pour over

Cracking pour overs

cracks in a wheat field
Cracks in the soil in a field after a dry spell. But there are many connections between coffee and soil.

Summer this year has so far been quite hot and dry. Perhaps you have seen the grass dying back. Or maybe you have noticed the cracks forming in the soil in your local parks and fields. Such cracking is the result of the very dry weather and hopefully you won’t find it in your coffee, but there is another effect concerning soil compaction that connects to brewing a morning coffee as well as farming it.

It’s about the rain. As each raindrop falls to the Earth, it makes an impact with the soil underneath. While a light drizzle is not going to have that much of an impact, a larger raindrop of diameter say, 5mm, is going to hit the earth at about 9m/s – and that could cause quite a stir. Each impact will shake off smaller sized particles of soil which dislodge and get stuck in the pores between the larger soil particles. So the smaller particles start to ‘clog’ the pores between the soil particles and reduce the ability of water to penetrate into the soil. And although it seems a small effect, the result of this clogging of the pores by the smaller soil particles is to reduce the water permeability of the soil by 200-2000 times*: a soil crust is formed.

lilies on water, rain on a pond, droplets
The impact of a drop? Each rain drop can have a significant effect on the soil surface

This crust not only reduces the amount of water that gets through to the roots (by reducing the soil’s permeability), it also acts as a barrier for seedlings coming up: while many seeds can get through quite strong layers, even Sorghum struggles to push through if it needs pressures of 13-18 Bar to break through this crust*. So even without any farming machinery causing further soil compaction, just the rain is going to affect how additional water goes through the soil and how plants can grow out of it.

We are getting to the coffee bit.

The crust strength is influenced by the number of small (clay-type) particles in the soil. Clay particles are less than 2 microns in diameter which is smaller than the grind size you would find in even a Turkish coffee grind. But if we were to grind very brittle coffee beans (that shattered into many smaller particles as well as the grind size we want), or we were to use a blade grinder leading to a large distribution of grind size in our freshly ground coffee, we may expect to see some effects like this while brewing.

optical microscope image in water
Two coffee grinds compared under a microscope. How does the uniformity of particle size in a grind affect the clogging of a pour over? Magnfied 5x

If we think about a pour over brew (as opposed to an espresso or an immersion type), the initial pouring of water over the grind bed will dislodge any smaller particles in the grind and clog the grind in the same way as the rain on the soil. So if we were grinding way too fine, or using a blade grinder, or had a preference for darkly roasted (more brittle) coffee beans, it is possible that our pour-overs would tend to ‘clog’ more than if we were using a uniform medium grind of more lightly roasted beans. Has anyone experimented with this?

But the second soil connection we may notice as we prepare our pour over is that after our initial pour, as we let the coffee ‘bloom’ and the CO2 bubbles out, we receive a lovely aroma. A wonderful coffee smell as the grind bed continues to out-gas. This may remind us of petrichor, which is that great, and distinctive, smell of rain. And it turns out that petrichor is formed by the rain hitting the soil surface and producing air bubbles as it falls. The air bubbles then burst releasing aerosols from the soil which are so familiar to us as the scent of rain. A similar process to the blooming of the coffee grind. But just as with the coffee grind, as the water continues to fall and particularly if the pour over clogs to leave us with a water layer on the surface of the coffee (or soil), the aroma will reduce (or at least change) as the mechanism producing the smell changes.

bloom on a v60
Blooming petrichor, or should it be coffichor?

On a farm or in a garden, the effect of this soil compaction can be reduced by practises such as mulching. In addition to reducing the impact of individual rain drops on the soil surface, the mulch reduces evaporation of the water from the surface and changes the albedo of the soil. All things that may help coffee farmers to grow healthier coffee plants. In our pour overs, it is probably not a good idea to add any form of mulch! But this does mean that we can experiment more with the grind!

There are many more connections between your coffee and the earth around us, what will you notice?

*Soil Physics, WA Jury and R Horton, Wiley and Sons Publishing, 2004

Time standing still at VCR, Kuala Lumpur

VCR chalkboard

A trip down memory lane via a new cafe. VCR in Bangsar, KL

One of the first science-based talks I gave was about how VCR tapes worked. Depending on how you viewed it (and whether you had to listen), this was either an achievement given that I was at school and didn’t really understand magnetism nor magnetoresistive devices, or a thing to be suffered through (for much the same reasons). So when I learned that a new café called VCR had opened in Bangsar in Kuala Lumpur, it prompted a series of fond (and a few embarrassing) memories.

