bloom

Drip coffee

The universe is in a cup of coffee. But how many connections to different bits of physics can you find in the time it takes you to prepare a V60? We explore some of those links below while considering brewing a pour-over, what more do you see in your brew?

1. The Coffee Grinder:

coffee at VCR Bangsar

Preparing a V60 pour over coffee. How many connections can you find?

The beans pile on top of each other in the hopper. As the beans are ground, the bean pile shrinks along slipping layers. Immediately reminiscent of avalanches and landslides, understanding how granular materials (rocks & coffee beans) flow over each other is important for geology and safety. Meanwhile, the grinding itself produces a mound of coffee of slightly varying grain size. Shaking it would produce the brazil nut effect, which you can see on you breakfast table but is also important to understand the dynamics of earthquakes.

Staying at the grinding stage, if you weigh your coffee according to a brew guide, it is interesting to note that the kilogram is the one remaining fundamental unit that is measured with reference to a physical object.

2. Rinsing the filter paper:

V60 chromatography chemistry kitchen

A few hours after brewing pour over, a dark rim of dissolved coffee can be seen at the top of the filter paper. Chromatography in action.

While rinsing the filter we see the process of chromatography starting. Now critical for analytical chemistry (such as establishing each of the components of a medicine), this technique started with watching solutes ascend a filter paper in a solvent.

Filtration also has its connections. The recent discovery of a Roman-era stone sarcophagus in the Borough area of London involved filtering the excavated soil found within the sarcophagus to ensure that nothing was lost during excavation. On the other hand, using the filtered product enabled a recent study to concentrate coffee dissolved in chloroform in order to detect small amounts of rogue robusta in coffee products sold as 100% arabica.

3. Bloom:

bloom on a v60

From coffee to the atmosphere. There’s physics in that filter coffee.

A drop falling on a granular bed (rain on sand, water on ground coffee) causes different shaped craters depending on the speed of the drop and the compactness of the granular bed. A lovely piece of physics and of relevance to impact craters and the pharmaceuticals industry. But it is the bloom that we watch for when starting to brew the coffee. That point where the grinds seem to expand and bubble with a fantastic release of aroma. It is thought that the earth’s early atmosphere (and the atmosphere around other worlds) could have been helped to form by similar processes of outgassing from rocks in the interior of the earth. The carbon cycle also involves the outgassing of carbon dioxide from mid-ocean ridges and the volcanoes on the earth.

As the water falls and the aroma rises, we’re reminded too of petrichor, the smell of rain. How we detect smell is a whole other section of physics. Petrichor is composed of aerosols released when the rain droplet hits the ground. Similar aerosols are produced when rain impacts seawater and produces a splash. These aerosols have been linked to cloud formation. Without aerosols we would have significantly fewer clouds.

4. Percolation:

A close up of some milk rings formed when dripping milk into water. Similar vortex rings will be produced every time you make a pour over coffee.

Percolation is (almost) everywhere. From the way that water filters through coffee grounds to make our coffee to the way electricity is conducted and even to how diseases are transmitted. A mathematically very interesting phenomenon with links to areas we’d never first consider such as modelling the movements of the stock exchange and understanding the beauty of a fractal such as a romanesco broccoli.

But then there’s more. The way water filters through coffee is similar to the way that rain flows through the soil or we obtain water through aquifers. Known as Darcy’s law, there are extensive links to geology.

Nor is it just geology and earth based science that is linked to this part of our coffee making. The drips falling into the pot of coffee are forming vortex rings behind them. Much like smoke rings, they can be found all around us, from volcanic eruptions, through to supernovae explosions and even in dolphin play.

5. In the mug:

Rayleigh Benard cells in clouds

Convection cells in the clouds. Found on a somewhat smaller scale in your coffee.
Image shows clouds above the Pacific. Image NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response

Yet it is when it gets to the mug that we can really spend time contemplating our coffee. The turbulence produced by the hot coffee in a cool mug prompts the question: why does stirring your coffee cool it down but stirring the solar wind heats it up?

The convection cells in the cooling coffee are seen in the clouds of “mackerel” skies and in the rock structure of other planets. The steam informs us of cloud formation while the condensation on the side of the cup is suggestive of the formation of dew and therefore, through a scientific observation over 200 years ago, to the greenhouse effect. The coffee cools according to the same physics as any other cooling body, including the universe itself. Which is one reason that Lord Kelvin could not believe that the earth was old enough for Darwin’s theory of evolution to have occurred. (Kelvin was working before it was known that the Sun was heated by nuclear fusion. Working on the basis of the physics he knew, he calculated how long the Sun would take to cool down for alternative mechanisms of heating the Sun. Eventually he concluded that the Sun was too young for the millions of years required for Darwin’s theory to be correct. It was the basis of a public spat between these two prominent scientists and a major challenge to Darwin’s theory at the time).

