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Pure Over Brewing

The Pure Over brewing balanced on my V60 jug. It may seem an odd thing to do, but brewing into a clear glass container that can then be poured into a mug makes a better coffee. Firstly, the brew speed can more easily be monitored, and secondly, any fines that do fall through the glass filter basket are left in the bottom of the jug and do not make it into the final coffee cup.

The kickstarter project promised an all glass coffee drip-brewer without the need for paper filters: great coffee and an elegant brewer, all without waste. But how does the Pure Over perform in practise?

Created by coffee loving glass blower, Etai Rahmil in Portland, Oregon, prototypes of the Pure Over were developed with “The Crucible”, a non-profit art-school in the area. The Pure Over was designed partly to avoid the need for disposable coffee filters during coffee brewing. Although the 275bn disposable filters/year claimed in the kickstarter video sounds an overly high estimate of the number of filters used (though do let me know if you have a referenced value for this), it is true that disposable filters do come with an environmental cost, which could build up and be appreciable. So, if we can reduce our impact on that, it would be a good thing to do.

Now available to purchase online, I got my Pure Over in the initial Kickstarter campaign. When it arrived in early March 2021, everything about it was elegant: the glass had an aesthetic to it that was quite striking. It is easy to understand how the inventors of the Pure Over can describe their ambition as “to make our world a more meaningful and beautiful place”. But there was an immediate puzzle: the holes for the filtration basket seemed larger than I would have expected, would this really work?

I have in the past tried changing a Chemex paper filter for a metal Kone in an effort to reduce my use of paper filters. However, I never got on with the Kone. The filter in the metal was fine enough that the coffee grounds became stuck in it and it was consequently a bit of a pain to clean. At the same time, I never managed to optimise the cup it brewed. I should say that some people have found metal filters and the Kone great products, but I was not one of them. Seeing the glass filter basket therefore made me concerned that this elegant brewer would be pleasing to the eye but never to the palette.

I am happy to say that I was wrong. The Pure Over can make a great cup of coffee and look good too, but it does have a couple of quirks.

The funnel from the Aeropress is brilliant at directing the coffee grounds into the base of the Pour Over. Filling the grinds over a bowl means that any fines that fall through the glass holes can be rescued and put back into the top of the coffee bed. Thereafter the bed is quite stable.

Firstly the grind. The temptation (mentioned in some of the user-reviews of the Pure Over) is to assume that because the holes in the glass filter basket are quite large, a fairly coarse grind would be preferable so that the coffee does not fall through. This is a mistake. The Pure Over works because the water filters through the coffee bed. When the grind is too coarse, rather than produce a thick matrix of coffee for the water to percolate through, the grounds cannot pack closely and what happens is that a large amount of empty space opens up through the coffee percolation ‘bed’. This means that the water flows through the coffee bed too quickly, barely extracting any of the flavour compounds. The resultant cup is weak and unpleasant. If the grind is too fine on the other hand, it will indeed fall through the holes which ultimately block during brewing and the coffee becomes over extracted. The answer is to use a fairly fine grind but not too fine, I use ever so slightly coarser than the grind I use for making V60s. Of course, some coffee grinds do fall through initially, but if you hold the Pure Over over a bowl while you put the coffee grounds into it, you can then catch those that fall through and put them back in at the top. As these are finer grinds anyway, this has the effect of blocking some of the holes (vacancies) that form in the coffee bed and enhances the extraction of the coffee.

Secondly, the part-filter, part-immersion style of the Pure Over means that the water temperature is critical. Because you are using a fairly fine grind within what is partially an immersion brewer, using water that is too hot can result in the coffee being over extracted and bitter. Therefore, in addition to playing with the grind size, it is important to experiment with the brew temperature.

Lastly, the Pure Over comes with a diffusion basket which slows the pour of the water and spreads it over the coffee grounds. This turns out to be important because if you pour the water directly from a kettle it can lead to cratering within the coffee bed and result in a non-uniform percolation through the bed.

The diffuser on top of the Pour Over. The design is supposed to reduce the speed at which the water lands on the coffee bed as well as distributing the pour across the whole coffee bed. There is a lot of physics here, it will have to wait for another post.

