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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

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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.

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General Home experiments Observations

A short (lived) black

coffee at Story
A black coffee with bubbles on top. The colours on a bubble are the result of light interference. But sometimes the top of the bubble could appear black. What is happening there?

The long black can be distinguished from the Americano by the order in which the espresso and the water are added to the cup. This in turn will affect the type of bubbles on the surface of the coffee. As a guess, the long black (espresso last) will have many more but smaller bubbles than the Americano (water last) which will probably have larger, but fewer bubbles. Perhaps this guess is wrong, this could be an excuse to get out and drink more coffee.

We are used to the coffee being black and the bubbles on the surface reflecting a rainbow of shimmering colours that change with the light and with time before they finally burst. We know the physics of the colours on the bubbles: they are the result of the interference of reflections from the outer and inner surface of the bubble cancelling out certain colours and adding to others dependent on the bubble skin’s thickness. But what about black bubbles? Or, if not entirely black, perhaps the cap of the bubble can, for a short while, appear black just before the bubble bursts?

It is easier to take a short break from coffee and look for this effect in soap films. Like the bubbles on coffee, soap bubbles are caused by the surfactant in the soap solution having a hydrophilic (water loving) and hydrophobic (water hating) end. The hydrophilic end of the surfactant can point into the water (coffee) leaving the hydrophobic end to form a surface. When this is agitated with air, the hydrophilic ends remain contacted with water resulting in bubbles which are thin layers of water surrounded by these surfactant molecules. In coffee the surfactant is not soap but is formed by the lipids and fatty acids. These bubbles are therefore slightly weaker than the soap based bubbles and so while they will form on a coffee, it is not easy to make a film of a coffee bubble in the same way as you can dip a wire loop into a soap solution and come out with a soap film.

However, we can use the stability of the soap film to investigate the colours in the coffee bubbles and watch the colours evolve with time. At this point, I would strongly encourage anyone reading to grab a solution of soap and a wire loop and start playing with soap films.

Soap film in a wire loop held by a crocodile clip.
A soap film in a wire loop showing reflected horizontal coloured bands that are the result of light interference.

Holding the wire loop so that the soap film is vertical with a light source shining at it, we can watch as the film changes from being uniformly transparent to having bands of colour form and move down the film. We watch as there is a red/green band and another red/green band and then on top of the bands there appears a white, or at least pale blue, almost white, band and above that a layer that doesn’t reflect the light at all. If we view the soap film against a dark background looking only at the reflected light, this top portion of the film appears black. Rotating the loop we can see that the bands effectively stay in the same position because it is gravity pulling on this soap film that is causing the film to be thicker at the bottom than at the top. And we recognise that the coloured bands are revealing that thickness change to us by the fact that they are changing throughout the film. If we are careful as we rotate the wire, we could even see vortex like motions as the layers settle into their new position relative to the frame including at the very top where there are swirls and patches of fluid that mix the black layer with the coloured bands. What is going on there?

In fact, this black layer is one of the thinnest things that they human eye can see, and it occurs because of a subtle piece of physics. All waves have a number of properties defined by the position of the peaks and troughs on the wave. The wavelength is the distance between two equivalent points on the wave. The amplitude is the height of the peak (or trough). And the phase is the position of the wave relative to the peak (or trough). When light is reflected at a surface of a material that has a refractive index greater than that which the light is travelling through (eg. air into water, soap, or glass), the reflected wave has a 180 degree phase shift relative to the incident wave. Each peak becomes a trough, each trough becomes a peak. When light is already travelling through water, soap or glass and gets reflected at the surface of the material that is effectively air, there is no phase shift and the light is reflected back with the same phase as the incident wave (a peak remains a peak and a trough a trough).

At the top of the soap film, the layer is so thin that the light reflected from the first surface (180 degree shift) overlays that reflected from the back surface (no phase shift) so that peak and trough cancel each other out and we see no light reflected whatsoever for any visible wavelength; the surface looks black.

As bubbles ‘ripen’ or age, they will become thinner at the top of the bubble. It is at this point that you may be lucky enough to see a region of the bubble from which no light is reflected, this is the black film.

