In search of origins

Amaje coffee
Buriso Amaje Coffee from Ethiopia via Amoret Coffee in Notting Hill. The Jimma 74158 and 74160 varietals are selections from coffee grown in the wild.

It was a goat herder named Kaldi, so the story goes, who first noticed the effect of coffee beans on the the energy levels of his goats. After telling the local abbot of his observations, the monks at the nearby monastery realised that this drink could help them stay awake during prayer and so the reputation, and consumption, of coffee spread from Ethiopia and then throughout the world.

While the details may be questionable, there is evidence that the coffee plant originated in Ethiopia. Coffee still grows wild in parts of Ethiopia and the oldest varietals are also to be found there. And so, when I realised that my latest coffee was an Ethiopian Natural of varietal Jimma 74158 and 74160, roasted by Amoret coffee in Notting Hill, I thought, why not do a coffee-physics review rather than a cafe-physics review? For there are always surprising links to physics when you stop to think about them, whether you are in a cafe or sampling a new bag of beans.

This particular coffee was grown by Buriso Amaje in the Bensa District of the Sidama region of Ethiopia. The varietals were selections from the Jimma Research Centre from wild plants that showed resistance to coffee berry disease and were also high yielding. Grown at an altitude of 2050m, the naturally processed coffee came with tasting notes of “Blueberry muffin, white chocolate” and “rose petal” among others. Brewed through a V60, it is immediately clear it is a naturally processed coffee, the complex aroma of a rich natural released with the bloom. Indeed, the bloom was fantastically lively with the grounds rising up with the gas escaping beneath them in a manner reminiscent of bubbling porridge (but much more aromatic). And while I lack the evocative vocabulary of Amoret’s tasting notes, the fruity and sweet notes were obvious, with blueberry a clear descriptive term while I would also go for jasmine and a slight molasses taste. A lovely coffee.

Brewing it again with an Aeropress, the tasting notes were different. We could start to ponder how the brew method affects the flavour profile. But then we could go further, how would this coffee taste if brewed using the Ethiopian coffee ceremony? Which leads to further questions about altogether different origins. Where did this come from and how do our methods of experiencing something emphasise some aspects while reducing others? Ethiopia offers a rich thought current if we consider how things originated because it is not just known for its coffee, Ethiopia is also home to some of the world’s oldest gold mines. Today, one of the larger gold mines in Ethiopia lies just to the North West of where this coffee came from, while a similar distance to the south east is a region rich in tantalum and niobium. We need tantalum for the capacitors used in our electronic devices. In fact, there is most likely tantalum in the device you are using to read this. While niobium is used to strengthen steel and other materials as well as in the superconductors within MRI machines. Where do these materials come from?

The Crab Nebula is what remains of a supernova observed in 1054AD. Explosions like these are the source of elements such as iron. Image courtesy of Bill Schoening/NOAO/AURA/NSF

Within the coffee industry there has been a lot of work done to demonstrate the traceability of the coffee we drink. But we know much less about the elements that form the components of many of the electronic devices that we use every day. And while this leads us into many ethical issues (for example here, here and here), it can also prompt us to consider the question even more fundamentally: where does gold come from? Indeed, where do the elements such as carbon and oxygen that make coffee, ultimately, come from?

The lighter elements, (hydrogen, helium, lithium and some beryllium) are thought to have been made during the Big Bang at the start of our Universe. While elements up to iron, including the carbon that would be found in coffee, have been formed during nuclear fusion reactions within stars (with the more massive stars generating the heavier elements). Elements heavier than iron though cannot be generated through the nuclear fusion reactions within stars and so will have been formed during some form of catastrophic event such as a stellar explosion, a supernova. But there has recently been some discussion about exactly how the elements heavier than iron formed, elements such as the gold, tantalum and niobium mined in Ethiopia.

One theory is that these elements formed in the energies generated when two neutron stars (a type of super-dense and massive star) collide. So when the LIGO detector, detected gravitational waves that were the signature of a neutron star collision, many telescopes were immediately turned to the region of space from which the collision had been detected. What elements were being generated in the aftermath of the collision? Developing a model for the way that the elements formed in such collisions, a group of astronomers concluded that, neutron star collisions could account for practically all of these heavier elements in certain regions of space. But then, a second group of astronomers calculated how long it would take for neutron stars to collide which led to a problem: massive neutron stars take ages to form and don’t collide very often, could they really have happened often enough that we have the elements we see around us now? There is a third possibility, could it be that some of these elements have been formed in a type of supernova explosion that has been postulated but never yet observed? The discussion goes on.

coffee cup Populus
Where did it all come from? Plenty to ponder in the physics of coffee.

The upshot of this is that while we have an idea about the origin of the elements in that they are the result of the violent death of stars, we are a bit unclear about the exact details. Similarly to the story of Kaldi the goat herder and the origins of coffee, we have a good idea but have to fill in the bits that are missing (a slightly bigger problem for the coffee legend). None of this should stop us enjoying our brew though. What could be better than to sip and savour the coffee slowly while pondering the meaning, or origin, of life, the universe and everything? That is surely something that people have done throughout the ages, irrespective of the brew method that we use.

As cafes remain closed, this represents the beginning of a series of coffee-physics reviews. If you find a coffee with a particular physics connection, or are intrigued about what a connection could be, please do share it, either here in the comments section, on Twitter or on Facebook.

Coffee quakes

ripples on coffee at Rosslyn, the City
From ripples on the surface, to listening to the sound your coffee makes. What links a coffee to an earthquake?

What do you hear when you listen to your coffee? Or a related question, what links your coffee to earthquakes and seismology?

In recent weeks I have been making coffee with milk, not often, but enough to notice something slightly strange. While heating the milk in a small saucepan, I have accidentally tapped the side of the pan while the milk was in it. The tap, perhaps unsurprisingly, produced a ripple on the surface of the milk propagating away from the point of tapping. But what was surprising was that a very short time later, a second ripple was generated, this time from the other side of the pan propagating back towards the original wave.

