This week, on Thursday, June 10th, 2021, there will be a solar eclipse. If you are at high latitudes in the Northern Hemisphere including parts of Canada, Greenland and Siberia, you will see a so-called ring of fire as the moon moves in front of the Sun. At lower latitudes the eclipse will be much more partial and in London we are expecting to see 20% of the Sun obscured by the Moon.
You can read more about solar eclipses on other websites such as here or here, on Bean Thinking, we are going to focus on the coffee links to the eclipse.
The first coffee link comes in how to view it. This website suggested a number of ways of viewing the eclipse, one of which was to use a colander. This suggests a perfect adaptation to a view via coffee: the Aeropress filter cap. The idea behind the method is that each of the holes provides a type of pin-hole camera to image the Sun. Knowing roughly where the Sun will be at 10.06am (BST = UTC+1), we can construct a device to hold the aeropress filter cap so that we can see 97 images of the Sun projected onto a piece of paper: 97 images of the Sun to be eclipsed over the following 2hours 18 minutes. The maximum eclipse is around 20% of the solar disc and occurs at approximately 11.15 (although the exact fraction obscured and timing depends on your location). The Aeropress Eclipse viewing device shown in the photo here has an added (smaller) pin hole which should provide a more focussed image of the Sun and so will provide a second way of imaging the eclipse.
A second coffee link comes with thinking about why this particular solar eclipse is not ‘total’ anywhere on earth but is instead described as annular. And to do this, we’ll think about a coffee bean. The amazing visual spectacle of a total solar eclipse occurs because the moon is 400 times smaller than the Sun but is (on average) about 400 times closer to the Earth. So when we think about looking at a coffee bean, held at arms length from our eye (about 60cm), it would totally obscure (eclipse) an object 3.2 m tall, 233.5 m away*.
The word “average” though hides an important detail that neither the Moon’s orbit around the Earth, nor the Earth’s orbit around the Sun are completely circular. On the 10th June 2021, the Moon will be two days past its maximum distance (apogee) from the Earth, and while the Sun is also nearly at its maximum distance, the distance ratio will mean that the Moon does not entirely obscure the Sun. Instead, if we return to our coffee bean analogy, it is the equivalent of stretching our arm 2 more centimetres and noticing that the object that was obscured is no longer completely obscured.
This will still make for a fantastic view if you are in Greenland, Siberia or happen to be at the North Pole where you will see a dark disc surrounded by a ring of Sun. For those of us further south, we will only see the Sun partially obscured by the Moon. Nonetheless, such an opportunity in any one particular location doesn’t come super-often (although worldwide there are often several eclipses per year, in London there will only be 42 partial eclipses in this current century). And in London, we have to worry about the weather too. So, if the weather is good for you, why not have a go viewing it, particularly if you adapt a piece of coffee brewing equipment to do so, and post your pictures of the effect here, or to Bean Thinking on Twitter or Facebook.
Finally, the timing of the eclipse is perfect for a mid-morning coffee, though maybe you’ll have to brew with something other than the Aeropress. Have fun.
*These figures have been calculated using a ratio of the size of the Moon to the Sun as 1:400.8 and an average distance of 1:389.2 (calculated from the average values). The distances on June 10 2021 mean that the distance ratio is closer to 1:377
Update to post, the day before (9 June 2021): This is the Aeropress viewing device in action, but 24h before the eclipse. Will the clouds stay away tomorrow?
Update 10 June 2021: It was cloudy in London and I couldn’t get the Aeropress filter cap method to work in the brief periods of sunshine during the eclipse. Suspect it was a problem with focus-distance/angle/remaining cloud cover at points. However, the smaller pinhole did work (see the blurry image below) and the clouds did mean that there was a natural filter that made a direct photograph possible (see below). Do share your images here if you managed to view it.
Many coffee mugs have an aspect of handedness to them: they have one handle and we tend to pick them up with our dominant hand. During zoom meetings, this point has recently been emphasised to me because of the design of some Ritzenhoff mugs. Picking up the mug with one hand, the image shown on the screen is quite different to picking up the mug with the other. Overall, about 90% of us are right handed with 10% being left handed (though it becomes a little bit more complicated than this). And while we can also have mugs with no handles and which don’t have any difference when picked up with the left or the right hand, this reflection on handedness in mugs could prompt a question, what about the coffee inside it, does it have directionality or “handedness” to it?