Which leads to some immediate questions. When we look carefully at the soap film, the boundary between the upper white band and the black film is quite sharp, it is not gradual as we may expect if the soap film were completely wedge shaped with gravity. It suggests that the top of the film is very thin and then suddenly gets thicker at the point where we start to see the colour bands. Moreover, the black film does not appear to mix with the thicker film just beneath it. As we watch, just before the soap film bursts, we get turbulence between the black layer and the thicker film, but the turbulent patterns appear like two fluids next to each other, not the same fluid in a continuum. And then, one final question. If we can’t measure the thickness of the black film with light (because it is all reflected as black) how can we know how thick this film is? If we rely on the light interference method, all we can say is how much thinner it is than the wavelength of light.

In fact, careful experiments have revealed two types of black film, which to us experimenting at the kitchen table would be indistinguishable. There is the common black film and the Newton black film. The Newton black film is effectively two layers of surfactant molecules only and is about 5nm thick (which is 5 millionths of a millimetre). The common black film is thicker, but is still much less than 100 nm thick. Investigating how these films behave is still an active area of research.

One last observation may prompt us to play for a bit longer with the soap films. Johann Gottlob Leidenfrost (1715-94) noted that if you put a sharp object such as a needle through the region of the soap film that showed the coloured bands, the film could self-heal and wouldn’t necessarily burst. If however you pierced the black region of the film, the film always burst entirely.

It seems that we could play endlessly with soap films, perhaps while watching the bubbles in our coffee. However you enjoy your coffee, have fun experimenting.

A couple more soap films showing reflected coloured interference bands. At the top, the film has become so thin that no light is reflected (clearly seen in the image on the right, where the lamp in the top left should be a circular reflection but is not reflected in the region above the coloured bands). In the image on the left, you can see what looks like turbulence or mixing just above the uppermost band.
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Home experiments Observations Science history

To err is human…

Press Room coffee Twickenham
A smaller V60. For one cup you would use less coffee, but the errors on the measurement will always be there.

Preparing a good V60 requires 30g of coffee (for 500 ml of water)*. This can be measured using a set of kitchen scales, but a first estimate can also be obtained, if you are using whole coffee beans, by timing the passage of the grind through the grinder. Using an Ascaso burr grinder, my coffee used to come through at an approximate rate of 1g/s, so that, after 30 seconds, I’d have the perfect amount of coffee. Recently however this has changed, depending on the bean, sometimes 30g is 40 seconds, sometimes just less than 30 seconds.

Clearly there is an error on my estimate of the rate of coffee grinds going through the grinder. This may be influenced by factors such as the hardness of the bean (itself influenced by the degree of roast), the temperature of the kitchen, the cleanliness of the grinder and, the small detail that the ‘seconds’ measured here refers to my counting to 30 in my head. Nonetheless, the error is significant enough that I need to confirm the measurement with the kitchen scales. But are the scales free of error?

Clearly in asking the question, we know the answer will be ‘no’. Errors could be introduced by improper zero-ing of the scales (which is correct-able), or differences in the day to day temperature of the kitchen (not so correct-able). The scales will also have a tolerance on them meaning that the measured mass is, for example, only correct to +/- 5 % Depending on your scales, they may also only display the mass to the nearest gramme. This means that 29.6g of coffee would be the same, according to the scales, as 30.4g of coffee. Which in turn means that we should be using 493 – 507 ml of water rather than our expected 500 ml (the measurement of which also contains an intrinsic error of course).

Turkish coffee
A Turkish coffee provides a brilliant illustration of the type of particle distribution with depth that Jean Perrin used to measure Avogadro’s constant. For more information see here.

The point of all of this is that errors are an inescapable aspect of experimental science. They can also be an incredibly helpful part. Back in 1910, Jean Perrin used a phenomenon that you can see in your coffee cup in order to measure Avogadro’s constant (the number of molecules in a mole of material). Although he used varnish suspended in water rather than coffee, he was able to experimentally verify a theory that liquids were made up of molecules, by the fact that his value for Avogadro’s constant was, within error, the same as that found by other, independent, techniques. Errors also give us an indication of how confident we can be in our determination of a value. For example, if the mass of my coffee is 30 +/- 0.4 g, I am more confident that the value is approximately 30 g than if the error was +/- 10 g. In the latter case, I would get new scales.