The first ripple had not yet travelled across the milk surface before the second ripple had been generated and travelled back towards it. Something was causing a vibration on the other side of the pan before the first ripple had had a chance to get there. Was the pan acting like a type of bell which, as I tapped it, started to resonate all around its circumference?

Assuming that the vibration of the tap travels at the speed of sound through the metal of the pan, it would take about 50 μs for the vibration to travel half way around the circumference of the pan (diameter 14cm, with a speed of sound in steel ~ 4500 m/s). But then, if the pan were resonating, the resonance frequency would depend on the speed of sound in the milk filling the pan, which would increase as the milk was warmed. Would we see evidence for this if we video’d tapping the pan as we heated the milk?

coronal hole, Sun
Observing periodic changes to the luminosity of stars can indicate the elements within them. Image credit and copyright NASA/AIA

Rather than watching the liquid within, we could also learn about the interior of a cup of coffee by listening to it. The “hot chocolate effect” is the classic example of this. The effect occurs when hot chocolate powder is added to warm water or milk and stirred. Think about the pitch of a sound made by tapping gently on the base of your mug while you make a cup of hot chocolate. Initially, adding the powder and stirring it will introduce air bubbles into the liquid. As you stop stirring the hot chocolate but continue to tap the base of the cup the air bubbles leave the drink. The cup is acting as a resonator, so the sound that you hear (the resonance of the cup) is proportional to the speed of sound in the liquid in the cup. As the speed of sound in hot water containing lots of air bubbles is lower than the speed of sound in hot water without the air bubbles, the note that you hear increases in pitch as the bubbles leave the drink. You can read more about the hot chocolate effect in an (instant) coffee here.

It is here that we find the first connection between coffee and earthquakes. Seismologists have been listening to the vibrations of the Earth for years in order to learn more about its interior. By observing how, and how fast, waves travel through the earth, we can start to understand not only whether the inside is solid or liquid, but also what the earth is made from. This is similar to learning about the air bubbles in our hot chocolate by listening to the sound of the mug. More recently, the seismologists have shown the effect of the Covid-19 related “lockdowns” on reducing seismic noise. Something that does not have an obvious coffee cup analogy.

But seismology is not just confined to the Earth. Vibrations of a different kind have also been used recently to learn more about the interior of stars, although here it is a mix of seeing and ‘listening’. Generally, when the surface of an object vibrates, it leads to compressions and expansions of the medium within the object. This is the essence of what sound is. But in a star, these compressions and expansions also result in changes to the luminosity of the star. So, by looking carefully at the frequency of the variation in brightness of different stars, it should be possible to work out what is going on inside them. It is a branch of physics now known as “Astroseismology”. Recent astroseismology results from NASA’s Kepler satellite have been used to challenge theories about how stars form and evolve. It had been thought that as a star develops, the outer layers expand while the core gets smaller. The theories proposed that this would result in a certain change to the rotation speed of the core of the star. The astroseismology observations have revealed that, while the gist of the theory seems right, the core rotates between 10 and 100 times slower than the theories would predict. As one astroseismologist said “We hadn’t anticipated that our theory could be so wrong…. For me, finding that problem was the biggest achievement of the field in the last ten years.”.

We now use strain gauges in electronic measuring scales. They were originally invented for an entirely different purpose.

Seismology and astroseismology offer clear links between listening to your coffee cup and earthquakes (or star quakes). But there is one more earthquake related connection to the coffee cup and it could be noticed by any of us who want to improve our home brewing technique.

To brew better coffee, we need to measure the mass of the coffee beans that we are using. Typically we will use a set of electric scales for this. Inside the scales is a device, called a strain gauge, that shows a change in its electrical resistance as a result of the pressure on it (from a mass of coffee for example). The scales translate this change in the electrical resistance to a mass that is shown on the display. One of the inventors of the strain gauge however was not thinking about measuring the mass of coffee at all. His interest was in earthquakes and specifically, how to measure the effect of the stresses induced by earthquakes on elevated water tanks. In order to do that he needed a strain gauge which led to the devices that you can now find in your measuring scales.

Two links between your coffee cup and earthquakes or seismology. Are there more? Do let me know of the connections that you find, either in the comments below or on Twitter or Facebook.

Filtering

When you prepare a filter coffee with a paper filter, you typically rinse the filter before starting the brewing process. As you do so the paper swells and can absorb several ml of water.

The other morning while preparing a V60, I noticed that the filter paper absorbed between 3-6g of water (3-6ml) each time I rinsed the filter before making a new coffee. My mind wandered to re-hydrating space food and the importance of water in the texture of the food we eat (and coffee we drink). And then I was reminded of a question I had been asked during these Covid-19 times: would a face mask that is damp work better, or worse, than a dry one for reducing the transmission of SARS-CoV-2, the virus that causes Covid-19?

The answer did not seem obvious. On the one hand, when we wet the paper filter while brewing coffee, the fibres within the paper swell and reduce the pore size of the filter. It seems likely that cotton fibres in a mask would behave similarly. This would have the effect of slowing and reducing the transmission of particulates through the mask. But on the other hand, we’re not thinking about particulates but about small amounts of viral material hosted in water droplets that are somehow exhaled. I decided on the “no idea” response at the time and put the question aside. Until the other morning while preparing coffee.

Unsurprisingly this question, and many like it are now the subject of intense research. I say unsurprisingly because a few years ago a new family of superconductors was discovered with (relatively) very high transition temperatures*. I was on holiday at the time but when I returned, it was to a large number of emails and ideas for experiments on these new materials that became known as the iron based superconductors. We had our first paper on these materials within a couple of months which, like all papers on this at the time, was uploaded, without peer review, to a pre-print server. Eventually most of the papers on the pre-print server got published in peer-reviewed journals, but this process was slow because it relied (and still does) on other scientists reading and taking the time to carefully respond to the points in your manuscript, then for you to address these points, for them to read it again and then, hopefully, ok the paper for publication. If you wanted to get the paper out and for a discussion to start, it had to be uploaded to the pre-print server.

canali Curators Coffee
Iron is a magnetic element. It was puzzling how a magnetic element could exist in a superconducting material and, moreover, seemed to make these materials even better superconductors than their non-magnetic counterparts.