Moving on, it is clear that this second branch of VCR (the first is in Pudu, in the main part of KL), aims to provoke such memories of times past. From the name of the wifi to the pulleys behind the counter and the wooden screen at the back of the café, various details around the café pull your memory in different directions. However the coffee is very much in the present. With three types of coffee available to try as a pour over as well as the standard espresso based drinks, this café has a lot to offer. The coffee is roasted by VCR themselves in their Pudu branch. There is also an extensive food menu with an interesting Chawan mushi as well as an intricate avocado toast (topped with pomegranate seeds, toasted quinoa and feta).

coffee at VCR Bangsar

Coffee and pour over jug. But is the number 68 or 89?

The friendly baristas were happy to advise on which coffee to match with which brewing device (though there seemed a marked preference for V60s on the days I visited). In total I tried 4 pour-overs, one with the Kalita Wave and the others by V60. These coffees were all excellent but very different. A couple were fruity, one was sweet and full bodied, one reminded me a bit of the local fruit durian, not I hasten to add because of its taste, but because the aroma from the cup was so different from the flavour of the drink. It was a great privilege to be able to try these different coffees consecutively and to really experience the variety of flavours in coffee. Great care was taken while making the pour over before it was brought over to the table, together with a jug of water, it also seemed to me that the baristas kept a discreet eye on me afterwards to ensure I enjoyed the coffee. So it was a good experience to have had the opportunity both to enjoy one of those pour overs and to observe the people and the surroundings of VCR when I had to wait for 1 hour for someone with no phone and no book. If you get the opportunity to do this I would very much recommend it. Find a comfortable café, order a coffee and then sit, without distractions, and watch what your mind notices and where it wanders for an hour.

An obvious place for a mind to wander would be to the mechanism of tape recording (and why mini-disks are the superior recording medium for the elegance of the physics involved). However, in an hour a mind wanders far further than the name. Supporting the cakes (and a display case for the 2nd place award of the brewers cup), was a table with a concertina type decoration around its edge. Was this a nod to the Kalita Wave brewing device? This is a significant difference between the V60 and the Kalita Wave: the ridges (or wave pattern) on the filter of the latter. How does coffee flow past these ridges? Does this difference in flow dynamics make a difference to the taste of the coffee?

variables grind size, pour rate, pour vorticity

It seems that there would be a lot of physics to observe in the fluid flow in a Kalita Wave filter.

A few weeks previously a friend had made a (lovely) coffee with her Kalita Wave. It was interesting to note the different dose of coffee she used and the way the grinds built up in the ridges (compared with my ‘normal’ V60). Why do the grinds end up in the ridges? Why is there a layer of dust on the blades of a fan? Why do some corners of a building collect more dust or leaves than others? Are these questions related and does it change the flavour of the coffee in the Kalita?

In fact, there are many subtleties in understanding how fluids move around solid objects. One of these is that at the interface of the fluid with the solid, the fluid does not flow at all, there is a stationary layer. Known as a boundary layer or Prandtl boundary layer (after the person who first suggested their existence, Ludwig Prandtl), realising these layers existed revolutionised the field of aerodynamics. The problem had been how to model the drag experienced by a solid object in a fluid flow. Although perhaps only of academic interest in terms of the flow of coffee around a Kalita filter or a spoon, by the end of the nineteenth century and particularly, with the invention of airplanes, how to calculate fluid (i.e air) flow around a solid (i.e. wing) object became very important for practical reasons.

vortices, turbulence, coffee cup physics, coffee cup science

Another cool consequence of boundary layers:
Vortices created at the walls of a mug when the whole cup of coffee is placed on a rotating object (such as a record player).

Prandtl introduced the concept of a boundary layer in 1904. The idea allowed physicists to treat the main body of the moving fluid separately to the layer, very close to the solid, that was dominated by friction with the solid. This meant that the Navier-Stokes equations (that are used to describe fluid flow and ordinarily do not have an analytical solution) are simplified for this boundary layer and can be quantitatively solved. Although simple, by the 1920s Prandtl’s layer (and consequently the solvable equations) were being used to quantitatively predict the skin friction drag produced by airplanes and airships.

The boundary layer allows us to understand how vortices form behind cylinders or around the corners of buildings. I suspect a mix of the boundary layer, turbulence caused by the coffee going over many of the ridges and the brick like stacking/jamming of the coffee grains would combine to explain the difference in the grind shape around the Kalita Wave and the V60 filters. What this does to the flavour of the coffee and whether better brewing would involve more agitation, I will leave to Kalita Wave coffee lovers to investigate. And when you do, I would love to hear of your results, either here on Facebook or Twitter.