But errors can also help us in more subtle ways. Experimental results can be fairly easily faked, but it turns out that the random error on that data is far harder to invent. A simple example of this was seen in the case of Jan Hendrik Schön and the scientific fraud that was discovered in 2002. Schön had shown fantastic experimental results in the field of organic electronics (electronic devices made of carbon based materials). The problem came when it was shown that some these results, despite being on different materials, were the same right down to the “random” noise on the data. Two data sets were identical even to the point of the errors on them, despite their being measurements of two different things.

A more recent case is a little more subtle but crucial for our understanding of how to treat Covid-19. A large study of Covid-19 patients apparently showed that the drug “Ivermectin” reduced mortality rates enormously and improved patient outcomes. Recently it has been shown that there are serious problems with some of the data in the paper, including the fact that some of the patient records have been duplicated and the paper has now been withdrawn due to “ethical considerations”. A good summary of the problems can be found in this Guardian article. However, some of the more worrying problems were a little deeper in the maths behind the data. There were sets of data where supposedly random variables were identical across several patients which suggested “that ranges of cells or even entire rows of data have been copied and pasted“. There were also cases where 82% of a supposedly random variable ended in the digits 2-5. The likelihood of this occurring for random variables can be calculated (it is not very high). Indeed, analysis of the paper showed that it was likely that these values too were either copy and pasted or “invented” because humans are not terribly good at generating properly random numbers.

A gratuitous image of some interesting physics in a V60. If anyone would like to hire a physicist for a cafe, in a 21st century (physics) recreation of de Moivre’s antics at Old Slaughters, you know how to contact me…

Interestingly, a further problem both for the Ivermectin study and for the Schön data comes when you look at the standard deviation of the data. Standard deviation is a measure of how variable is the measured outcome (e.g. duration of time a patient spent in hospital). For the ivermectin study, analysis of the standard deviations quoted on the patient data indicated a peculiar distribution of the length of hospital stay, which, in itself would probably just be a puzzle but in combination with the other problems in the paper becomes a suggestion of scientific fraud. In Schön’s data on the other hand, it was calculated that the precision given in the papers would have required thousands of measurements. In the field in which Schön worked this would have been a physical impossibility and so again, suggestive of fraud. In both cases, it is by looking at the smaller errors that we find a bigger error.

This last detail would have been appreciated by Abraham de Moivre, (1667-1754). As a mathematician, de Moivre was known for his work with probability distribution, which is the mathematics behind the standard deviation of a data set. He was also a well known regular (the ‘resident’ mathematician) at Old Slaughters Coffee House on St Martin’s Lane in London[1]. It is recorded that between 1750 and 1754, de Moivre earned “a pittance” at Old Slaughters providing solutions to games of chance to people who came along for the coffee. I wonder if there are any opportunities in contemporary London cafes for a resident physicist? I may be able to recommend one.

*You can find recipes suggesting this dosage here or here. Some recipes recommend a slightly stronger coffee amount, personally, I prefer a slightly weaker dosage. You will need to experiment to find your preferred value.

[1] “London Coffee Houses”, Bryant Lillywhite, 1963

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General Home experiments Observations Science history

Up in the air with a Pure Over Brewer

The diffuser sitting on top of the Pure Over coffee brewer. The holes are to ensure that the water falls evenly and slowly onto the grounds below.

The Pure Over is a new type of coffee brewer that is designed to brew filter coffee without the need for disposable paper filters. The brewer, which is completely made of glass, is a perfect size for brewing one cup of coffee and, as promised, makes a lovely cup without the need for wasteful paper filters. Generally, for 1-cup filter coffees, the Pure Over has become my go-to brewing method, although it does have a few idiosyncrasies to it that are helpful to be aware of while brewing.

An advantage of this brewing device is that it provides a large number of opportunities for physics-watching, including a peculiar effect that connects brewing coffee to an air balloon crash into the garden of a London Coffee House. It concerns a feature of the Pure Over that is specific to this particular brewing device: the ‘diffuser’ that sits on top of it.

