raindrops – Bean Thinking

A Need for Roots, Hermanos Coffee, Portobello Road

Hermanos Colombian Coffee Roasters on Portobello Road. There was a fair amount of graffiti, which could offer further avenues of thought but is probably owing to the Notting Hill carnival which had happened just before my visit.

There is nothing quite like wandering through a market street in the hour or so before the market opens. It feels as if you are there as part of the city is waking up, things moving into place, ready to start the hustle and bustle of the day. It is one of the things I like to do when exploring a new city: wake up earlier in order to walk around and try to find breakfast, listening for the character of the place. Sometimes though, it is good to try this in your own city, it is a chance to see your home, your space, in a new light; a different aspect of its essence. So it was that I ended up wandering along Portobello Road shortly after 8am one week day morning. Market stalls were being moved into place, office workers were cycling or scooting on their way to work and a small little coffee shop with an open door seemed to invite customers in to sample their coffee.

Hermanos Colombian Coffee Roasters cafe can be found at 127 Portobello Road. Actually, it can also be found at 7 other locations including Kings Cross and Victoria Stations. Does this qualify as a coffee chain? Regardless of the number of cafes, Hermanos Coffee on Portobello Road is somehow clearly integrated into that community, so much so that this initially appeared to be a small scale single shop embedded within Portobello. When you sit to drink a coffee while the street around you ‘wakes up’, it seems as if people around drop their guard a bit, you see glimpses into relationships that later in the day will be hidden by the rush of people. Perhaps it was the time of day that made this cafe seem particularly friendly, I stepped inside to order my coffee.

Perhaps unsurprisingly, the coffee is roasted by Hermanos Colombian Coffee Roasters and they have a large selection of their coffee (and coffee making equipment) for sale on your left as you walk into this small cafe. The counter is just ahead of you (also on your left) with the menu behind the counter showing the usual selection of coffees available. I am fairly confident that they also offered a pour over coffee on the menu, but it being the start of the morning rush, I didn’t want to preoccupy the barista with making a filter for me and so I ordered a long black. Beyond the counter, there were a few seats with people already inside enjoying coffee and conversation before the start of the day. As I indicated that I wanted to sit outside, the person behind the counter asked me if I would prefer “china or a take-away cup”. It’s a small detail but it was nice to be asked. I picked up my cup of coffee from the end of the counter, wandered outside and took a seat waiting to watch what went on for the next half hour or so.

Coffee on a wooden table. Looking at the lines formed by the wooden panels, you can see which one was wonky.

I listened as the barista spoke to each new customer, some of whom were clearly regulars. “Back to hot coffees as the weather turns colder then?” I heard, as a customer walked in off the street. I then watched as the barista came outside, took a short breather during a lull in customers and greeted the person who was opening the hat store next door. Another person walked into the cafe to pick up his bag that he had left there briefly as he knew it would be ‘safe’. This is (unfortunately) not something that you would normally assume of Portobello Road. It was not just the community aspect that jumped out at the interested observer of this cafe. The science started to appear everywhere. This is of course in one sense always true. The mere fact of seeing something involves multiple elements of physics and biology without thinking about anything deeper. However, some cafes produce something in the physicist akin to writer’s block, it can be hard to link to what is around you. This is very far from the case at Hermanos.

The recent rain on the wooden table top suggested the phenomenon of coffee rings and what makes a surface wettable, while the table itself with one wonky wooden slat immediately prompted considerations on defects in crystal lattices. One of the many people who rushed by was talking to someone on a mobile phone set to speaker. This meant that the entire conversation was audible to the nearby coffee drinker. You could ponder what they were talking about or you may start to think about why it is that someone through a phone line sounds different to that same person speaking in front of you. Every sound is made up of several frequencies of sound wave. The more complex the sound, the greater the number of frequencies used to convey it. When you speak to someone face to face, all of those frequencies will be transmitted from the speaker to the listener. When a sound is transmitted through a phone line, it is necessary to limit the frequencies that the phone line can carry. For some sounds, some of the higher frequencies that contribute to that sound’s ‘sound’, will get blocked off by this bandwidth limitation. Consequently certain sounds, like “s” and “t”, will sound slightly different over a phone line than when speaking face to face. The conversation will be perfectly intelligible, but we pick up on subtleties in people’s voice and know that they sound slightly different through the phone network.

The Hermanos signboard photographed with the hat store in the background.

Elsewhere, the sign of the cafe puts a square around the “H” of “Hermanos” which prompts recollections of the element hydrogen which has that symbol on the periodic table. It was an appropriate connection because the raindrops on the signboard were reflecting the Sun which is mostly made of hydrogen being formed into helium through nuclear fusion. Looking again at the sign, the reflections and the table, it was clear how our eyes interpret lines and angles into information about distance. These are observations that scientist-artists of the past have used to formulate the rules of perspective. It caused me to look again at the wonky wooden slat and think about how I knew it was wonky (without putting my coffee on top of it). Perspective then surfaced again as the worker in the hat stall next door brought out the days collection of hats and goods. As he was standing behind a hat stand arranging a viewing table, it appeared that he was wearing one of the hats on the stands. Only the fact that he moved and the hat didn’t showed that actually my eyes had been deceived. A parallel meaning of perspective was evident around as the street changed while I was there: by the end of my coffee, the shops were open, parents were rushing children to the school around the corner, the street was alive. What would I think of this cafe if, rather than enjoying my coffee at around 8.30am, I was trying to drink it in the middle of the Saturday tourist rush?