Clearly, in order to keep up with scientists worldwide, we were looking at the pre-print server every morning looking for new ideas and new observations (and if anyone had done the same as we were trying to do at that precise moment but ‘beaten’ us to it). We had to be careful while assessing the claims in the pre-print papers. Some of the pre-prints were eventually withdrawn as they had made overblown claims (admittedly very few). Many were revised and had their claims either subtly altered or brought down a bit from hyperbole before being published in the journals. But none of this mattered to the world outside the lab because while exciting to us, and while the temperature of the transition was, from a physics perspective, very high, for the general public it would have been hard to get excited about materials that went superconducting below about 50 K or, in more common units, -223 C.

This side-story matters because, like our superconductors, the pandemic is the subject of intense research with much of it being uploaded to pre-print servers first so that scientists world wide can get into a conversation about the latest results. However, unlike our superconductors, the general public cares a great deal about a pandemic that is affecting us all and about the scientific rationale for measures such as mask-wearing, social distancing etc. While it is tempting to read the pre-prints, as I am not working in the field, it is not possible for me to read the papers on pre-print servers and be able to have a good guess as to whether the claims are reasonable, over blown or under-evidenced. So, I try to rely only on papers that are past the point of peer review and published in scientific journals. There is something very disheartening about reading an interesting newspaper report that near the end says “the study, which has not yet been peer-reviewed…”. Will the interesting study hold up? It is difficult, from outside the research area, to tell.

However, we need to get back to the masks and the filters. Was there a study, in the peer-reviewed and published literature, that looked at whether moistened masks performed better than non-moistened masks?

Masks: can we set up an experiment to see how effective ours are relative to the fitted N95s that are not available to most of us?
Masks: can we set up an experiment to see how effective ours are relative to the fitted N95s that are not available to most of us?

In fact, there is a lot of research on the effectiveness of masks. The research includes computer modelling, imaging of real people breathing/talking/coughing with and without masks and more reproducible tests where the mask material is tested using the conditions of a simulated sneeze. This last study also tested whether that simulated sneeze is contained better by a cloth mask (with filtration down to PM 2.5) or a damp cloth mask (with the same nominal filtration).

The different types of research are needed because they answer different types of question. How effective each type of mask is will depend on the type of material (tested with the simulated sneeze) and the way that people wear them (tested by the imaging of people wearing masks). While the computer modelling suggests what may happen in more ‘real life’ environments such as being outdoors with a gentle wind blowing.

In terms of the initial question about the damp masks, it turns out that the fact that the fibres in the mask swell with the water does indeed help reduce the droplet transmission through the mask material. But the authors caution that if the mask is worn for a longer period of time, the damp mask may get saturated with virus loaded droplets and so the mask would need to be changed (and refreshed with fresh water) frequently in order for it to be effective against transmission of the virus loaded droplets. (It’s also noteworthy that the effect of the damp mask was only tested for one mask type that may not be typical of what the general public wears). However, for most of us it would not be practical anyway to wear a damp mask. Moreover, if we were having to change the mask frequently, it may not be helpful for us at all. But the good news is that the imaging studies show that we don’t have to do either.

A fantastic report in Scientific Advances showed two things. First, that most masks that we wear properly give a significant benefit for the people around us. And secondly, they provided an experimental set up that can easily and relatively cheaply be replicated by people with a little technical knowledge and a mobile phone. However, given that ‘relatively cheaply’ still means about $200, I’ll take their results instead, if you don’t mind spending the money on a laser and some lenses (or happen to have some lying around), please do let me know how you get on.

Press Room coffee Twickenham
Another paper filter, this time at the Press Room, Twickenham. When we add water to a (dry) paper filter, the fibres within it swell and expand making it a better filter. Would the same happen with masks?

The authors took several of the types of face mask being worn by the public and imaged the droplets coming from a person speaking through each of them. The masks tested included surgical masks, N95 masks, and hand-made masks with 2-layers of cotton or 2-layers of cotton with an extra polypropylene layer in the middle. All of these masks reduced the droplets transmitted through the mask significantly. Indeed, relative to no-mask, some home-made multiple cotton layer masks cut the droplets by nearly a factor of 10. The exceptions were bandanas and neck gaiters. The bandanas that were tested only cut the droplets getting through by a factor of 2, but the gaiters were worse. Speaking through the neck gaiter that they tested, the authors observed that the number of droplets getting through the gaiter actually increased relative to speaking wearing no mask. While this seems counter-intuitive, they suggested that this was likely because the gaiter was breaking up the larger droplets into multiple smaller droplets and so their equipment, which just measured the number of droplets, measured an increase relative to someone wearing no mask.

The problem here of course is, as the computer simulations showed, smaller droplets stay in the air for longer, larger droplets tend to fall with gravity. Something else that we know by thinking about our coffee.

So the final conclusion? Yes, it is possible that a damp mask may be better than a dry one though there are caveats on that result. But in actual fact, most masks that we wear in an indoor environment will help to protect other people (though maybe be careful with the gaiter materials). And a second conclusion? Perhaps preparing a coffee should be a time of escape from the concerns of coronavirus and really, next time, I should just enjoy the moment and think about re-hydrating space food.

*Actually, the iron-based superconductors had been discovered a couple of years previous to the excitement. But at that point, the reported transition temperatures were low enough that even the superconducting field was curious but not excited.

Coffee and the stars

cold mug
There are many ways that gazing at a cup of coffee can help with sky gazing.

There is a problem looming on the horizon concerning how astronomers can continue to look at the sky as the effects of global climate change become more pronounced. Some of these issues are an extension of those that have been affecting amateur astronomers since the invention of telescopes. Fortunately for those with portable telescopes, many of the issues can be minimised, but some effects will be a problem for our larger observatories. And of course, for this website, we can gain an insight into what the problems are by gazing more closely at our coffee.