The glass diffuser has five small holes at the bottom of it which are designed to reduce the flow of the water onto the coffee bed so that it is slower and more gentle. In order to avoid the paper filters, the Pure Over features a filter made of holes in the glass at its base. This filter does surprisingly well at keeping the coffee grounds out of the final brew, but it works best if the coffee bed just above it is not continuously agitated. The idea of the diffuser is that the coffee grounds are more evenly exposed to the water, with the grounds closest to the filter being least disturbed and so the coffee is extracted properly.

As water is poured from a kettle through the diffuser, the water builds up in the diffuser forming a pool that slowly trickles through the holes. Initially this process proceeds steadily, the water is poured from the kettle into the diffuser and then gently flows through and lands on the coffee. At one point however, the pressure of the steam within the main body of the brewer builds until it is enough to push the glass diffuser up a bit, the steam escapes and the diffuser ‘clunks’ back onto its base on top of the pure over. Then, this happens again, and again, until there is a continuous rattle as the steam pressure builds, escapes and builds once more.

The ideal gas laws, such as that found by Jacques Charles, relate the volume and pressure of a gas to its temperature. The application of the laws helped to improve the design of steam engines such as this Aveling and Porter Steam Roller that has been preserved in central Kuala Lumpur, Malaysia.

The pressure of the steam builds until the force exerted upwards by the rising steam is greater than the weight of gravity pulling the diffuser down. Once enough gas escapes, the pressure is reduced and so the steam no longer keeps the diffuser aloft which consequently drops with a clunk. The motion could take our thoughts to pistons, steam engines and the way that this steam movement was once exploited to drive our industrial revolution. Or you could go one stage earlier, and think about the gas laws that were being developed shortly before. There’s Boyle’s Law which relates the pressure of a gas to its volume (at constant temperature). That would perhaps partially explain the behaviour of the pure over. But then there’s also Jacques Charles and his observation that the volume of a gas is proportional to its temperature (at constant pressure). This too has relevance for the pure over because as we pour more water in from the kettle, we warm the entire pure-over body and so the temperature of the gas inside will increase. Consequently, as the amount of hot water in the pure over increases, the temperature goes up, the volume of that gas would increase but is stopped by the diffuser acting as a lid. This leads to the pressure of the gas increasing (Boyle) until the force upwards is high enough, the diffuser lid rises upwards on the steam which escapes leading the pressure to once again drop and the diffuser top to go clunk and the whole cycle begins again.

Of course, we know that Boyle’s law is appropriate for constant temperature and Charles’s law is appropriate for constant pressure and so the laws are combined together with the Gay-Lussac/Amonton law into the ideal gas laws which explain all manner of things from cooling aerosols to steam engine pistons. And yet, they have another connection, which also links back to our pure over, which is the history of hot air balloons.

Charles discovered his law in around 1787, a few years after the first non-tethered hot air balloon ascent, in Paris, in June of 1783. The hot air balloon is a good example of the physics that we can see in the pure over. Although Charles must have suspected some of the physics of the hot air balloon in June, he initially decided to invent his own, hydrogen filled balloon which he used to ascend 500 m in December of 1783. Hydrogen achieves its lift because hydrogen is less dense than air at the same temperature. However, it is the hydrogen balloon that links back to coffee and coffee in London.

hot air balloon
The ideal gas laws also contribute to our understanding of the operation of hot air balloons. We are familiar with them now, but how would such an object have been perceived by observers at the time of the first flights?

The first balloon flight in England took place using a hydrogen, not a hot-air, balloon in 1785. The balloon was piloted by Vincenzo Lunardi who was accompanied by a cat, a dog and, for a short while, a pigeon (before it decided to fly away). But it was not this successful flight that connects back to coffee, it was his maiden flight on 13 May 1785. On that day, Lunardi took off from the Honourable Artillery Company grounds in Moorgate, flew for about 20 minutes and then crashed, or as they said at the time “fell with his burst balloon, and was but slightly injured”(1) into the gardens of the Adam and Eve Coffee House on the junction of Hampstead Road and, what is now, Euston Road. In the 1780s the Adam and Eve coffee house had a large garden that was the starting point for walks in the country (in the area now known as Somers Town)(2). Imagine the scene as, quietly appreciating your tea or coffee, a large flying balloon crashes into the garden behind you.