When you have optimised these parameters (grind size, water temperature and speed of pour through the diffusion basket), the resultant cup is very much worth it. I found the coffees I made with it to have a character similar to the character that was apparent when I brewed with the V60 and different to the character that the coffee acquired when I brewed the same coffee with an Aeropress. The oils and some fines do come through, which is why I brew the Pure Over into my V60 jug and then pour that into my mug. This has the dual benefit of my being able to see how fast the coffee is filtering through the Pure Over basket and it resulting in a ‘cleaner’ cup as the fines are left at the bottom of the jug when I pour the coffee into the cup.

Over all, a really good cup of filter coffee without a filter. You can read another review of it in Barista Magazine here.

There is additionally a lot of physics involved in how the coffee brews. Although I didn’t mention it here, there is a link to traffic jams and filtration, a link to some novel methods now used in the organic farming of coffee beans and a connection to steam engines. There are also other links that I think do help to contribute to a more meaningful and beautiful world, so please do return in future weeks for an exploration of some of the physics involved in this interesting new addition to coffee brewing.

Coffee Elephants

coffee Coromandel Coast, Indian Shade grown coffee
The coffee from Coromandel Coast. Chocolate, ginger and nougat. I got the chocolate and the nougat, though the taste profile changed quite significantly between brewing by a V60 or an Aeropress

The coffee from Coromandel Coast arrived in a box, in bags that were suitable for industrial composting, each printed with an elephant on the packaging. The elephant is the logo of Coromandel Coast and is a nod to the fact that all of their coffees (which include single origins and blends) originate in India. All of the coffees have been shade grown which helps with the carbon footprint of the coffee, hence the slogan “Climate solution in your cup”. Which means that it would have been easy to do a coffee-physics review based on the different ways that coffee can be grown and why shade grown coffee can be part of a climate solution for coffee. But that would have been too quick; one of the motivations for cafe-physics reviews (and the related coffee-physics reviews) is to slow down and explore how sitting down and contemplating a cafe (or just coffee) can lead to so many different but connected thought trains. Given that your attention is drawn to the issues of climate change, and what you can do, from the instant you order from Coromandel Coast, this seems to be too obvious, even if an incredibly useful, thought train. So, if you would like to follow that thought train while contemplating the coffee you are drinking, you can read more about the environmental impact of coffee growing here or here and the importance of shade grown coffee here. An alternative thought train may be provided by the elephants.

I purchased two coffees from Coromandel Coast: Ganga and Chalukya. The Ganga was a washed catuai peaberry coffee with tasting notes of “chocolate, ginger and nougat”. The chocolate definitely comes through when brewed in the V60 and the pureover while the Aeropress produces a somehow cleaner taste profile that I find characteristic of washed coffees. Coromandel Coast was established in 2017-8 and is both a coffee roaster and a cafe based in Croydon. All of the packaging is recyclable or compostable, including the box it arrives in which is additionally re-usable (and will be reused again a couple of times before it is eventually recycled).

The elephant stamp. Is every copy identical? Could we use one elephant to understand the others?

The ink-stamped elephant on the box is a nice touch and echoed on the coffee bags. You could perhaps start to think about ink printing, dyes and the invention of the printing press, there are plenty of thought-paths that open themselves out. But a chance conversation over the coffee provided a different direction into the ways in which physics is taught at schools.

It appears that the school of my interlocutor that day initiated the physics course with a very boring set of classes on units. I was asked that morning: why would the teacher have started teaching physics with such a boring set of lessons? But I wondered a separate question, how can units be boring? How sad that they were made to be so. For although they are of fundamental importance in how we explore and understand our world, and could perhaps be quite dry, they can also link elephants to the Sun and to the work we now do to understand coffee better. For if we start with elephants, it was a favourite unit of my physics teacher. Used for all manner of things when we omitted to include the units in our answers. Consider the coffee: it comes in bags of 250 what? 250 elephants? or 250 grammes? The elephant became a unit of frustration for the lack of stated proper units. But we can push the Coromandel Coast elephant link a bit further, for each elephant on the packet is an ink-stamped copy. They are different but identical, they serve as a standard.

neon sign, light emission
Light is emitted from different chemicals at certain, definite wavelengths. This is an effect you will have seen on many a high street in these neon signs where the colour is determined by the composition of the gas within the sign. We can use the reverse of this to identify chemicals based on what wavelengths they absorb. But to do that, we need to know that we are all measuring in the same units.

