The Dark Arts at Amar, Chelsea Green

Amar Cafe, Drinking coffee on Chelsea Green, Colombian Coffee
Amar Cafe on Chelsea Green. The small terrace area outside is on the spot of two car parking spaces.

Amar Cafe means to love coffee. Though this is in Spanish. In Bengali it apparently means “my cafe”. This could perhaps lead to a short meander onto a language inspired thinking trail about how a cafe that you love to frequent becomes “yours” in a certain sense. Though to return to the coffee, the cafe itself is a small space located on Chelsea Green. There are other branches of Amar cafe in telephone boxes around London and in Stratford upon Avon. A couple of parking spots just outside the cafe have been converted to an outside terrace complete with small olive trees at which you can enjoy your coffee in the fresh air of the Green. Although this is clearly a Covid-19 related temporary measure, it would be good if some of these outside places can remain on a more permanent basis. They do add to the character of the Green. Amar cafe specialises in Colombian coffees that they source themselves. All of the usual espresso based drinks are available as well as the option of a pour over. A small selection of pastries including empanadas are available for breakfast. There are about four tables outside and a couple of tables (along with a window bench) inside. As you enter the cafe, the bar is immediately in front of you. Outside, the cafe is painted a bright yellow colour which makes it stand out among the independent shops that are in this little quarter of Chelsea.

On the two occasions that we have visited, I have enjoyed a really well made pour over each time. Although I am not good at generating my own tasting notes, I would say that the coffee was sweet and syrupy, with a fruitiness and complexity that was very enjoyable. It was presented in a V60 jug together with a black, handle-less, porcelain cup.

V60 at Amar Cafe, Chelsea
Carafe of coffee and cup. The blackness of the coffee is similar to the blackness of the cup. On the carafe, condensation has formed on one side only. How did that happen?

Gazing into the filter coffee, there were patterns on the surface of the coffee that you could see reflected at different angles. But looking more deeply, it looked black, within a black porcelain cup. Where did the cup end and the coffee begin? How could you see black on black? Which was blacker?

A material appears black to us when it absorbs the majority of the visible light incident on it and doesn’t reflect or emit any visible light back. Until 2019, the world’s blackest material was “Ventablack”, but even this material only absorbed 99.96% of the light shining on it. Late in 2019, a new material was discovered that absorbs 99.995% of light shone onto it. And not just that, it absorbs 99.995% of light from the ultraviolet to the THz (between infrared and microwave). The material is truly black. But the inventors of this material were not trying to make a black material, they weren’t even really interested in optics. The invention came courtesy of a collaboration with an artist.

At the time, Diemet Strebe was the artist in residence at MIT. The scientists at MIT who would go on to discover this new black material, were interested in the electrical properties of carbon nano tubes (CNTs). CNTs are a layer of carbon atoms (arranged in the hexagonal structure familiar for layers of graphite) wrapped into a tube. Each tube may be just a few nanometers diameter but they can grow hundreds of micrometers long. Depending on how they are wound into a tube, CNTs can have a very low electrical resistance. But this electrical advantage is lost when you try to attach them to a metal like aluminium because the surface of aluminium always oxidises. Unlike aluminium, aluminium oxide is a brilliant electrical insulator. Which is great if you want to study effects in which the electrical current is blocked, but terrible if you want to utilise these fantastically conducting CNTs. This was the problem that the scientists at MIT wanted to solve. Their solution was to remove the oxide using salt water and then deposit the CNTs on top. (When phrased like this it sounds such a simple idea, “why did no one do this before”, but there are many experimental steps needed to be able to grow CNTs onto substrates such as aluminium and it has taken many years to get to this point). Once the CNTs were deposited, the authors found that not only did they have a good electrical conductor, the material was really black.

Coffee love. Some evidence of foam ripening at the surface of an oat milk latte.

What happened next is where the art comes in. Professor Brian Wardle who led the study was quoted as saying “Our group does not usually focus on optical properties of materials, but this work was going on at the same time as our art-science collaborations with Diemet. So art influenced science in this case.”