The Adam and Eve is no longer there, in fact, its original location now seems to be the underpass at that busy junction, and the closest coffee house is a branch of Beany Green. However there is one, last coffee connection and it brings us back to the pure over. The pressure of the steam under the diffuser needs to build until the upwards force of the steam can overcome the gravitational force down of the weight of the glass diffuser. In the same way Lunardi had to have enough lift from the hydrogen balloon to compensate for the weight of the balloon and its passengers. Lunardi had wanted to be accompanied by another human on the day of his successful flight. Unfortunately, the mass of two humans in a balloon was too much for the balloon to accommodate which is why, the human was replaced by the dog, the cat and the pigeon.

Which may go some way to illustrate how far the mind can travel while brewing a cup of coffee, particularly with a brew device as full of physics as the Pure Over.

1 London Coffee Houses, Bryant Lillywhite, George Allen and Unwin publishers, 1963

2 The London Encyclopaedia (3rd edition), Weinreb, Hibbert, Keay and Keay, MacMillan, 2008

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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).

Categories
Home experiments Observations

Viewing an eclipse, the coffee way

NASA image of annular eclipse from space
A different perspective? This is the view looking towards Earth of the 2017 Annular solar eclipse over South America. Taken by the EPIC DSCOVR project of NASA.

This week, on Thursday, June 10th, 2021, there will be a solar eclipse. If you are at high latitudes in the Northern Hemisphere including parts of Canada, Greenland and Siberia, you will see a so-called ring of fire as the moon moves in front of the Sun. At lower latitudes the eclipse will be much more partial and in London we are expecting to see 20% of the Sun obscured by the Moon.

You can read more about solar eclipses on other websites such as here or here, on Bean Thinking, we are going to focus on the coffee links to the eclipse.

The first coffee link comes in how to view it. This website suggested a number of ways of viewing the eclipse, one of which was to use a colander. This suggests a perfect adaptation to a view via coffee: the Aeropress filter cap. The idea behind the method is that each of the holes provides a type of pin-hole camera to image the Sun. Knowing roughly where the Sun will be at 10.06am (BST = UTC+1), we can construct a device to hold the aeropress filter cap so that we can see 97 images of the Sun projected onto a piece of paper: 97 images of the Sun to be eclipsed over the following 2hours 18 minutes. The maximum eclipse is around 20% of the solar disc and occurs at approximately 11.15 (although the exact fraction obscured and timing depends on your location). The Aeropress Eclipse viewing device shown in the photo here has an added (smaller) pin hole which should provide a more focussed image of the Sun and so will provide a second way of imaging the eclipse.

A second coffee link comes with thinking about why this particular solar eclipse is not ‘total’ anywhere on earth but is instead described as annular. And to do this, we’ll think about a coffee bean. The amazing visual spectacle of a total solar eclipse occurs because the moon is 400 times smaller than the Sun but is (on average) about 400 times closer to the Earth. So when we think about looking at a coffee bean, held at arms length from our eye (about 60cm), it would totally obscure (eclipse) an object 3.2 m tall, 233.5 m away*.

Eclipse viewer
An aeropress based device for viewing the eclipse. The strings attached to the cardboard flap at the top allow the angle of the aeropress filter cap to be fixed at different points. The camera is at the approximate point where the images will be projected onto paper.

The word “average” though hides an important detail that neither the Moon’s orbit around the Earth, nor the Earth’s orbit around the Sun are completely circular. On the 10th June 2021, the Moon will be two days past its maximum distance (apogee) from the Earth, and while the Sun is also nearly at its maximum distance, the distance ratio will mean that the Moon does not entirely obscure the Sun. Instead, if we return to our coffee bean analogy, it is the equivalent of stretching our arm 2 more centimetres and noticing that the object that was obscured is no longer completely obscured.