It’s time to make a hot coffee. Or a tea. In fact, for some of the following observations a cup of green tea or a herbal tea would be perfect. You are after a brew that is light and allows you to see through to the bottom of your mug. But if you want to keep with coffee, worry not, there are still important clues to be seen above the coffee (and you can always use the spare brewing water to pour plain hot water into a cold cup).

If you have made a tea, you should be able now to look into your tea to the bottom of the cup. If it is a sunny day, or if you have a light on behind you, you will hopefully be able to see lines of light starting to form and then dancing around the base of the cup. If you have made a coffee, this will be more difficult for you to see. In addition to pouring any spare brew water into a cup to see the same effect in plain water, you could also look at the top of your cup and notice how the steam is making the air above more turbulent, changing the way you see things on the other side of the mug (is there an allegory there?).

The dancing light patterns and turbulent steam clouds are similar to conditions in the atmosphere that can make observing the stars difficult for amateurs and professionals alike. It is perhaps easier at first to think about the keen amateur astronomer who takes their telescope from the warmth of their indoors to the cold of a cloudless night. We can perhaps immediately see analogues with the (hot) tea in the (cold) cup and the steam clouds above the coffee.

Shortly after pouring hot tea into a cool cup you should be able to see these bright lines dancing over the base of the cup. They indicate how the refractive index of the tea changes as a function of temperature and so show the convection zones within the tea cup.

We can start by thinking about the turbulence in the air movement of the atmosphere being similar to the turbulence in the steam clouds above the cup. It is hard to focus on point objects through the steam clouds; the star light twinkles as it travels through our atmosphere. But then, just as we see the light patterns form in our tea cup as regions within the tea that have ever-so-slightly different temperatures mix in a convective pattern, so the hot air within the tube of the telescope will mix with the air at the edge of the tube that has been cooled by contact with the night-temperatures. The refractive index of air and water varies as a function of temperature (fluid density). And so with the telescope as with the tea cup, these regions of hotter and cooler fluid (air and tea respectively) have different refractive indices, meaning that any light travelling through those regions gets bent by different amounts as a function of the temperature of the medium it flows through. In the tea cup, this means that we see bright lines dancing across the bottom of the cup that trace the convection zones in the tea. In the telescope we would get a wobbly image.

For the amateur with their portable telescope the solution to the convection problem, if not the atmospheric turbulence, is relatively simple. Take your telescope outside for a good amount of time so that the air inside the tube can reach a similar temperature to the air outside. Convection will subside and the image will be more stable. If we wanted to drink cold tea, we could see the same thing with our tea cup: leave the tea to cool to room temperature and those dancing light lines on the bottom of the cup should subside (this is admittedly a thought experiment on my part. I have generally finished the tea before reaching this point).

But unfortunately, similar phenomena also affect professional observatories, and a recent study suggests the problems are likely to get worse as the effects of global climate change become increasingly apparent. One of the first problems is exactly the same as for the portable telescopes: the telescopes are frequently warmer than their surroundings. Observatories such as the European Southern Observatory facility in Cerro Paranal, Chile, have in the past compensated for this by cooling the domes housing the telescopes during the day to match that of the air outside. The problem is that the feedback circuits do not work to cool to a temperature higher than 16C and, as the atmospheric temperatures rise, so it becomes harder to maintain the temperature equilibrium between the telescope and the atmosphere. As the atmosphere becomes warmer, it also becomes more turbulent, causing further problems for observations done with ground based telescopes.

Edmond Halley, Canary Wharf, Isle of Dogs, view from Greenwich
The view towards the Isle of Dogs (and Canary Wharf) from Greenwich. In the 17th century it was thought that the Isle of Dogs floated on the tidal Thames because of how it seemed to rise and fall with the tide. The reality is far more interesting and involves the same physics that affects tea and telescopes. You can read about that aspect here.

More difficult however is the effects of water vapour in the atmosphere for observations being made in the infra-red. As the atmospheric temperature increases, so the water vapour content in the atmosphere will increase. One measure of the water vapour in the atmosphere is known as the integrated water vapour (IWV). The IWV is the total water vapour in a column of air stretching vertically from the Earth’s surface to the top of the atmosphere. High IWV levels affect observations in the infra-red and are particularly frequent during El Nino events. It is not just that climate change will cause there to be, on average, more water vapour in the atmosphere. It is known that the frequency of El Nino events is increasing as a consequence of the effects of the climate change we are already seeing. This will lead to more frequent occasions when the observing conditions are unfavourable for ground based telescopes.

The authors of the study conclude that we will need to think about the effects of climate change on the local conditions before we can build any new ground based observatories. We will need to adapt to the new conditions that climate change forces on us. As to how we can minimise the effects of climate change altogether, that will require gazing into our coffee and tea and thinking a lot more deeply. There are things we can do, individually and collectively. Is it too much wishful thinking to wonder if we will start to do them in 2021?

Gallery of fluid motion, 2020

Ever wondered about the shape of the splash formed as your pour over coffee drips into the brew? Or considered what it looks like if you blast falling drops with a high powered laser? Well, now you can discover these and more in this year’s videos submitted to the annual meeting of the American Physical Society, Division of Fluid Dynamics. You can see the full gallery here, and this year’s prize winners here, but below are some of 2020’s entries that have a particular relevance for coffee or cafes.

I hope you enjoy watching some beautiful physics.

Brewing a pour over? Watch the drips

As you watch each drop falling into the coffee below, some produce a splash, some bounce on the surface and some just fall without much effect. Watch drops entering a puddle of water in slow motion to see what happens as each drop splashes:

Rain falling into puddles, or coffee into your V60 brew?

Mocha Diffusion: An experiment you can do in your kitchen

Mocha diffusion is a process for decorating ceramics, but if you have some food colouring and some time, perhaps you can do similar experiments at home.

Mocha Diffusion: from ceramics to the kitchen

Levitating boats with a beat

If you came along to the first Coffee & Science evening at Amoret coffee in June 2019, you will have an idea what this is about. For those of you who didn’t make it, a similar experiment can be done at home (instructions here) and while we didn’t adapt it at the time to the artificial boats shown here, there is no reason that you cannot improve upon that experiment and replicate this one. But if you do, please do let me know how you get on.

