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.



Sugar castles at Iris and June, Victoria

Iris and June, Victoria, coffee in Victoria

The exterior of Iris and June

This post has been a long time coming. Over the past few months I’ve been popping into Iris&June to get take away coffee now and then and have got quite fond of the friendly service and good coffee. What I have not really had the opportunity to do (until recently) was sit and enjoy a coffee inside. Fortunately that’s now changed and I can add Iris&June to the Daily Grind.

So, how is I+J? Well, it is a 5 minute walk from Victoria train station and a welcome break for good coffee. They serve Ozone based espresso, with a brew bar which features guest roasts (also from Ozone) made with the V60 or Aeropress. There are a good looking selection of cakes on offer, though sadly, on the day that I could sit inside with my drink, they all had nuts in them. Hopefully another time.

Sugar jar, I&J, I+J

A jar of sugar at Iris and June

I took a seat on the cushioned bench near the wall and started to look at what was going on. It is the sort of place that is very good for people watching. My eye though was drawn to what was on my table: a jar of sugar. It is not that I take sugar in my coffee, it is that I was reminded of a tutorial I once had as a student. I cannot remember the exact conversation but it concerned piles of sand. My tutor (a theoretical physicist) had said something along the lines “Ah yes, well, of course, everyone knew the maximum angle that a pile of sand could make before it became unstable and then how it started to collapse…. Until of course someone measured it.” [laughed] “We’d got it entirely wrong.”

This ability to laugh at what we do not know, (or what we assume we do know and then measure it and find out that in fact we do not)  is one of the pleasures of physics. We are trying to understand the world we live in, we have not yet got there. Sometimes it is the smallest things that are not yet understood, such as how and why (dry) sand forms avalanches as it is piled up. Yet these small things can turn out to have big consequences (as was also the case for the understanding of coffee stains). In this case, the experiment had tested the way that a pile of sand collapsed in response to different shaped grains of ‘sand’. It had relevance then (and continues to have relevance now) not only in terms of granular dynamics: how do we predict landslides/avalanches? But also in terms of crucial theoretical models about how these processes behave. Theoretical models that are applied to systems as diverse as knowing how electrical devices (resistors) work to understanding the noise on the luminosity of stars. Realising that we were wrong enabled us to probe the question more deeply and thereby to understand it more.

There are similarities between sugar and sand, but also key differences. Although it was tempting to start building sugar castles in the sugar jars on the tables at Iris and June, I was aware of the impression that I may have made to those who go to I+J to people watch (see above). I will therefore leave it as a home experiment. How steep a sugar castle do you think you can make? And how steep can you in fact make it, what is the role of water in building sand castles?

Please leave any reports of experimental results for how steep you can make a pile of sugar in the comments section below and feel free to send me your sugar-castle pictures.

Iris and June is at No 1 Howick Place, SW1P 1WG