Thinking about how black the material was, the team decided to measure its optical absorption, which is when they discovered that they had broken the record previously held by Ventablack. And it was then that the art came back in. Strebe took a $2m natural yellow diamond and covered it with this ultra-black material. The result is striking (link). The composition, called “The Redemption of Vanity” could cause us to pause and ponder what we value as a society. What makes a sparkling diamond so valuable? How do we start to see objects by the fact that we can’t see them at all? If we extend this contemplation to our surroundings of Chelsea Green, we may wonder at this small little triangle of grass with its couple of benches. What does it reveal or hide? Does it help us to know that this is the last remnant of Chelsea Heath*, a bit of common land in which occupants of the surrounding manor houses down on (what is now) Cheyne Walk had the right to graze their livestock. Throughout this green, cows wandered up from the Thames as recently as the seventeenth century. A part of London that has disappeared, obscured by modern buildings yet held in memory by the street names and, the names of housing blocks.

As for why this material absorbs so much light, it is still an open question. It is known that the arrangement of the CNTs (including their alignment) can make a material coated in them very black. Even Ventablack is made from CNTs. It is a question that will probably continue to be discussed over many coffees. But is it a question that we would be asking again at all if it weren’t for the interaction in this case of art and science? Another point of contemplation that we can enjoy while looking into our coffee and just wondering ‘why’?

Amar Cafe is on Chelsea Green, 15 Cale Street, London, SW3 3QS and at several other telephone box locations.

*London Encyclopaedia, 3rd edition, Weinreb, Hibbert, Keay and Keay

Under pressure

What do you notice about this iced latte? The cup is rich with physics, but for this post, the important bit is the floating ice on top.

A coffee should be a time for relaxation, for reflection. As we come to the end of summer here in the northern hemisphere, we may want to enjoy one last iced coffee before we return to the warming coffees of winter. If on the other hand you are reading this from the southern hemisphere, the equatorial region, or some time after it was originally posted, you may be just starting to enjoy your iced coffees again. Either way, ice is remarkable and it is good to make some time to enjoy it. One of the things that makes it remarkable is what seems to be its very ordinariness: it floats.

Ice floats because the solid form of water is less dense than the liquid form. This is actually fairly unique to water. Most liquids get more dense as you cool them. As they transform into solids, they get denser still. This would mean that if you were to cool a liquid until it starts to solidify, the solid would sink, not float, on the liquid. If water were like most other liquids, all the ice in our iced-coffee would be at the bottom, not jiggling at the top. In addition to what would be an almost aesthetic problem for the coffee, this has consequences for life itself. When a lake or a pond freezes over, the fish and other aquatic life, can survive under the ice in the denser water. This odd property of water has helped life to evolve.

The reason for this strange behaviour lies in the way that water molecules bond together. Each water molecule can bond to a neighbour through a hydrogen bond. This optimises the structure to a layered form of well spaced hexagons (link here for an interactive model of water ice). Each corner of the hexagon is an oxygen atom. The size of the hexagon means that, if they weren’t arranged into a regular lattice, the water molecules could get closer together than they do in the solid phase. Which is another way of saying that the liquid can get more dense than the solid. Ice will float on water.

The layered structure of the ice crystals also means that each hexagonal face will tend to glide over the one below it or above it. It is this property of ice that means that we can determine the direction of glacial flow in centuries past. When fresh snow falls on top of a glacier, the density of the snow layer is about 50-70 Kg/m3. For comparison, the density of water is 1000 Kg/m3. Although each snow crystal is hexagonal, they have random orientation as they fall. As new snow falls, it pushes down on the old snow and compacts it until, about 80m down into the glacier, the density of the (now) ice is 830 Kg/m3. As the depth increases still further, the density increases to 917 Kg/m3 which is as dense as a glacier can be but is still much less than the density of water; a glacier would float. When the snow crystals are pressed down, the hexagonal layers of ice will glide past each other in the direction of push and the crystals will re-orienate. They will also grow as they merge with other crystals and as a result of the heat from the bedrock beneath them. This means that deep in the glacier, more of the crystals will be orientated in the direction of the push. Taking a vertical core of ice and looking at the orientation of the crystals in 0.5mm thick cross sections therefore reveals how they have been pushed as a function of depth. This in turn reveals which way the glacier has flowed in the past.