Another experiment you can do at home.

Can we do this with coffee?

How does a liquid flow down a string? In this experiment, the authors varied the diameter of the liquid flow and the position of the string to show some beautiful effects with fluid flow. It would be tricky to adapt this to coffee as I think that in order to see the effects shown here, you would need to have a very viscous fluid. On the other hand, why not try and let us know how you get on.

Coffee on a string?

Coronavirus and masks

It is 2020 after all. There were quite a few videos imaging the air flow around breathing, talking and coughing people. Some of the videos compared types of mask, some imaged singers in addition to the coughing people. You can see other videos in the full gallery here. But, as many of us are having to, or deciding to wear masks while we pop into get a coffee, you may want to see the effect that they have on the air flow surrounding the mask wearer.

To mask or not? Is it even a question?

And finally. Don’t try this at home

Ever wanted to smash a droplet with a highly focussed laser? Now you don’t have to but can watch what happens here:

Smashing a falling droplet with a laser. Why not.

Smelling collectively

You can see the steam rising above the cup in this coffee at Carbon Kopi. But you will have to imagine the aroma.

It is hard to choose the best thing about coffee, so many aspects combine to make a good cup. But one of the key things about drinking coffee, particularly if you have had a difficult meeting or have just come in from the cold, is the aroma that wafts up as you grind the beans, add water to bloom the coffee and then brew. In happier times, we may be walking down the street preoccupied about something that is going on and then suddenly get hit by a fantastic aroma that signals our proximity to a good cafe. We perhaps ‘follow our noses’ to the source of the smell and then breathe in the scents as we enter the cafe. Which brings us, in a round about way, to moths and a recent paper that appeared in Physical Review E.

It is not that moths have been shown to have a particular liking for the smell of coffee. That may be an area of future research for somebody. But they do need a very good sense of smell because they need to be able to ‘follow their noses’ in order to find the source of a smell that they are interested in (typically a pheromone released by a female moth). This female moth may be located 100s of metres away from the male and probably does not emit that much odour, so how do the male moths find her?

In a similar manner to our approach to the aromatic coffee shop, the moths first travel against the wind, aware in some sense that the smell is carried downstream. If they lose the scent, they then fly perpendicular to the wind flow in an attempt to sniff the aroma once more. This pattern of zig-zagging flight allows them to approach the source of the smell fairly quickly*.

Eggs of a large cabbage white butterfly. No real links with coffee and few with moths, but the adult pair may well have had to find each other using the sense of smell.

It’s a clever method that is perfect if the wind flows in one direction without any turbulence. But how many times have you watched as leaves have been swept up in the wind flow and danced a swirling vortex pattern before falling back to the ground? Or, as you approach the side of a tall building, you get hit by a gust of wind that seems to come in a number of directions all at once because of the way that it is being affected by the presence of the building wall? We can see a similar thing in babbling streams and in our coffee as the convection currents swirl in vortices. The real world is not so simple as a linear wind flow, in the real world the wind is turbulent.

And yet still the moths find their way to the source of the smell that they are seeking. How do they do it, and could we design a robot (or robots) to emulate the moths in order to find, for example, chemical leaks? It was these questions that were addressed by the recent paper in Physical Review E. In the study they used mathematical calculations to look, not at the behaviour of an individual moth, but at the behaviour of a swarm of moths, a group of moths all searching for a mate.

In the computer model, each individual moth could discern the wind speed and direction and also detect odour molecules. So, left to their own devices, the individuals in the model would follow the zig-zag pattern of individual moths observed in nature (this was a deliberate element of the model). But the model-moths were given another ‘sense’: the ability to see the behaviour of their fellow model-moths. Which direction were the others going in? How fast were they moving?

The model-moths were then provided with one final behaviour indicator, a parameter, β, which was called a ‘trust’ parameter. If β = 0, the model-moths did not trust what the others were doing at all and relied purely on their own senses to reach the prize. Conversely, if β = 1, the model-moths completely lacked confidence in their own ability to discern where the smell was coming from and followed the behaviour of their peers.

We find our way to a cafe via visual cues or perhaps the sounds of espresso being made. But can we also follow the aroma?

Running the model several times for different wind conditions including a turbulent flow, the authors of the study found that the moths reached the destination smell best if they balanced the information from their own senses with the behaviour of their peers. In fact, the best results were for a trust factor, β ~ 0.8-0.85 meaning that they trusted their peers 80-85% of the time and relied on their own decisions 10-15% of the time. If they did that, they reached the smell source in only just slightly longer than it would take a moth to fly directly to the source of the smell in a straight line. An astonishingly quick result. As the authors phrased it, the study indicated that you (or the moths) should “follow the advice of your neighbours but once every five to seven times ignore them and act based on your own sensations”.

Now it would be tempting to suggest that this study has no relevance for us individuals finding a coffee shop and minimal relevance to coffee. But that I think would be premature. For a start, a similar result was found when the question was not about moths but about the best way for a crowd of people to leave a smoke filled room. If everyone behaved individualistically, or conversely, if everyone behaved in a purely herd like manner, the crowd took longer to escape the room than if people balanced their individualistic needs with a collective behaviour. It is a push to suggest that the same thing may be relevant for us finding cafes, but who knows what may happen post-lockdown(s) as we collectively attempt to find a well made flat white to enjoy outside our homes. Maybe we too need to trust our own senses some of the time but be open to taking the advice of those around us too.

*You can read more details in the paper Durve et al., Phys Rev E, 102, 012402 (2020)

Packaging, all about substance

The OK Vincotte or OK Compost HOME labels are for items that are suitable for “home” composting. This label was on a coffee bag from Amoret Coffee

Who would have thought that buying coffee to drink at home could be such a moral minefield? There are issues of sustainability: for the people involved in the coffee process through to the planet. Issues of transportation and the balance between supporting local independents or larger companies with different sustainability policies. And in amongst all this are issues of packaging the final product. How does your freshly roasted coffee arrive? Is it in a bag that you have no choice but to dispose of in the ordinary rubbish, or in a bag (or even bottle) that can be re-used and recycled or composted?