And the standards are important for units because we need to know that we are all measuring the same thing. When Anders Angstrom was measuring the absorption and emission spectra of the Sun and of different gases, he quoted the absorption lines in units of 1/10 of a nanometre (a unit now called the Angstrom). Different gasses will absorb (or emit) light at very specific frequencies or wavelengths. Being a very careful experimentalist, Angstrom had ensured that his measurements of the wavelengths that were absorbed or emitted were checked against the standard measure of length of the day, the metre. But at the time, the metre was defined by the length of a metal rod stored in Paris. All other standards of the metre were copies of this original one, including the metre kept at Uppsala where Angstrom was doing his experiments. An issue with metals is that they will age. With time you will get some shrinkage and some expansion owing to the formation of oxides etc. on the metal. The metre in Paris had aged in a different way to that in Uppsala which was just a tiny bit shorter than the Paris metre*. These differences would not be noticeable were Angstrom measuring the size of elephants, but instead he was concerned with measurements that were one ten-billionth of a metre. And at this scale, it mattered a great deal. Angstrom was aware of the systematic error in his results but it wasn’t until after his death that the error was fully hunted down and corrected for.

The position of the lines that Angstrom had been measuring reveal the chemical composition of the gases, and so knowing whether a line appears at 700 or 710 nm, reveals information about the chemical studied. We still use these spectroscopy techniques, not just for understanding gases, but also for checking the composition of medicines and for understanding the differences between Arabica and Robusta coffees. Which brings us back to the coffee, for while we no longer use a physical measure of length as our standard metre, we still use a standard definition of the metre that allows us to compare coffees and stellar spectra. It also allows us to appreciate the beauty in the uniformity of an ink-stamped elephant on a box housing an interesting and flavourful, climate sensitive, coffee.

You can order from Coromandel Coast here, or (post-lockdown) visit the cafe at Filtr, 53 Limpsfield Road, S. Croydon,, CR2 9LB

*To read more about the history of the definitions of units including the metre, click here. This anecdote was originally recorded in a book that I do not have physical access to at the moment owing to coronavirus restrictions. As soon as I get the name/author of the book, I’ll include it here.

Thought bubble

inverted Aeropress and coffee stain
A problematic inversion with the Aeropress. This brew method offers plenty of physics connections for those who look.

The Aeropress is not a brewing technique that creates many bubbles on the surface of a coffee. Unlike the crema of an espresso or the iridescent bubbles on top of a black coffee prepared using a cafetiere, the surface of an Aeropress coffee could be thought of as a bit, well, dull. The paper filter within the Aeropress removes many of the oils while this calm brew method generally does not create the turbulence needed to produce bubbles that cling to the side of the resultant cup. Yet it is this brew method that can provide a bubble link to climate change and coffee roasting, and to see why, we need to pay careful attention to our brew.

Although there are many techniques for brewing with the Aeropress (you could try the guide here or here), one step common to most brew guides is that you will need to rinse the paper filter in the basket before you brew. The rinsing step removes a potential paper-y taste from the filter as well as helping it to stay fixed in position (the reason for this could be the subject of another post). Importantly for this particular post though, it also traps air within the holes of the filter, which you can see in the photograph.

The bubble is trapped owing to the strong surface tension of the water dripping from the basket. You could perhaps test this by adding soap to your brewing water in order to reduce the surface tension and watching to see if the number of trapped air bubbles you produce decreases. Or perhaps there are limits to what you are prepared to do with coffee in order to see some physics. Whichever way, the fact that the bubble is there at all can lead us down several thought alleys.

Perhaps we start to think about air that is trapped within water. In a way, this air is characteristic of what is around us now: the pollutants, the oxygen level etc. Which, while it may seem an obvious statement has an immediate consequence. Air that is trapped in water that is then frozen remains as a record of the composition of the air at the exact point of time that it was trapped. So if layers of ice form trapping layers of bubbles of air, and this happens for many years, we can analyse the composition of the trapped air bubble to discover what the atmosphere was like 100, 1000 or 100 000 years ago. This offers a way of understanding how concentrations of carbon dioxide, for example, have varied over the millennia.

