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Language

Tasting notes on a coffee. Do each of us hear the same meaning even as we use the same words?

How should we describe the flavour of coffee? First used in 1995 and redeveloped in 2016, the Speciality Coffee Association (SCA) coffee tastes flavour wheel was designed to give coffee professionals a common language to describe the flavours they were experiencing. You can find copies of the colourful wheel in many coffee shops, or sit at home and explore your coffee with it directly from the SCA’s website. Ideally it would help us to discuss different coffees, to compare and to consider which we find fruity, spicy or tasting of chocolate. But how common is our language really?

The wheel has come in for particular criticism for its cultural, or geographical, specificity given the global interest in good coffee. Flavours that are well known in North America may not be so common elsewhere. And so the wheel has been adapted to include more diverse flavour terms both in Taiwan (where terms include jujube and many other fruits) and in Indonesia (where there is a strong emphasis on spices). However, the problems go deeper than this. As a keen blackberry forager, I know that the flavour of blackberries is very variable, both between locations and depending on the time of year that you pick (during the first fruits of July or towards the end of the season in September/October). Nonetheless, ‘blackberry’, is a flavour referenced on the wheel. The developers of the wheel anticipated this problem and knew that we need to have a common understanding of the flavour ‘blackberry’ in order that it can be useful. They therefore referenced the flavour blackberry to one particular type of blackberry jam. This helps and serves as a good control, but only if we all know that ‘blackberry’ really refers to a type of blackberry jam.

The issue seems to go beyond the idea that people ‘don’t understand’ how the terms are meant to be used, it is more that we are using the terms in different ways. The issue is not confined to coffee, there are many examples in science too. The term ‘theory’ for example has a specific meaning within scientific practice that is different from that used in every-day language. For scientists a theory represents a description of the world that is backed by experiments and experimentally testable predictions. Anthropogenic climate change is a ‘theory’ backed by a large amount of evidence, it is our best way of understanding what is going on with the climate. Here ‘theory’ means ‘what we think is really happening’. It is very far from the idea of ‘theory’ found even in the dictionary where one definition is of “a speculative (esp. fanciful) view”. And this gives us a problem because if we talk scientifically of a ‘theory’, as how we think the world is working, we may be heard by others as if we are just making these wild ideas up and, a few years down the line, a new theory will take this one’s place. Indeed, some scientists have argued that the problem has got so bad, we should just get rid of terms such as ‘theory’ altogether.

There are some words that we do not understand or deliberately use in ambiguous ways. There are however many words that we use which do not mean the same thing to another group of people. (Sign at White Mulberries, 2015)

It is not an issue easily solved by education, because education can imply that one group (which is typically not ‘us’) needs to be informed about the correct meaning of the word. Indeed, the issue is not that one group does not understand the correct meaning, the issue is that we are using different languages while utilising the same words. Another word that demonstrates this point is ‘strength’*. The dictionary has ‘strength’ as ‘being strong’ and strong as “having power of resistance, not easily broken or torn or worn…. tough, healthy, firm, solid….” This comes close to how the word may be used in a scientific context as ‘tensile strength’, which is the amount of load (force) that a material can support without fracture. You can also see how the word could be understood within the world of coffee as the amount of total dissolved solids in a particular brew. Nonetheless, both of these are different from how the word is used in English and can be applied to coffee as being about the degree of roast of a coffee.

If it were just a question of occasional misunderstandings this may be tolerable but once again, things become more complicated as we look deeper. As alluded to with the ‘climate change theory’, it can have consequences for our behaviour: are we likely to make the changes needed if we can convince ourselves, on one level at least, that climate change is just a theory? But with other fields, it can also have an effect on our emotional response to a story. The term ‘migrant’ and ‘migration’ refers only to movement of persons within geography. The term can apply to international movement or even to movement within a country or region. Importantly, within a geographical context, the term is “value neutral”; it is merely a descriptive term. We do not have to look too far in the reporting around us to find that the word ‘migrant’ in particular is not taken to be value neutral within common usage. This is a difference in usage that could have profound political and ethical repercussions.

