has been used as an invisible ink even as recently as Mary, Queen of Scots and the American Revolution**.

coffe ink example
Coffee ink made for writing with a fountain pen (including recipe). Perhaps a yellow-ish paper was not the best medium to use to showcase the ink. An inadvertently (almost) invisible ink.

This idea, of making the invisible visible, by adding a key ingredient seems to form a nice metaphor for the societal aspect of the work of Second Shot. On their website they describe this aspect as:

We’re changing perceptions on homelessness by being a destination that serves some of London’s best coffee, alongside a unique community atmosphere, amazing food, and just so happens to be changing lives.

We employ people who have been affected by homelessness, train them up and transition them on to long term employment elsewhere, helping them on their individual journey taking them from where they are, to where they deserve to be.

Key ingredients of training, and accompaniment on individual journeys that combine to change our perceptions but that are not realised by us as we consume great coffee at this friendly cafe: Making the invisible visible, but doing it without us even realising that we have received a hidden message.

Second Shot’s second branch is at 49 Church St. NW8 8ES

*”Make Ink: A forager’s guide to natural ink making”, J Logan, Abrams New York, 2018

** “Prisoners, Lovers, & Spies: The story of Invisible Ink from Herodotus to al-Qaeda”, K Macrakis, Yale University Press, 2014

Seeing the unseen at Scarlett, Angel

Coffee Angel, Scarlett, roasters, coffee in Islington
Coffee at Scarlett, Angel

Although first alerted to Scarlett coffee in Angel by Double Skinny Macchiato last summer, we managed to visit during the one week of their summer holiday (and so we revisited Katsute100 around the corner instead). Nonetheless, it remained on the list and a few weeks ago we turned up for a mid-afternoon coffee at this inconspicuous looking venue on a side street just around the corner from Angel tube.

The roaster at the back of the cafe forms an immediate impression. With the large, communal table at the front of the cafe, backed by stairs leading up to the roaster, this is a place where coffee is taken seriously. The counter (on the left as you enter) offered a range of cakes and edibles but having recently come from lunch in Chapel Market, we passed on this on this occasion. Above the counter there were about 5 lights hanging down forming what looked like a giant Newton’s Cradle. Just too high for me to reach unfortunately.

I enjoyed my long black as I started to take in the surroundings of this cafe. Various people and regulars came and went, suggesting that this is a friendly local haunt for many. Noticing the number of different roasted coffee beans for retail, it was clear that this is a venue that you could return to for a different coffee experience each time. Each time exploring an aspect of the flavour of the coffee and building on the experience of coffee tasting that you have enjoyed before. It is definitely on the list for a repeat visit.

Interior Scarlett
One of the light fittings at Scarlett in Angel. Cube outlines drawn on paper can form an optical illusion where you can’t work out if the cube is coming out at you or going into the paper.

Above our heads, the lights were framed by the outline of a cube. Fantastic for optical illusions, these cubes offer us an opportunity to think about how we perceive depth and direction; how our eyes work and perhaps, more fundamentally, what it even means to see an object (as with Berkeley’s “New Theory of Vision”). Then, while looking through the menu, it became clear that here too there was an optical illusion of sorts. For the price list was not written on the board so much as cut out of it (see the photo below). The price you could read off the menu was, in some sense, precisely the information that was not actually on the board. Our brain makes patterns of that which we don’t see and, together with our assumptions about what should be there, we form an idea of the price we have to pay.

It is a similar thing with many algorithms in use around us now. Such tools can be immensely helpful, offering us suggestions for coffees we may like to try (based on our buying habit) or routes that we may like to take to get us to our destination. And yet, are there problems hidden in the assumptions that some of these algorithms make? What information are we getting based on elements in the programme that we do not see?

In her excellent book “Weapons of Math Destruction”, Cathy O’Neil explores some of the more dangerous ways that our biases and assumptions (particularly those that we don’t see in ourselves) can impact the results of algorithms that have been written to optimise processes from the sorting of job applications to determining the length of time a given convicted criminal will serve for an offence. In an example relevant for cafes, O’Neil related an example of how Starbucks had used an algorithm to determine which baristas and managers should work which hours, including who should close the shop at night and who should open it in the morning.

Scarlett menu
The menu at Scarlett. Apart from the filter coffee, the prices and information for each coffee is revealed by what is absent from the board rather than what is printed onto it.

The algorithm was programmed to calculate the most efficient use of the cafe’s time and money, specifically prioritising the profit that the company made. One measure of this was “revenue per employee hour”. This had the consequence that staff members were frequently in a position where they were told that they had to do both the (late night) closing and (early morning) opening of the shop and were given very few days notice of this expectation. Clearly this impacted the lives of their staff and affected their ability to arrange child care, support themselves through further education and other consequences. Eventually Starbucks was forced to amend this algorithm but change comes hard: how do you ask a computer to measure “fairness” to an employee (a subjective term) when you can use revenue per employee hour which is measurable, quantifiable and therefore ‘accurate’?

Perhaps you think that the link back to Scarlett here is obvious: That if you choose to drink your coffee in friendly neighbourhood cafes where cafe owners and baristas work to patterns formed by encounter rather than algorithm it would be better than a place which is run assuming all workers are cogs in a profit machine? Perhaps. But the link back to Scarlett in my mind is not that at all.

If you look at the front of Scarlett, or its webpage, and assume that the pink bird is a funny looking flamingo, you may make a series of assumptions about what you think the cafe will be like and why the owners have a bird on their front door. If you found out that the bird was actually a Scarlet Ibis and associated with the coffee growing regions of South America, your ideas about the cafe and the owners may be different. For a general customer, looking for somewhere to enjoy a great coffee, perhaps these assumptions and ideas do not matter so much. But if we are ever in a position to feed our assumptions into an algorithm, these hidden (to our own conscious) assumptions could matter a great deal.

Scarlett is at 30 Duncan Street, N1 8BW

“Weapons of Math Destruction – how big data increases inequality and threatens democracy” by Cathy O’Neil, Penguin Books, 2016

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