An example of air bubbles within the Aeropress filter. In addition to the long bubble caused by incorrect filter placement, you can see two air bubbles in the hollows of the plastic basket under the paper filter (circled with a dotted red line).

But maybe your mind stays with the coffee: what about air bubbles within a coffee bean? In order to turn the green coffee bean into the aromatic substance that we all appreciate, it needs to be roasted. Roasting coffee is a fantastic mix of science and art: using the knowledge of what happens during roasting and applying (and playing with) that knowledge to produce great tasting coffees. At its core, the roasting process involves heating the beans for a certain amount of time in order for the water to come out of the green bean, the sugars to turn in the Maillard reactions and for the various aromatics to develop chemically. The green bean also undergoes physical changes. The colour is altered, the bean expands and the internal gases (first water, then carbon dioxide) build up pressure within the bean and then crack open some of the cell structures during roasting. And while this sounds fairly simple, there are ‘arts’ involved in roasting: how long do you let the beans dry? How fast do you take the bean through the Maillard processes? Do you let the beans cool slowly or cool them really fast to stop any further chemical reactions immediately? Each of these has effects on the final flavour of the bean, some which are fairly similar across the industry, some which rely much more on the creativity and discernment of the roaster.

There are obvious analogues to materials physics and materials chemistry. In order to make the different materials that are studied, raw materials are often heated to a high temperature and left for a significant time before either being cooled slowly or suddenly, by quenching. There is the science: the temperature at which different reactions occur and the way that materials form together in order to produce grains that get larger as they are heated for longer. And then there is the art, how fast to heat, how long to leave it for, whether to cool or quench, even what gas should be used to flow over the forming compounds. Small differences in how the materials are heat treated can have large consequences on the applicability and strength of the final material, with applications from gear cogs to airplane engines.

Kamwangi and Gelana coffee under the microscope
A fluorescence microscope image magnified 20x of two types of coffee after roasting. The microstructure (including pore development) will depend on the type of coffee as well as the style of roasting.

To return to the coffee roasting, the effect of the temperature has a similar marked effect on the microstructure of the resultant bean which will have consequences for how the roast ages. For example, a study about 20 years ago showed the differences between coffee beans roasted to an equivalent level (measured by moisture loss and colour analysis of the roast) at two different temperatures. The physical properties of the final roasted beans were very different. Not only did the higher temperature (260C) roasted beans show a larger volume increase compared to the low temperature (220 ) roasted beans, the pore structures of the beans were also different. For the higher temperature roasts, larger micropores had opened up within the cell walls of the roasted coffee. These pores connected to regions deep within the bean that would otherwise be cut off from the air: trapped bubbles within the bean that, with the higher temperature roasting, now have a way of escaping to the outer surface. Indeed, one day after roasting, the authors of the study saw, under a microscope, many tiny spots of coffee oil seeping from the interior of the higher temperature roasted bean and to the surface.

This has consequences for how the bean will age after roasting and so how we as consumers will appreciate the drink. Roasting is a dark art indeed, and one that I’m grateful for the many skilful practitioners that we now have around. Roasters who help us to appreciate the flavour of our coffee, as well as the directions of thought it takes us on.

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.

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.

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.

Listening

kleen kanteen, refillable water bottle
What do we hear as we fill a water bottle?

Filling a re-usable water bottle from the tap, the sound starts off as a low hum, then rises in pitch before a sudden change in note as the water spills over the spout because you have over-filled it. Texturing milk in a pitcher, the sounds change as the bubbles form and break, ready for pouring as latte art. How often do we know what is happening by listening to the sound something makes?

And yet these sounds are revealing more than just when the bottle is full or the milk can be poured. They are teaching us, if we listen carefully, about the physics of what is going on within the water bottle, within the milk pitcher and even within coffee grinds as we bloom the coffee. Consider the water bottle. It is a classic resonator, the basis of many musical instruments. As we fill the bottle, the liquid level acts as an end point to the bottle, reducing the volume of air in the bottle as the water fills it. The note that we hear coming out of the bottle corresponds to the frequency of the sound wave that is resonant in the empty volume of space. As the frequency is inversely proportional to the (square root) of this volume, when the volume decreases (ie. the bottle is filled) the frequency increases, so the note that we hear will go up. The bottle is acting as an approximation to a Helmholtz resonator. You can read about how this can be used for experiments with coke bottles here, or more of the physics (and the maths behind it) here.