As many of us are buying more coffee on-line at the moment, I thought that it may be helpful to have a list of roasters who have gone to some effort in thinking about the sustainability of their final packaging. Of course many other issues are involved in your decision about which coffee to purchase. This list is only intended as a place to collate information on coffee bean packaging. The list is not definitive, so if you know of a roaster (or if you are a roaster) who is not currently featured on this list but you think ought to be, please let me know as I will be updating the page regularly. Similarly if you notice a mistake, please get in touch (e-mail, Twitter, Facebook).

One more caveat. We each need to decide what we consider a ‘good’, or sustainable packaging. The issue is highly complex. Some of us will have the ability to compost at home, some will have access to an industrial composting bin, some will go to supermarkets regularly and would prefer to recycle plastic together with other plastic bags. And then of course there is the problem that packaging is just one part of a whole relationship between farmer, supplier, roaster, customer and planet. It requires thought and consideration on our part as consumers, on the part of the coffee roasters and, I think, it requires kindness on all our parts, appreciating the efforts of those who are trying to improve things while recognising that there is currently no perfect solution.

In alphabetical order:

Compostable (Home)

Very few items marked “compostable” are, in reality, “home” compostable. Properly home compostable items are certified by the “Ok Compost, Vincotte“/OK compost-HOME labels.

Amoret – Notting Hill, London and online. Coffees (including directly traded coffees) are supplied in bags certified as home compostable (OK Compost). Owing to supply problems during the pandemic, some bags of coffee have been packaged in EN13432 (industrially) compostable bags instead but a recent addition of a new supplier should hopefully solve these supply problems.

Coromandel Coast – online. Shade grown coffee from India, coffee orders online come in a Natureflex bag within a recyclable cardboard box. Natureflex is certified as ASTM D6400 but also listed as “home” compostable and indeed composted in my worm bin composter in 17 weeks (packaging was from Roasting House in that instance).

Roasting House – online. Delivery by bike in the Nottingham area. Ground coffee is supplied in home compostable packaging. Whole beans are supplied in recycled and recyclable bags (see below). You can read more about their latest packaging policies here.

Compostable (Industrial)/Biodegradable

Most packaging that is marked compostable (or biodegradable), but that is not marked as home compostable, will require specialist facilities to compost/degrade such as industrial composting. Compostable items should be certified by (BS) EN 13432 and/or ASTM D6400.

Coromandel Coast – Croydon and online. Bags of coffee purchased in Filtr, the coffee shop associated with Coromandel Coast in Croydon, are supplied in industrially compostable packaging. For coffee purchased online see above.

Dear Green – Glasgow and online. “Together we can all make a difference”. Next year, COP26 will go to Glasgow and Dear Green are ready, organising the 2018 Glasgow Coffee festival to be re-usable cup only. Coffee is supplied in biodegradable packaging.

Glen Lyon Coffee – Perthshire and online. Glen Lyon coffee made a commitment to zero waste in 2017 and use OK Compost Industrial certified coffee bags for their 250g and 500g packaging. 1kg bags are designed to compost within 3 months in a home composting environment. They offer a ‘drop box’ for customers to return their bags for composting. You can read further details about their dedication to sustainability here.

Recyclable

Several roasters have opted for recyclable packaging and quite a few are using the Dutch Coffee Pack bags which are additionally carbon neutral (via offsetting which you can read about here). Be careful with the “recyclable” label as it may, or may not, be suitable for collection with your household waste. Look for the recycling labels on the bags. PET plastic (label 1) is often collected with the street based collections but LDPE (label 4) should be taken to a supermarket where they provide recycling for plastic bags.

Atkinsons – Lancaster, Manchester and online – Established in 1837 as a tea merchant in Lancaster, Atkinsons now sell tea and coffee using recyclable packaging which is also carbon neutral.

Casa Espresso – online – Great Taste award winner for 3 years in a row, coffee is supplied in recyclable and carbon neutral packaging.

Chipp Coffee – online – In addition to using recyclable packaging, you can read more about the ethical and sustainability policies of Chipp Coffee here.

Fried Hats – Amsterdam and online – recyclable but also re-usable. The coffee comes in bottles that can be re-used before ultimately being recycled.

Good Coffee Cartel – online – Coffee in a can, but this time a re-usable and recyclable can containing speciality coffee beans.

Manumit coffee – online – “Manumit”, a historical verb meaning to set a slave free. Manumit coffee works with people who have been subject to exploitation and modern slavery so that they can rebuild their lives. Their coffee comes in recyclable and carbon neutral, Dutch Coffee Pack packaging

New Ground – Oxford, Selfridges and online. Providing opportunities for ex-offenders to develop new skills and employment, coffee is provided in recyclable packaging.

Paddy and Scotts Suffolk based, various outlets and online – Coffee packaging is described as PET recyclable or compostable. Check labelling on package. More info here.

Rave Coffee – Cirencester and online – Rave coffee have been conscientious in describing the reasoning behind their policy of using recyclable (LDPE (4)) bags, You can read about the rationale here.

Roasting House – For whole beans, Roasting House supply the coffee in (recycled and) recyclable paper packaging. Ground coffee is supplied in home compostable packaging (see composting section).

Steampunk coffee – online. I’m reliably informed that their coffee is supplied in recyclable packaging but have been unable to confirm.

This list will be updated regularly. Please do get in touch if you would like to suggest a coffee roasting company who should be included: email, Twitter, Facebook.

A demon in your coffee

Americano, Maxwell's demon
So innocent looking. But could an imaginary demon lurking within help us to understand more fully a major theory in physics?

Is there a way of preparing an Americano that can reveal a particularly knotty problem in physics with implications for information theory?

The question arises out of a field of physics, developed through the nineteenth century, that deals with energy and temperature: thermodynamics. It is the theory that describes how a hot coffee, left in a cold room, will eventually cool to the temperature of the (ever so slightly warmer) room. And though this may seem a trivial example, the theory is immensely powerful with applications from steam engines to superconductors. But it is back with the cooling coffee that we may find a demon, and it is worth finding out a bit more about him.