 

Waiting for the drop at Kurasu, Kyoto (Singapore)

Kurasu Kyoto Singapore, coffee Raffles City

The sign towards the entrance at Kurasu Kyoto, Singapore

Kurasu Kyoto, in Singapore, was recommended to me as a great place to experience pour-over coffee. Although they will serve espresso based drinks too, it is the pour over coffee for which they are famous. The Singapore branch is at the front of a shared working space in an office block. Entering from the street, you have to go up one level before the smell of the coffee will guide you to the café.

Ordinarily, coffee chains would not be featured on Bean Thinking. However, despite it’s name, this is a ‘chain’ of only two outlets, the original branch in Kyoto, Japan and this one in Singapore. The menu featured several coffees with their differing tasting notes together with a few other drinks. Coffee is shipped from Japan weekly as well as being locally roasted in Singapore. It is very much a place to enjoy your coffee while sitting on the comfortable chairs before getting back to work (or perhaps, a place to meet potential colleagues over a refreshing cup of coffee). And it is highly likely you will enjoy your coffee which is prepared for you as you wait.

coffee machine, V60 Kalita

The bar and some of the coffee equipment in the cafe space at Kurasu Kyoto Singapore

There is no hint of automation here. Each cup of coffee is prepared carefully and individually by the barista behind the bar. V60 or Kalita, it was somewhat mesmerising to watch the pour over being prepared, rhythmically, carefully, by hand. Indeed, automation seems almost alien to this place where the act of making coffee is truly artful. Once prepared, the coffee is brought to your table in a simple ceramic mug for you to taste for yourself and see how your tasting notes compare.

As I was watching, two thoughts occurred to me, the first of a directly scientific nature, the second more about our society. Firstly watching the barista slowly prepare the pour over, it is difficult not to be reminded of the pitch drop experiment.

You may remember the story from 2013 and then again in 2014. Two experiments that had been set up in 1944 and 1927 respectively finally showed results. The experiments were (indeed are, they are still going) very similar and concerned watching pitch (which is a derivative of tar) drop from a funnel. Pitch is used to waterproof boats and appears to us almost solid at room temperature although it is actually a liquid but with an extremely high viscosity. To put this into perspective, at room temperature coffee has a viscosity similar to water at about 0.001 Pa s, liquid honey has a viscosity of about 10 Pa s, but this tar has a viscosity of 20 000 000 Pa s. The experiments involved pouring this tar into a funnel and then waiting, and waiting, for it to drip. Both experiments seem to drip only approximately once a decade but until 2013 (and 2014 for the other experiment), the actual drop had never been seen. Both experiments are now building their droplets again and we await the next drop in the 2020s.

Imagine waiting that long for a drip coffee.

coffee Kurasu Kyoto Singapore

Apparent simplicity. The coffee at Kurasu Kyoto Singapore

But then a second thought, there is currently a lot of angst, particularly about automation and our environmental and/or political situations, as if they are something from outside ourselves being imposed upon us. To some extent it is true that we are not in control over many things happening around us. But in our feeling of powerlessness, are we resigning more than we ought to of our responsibility for the power that we do have? It was something that deeply concerned Romano Guardini in his essay “Power and Responsibility”¹. To use the example of automation and the pour over. Guardini argues that people become poorer as they become more distant from the results of their work (e.g. by automating the pour over coffee with a machine). And that the better the machine, the “fewer the possibilities for personal creativeness”¹ that the barista would have. For Guardini, this has consequences for the human being for both barista and customer. The barista clearly loses the element of their creativity when preparing a pour over with a machine but the customer too is affected by the loss of a personal contact, possible only through individually created things. Rather than celebrating each other as individuals we become consumers with tastes “dictated by mass production”¹ and people who produce only what the “machine allows”. To respond to the challenges of our contemporary society involves discovering where we each have responsibility and exercising it, no matter how small or large that responsibility seems (to us) to be.

Which is somehow resonant with the interview that one of the Kyoto based baristas at Kurasu Kyoto gave that was recently circulated by Perfect Daily Grind. Asked what was her preferred brewing method, she replied it was the V60 because of the control that the individual barista could gain over the flavour of the cup merely by tweaking some of the details of the pour. A knowledgable art rather than a technology. And it is precisely this knowledgable art that you can see carefully and excellently practised in the Singapore branch.