Sun-dog, Sun dog
A ‘rainbow’ of colour as seen in a ‘sun dog’ observed in central London. Note the order of the colours.

The structure of ice has one other surprise for those of us who are enjoying more coffee outside. Depending on the weather conditions, high up in the atmosphere, hexagonal ice crystals form. Because they are hexagons, they are, in effect, a section of a 60 degree prism. This means that light entering through one face, will be refracted twice to emerge from the crystal at 22 degrees relative to where it came from. If there are enough of these crystals high in the atmosphere, a bright circle will form around the Sun. For reasons that are probably obvious, it is known as the 22 degree halo. It seems fairly difficult to observe this halo. What is far more common to see are two bright regions of light at the 9 o’clock and 3 o’clock positions on the halo. In addition to being brighter than the rest of the light circle, these two regions often appear like a ‘rainbow’, but with the red on the inside of the halo and the blue on the outer edge. Known as “sun dogs” or parhelia, they too are a consequence of the ice crystals. As the ice crystals fall, they are more likely to fall flat so that each hexagonal face is horizontal. More of these ice crystals means that there is going to be more light refracted at the position horizontal to the Sun and so the light there is intense. They appear as separated colours for the same reason that the colours disperse with a prism: each wavelength of light has a very slightly different refractive index and so gets ‘bent’ by a slightly different amount. The ice crystals are bending the red a bit less than the blue.

This is a good time of year to keep an eye out for Sun dogs and haloes. And if we can do so while enjoying a well made iced coffee with the ice cubes floating at the surface, all the better.

Please do share any photographs you have of coffee with 22 degree haloes or sun dogs, either here or on Facebook or Twitter.

A return to Pritchard & Ure

A view from the terrace at Pritchard & Ure, overlooking the garden centre.

It is always great to realise that we have enough time to head across town to enjoy a coffee at Pritchard & Ure. If you haven’t yet tried it, Pritchard & Ure is a lovely spot in Camden Garden Centre (near Camden Road overground station). I first visited back in 2018 and ordinarily, I would not do a second cafe-physics review. But then 2020-21 have not been ordinary either and Pritchard & Ure too has changed. Back in 2018, a swaying pendulum prompted thoughts on how we knew that the Earth rotates. Since then, the world has moved in a different way.

In the case of Pritchard & Ure, this is reflected in a definite physical change to the cafe: a new terrace has been built overlooking a semi-outside section of the garden centre. This bit of the garden centre is sheltered from the rain by a permanent roof, almost like a permanent umbrella (see picture). The cafe on the other hand is protected from light rain and wind by a series of garden umbrellas. Apparently the indoor section of the cafe remains open if the weather becomes too awful (or presumably in autumn/winter). But in these times when it is good to be able to socialise outside, the new terrace offers a perfect place to do it. Accordingly, I took the opportunity to have an oat milk latte. While black coffee is normally a good test of the coffee in a cafe, I knew Pritchard & Ure served great coffee from my previous visit. Roasted by Workshop, the coffee is still offered in either a 6oz or 8oz size. But it’s been a while since I had enjoyed a properly made latte in a cafe and so why resist? We also enjoyed a spot of brunch, all while admiring the number of plants (and cacti) on view.

Can there be too much physics in one picture? Let me know what you see.