This is in fact not an unreasonable question as it is about how light interacts with the coffee. But to see how the coffee could show a handedness, it is worth a brief diversion into the nature of light. Light consists of an oscillating electric (and magnetic) field which oscillates perpendicularly to the direction that the light is travelling in. Apart from the fact that the oscillations are perpendicular to the direction of travel, these oscillations can be in random directions, a situation in which we would say the light is ‘unpolarised’. If however the electric field oscillates in one direction only, the light is said to be polarised. We can find out how the light is polarised by using a pair of polarised sunglasses or a piece of polaroid and rotating it to see how the intensity of the light changes as the polaroid is (or sunglasses are) rotated.
This so called linear polarisation is only one type of polarisation however. If you imagine viewing the electric field of the light head-on coming towards you, it could also rotate rather like a corkscrew. And just like a corkscrew, it could either rotate clockwise or anticlockwise; this is circularly polarised light. When light interacts with, or reflects from some chemicals, it can turn from being unpolarised to left or right circularly polarised. We’d say that it has chirality or ‘handedness’, and it is this effect that we are asking about in coffee. One fantastic example of a surface that reflects (mostly left) circularly polarised light is the shells of certain beetles in the Lomaptera and Hybosoridae families. Here, the brilliantly shimmering colouring of these green and occasionally other coloured beetles is entirely structural, meaning that there is no pigmentation on the shell, the colour is caused by how the light interacts with the (colourless) layers of the shell. In the case of the beetles it is because the shell is made from layers of strongly linearly orientated chitin molecules. Because the beetle shell is composed of many layers each twisted slightly from the one beneath it, the light ends up interacting with a corkscrew type reflecting surface that gives the reflection a left circular polarisation.
While this is a cool effect in beetle shells, the consequences of this handedness in nature can be catastrophic. Some molecules have an intrinsic ‘handedness’ to them, so although two molecules have the same chemical composition, they are the mirror reflection of each other and so not identical. It is like the cartoon molecule in the image below. Both ‘molecules’ contain the same number of coloured circles but their positioning means the molecule on the right is not the same as the one on the left. In some cases, these molecules will interact with light differently, one will polarise the light with a left circular polarisation and the other a right circular polarisation. As the molecules are chemically identical but do not map onto each other (they have ‘handedness’) they are called enantiomers. Years ago I had a summer job at Pfizer in Sandwich, Kent, UK, analysing various candidate drugs to check both that they were chemically pure and that they were what they were thought to be. One of the tests that I had to do repeatedly was polarimetry which measures the optical activity of the molecules in a sample. In short, this measures whether the chemical in the sample shows a handedness and if so, how much. It may at first sight seem not to make too much of a difference, after all the molecules are chemically the same. However it makes a large difference, not just to the way that light interacts with the molecules, but to the way that our bodies do too.
In the late 1950s and the early 1960s, the drug thalidomide was prescribed for, among other things morning sickness. Thalidomide is an example of a drug in which there are two enantiomers which, ordinarily exist in equal amounts. The problem was that one of these enantiomers (the s-enantiomer) was teratogenic which means that it caused birth defects in forming embryos. It was suggested in the 1970s that if the r-enantiomer of thalidomide had been isolated from the mix and given without any s-enantiomer present, the birth defects could have been avoided. While this conclusion has since been questioned, nonetheless, now all drugs are tested to ensure that this problem can never happen again, and part of that test involves looking at the optical handedness of the drug sample with a polarimeter.
A few years ago a study revealed why you were more likely to spill an Americano than a latte. It was found that a layer of bubbles on top of a liquid could reduce the amplitude of any ‘sloshing’ produced as you walked with the cup. As the latte has more bubbles than an Americano (or long black), the Americano would slosh more and so spill more easily: if you want to grab a take-away coffee, either grab a lid or order a cappuccino.
It seemed that the bubbles were reducing the amplitude of the slosh because they were causing friction at the sides of the vessel holding the liquid (we would probably say the coffee cup). This friction reduced the energy of the vibration and so decreased the amplitude of the slosh. Without any bubbles, the researchers had produced a ‘slosh’ with an amplitude of 1cm (in their vessel which was about 7cm across). As they added layers of bubbles, this amplitude decreased until at bubble layer thicknesses of five bubbles and more, the amplitude of the slosh was 0.1cm. Bubbles reduced the amplitude of the slosh by a factor of ten.