Similarly with the milk pitcher, the changing musical note is telling us about the changing conditions within the pitcher, though in this case it gets quite complex. Firstly, as the steam wand is introduced to the pitcher, air is introduced to the milk which “stretches” it. This builds the volume of the milk in the pitcher and introduces air bubbles into the liquid. The combination of the volume change and the introduction of air is going to affect the sound that the jug would make, but the sound you hear, the ‘hiss’ is most probably dominated by the sound of the steam leaving the steam wand. After a short while, the barista will lower the steam wand further into the milk in order to heat the milk in the pitcher. Treating the pitcher again as an approximation of a Helmholtz resonator, we know that the frequency that we hear from the pitcher increases as the speed of sound inside the resonator increases. As the speed of sound in the milk increases with temperature (assuming that it is mostly made of water), to a first approximation we expect the note that we hear to increase in pitch with time. So after the hiss, we will hear a note which rises in pitch as we continue to warm the milk. Is this what we hear?

Latte art scutoid tulip
What sounds do we hear as milk is frothed in a cafe?

Together with other species, we use the information that sound gives us to understand much of the world around us. “Listening” famously helps bats to navigate and hunt and also, helps us to understand more about what occurs in the ocean. Indeed, it has even been suggested that we should listen to the sounds recorded as space probes land on different planets or moons in order to gain further information about what could be hidden just out of view of the camera. Of course, the sounds on another planet may not sound exactly as they do on Earth. Prof. Tim Leighton of the University of Southampton has calculated (and then synthesised) what a methane-fall (like a waterfall but of liquid methane) would sound like on the surface of Saturn’s moon Titan. You can hear the recorded waterfall on earth here, and the simulated methane fall on Titan here. Provided we know what we are listening for and to, better listening can improve our understanding of our surroundings.

the broadcasting equipment at the WW cafe Hackney
London Fields Radio at the Wilton Way cafe. Sounds enter our conscious in a variety of ways, but how many of them do we listen to?

An example of where better listening may improve our understanding of our surroundings comes with bread. One common way of knowing when bread is properly cooked is to tap its base and listen to when it sounds hollow. We can assume that this is because the bread crust acts as the walls of a resonator with the large number of air bubbles that form during cooking (and which make the structure of the crumb) being the bit where the sound wave resonates. The hollow sound shows that what is inside is solid, whereas if it were still dough-y, it would damp the resonance (no pun intended) and make it dull sounding. If this assumption is correct, the note that is made by tapping the bread will decrease as the bread cools and the speed of sound in the air in the bread decreases. But can we also get information about the crumb structure of our loaf by listening to the pitch of the loaf as we tap it? Would not the frequency of the resonance (ie. the sound) change depending on how open the bread structure is (a large, open loaf would perhaps have a lower ‘note’ than a loaf with a small crumb which may have a higher note). Is the bread ‘telling’ us more than just that it is cooked? Experimenting bakers, it’s over to you.

Bloom

bloom on a v60
Blooming – the petrichor moment of brewing a coffee.

One of the great moments while brewing coffee happens as you add a small amount of hot water to the coffee grounds and an intense aroma rises towards you. Together with the sight of the bubbles of carbon dioxide escaping the just-ground coffee and the sounds as the grind expands, cracks and the bubbles pop, it is a multi-sensory experience.

It is also a very good point to stop what we are doing, and think for 30 seconds, or a minute. Which means also that it’s a perfect time to experiment with your coffee. Istobiii is inviting us all to an experiment to try what he is calling “cold bloom”. You can watch his invitation to the experiment here.

Does blooming your coffee with cold (or tepid) water produce better coffee? What would be the difference between blooming with colder water compared with just boiled? And why do we bloom anyway?

Given that Istobiii is suggesting extending the bloom time with the colder water to 1.5-2.5 minutes, we have plenty of time to think. Do give it a try, and have fun experimenting.