This will still make for a fantastic view if you are in Greenland, Siberia or happen to be at the North Pole where you will see a dark disc surrounded by a ring of Sun. For those of us further south, we will only see the Sun partially obscured by the Moon. Nonetheless, such an opportunity in any one particular location doesn’t come super-often (although worldwide there are often several eclipses per year, in London there will only be 42 partial eclipses in this current century). And in London, we have to worry about the weather too. So, if the weather is good for you, why not have a go viewing it, particularly if you adapt a piece of coffee brewing equipment to do so, and post your pictures of the effect here, or to Bean Thinking on Twitter or Facebook.

Finally, the timing of the eclipse is perfect for a mid-morning coffee, though maybe you’ll have to brew with something other than the Aeropress. Have fun.

*These figures have been calculated using a ratio of the size of the Moon to the Sun as 1:400.8 and an average distance of 1:389.2 (calculated from the average values). The distances on June 10 2021 mean that the distance ratio is closer to 1:377

Update to post, the day before (9 June 2021): This is the Aeropress viewing device in action, but 24h before the eclipse. Will the clouds stay away tomorrow?

The Aeropress Eclipse viewer in action. The images of the Sun are projected onto the cardboard behind the filter cap.

Update 10 June 2021: It was cloudy in London and I couldn’t get the Aeropress filter cap method to work in the brief periods of sunshine during the eclipse. Suspect it was a problem with focus-distance/angle/remaining cloud cover at points. However, the smaller pinhole did work (see the blurry image below) and the clouds did mean that there was a natural filter that made a direct photograph possible (see below). Do share your images here if you managed to view it.

Although there were brief periods without cloud, focussing issues etc. meant that I couldn’t get the Aeropress filter cap viewing method to work. Maybe for the next one!
A smaller pinhole did give an image of the Sun being eclipsed (lower blurry bright image)
The fact that it was cloudy did mean however that I could take a photograph of the eclipsed Sun directly. This was at about 11.10am (5 minutes or so before the maximum point of eclipse)
Categories
Uncategorized

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?

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Uncategorized

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.

Categories
Coffee cup science General Observations Science history Sustainability/environmental

Pure Percolation

Pure over boxed
The Pure Over in its box. The glass base is designed with an inbuilt filter, avoiding the need for disposable paper filters but making the physics of percolation unavoidable.

It was entirely appropriate that the first coffee I tried in the Pure Over coffee brewer was the directly traded La Lomita Colombian from Ricardo Canal via Amoret Coffee. Ricardo was a special guest at one of the Coffee and Science evenings we held at Amoret Coffee in Notting Hill (pre-pandemic) where, among other things, he spoke about how he is using Biochar on his coffee farm. Biochar is a porous, charcoal based material that can help to provide the coffee plants with nutrients as well as water, thereby reducing the amount of fertiliser the plants need. To understand how it works, we need to understand a bit about percolation, which of course we also need to understand in order to brew better coffee in the Pure Over. Indeed, there are enough similarities, and an extension to a quirk of how espressos are brewed, that it is worth spending a little more time thinking about this process and the connections revealed as we brew our coffee.

Percolation recurs in many of the brew methods we use for making coffee. The V60, Chemex, Kalita wave, percolators and the espresso itself, all rely at some point on water flowing through a bed of ground coffee. The flavour of the resultant cup is dependent on the amount of coffee surface that the flowing water is exposed to together with the time that it is in contact with the coffee. What this means is that grain size, or the degree to which you grind your coffee, is critical.

Playing with brewing coffee, we know some things by experience. Firstly, frequently, the flow through a coarse grind of coffee will be quite fast (probably too fast to make a good cup). Secondly, we know that for any particular brew method, the more water we pour into the brewer, the faster the water initially comes through. We also know that we can affect the flow rate of water through the coffee if we increase the area of the coffee bed, or decrease its thickness. These observations were quantified into an equation by Henri Darcy in 1856. Darcy’s work had been as an engineer, designing and building public works such as the aqueduct that brought drinking water into the city of Dijon in the 1840s. Darcy received significant recognition at the time for his work including the Légion d’honneur, but it is more for a later set of experiments and particularly for his equation that we remember him today. In the 1850s Darcy was working on the problem of water purification. Passing water through a bed of sand is still used as a method of purifying the water today. Darcy used a series of cylinders filled with sand to investigate how quickly water trickled through the sand bed in order to come up with a proper quantification of those things that we too know by experience with our coffee filters. You can read about the mathematics of Darcy’s equation here.

espresso puck
An espresso puck. The compact structure nonetheless allows water to percolate through it at high pressure.