A few years on and a different set of researchers wondered about the implications of this research on the break-up of ice sheets. The concern was that as the Earth’s ice melted, winds could generate larger amplitude waves on the (now liquid) water surface which, as they impacted the remaining ice sheets could cause them to fracture and crack, thereby accelerating the rate of ice-loss in the polar regions. And yet there is a question. If the ice cracks up and starts floating as icebergs on the water’s surface, could this affect the amplitude of the waves generated by the wind? Could the floating icebergs act similarly to the bubbles of the latte in the earlier study?
Now of course, there are no container walls in the sea but there are plenty of other mechanisms by which a layer of floating objects may reduce the amplitude of a vibration. In particular, if there are a group of floating objects on the surface of a body of water, as the wave moves up and down the objects move up and down with it, but they also move horizontally. As they move horizontally, away from and then towards each other, there has to be a localised liquid flow into and out of the space between the particles and this offers a way of transferring energy from the amplitude of the wave into a different water movement. This effect increases as more layers of floating objects are added to the water, just as with the latte study. The reduction in the wave height is dependent on the thickness of this layer and, surprisingly, not on the size of the floating objects themselves.
Thinking about these results can help us to understand how mangrove swamps help to protect the coastline during storm surges. During the 2004 tsunami, it was shown that villages behind mangrove swamps in a certain region of India suffered less damage during the surge than villages on unprotected areas of coastline. The mangrove swamps were reducing the height and energy of the surge to make it less destructive. What was it about the mangroves that acted as a coastal defence? Studies since 2005 have emphasised the importance of the aerial root structure of different species of mangrove tree, as well as the density and height (age) of the trees. As the water surges past these roots or branches, they are moving and causing friction for the incoming water, causing localised water flows and removing the energy from the incoming wave. In a sense they are reducing the amplitude of the incoming wave in a way we can understand by contemplating our sloshing latte. This has obvious implications for coastal defence and accordingly authorities around the world have been planting mangrove swamps to protect coastal areas.
These recent efforts for replanting mangrove swamps though come with a history of a 35% reduction worldwide in the area of mangrove swamps between 1980 and 2000. This becomes a further problem because the mangrove swamps have been shown to be excellent carbon sinks, offering a way to reduce atmospheric carbon dioxide and trap it within biomass. A possible sign of hope however is that the existing effects of climate change are causing a growth in the area of coastal mangroves as salt marsh gives way to mangrove in latitudes that have previously been too cold for the mangrove trees to survive the winter. This growth in mangrove swamp offers both a level of coastal protection and a possible negative feedback mechanism for the effects of climate change, though it is unclear what the effects would be of the changing eco-system on the diversity of life in the coastal regions.
It was entirely appropriate that the first coffee I tried in the Pure Over coffee brewer was the directly traded La Lomita Colombian from Ricardo Canal via Amoret Coffee. Ricardo was a special guest at one of the Coffee and Science evenings we held at Amoret Coffee in Notting Hill (pre-pandemic) where, among other things, he spoke about how he is using Biochar on his coffee farm. Biochar is a porous, charcoal based material that can help to provide the coffee plants with nutrients as well as water, thereby reducing the amount of fertiliser the plants need. To understand how it works, we need to understand a bit about percolation, which of course we also need to understand in order to brew better coffee in the Pure Over. Indeed, there are enough similarities, and an extension to a quirk of how espressos are brewed, that it is worth spending a little more time thinking about this process and the connections revealed as we brew our coffee.
Percolation recurs in many of the brew methods we use for making coffee. The V60, Chemex, Kalita wave, percolators and the espresso itself, all rely at some point on water flowing through a bed of ground coffee. The flavour of the resultant cup is dependent on the amount of coffee surface that the flowing water is exposed to together with the time that it is in contact with the coffee. What this means is that grain size, or the degree to which you grind your coffee, is critical.
Playing with brewing coffee, we know some things by experience. Firstly, frequently, the flow through a coarse grind of coffee will be quite fast (probably too fast to make a good cup). Secondly, we know that for any particular brew method, the more water we pour into the brewer, the faster the water initially comes through. We also know that we can affect the flow rate of water through the coffee if we increase the area of the coffee bed, or decrease its thickness. These observations were quantified into an equation by Henri Darcy in 1856. Darcy’s work had been as an engineer, designing and building public works such as the aqueduct that brought drinking water into the city of Dijon in the 1840s. Darcy received significant recognition at the time for his work including the Légion d’honneur, but it is more for a later set of experiments and particularly for his equation that we remember him today. In the 1850s Darcy was working on the problem of water purification. Passing water through a bed of sand is still used as a method of purifying the water today. Darcy used a series of cylinders filled with sand to investigate how quickly water trickled through the sand bed in order to come up with a proper quantification of those things that we too know by experience with our coffee filters. You can read about the mathematics of Darcy’s equation here.