There are four laws of thermodynamics (the original three and then what is known as the ‘zeroth’ law). But it is the second that concerns us here. It can be phrased in a number of different ways but essentially says that there is no process for which the only result is the transfer of heat from a cold object to a hot one. To think about our coffee, the coffee will cool down to the same temperature as the room, but as the law describes, the room cannot get colder by giving its heat to the coffee cup (so the coffee gets hotter)!

It is in fact, one of the few places in physics where there is a ‘direction’ to time. For most of the laws of physics, time could run in the opposite direction without changing the effect, but not so for this one. The second law of thermodynamics is a definite provider for an arrow of time.

coffee clock, Rosslyn coffee
The clock at Rosslyn Coffee in the City of London. But the image alludes to a fundamental truth: the way that coffee cools is one of the few areas of physics for which it matters which way time ‘flows’.

But that is a digression. We ought to return to the demon in the coffee. The second law of thermodynamics seems to be based on our common sense (though perhaps that is because our common sense is formed within the laws of physics that determine the second law of thermodynamics). But with confidence in our common sense to understand the second law of thermodynamics, let’s do a thought experiment in which we make a strange type of Americano. Imagine a cup of coffee with an impermeable partition cutting through it. Into one half of the cup we pull a lovely, single origin, espresso. The crema rising onto the surface with some brilliant tiger striping on show. Into the other half of the cup we pour some water, initially at the same temperature as the coffee. We drill a small hole in the partition and watch what happens. Of course we know what happens. Ever so slowly, the coffee starts to get into the water and the water into the coffee until we are left with a balanced Americano on both sides with both sides at the same temperature.

Great, but now let us introduce the demon. Actually, he’s called “Maxwell’s Demon” because it was Maxwell who first proposed him (in ~1871), but we can call him anything we like. Perhaps he’s not a he at all. Our demon sits next to the small hole we have made in the partition and watches as the molecules travel towards the hole from the water’s side and the side holding the coffee. This demon is a bit of a trouble maker and so any fast moving molecules (hot) from the water he allows to get into the coffee and any slow moving molecules (cold) from the coffee he allows to get into the water. He does not allow slow molecules from the water into the coffee or fast molecules from the coffee into the water. Just to add to the mix, any coffee solubles he returns to the coffee allowing only water molecules through the hole in the partition.

If our demon exists, we would end up with a lot of very fast molecules on the coffee side (which will therefore be hotter) while the water would hold slower molecules (and be colder). We’d have a very hot espresso on one side of the partition and some luke warm water on the other. It’s not only a terrible Americano but a violation of the second law of thermodynamics! Which is worse?

Although he was proposed as a thought experiment, it is a problem with serious implications for the second law of thermodynamics (which otherwise seems to be a very good model of how things work). Because while we may not seriously consider an actual demon in the coffee, what stops some mechanical tool that we make from violating the second law, if the demon, in principle, could exist? Could the second law be wrong? Could there be a way of getting heat into our coffee from a cold room?

3D hot chocolate art on an iced chocolate, Mace, Mace KL, dogs in a chocolate
Art on a hot chocolate at Mace in KL. Well, what is your mental image of Maxwell’s demon?

The consensus has been that even were the demon to exist, ultimately he is powerless against the second law which does not get overturned by his presence. Because even if we could end up with a super hot espresso on one side of the barrier and cold water on the other side, this is not the whole system; the whole system includes the demon. And the second law applies to the whole system not the system minus the demon. So when we consider the energy (and entropy) of the demon in doing the work necessary to decide which molecules to let through and which to filter out, we find that work is done on the system (by the demon) and the entropy, the disorder if you like, of the whole system has increased (which is another way of phrasing the second law). Calm is restored, we get our Americano back, the laws of physics as we understand them are retained.

But Maxwell’s demon has not been completely exorcised yet, or at least, he is proving to be quite helpful. Because it turns out that there are methods for which the energy cost for the demon is minimal and the argument above no longer works. It seems we are back to square one. But even in that situation, it was realised that the demon has to record, make a note of, which molecules are fast and which are slow, which are coffee and which are water. It has led to an understanding that information has to be part of our consideration of thermodynamics. And as our ability to manipulate nanostructures and individual atoms improves, so experiments are able to explore how information ties into thermodynamics and why Maxwell’s demon still has not undone the second law yet. But it is here that we encounter another demon, the one that is found in the details, so if you are interested you can read more about it here.

Missing matter

soya latte at the coffee jar camden
Not one made by me! But instead a soya-latte at the Coffee Jar a couple of years ago.

During these strange times of working from home, perhaps you, like me, have been preparing a lot more coffee. For me this has included, not just my regular V60s, but a type of cafe-au-lait for someone who used to regularly drink lattes outside. My previous-latte-drinker turns out to be a little bit discerning (the polite way of phrasing it) and so prefers the coffee made in a similar way each day. Which is why I’ve been weighing the (oat) milk I’ve been using.

So, each morning to prepare a coffee, I’ve been using a V60 recipe from The Barn and then, separately, weighing out 220g of refrigerated oat milk into a pan that I then heat on the stove. Generally I heat the milk for just over 5 minutes until it is almost simmering whereupon I pour it into a mug (with 110 – 130g of coffee inside – depending on the coffee). Being naturally lazy, I keep the cup on the scales so that it is easier to pour the milk in and then, completely emptying the pan into the coffee, the scales register an increase of mass (of milk) in the cup of 205-210g. Which means about 10-15g of milk goes missing each morning.

Now clearly it is not missing as such, it has just evaporated, but it does prompt a question: can this tell us anything about the physics of our world? And to pre-empt the answer, it actually tells us a great deal. But to see how, we need to go on an historical diversion to just over three hundred years ago, when Edmond Halley was presenting an experiment to the Royal Society in London. The experiment shares a number of similarities with my heated oat milk pan. It was later written into a paper which you can read online: “An estimate of the quantity of vapour raised out of the sea by the warmth of the Sun; derived from an experiment shown before the Royal Society at one of their late meetings: by E Halley“.

lilies on water, rain on a pond, droplets
Coffee, evaporation, clouds, rain, rivers, seas, evaporation. Imagining the water cycle by making coffee.