Jonathan's coffee house plaque
The site of Jonathan’s in Exchange Alley. Where are our modern equivalents? Places where we can meet to encounter and listen to each other?

So if education is not the answer what could be? Perhaps it was unfair to rule out education so quickly, it depends on how we understand the word education itself. If we understand it purely as being about communicating to somebody, we won’t get very far. If however we understand education to be a flow of knowledge, in both directions, and communication as not being ‘communicating to’ but ‘listening with’, then we can start to speak and understand each other’s language more fluently. We need to regain a forum in which we can really learn from each other and hear what the other is saying. And then, for them to hear us too: we need an encounter, a dialogue, a conversation. Perhaps we need a return of the coffee houses, or even the Salons of old, or failing those, a new way of encountering the other on social media. How can we encounter each other in 280 characters? How do you encounter others?

*WIth thanks to Amoret Coffee for suggesting this one over on Twitter (and to all the other people who contributed to that Twitter discussion for the many fascinating and thought provoking suggestions of such problematic words).

Is nature even handed?

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?

These Ritzenhoff mugs each have a different character, but they also each show a handedness. The face would appear to the drinker if picked up with the right hand or to the viewer if picked up with the left.

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.

We encounter polarised light all of the time, although we may not necessarily realise it. The reflections of light off the surface of a cup of coffee are partially polarised, and if viewed above a certain angle, known as the Brewster angle, the polarisation is completely in the plane of the reflection. The same is true of reflections generally, while the light scattering caused by the effect that makes the sky appear blue also polarises the (otherwise unpolarised) sunlight. Perhaps for reasons such as these, Sir Lawrence Bragg in one of his lectures to the Royal Institution said “I’ve always found it useful to carry round a piece of polaroid with me”. A life lesson that I fully intend to take on board.

When light is reflected from a surface, including from the surface of a cup of coffee, the reflected light is partially polarised.

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.

If you imagine each of these circles as representing different atoms, these two molecules are not quite the same. Though they are the same compositionally, one is the mirror image of the other, they are enantiomers.

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.

What does this mean for coffee? Does coffee contain any handedness? The chemistry of coffee is complex, with up to 900 volatile aromatic compounds and then further chemicals dissolved within the brew. We can get an answer to the question though by just looking at some of the main compounds in coffee: caffeine, the various thiols that create the aroma and substances such as caffeic acid that contribute to the flavour. Caffeine itself has no chiral centre, meaning it is even handed however the same is not true of the thiols nor necessarily the acids, both of which can contain some degree of chirality or handedness. For the case of the aromatic thiols, this may even be important as we do not seem to sense the two types of molecule in the same way. Handedness matters. Some researchers have even looked at how roasting affects the amount of different enantiomers in robusta and arabica coffee. All of which shows that, just as our own coffee mugs reflect our handedness in zoom calls, so too the coffee has a handedness when it interacts with light.

Now who thought that coffee was balanced?

From a latte to a mangrove swamp

Latte art scutoid tulip
Pretty to look at as well as preventing coffee spill. What is there not to like about latte art?

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?

Personally I prefer drinking a pour over coffee without milk and in a proper cup. But if you are going to take-away, maybe you need to order a cappuccino instead.

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.

Thames, Canary Wharf
The shore of the Thames at low tide. How does the coastline affect the wave dynamics of our water ways? What concentration of plastic bottles and littered take-away cups would result in an alteration of the wave dynamics at the shore line?

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.

There is perhaps one last point to notice before we finish our coffee. There are regions of the ocean that now contain hundreds of square kilometres of floating plastic waste. Even close to our own shorelines and in our river network, plastic waste litters the water. What effect (if any) are these having on the wave dynamics at sea and in our rivers? One more thing to ponder as we carefully walk along sipping our take-away.

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.