Darcy found that the flow rate of water through the sand bed increased when the porosity of the bed was higher (fine, dense sand would delay the flow of the water more than coarse, loosely packed sand). If there was a greater pressure on the water at the top of the bed (ie. more water is on top of the sand), the flow rate through the bed would increase too. Conversely the flow would get slower as the water was made more viscous. This is something we too know from experience: try to pour honey through the coffee grounds and it just won’t work.
For us to apply Darcy’s insights into making better coffee, it means that we need to think about the grind size. Too coarse and there will be lots of empty space through the bed of grounds: the porosity is high, and the water will flow straight through. Too fine and the flow rate will decrease so much that rather than just the sweet and slightly acidic solubles that first come out of any coffee extraction*, there could be too much of the bitter organic compounds that come out later, changing the character of the cup. With coffee we have an additional concern. Unlike sand, coffee grinds will swell, and splinter, as water is added to them, closing up any narrower paths and lengthening the brew time. This also means that, unless we properly wet the grounds prior to filtering our coffee, the extraction will be non-uniform and not reproducible. Another reason to bloom coffee thoroughly before brewing.
There is one more factor in brewing our coffee however that Darcy’s equation, which is valid for more stable systems, overlooks. Darcy assumed a constant flow rate of water through the sand bed, but coffee is different. In his book about espresso*, Illy showed that the flow of the water through an espresso puck was not constant over time. Something really interesting was happening when you looked carefully at an espresso puck. Ground coffee can come in a large distribution of sizes. In addition to the grind that we are aiming for, we also get a whole load of smaller particles called ‘fines’. Sometimes this is desirable, but with espresso, and by extension with our filter coffees, these fines add a twist to the physics of the percolation. As the espresso water is pushed through the coffee puck, the fines get pushed down through the puck between the ‘grains’ of the coffee grinds. This reduces the flow rate of the water until the point at which they get stuck. This will have the effect of increasing the contact time between the coffee and the water and so allowing more flavour solubles to be extracted. But crucially, these fines remain somewhat mobile. If you were to turn the whole espresso puck upside down (and Illy had a machine that allowed him to do this in-situ), the fines would again go on the move. Migrating from the new top of the puck to the new bottom. Filling the voids between the slightly too coarse grains. Complicating the simplifications in Darcy’s equation, but adding flavour to our brew.
Which leaves the connection between the farming method and the coffee. Biochar is formed by burning carbon containing waste (such as plant matter) in a low oxygen environment. Burying the resultant charcoal is therefore a way of storing carbon, and preventing its release into the atmosphere, for many years. But it is not just good for carbon storage. The buried charcoal is highly porous and traps nutrients within its structure so that the plants growing near it can be fertilised more efficiently. Moreover, the fact that it is porous, just like the coffee or sand beds, means that it traps water for a long time. Consider how long it takes a used filter full of coffee grounds to completely dry out! The water gets trapped within the porous structure and does not evaporate easily. This aspect of the biochar means that, as well as nutrients, the plants that grow nearby get a good source of reliable water. The ancient civilisations of the Amazon region used something similar to biochar in their farming techniques resulting in soil now known as “Terra Preta”, an extremely rich form of soil that improves plant growth. On his farm, Ricardo is going fully circular and making his biochar out of old coffee trees. The old trees thereby giving new opportunities to the fresh growth. It is a carbon capture scheme that reduces the need for fertilisers and that relies on percolation physics to work to best effect for the plants.
It seemed a moment of perfect coffee-physics poetry to use coffee grown on a farm using these techniques while initially experimenting with my own, percolation sensitive, Pure Over brewer. Percolation physics and interconnectedness all in one cup.
*Illy and Viani (Eds), “Espresso Coffee”, 2nd Edition, (2005)
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.
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.
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.
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 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.
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.
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.
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.
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.
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.
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.
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.
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?
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.”.
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.
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.
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?
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.
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.
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.