Halley heated a pan of water to the temperature of “the Air in our hottest summers” and then, keeping the temperature constant, placed the pan on a set of scales to see how much water was lost over 2 hours. The temperature of the air in “our hottest summers” cannot have been very high, perhaps 25-30C and there was no evaporation actually seen in the form of steam coming from the pan (unlike with my milk pan). Nonetheless, Halley’s pan lost a total of 13.4g (in today’s units) of water over those two hours.

Halley used this amount to estimate, by extrapolation, how much water evaporated from the Mediterranean Sea each day. Arguing that the temperature of the water heated that evening at the Royal Society was similar to that of the Mediterranean Sea and that you could just treat the sea as one huge pan of water, Halley calculated that enough water evaporated to explain the rains that fell. This is a key part of the water cycle that drives the weather patterns in our world. But Halley took one further step. If the sea could produce the water for the rain, and the rain fed the rivers, was the flow of the rivers enough to account for the water in the Mediterranean Sea and, specifically, how much water was supplied to the sea compared to that lost through the evaporation? Halley estimated this by calculating the flow of water underneath Kingston Bridge over the Thames. As he knew how many (large) rivers flowed into the Mediterranean, Halley could calculate a very rough estimate of the total flow from the rivers into the Mediterranean.

Grecian, Devereux, Coffee house London
A plaque outside the (old) Devereux pub, since refurbished. The Devereux pub is on the site of the Grecian Coffee House which was one of the places that Halley and co used to ‘retire’ to after meetings at the Royal Society.

The estimates may seem very rough, but they were necessary in order to know if it was feasible that there could be a great water cycle of rain, rivers, evaporation, rain. And although Halley was not the first to discuss this idea (it had been considered by Bernard Palissy and Pierre Perrault before him), this idea of a quantitative “back of the envelope” calculation to prompt more thorough research into an idea, is one that is still used in science today: we have an idea, can we work out, very roughly, on the back of an envelope (or more often on a serviette over a coffee) if the idea is plausible before we write the research grant proposal to study it properly.

So, to return to my pan of oat milk simmering on the stove. 15g over 5 minutes at approaching 100C is a reasonable amount to expect to lose. Only, we can go further than this now because we can take the extra data (from the thermostats we have in our house and the Met Office observations for the weather) of the temperature of your kitchen and the relative humidity that day and use this to discover how these factors affect the evaporative loss. Just as for Halley, it may be an extremely rough estimate. However, just as for Halley, these estimates may help to give us an understanding that is “one of the most necessary ingredients of a real and Philosophical Meteorology” as Halley may have said before he enjoyed a coffee at one of the Coffee Houses that he, Newton and others would retire to after a busy evening watching water evaporate at the Royal Society.

A three coffee puzzle

Second shot coffee and cake
How would you describe the gravitational attraction between a Long black, a hot chocolate and a piece of cake?

Not a question of how many coffees are acceptable before lunch, but an astronomical conundrum with consequences for your cup.

It starts with gravity. Perhaps you remember that Newton came up with a set of equations describing the laws of gravity. You may even remember the essence of those equations, that the force between two masses is proportional to their product and inversely proportional to the square of the distance between them. If we wanted to phrase it mathematically, the force, F, is given by:

F = GMm/(r x r)

Where G is a constant and r the distance between the masses M and m.

Which is all very well, but suppose we have three masses, or four? M, m and M’, m” for example. If we happened to drop an apple (mass = m) between the moon (mass = M*) and the Earth (mass = M), how exactly, and where exactly, would it fall? How do we add an extra mass into the equation?

It is one of those problems that can seem far removed from your coffee cup, but in fact, the connection is quite close.

The Orion Nebula, M42, can just be seen with the naked eye in the sword of Orion, it is known as a birth place for stars. This image was obtained using the Hubble Space telescope. A separate dust cloud also in Orion was observed for 11 years as a possible host for planetary formation. Credit:
NASA
ESA, M. Robberto ( Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team

But although you may not often drop an apple somewhere between the Earth and the Moon, the question became relevant recently when astronomers observed a dusty disc, the sort of environment that is capable of planet formation, surrounding a three star system. The stars are found in the constellation Orion, which is visible in the evening at this time of year (autumn/winter) from the Northern Hemisphere.

Although these dusty discs are thought to be a host to planetary formation, astronomers have yet to observe any planets actually forming out of the dust. It is thought that in some cases, the gravitational perturbations caused by multiple stars at the heart of the dust clouds could lead to the formation of planets. And so the system in Orion, with three stars in the centre of the dust cloud was perfect to observe the effect of the three stars on the integrity of the disc. Over 11 years, the astronomers recorded the system and then included modelling into understanding how the planetary disc was breaking up. But of course, to do this, they would have needed to understand how the gravitational force is affected by having 3 or more interacting masses.

To solve the problem requires mathematical functions known as a “Bessel functions”. These functions were first described by the astronomer Friedrich Wilhelm Bessel in 1817 who used them for exactly this sort of problem. But they don’t just apply to describing the gravity between three or more objects. They can be used amongst other things to understand heat transfer, to model the microwave fields in a microwave oven and to understand vibrations on your coffee.

The beat of a drum or the resonance on our coffee – the mathematical description of the resonance patterns on coffee is shared with the mathematical description of the gravitational force between three or more objects.

Because when you see a series of concentric circles on the surface of your coffee where the table underneath the cup is vibrating, or when you see more complex patterns as you drive a take away cup over a rough table surface, these patterns can be described using exactly the same Bessel functions as would have been used to model the star system in Orion.

And so there is a direct link between the maths describing the planetary formation in a star system visible in our night sky and the patterns of your coffee cup. But if you want to drink your coffee while gazing at Orion, you may want to stick to decaff, or wake before dawn.

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