Darcy found that the flow rate of water through the sand bed increased when the porosity of the bed was higher (fine, dense sand would delay the flow of the water more than coarse, loosely packed sand). If there was a greater pressure on the water at the top of the bed (ie. more water is on top of the sand), the flow rate through the bed would increase too. Conversely the flow would get slower as the water was made more viscous. This is something we too know from experience: try to pour honey through the coffee grounds and it just won’t work.

For us to apply Darcy’s insights into making better coffee, it means that we need to think about the grind size. Too coarse and there will be lots of empty space through the bed of grounds: the porosity is high, and the water will flow straight through. Too fine and the flow rate will decrease so much that rather than just the sweet and slightly acidic solubles that first come out of any coffee extraction*, there could be too much of the bitter organic compounds that come out later, changing the character of the cup. With coffee we have an additional concern. Unlike sand, coffee grinds will swell, and splinter, as water is added to them, closing up any narrower paths and lengthening the brew time. This also means that, unless we properly wet the grounds prior to filtering our coffee, the extraction will be non-uniform and not reproducible. Another reason to bloom coffee thoroughly before brewing.

There is one more factor in brewing our coffee however that Darcy’s equation, which is valid for more stable systems, overlooks. Darcy assumed a constant flow rate of water through the sand bed, but coffee is different. In his book about espresso*, Illy showed that the flow of the water through an espresso puck was not constant over time. Something really interesting was happening when you looked carefully at an espresso puck. Ground coffee can come in a large distribution of sizes. In addition to the grind that we are aiming for, we also get a whole load of smaller particles called ‘fines’. Sometimes this is desirable, but with espresso, and by extension with our filter coffees, these fines add a twist to the physics of the percolation. As the espresso water is pushed through the coffee puck, the fines get pushed down through the puck between the ‘grains’ of the coffee grinds. This reduces the flow rate of the water until the point at which they get stuck. This will have the effect of increasing the contact time between the coffee and the water and so allowing more flavour solubles to be extracted. But crucially, these fines remain somewhat mobile. If you were to turn the whole espresso puck upside down (and Illy had a machine that allowed him to do this in-situ), the fines would again go on the move. Migrating from the new top of the puck to the new bottom. Filling the voids between the slightly too coarse grains. Complicating the simplifications in Darcy’s equation, but adding flavour to our brew.

Watch House coffee Bermondsey
There is a fountain on the wall (right hand side) of the Watch House cafe in Bermondsey. Many public fountains in London date from the 1850s emphasising just what a problem access to drinkable water once was.

Which leaves the connection between the farming method and the coffee. Biochar is formed by burning carbon containing waste (such as plant matter) in a low oxygen environment. Burying the resultant charcoal is therefore a way of storing carbon, and preventing its release into the atmosphere, for many years. But it is not just good for carbon storage. The buried charcoal is highly porous and traps nutrients within its structure so that the plants growing near it can be fertilised more efficiently. Moreover, the fact that it is porous, just like the coffee or sand beds, means that it traps water for a long time. Consider how long it takes a used filter full of coffee grounds to completely dry out! The water gets trapped within the porous structure and does not evaporate easily. This aspect of the biochar means that, as well as nutrients, the plants that grow nearby get a good source of reliable water. The ancient civilisations of the Amazon region used something similar to biochar in their farming techniques resulting in soil now known as “Terra Preta”, an extremely rich form of soil that improves plant growth. On his farm, Ricardo is going fully circular and making his biochar out of old coffee trees. The old trees thereby giving new opportunities to the fresh growth. It is a carbon capture scheme that reduces the need for fertilisers and that relies on percolation physics to work to best effect for the plants.

It seemed a moment of perfect coffee-physics poetry to use coffee grown on a farm using these techniques while initially experimenting with my own, percolation sensitive, Pure Over brewer. Percolation physics and interconnectedness all in one cup.

*Illy and Viani (Eds), “Espresso Coffee”, 2nd Edition, (2005)