As before, obvious thought trains went in the direction of the science of plants and ecology. The large number of cacti just below our table was particularly suggestive of the changing conditions of our planet and the tendency for some areas of our world to be subject to more drought. The flowering plants too could prompt reflections on insects and how climate change is affecting them, including the possibility of mass extinctions. The past couple of weeks have seen Extinction Rebellion back in London as we prepare for COP26. One action that they took was an occupation of the Science Museum. The museum was targeted because Shell sponsor some of the exhibits including the “Our Future” exhibit about climate change. Extinction Rebellion have written an open letter to the Science Museum arguing, amongst other things that Shell gains “prestige and implied endorsement by the Science Museum group”. This is despite Shell’s own business plans not being “in line with limiting warming to 2C“. The museum disagrees with the principle of boycotting sponsorship by Shell on the grounds that such companies have the “capital, geography, people and logistics” needed in order to fight climate change. They also argue that some of these exhibits which help to inform the public about issues such as the science around climate change are only possible because of the financial muscle of companies such as Shell. It is a tough ethical cookie. One where we may have to try to read about the arguments and yet withhold judgement, knowing that most of us do not know enough, or have not thought deeply enough, to comment authoritatively.

The canal system built during the eighteenth and early nineteenth century required significant engineering expertise. This is a view from inside a loch on a canal within the M25 that surrounds London as the water fills through the gates, showing the loch gates and the walls of the canal.

A somewhat similar issue concerns the site of the garden centre itself. At the beginning of the 19th Century, the land belonged to William Agar (hence Agar Grove just north of the garden centre). Agar himself lived in Elm Lodge which was approximately where Barker Drive is now. He was involved in a dispute with the Regents Canal Company. He did not want the new canal to cut through his land. Finally, at the end of 1817 he relented and now, the canal cuts NW to SE just west of Pritchard & Ure. Was Agar a NIMBY (not in my backyard) or was his objection more complex? It’s another issue on which we have to suspend judgement. Though maybe this is easier to do as the case is over two hundred years old. Would we be so balanced if the Regents Canal were being built now and we wanted to react quickly on Twitter? What if the Regents Canal were HS2?

A more physics-based issue of balance could be seen in the umbrellas arranged over the terrace. They were supported not centrally but from the side, so the umbrella could be easily placed above the tables without the supports getting in the way. Immediately we could make connections to counterbalances and cranes. How is it physically possible that such a weight can be held by an outstretched (mechanical) arm? The weights of the flower pots standing on the umbrella bases may give us a clue.

There were many opportunities to think about issues of physics or balance on this terrace. It was a reminder of how good it is to go to a different cafe, put aside the smart phone, and just sit, enjoy a well made coffee and ponder about any subject that strikes your mind. Pritchard & Ure is a perfect place to do this, it remains a friendly space with good coffee (and food) at which you can enjoy thinking. And now, with the outside terrace, there is even more reason to go there as it is rare to find a cafe close-ish to central London with a large outdoor, and socially distanced, seating space.

Pritchard & Ure is at 2 Barker Drive, NW1 0JW

Activated Roasting

Brazil nut effect
Transforming green beans into the coffee we all recognise. Maillard reactions are behind some of the chemistry involved in coffee roasting. But how can we determine how fast a reaction will occur?

Coffee roasting is a complex process involving chemistry, physics and art. The experience and skill of the roaster turns the unpromising looking green beans into fragrant coffee beans that we can appreciate. Activated by the heat, many chemical processes occur as the aromatic volatiles are formed, compounds in the bean are transformed and the bean changes colour to that deep brown appearance with the smell that we associate with coffee. One of these processes are the Maillard reactions.

Maillard reactions transform “reducing sugars” such as glucose and fructose into the browning melanoidins (via a couple of intermediary steps). They are responsible not just for the colour and aroma of coffee, but also for the crust of a freshly baked loaf of bread, the transformation of a steak or just browned (not caramelised) onions and all manner of culinary processes. In coffee, the Maillard reactions usually start to become noticeable above 140C. At higher temperatures you also get caramelisation. But even at room temperature, or at body temperature, some Maillard reactions occur, just very slowly. Maillard reactions have even been implicated in the formation of certain cataracts. What is it that determines how fast the Maillard reactions occur?

