coffee under the microscope

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

Before adding any water, the cell structure in the Kamwangi is more open, the Gelana appears ‘closed’.
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.

Last week, a new study was published that explored the mathematics behind brewing the perfect filter coffee. The research, summarised here, modelled the brewing process as being composed of a quick, surface extraction from the coffee grounds, coupled with a slower brew, where the water was able to get into the interior of the coffee grind. It was an interesting study and the authors are now looking at grind shape and the effect of how you wet the grounds. However, what struck me was that the authors mentioned scanning electron microscopy (SEM) images of ground coffee. A lovely idea, what does coffee look like when magnified hundreds (or thousands) of times?

So here are a few images that I found shared under Creative Commons Licenses. I hope you find them as fascinating as I do.

1) A green coffee bean:

Green coffee bean under the microscope — A green coffee bean. Sadly no details as to magnification. Image shared under CC license from Nestle, Flickr

2) Instant Coffee

Instant coffee from Nestle — Spray dried instant coffee from the Nestle, Flickr account. Image shared under CC license.

3) Roast and ground coffee (fluorescence microscopy image)

ground coffee, fluorescence image — Ever wondered what your coffee looked like when magnified many times? This image using fluorescence microscopy is of roasted coffee. Note the similarities between this image and the following one (which has a scale bar). Image shared under CC license from Nestle, Flickr

3b) More ground, roasted coffee, this time from Zeiss

Zeiss roast coffee — Scanning electron microscope image of roast (and ground) coffee magnified 750x. Image from Zeiss, Flickr, Todd Simpson, UWO Nanofabrication Facility. Shared under CC license. (To put the scale bar in perspective, it is the size of the smallest particles in an espresso grind. Clearly, the grind here is quite coarse).

4) Finally, an image of tea, just to keep this article tea-coffee balanced:

Green tea under the microscope — Green tea as seen under the microscope by the scientists at Nestle. Shared under CC license Nestle, Flickr.

If you come across any great images of coffee (or tea) under the microscope, please do share them. In the meanwhile, enjoy your coffee however you brew it.