Kurasu Kyoto (Singapore) is at 331 North Bridge Road, Odeon Towers, #02-01

“Power and Responsibility” in “The End of the Modern World”, Romano Guardini. ISI books, (2001)

 

Now you see it now you don’t at Bond St Coffee, Brighton

Outside Bond St Coffee Brighton

Bond St on Bond St, Brighton

A couple of weeks back, I tried the lovely Bond St. Coffee in Brighton on the recommendation of @paullovestea from Twitter. It was a Saturday with good weather and it turns out that this particular café is (understandably) very popular and so, sadly, to begin with we could only sit outside. That said, it was a lovely spring day (sunny but a bit chilly) and so it was quite pleasant to watch the world go by (or at least Bond St) while savouring a well made pour-over coffee. All around the café, the street decoration hinted at times past. Across the road what was obviously a pub in times gone by has turned into an oddities store. Air vents to a space underneath the window seating area in Bond Street café itself suggested an old storage space. A seat in the window appeared to have been re-cycled from an old bus seat.

But it was the pour-overs at Bond St. Coffee that had been particularly recommended and they certainly lived up to expectations. I had a Kenyan coffee roasted by the Horsham Roasters. The V60 arrived at our bench seat/table in a metal jug together with a drinking glass. The angle of the Sun caught the oils on the surface of the coffee, reminding me of Agnes Pockels and her pioneering experiments on surface tension. Pouring the coffee into the glass I thought about the different thermal conductivities of glass as compared to metal and how I had put both down on the wooden bench. How was heat being transferred through these three materials? And then, as I placed the metal jug back on the bench I noticed the reflections from the side of the jug and thought, just why is it that you can see through the colourless glass but the metal is grey and opaque?

Metal jug and transparent glass

Metal jug, glass cup. V60 presentation at Bond St Coffee

On one level, this question has a simple answer. Light is an electromagnetic wave and a material is opaque if something in the material absorbs or scatters the incoming light. In a metal, the electrons (that carry the electric currents associated with the metal’s high electrical conductivity) can absorb the light and re-emit it leading to highly reflective surfaces. In glass there are no “free” electrons and few absorbing centres ready to absorb the light and so it is transmitted through the glass.

Only this is not a complete answer. For a start we haven’t said what we mean by ‘glass’. The glass in the photo is indeed transparent but some glasses can be more opaque. More fundamentally though, there is a problem with the word ‘opaque’. For us humans, ‘visible’ light is limited to light having wavelengths from about 400nm (blue) to about 780nm (red). ‘Light’ though can have wavelengths well below 400 nm (deep into the UV and through the X-ray) and well above 780 nm (through infra-red and to microwaves and beyond). We can see the spread of wavelengths of light visible to us each time we see a rainbow or sun dog. Other animals see different ranges of ‘visible’ light, for example, bumble-bees can see into the ultra-violet. So, our statement that glass is transparent while metal is opaque is partly a consequence of the fact that we ‘see’ in the part of the spectrum of light for which this is true.

Sun-dog, Sun dog

Sun dogs reveal the spectrum of visible light through refraction of the light through ice crystals.

For example if, like the bumble-bee, we could see in the UV, some glass may appear quite different from the way it does to us now. Even though the glass in the photo lacks the free electrons that are in the metallic jug, there are electrons in the atoms that make up the glass that can absorb the incident light if that light has the right energy. There are also different types of bonds between the atoms in the glass that can also absorb light at particular energies. The energy of light is related to its frequency (effectively its colour*). Consequently, if the energy (frequency/ wavelength) of the light happens to be at the absorption energy of an atom or an electron in the glass, the glass will absorb the light and it will start to appear more opaque to light of that colour. Many silicate glasses absorb light in the UV and infra-red regions of the electromagnetic spectrum while remaining highly transparent in the visible region. High purity silica glass starts to absorb light in the UV at wavelengths less than approx 160nm†. Ordinary window glass starts to absorb light in the nearer UV†. In fact, window glass can start to absorb light below wavelengths of up to ~ 300 nm, the edge of what is visible to a bumble bee: The world must appear very different to the bumble bee. At the other end of the scale, chalcogenide based glasses absorb light in (our) visible range but are transparent in the infra-red.

Looking at how materials absorb light, that is, the ‘absorption spectrum’, enables us to investigate what is in a material. It is in many ways similar to a ‘fingerprint’ for the material. From drugs discovery to archaeology, environmental analysis to quality control, measuring how a material absorbs light (over a wider range of frequencies than we can see) can tell us a great deal about what is in that material.

Perhaps you could conclude that whether something is opaque or crystal clear depends partly on how you look at it.

 

Bond St Cafe is on Bond St, Brighton, BN1 1RD

*I could add a pedantic note here about how the colour that we see is not necessarily directly related to the frequency of the light. However, it would be fair to say that a given frequency of light has a given ‘colour’ so blue light has a certain frequency, red light a different frequency. Whether something that appears red does so because it is reflecting light at the frequency of red light is a different question.

†”Optical properties of Glass”, I Fanderlik, was published by Elsevier in 1983.