The rate at which a chemical reaction takes place is determined by an energy known as the activation energy. The activation energy is the energy that the molecules would need to overcome in order to react together. It may be the result of having to overcome a repulsion between the molecules getting close together, or it may describe an energy needed to transfer electrons from one chemical to the next. Molecules can gain this energy from heat which means that at higher temperatures, more molecules have the energy for the reactions to occur. We could rephrase this to say that the rate of the reaction is greater at higher temperatures. This is expressed mathematically with the Arrhenius relation. In a fantastic illustration of the connectedness of things, this same Arrhenius relation can be used to describe many other phenomena such as how fast water evaporates from a coffee cup, how quickly milk goes off and even how long semiconducting devices will last before failing.

The Arrhenius equation also describes how quickly steam will evaporate from a coffee cup. As you can see above the cup here at Carbon Kopi

Although the reactions are faster at higher temperatures, there is no defined temperature below which they stop. Instead, the rate just decreases to such a point that the reactions happen rarely. Perhaps you could observe some of the chemical changes of roasting coffee at room temperature if you waited long enough. But before that point, other reactions with lower activation energies would occur or fungal growth may happen that would turn the beans rancid. Best to follow the roasting recipes.

Yet for coffee there is an additional complication before the Maillard reactions can happen. Unlike the situation where all the chemicals are together and able to react, the chemicals in the coffee bean exist within a structure. The molecules are not necessarily in the same place as each other; they need to move across the bean, including perhaps through the cell walls. And as the bean is heated, there are structural transitions that make it easier (in some cases) and harder (in others) for the chemicals to meet each other in order to react. What exactly happens when coffee is roasted?

To track what was going on Loong-Tak Lim and colleagues at the University of Guelph looked at how parameters such as the lightness of the roast or the weight of the bean varied as a function of roasting time. They roasted a lot of (small batch) coffee. Impressively, they also managed to put a thermometer right into the middle of a green coffee bean to track the temperature of the interior of the bean rather than the atmosphere in the roaster. The unfortunate detail was that they had to glue the thermometer in place.

Roasting coffee at four temperatures (220, 230, 240 and 250C), they showed how the degree of roast (indicated by the lightness of the bean) varied with roasting time and temperature. Unsurprisingly, a higher roast temperature produced a darker roast more quickly. But there were surprises too.

When they plotted the lightness of the roast as a function of time, they saw not one reaction with one activation energy but two. The two regions were quite distinct indicating that something chemically significant happened to the roasting process at around the point indicated by a “medium” roast. The activation energy of the first stage was 59.7 kJ/mol while the second stage had an activation energy of 170.2 kJ/mol. Whereas the first stage was over pretty quickly, the higher activation energy of the second stage meant that it happened far more slowly.

Don’t they look great? Roasting coffee connects to a vast range of concepts in physics and chemistry. Perhaps now is just a time to appreciate them.

The same sort of two step process was seen when they looked at how much mass the bean was losing as it was roasted. A lot of mass was lost early in the roast but as the roast degree went on, so the reaction slowed.

What caused the rapid slowing down of the second stage? One of the suggestions was that it was associated with the moisture loss as the green beans dried. A second suggestion was that a structural transition in the bean (of which there are many at these temperatures) hindered the reaction dynamics. This highlights a difference between coffee roasting in the lab and in the cafe. In the lab, the beans were rapidly heated to a set temperature at which they were held until the end of the experimental roasting time. In contrast, to produce great tasting coffee, many roasters will tweak the temperature-time profile of the roast so that a lot of the drying occurs before the Maillard reactions are allowed to ramp up. In a sense, the science is behind the experience here. To find out what is going on most parameters have to be kept constant while only one or two are varied. It doesn’t make great coffee but, hopefully, it is the start of a journey to understanding what is really happening as the beans seem to magically transform into something we can drink.

Meanwhile for those of us who neither roast nor experiment with the coffee but rather just enjoy the results of other people’s work, we can admire the connections that are being illustrated through working out what exactly happens as we roast the coffee. From the vastly disparate subjects covered by the Arrhenius equation, to the fact that the structural transitions that affect the coffee roast also occur in ceramics and magnetic materials. You will often hear it said that “everything is connected”. For coffee at least, this is yet another case where that appears to be true.


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.