Extraction yield

Espressos in the evening

Where it all happens. Amoret Coffee in Notting Hill.

Two weeks ago saw the latest in the series of “coffee and science” evenings at Amoret Coffee in Notting Hill. Designed to be informal (and hopefully conversational), each evening explores a different aspect of the interaction and connections between coffee and science (or more specifically, physics). This time, we were also very fortunate to be joined by Ricardo of La Lomita coffee farm in Columbia.

Last time we had investigated foam and so this time we looked more at the base of the cappuccino: espressos. We started off with Sadiq of Amoret preparing a pour over (this time of an Ethiopian) in order for us to feel coffee focussed before leaping into a discussion of the extraction of espressos. And an experiment! How does the extraction of the espresso vary with the strength? We were exploring the extraction-strength relation described on Barista Hustle. Three espressos were prepared by Sadiq: one that was spot on, one that was under extracted and one that was left for too long to percolate through the puck. How did they taste and compare? While various participants took to the very important, but ultimately subjective, taste tests, Sadiq used the Total Dissolved Solids meter to explore how ‘strong’ the coffee was in terms of the percentage of dissolved solids. The extraction on the other hand is a function of the time of the brew and as more water goes through the espresso puck and the shot pull time gets longer, the strength of the coffee (as measured by the percentage total dissolved solids) can get relatively lower as the espresso yield (the size of the drink) gets larger.

straw, water, glass, refraction
The total dissolved solids meter uses the different refractive indexes of coffee and water to measure the amount of coffee dissolved in the beverage. The refractive index is what causes a straw to appear to bend when it is put in a glass of water.

A note on the physics here: the total dissolved solids meter uses the refractive index of the coffee to evaluate the ‘strength’. According to Illy*, the refractive index of a strong espresso is 1.341 at 20C. In comparison water has a refractive index (at 20C) of 1.333. Assuming light enters the coffee at an angle of 20 degrees, this means that the difference in the refraction of the light between coffee and ordinary water is 14.78 – 14.87 = -0.09 degrees. A pretty sensitive meter.

We followed this up with an exploration of crema. What, if anything, does crema tell you about a coffee? Does it even matter? I was impressed by the fact that some members of the group could recognise the Nicaraguan from the Ethiopian espresso just from the way it looked; the Nicaraguan had a different crema effect and coloration than the Ethiopian. Among other factors, the colour of the crema will be influenced by the number of suspended small particles in the coffee. A detail that brought us back to a link with Prof Jan Cilliers who had come along last month. A review paper on the science of cremas included a reference to Jan’s work on froth flotation. A connection between coffee cremas and the froth flotation technique used in mining, an excellent point for an evening of interconnectedness!

The ancient Greeks considered the circle to  be the perfect shape. I'd suggest they were nearly right. The perfect shape has to be a cylinder.
The ancient Greeks considered the circle to be the perfect shape. I’d suggest they were nearly right. The perfect shape has to be a cylinder.

By this time we had moved upstairs at Amoret and the discussion continued about extraction techniques and percolation. Which linked very nicely to the work that Ricardo of La Lomita is doing at his coffee farm in Columbia. Ricardo uses biochar around his younger plants. Biochar is charcoal, formed by burning old plant matter (in Ricardo’s case, old coffee trees) in a low oxygen environment. This leaves the carbon of the trees intact and so acts as a way of sinking carbon (for many years) into the soil and avoiding its escape as CO2 into the atmosphere. In addition to this, the percolative structure of the charcoal traps nutrients within the structure giving the coffee plants every chance of success in their growth. As a last point, the way that the biochar holds and stores water (think about how an espresso puck remains damp or a V60 filter keeps the water for an age), means that the coffee plants are more resistant to drought, which is an increasing problem for coffee farms in a time of climate change.

More evenings are planned for early in 2020, do join us if you can. There were some excellent suggestions for topics for future events, so together with a few that we were thinking about already, there is plenty to think about for next year! However, if you have a question about the physics of coffee, have noticed something in coffee that you would like to explore or just generally want to think more about one or another aspect of coffee, do tweet, FB or email me your suggestions. Looking forward to 2020 already.

*Illy and Viani (Eds), “Espresso Coffee”, 2nd Ed (2005)