 

Of course there is much more. Many other links that take your coffee to the fundamental physics describing our world and our universe. Which ones have you pondered while you have dwelt on your brew?

Coffee under the microscope

Inside Coffee Affair

There are many great cafés in London serving excellent coffee but inevitably a few stand out. One such café is Coffee Affair in Queenstown Road railway station which ‘inhabits’ a space that really encourages you to slow down and enjoy your coffee while just noticing the environment. An ex-ticket office that whispers its history through subtle signs on the parquet floor and in the fixings. The sort of place where you have to stop, look around and listen in order to fully appreciate it. And with a variety of great coffees on hand to sample, this is a café that is a pleasure to return to whenever I get the opportunity.

So it was that a few weeks ago, I happened to wander into Queenstown Road station and into Coffee Affair. That day, two coffees were on offer for V60s. One, an Ethiopian with hints of mango, peach and honey, the other, a Kenyan with tasting notes of blackcurrant and cassis. But there was an issue with them when they were prepared for V60s. The Ethiopian, “Gelana Abaya”, caused a considerable bloom but then tended to clog the filter cone if due care was not taken during the pour. The other, the Kenyan “Kamwangi AA”, did not degas so much in the initial bloom but instead was easier to prepare in the V60; there was not such a tendency to clog.

What could be going on?

So we had a look under the microscope at these two coffees. Each coffee was ground as if it was to be prepared in a V60 and then examined under the microscope. Was there any difference between the appearance of the Gelana compared to the Kamwangi? A first look didn’t reveal much. Magnifying both coffees at 5x, it could be said that the Kamwangi had more ‘irregular protrusions’ on the ground coffee compared to the smoother Gelana, but it was hard to see much more:

coffee under the microscope

The samples of ground coffee imaged under an optical microscope at 5x magnification. Kamwangi is on the left, Gelana on the right. “500 um” means 500 micrometers which is 0.5 mm.

So, the microscope was swapped to image the coffee in fluorescence mode. It was then that the cell structure of the coffee became clear. Here are the two coffees magnified 10x:

Fluorescence microscopy 10x, Ethiopian, Kenyan, Kamwangi, Gelana

Fluorescence microscope image of the two coffees at 10x magnification. Note the open structure in the Kamwangi and the more closed structure in the Gelana.

and at 20x

Kamwangi and Gelana coffee under the microscope

A fluorescence microscope image magnified 20x – not ‘um’ means micrometers (1/1000 of a mm), so the scale bar represents 1/10 mm.

So there is perhaps a clue in the cell structure. It seems as if the Kamwangi structure is more open, that somehow the cells in the Kamwangi break open as they are ground but the Gelana somehow keeps its cells more intact. Could this be why the Gelana blooms so much more?

Which naturally leads to a second experiment. What happens when you look at these two coffees in water under the microscope? Here the fluorescence images didn’t help as all you could see were the bubbles of gas in each coffee but the optical microscope images were of more interest.

optical microscope image in water

The two coffees compared under the microscope while in (cold) water. Magnfied 5x

‘Bits’ broke off the Kamwangi as soon as water was added but in comparison, there were far fewer bits of coffee breaking off the Gelana grains.

So what do you think has happened? If you remember our question was: when these two coffees were prepared with a V60, the Gelana bloomed a lot but then clogged in the filter (without extreme care while pouring the filter). Meanwhile the Kamwangi did not bloom so much but also did not clog the filter, what could be happening?

From the microscope images, it appears that

  1. Before adding any water, the cell structure in the Kamwangi is more open, the Gelana appears ‘closed’.
  2. When water is added, there are many more ‘bits’ that come off the Kamwangi whereas the Gelana does not show so much disintegration on the addition of water.

If pushed for a hypothesis, I wonder whether these two observations are linked. What is happening is that the cell structure in the Kamwangi is, for whatever reason, fairly fragile. So as soon as it is ground, the cells break up and a lot of the carbon dioxide is released. Consequently when water is added to it, the bits of broken cell quickly disperse through the water and it doesn’t seem to ‘bubble’ that much. In comparison, the Gelana cell structure is tougher and the cells only open up when water is added. I wonder if this means that the ground Gelana coffee will swell rather than break up and so ‘jam together’ as each grain tries to expand rather like trying to inflate many balloons in a bucket. They will push against each other and prevent water from easily percolating through the ground coffee.

Sadly, many more experiments would be required before we could see if there’s any truth in this hypothesis however that does provide a great excuse, were one needed, for many return trips to Coffee Affair. Meanwhile, what do you think? Do any of the images stand out to you and why? What do you think could be the cause of our V60 coffee mystery? I’d love to hear your thoughts so please let me know either here in the comments section (moderated and experiencing a lot of spam at the moment so please be patient), on Facebook or on Twitter.