The coffee was good. A very drinkable cup with which to enjoy my time on Portobello Road. The cafe was equally good, showcasing many different aspects of what being a cafe is about. Hermanos Coffee is definitely worth a visit and I hope to go again next time I have the urge to explore Portobello Road before the crowds arrive. It seems appropriate however to conclude this cafe-physics review with a quote from the Hermanos brothers themselves about their cafes and coffee. They say “We understand that our journey at Hermanos is also the collective journey of so many, who each in their own way contribute to and benefit from the world of coffee.” A journey that is so much easier to appreciate if you pause to sit and people watch at this lovely little cafe.

Hermanos Coffee is at 127 Portobello Road, W11 2DY and at multiple locations around London.

The impact of water on coffee

lilies on water, rain on a pond, droplets — What is the crater shape produced by falling droplets of water on freshly ground coffee?

Recently there has been considerable discussion about the impact of water on the taste of your coffee. Although this is interesting not only from a chemistry perspective, but also an experimental design and an environmental one, Bean Thinking is probably not the best place to explore such effects of chemistry on coffee taste. If you are interested, there is a recent article about it in Caffeine Magazine, click here. Instead, on Bean Thinking, the idea would be to go a little more fundamental and ask instead what is the impact of water on coffee? What effect does dripping water have on the craters produced in freshly roasted coffee grinds?

You may have noticed craters produced by rain drops on sand or paused while preparing your drip brew to think about the different ways that water percolates through a filter compared to an espresso puck. But have you stopped to consider what determines the shape of the crater that is produced as a falling droplet impacts a loose bed of granular material (such as coffee). Perhaps you have looked at images of the Chicxulub crater on the Yucatan peninsula and wondered about asteroid impacts on the Earth or craters on the Moon but what about something closer to home? What if the impacting object were liquid and the impact surface more sand like? It’s a problem that affects how rain is absorbed by soil as well as the manufacture of many drugs in the pharmaceutical industry. But it is also something that we could experiment with in coffee. Is there a difference between craters formed in espresso pucks compared to those in the coffee in the filter paper of a V60?

bloom on a v60 — Bubbles in a V60 filter – but what is the impact of individual drops of water on the dry grains of coffee? The ultimate in slow coffee.

Recently, a study appeared in Physical Review E that investigated the crater shapes produced by water droplets on a bed of dry glass beads (imitating sand). The effect of the impact speed of the water droplet as well as the packing density of the granular bed (sand/coffee) was studied. A high speed camera (10 000fps) was used in combination with a laser to reveal how the shape of the craters changed with time, from the initial impact right through until the crater was stable. The authors came up with a mathematical model to consider how the energy of the falling droplet was distributed between the impacting drop and the sand bed. Does the droplet of water deform first or does the energy of the impact go into displacing the sand and so forming the crater?

Perhaps unsurprisingly, when drops of water fell onto dense beds of sand (think espresso pucks but not quite so packed), the craters produced were quite shallow. It would take a lot of energy to displace the densely packed sand but not quite so much to deform the droplet. But when the drops fell onto looser sand beds (think drip brew coffee) the crater produced formed in two stages and depended on the velocity of impact. A deep crater was formed as the drop first impacted the sand. Then as the camera rolled, the sides of the crater started to avalanche producing much wider craters that had different shapes in profile (from doughnut to pancake type structures). For looser beds of sand, the faster the impacting drop, the wider the final crater. You can read a summary of the study here.

So what would happen for craters produced during making an espresso compared to those produced making a drip brew? A first approximation would be that the espresso coffee is more densely packed, so the craters should be shallower and less wide than those produced in the loose packed filter coffee. However then we need to think that the water used in making espresso is forced through the puck with high energy. In contrast, in drip brewing techniques, the water used has a lower impact energy, (it could be said that the clue is in the name). So the energy of the impact would form larger craters in the espresso pucks and smaller craters in the drip brewers, an opposite expectation from that of the packing densities, which effect wins?

coffee ground in a candle holder — Early experiments with coffee grind craters: There are advantages to working with glass beads and high speed cameras.

But is there anything else? Grind size! Espressos are made using finely ground coffee beans, with a typical “grain size” being about 10μm (0.01mm). Drip brewed coffee is somewhat coarser, a typical medium grind being compared to grains of sand (which vary between 0.05-2mm, 50 – 2000μm but we’d expect ‘medium’ ground coffee to be at the lower end of that). This is fairly similar to the ‘sand’ used in the study in Phys Rev E which used grains of size 70-110 μm. A slightly earlier study had shown how the crater shape depended on grain size for ‘sand’ ranging from 98 to 257 μm. That study had revealed that how the water interacted with the different grain sizes depended in turn on whether those grains were hydrophilic (wettable) or hydrophobic (water proof). It is probably safe to assume that the coffee used in an espresso grind has the same hydrophilic properties as the coffee used in drip brew but even so, we still have those three variables to contend with, packing density, impact energy and grind size. So, happy experimenting! Let’s find out how the impact craters left in coffee change with preparation method. And whatever else, it’s a perfect excuse (if one were really needed) to drink more coffee while slowing down and properly appreciating it.

With thanks to Dr Rianne de Jong for pointing me in some interesting directions (not all of which fitted in this piece) towards the interaction of water with coffee, more coming soon I hope.