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Dialing in espresso to produce a tasty shot is a tricky and often laborious process. Anyone who has worked behind the bar or wrangled with their home espresso machine can attest to how many times they have had to adjust their grind and pull shots to get espresso to taste just right.
When fiddling with a grinder, making the settings finer slows the rate at which water can move through the grounds; a coarser setting does the opposite. But how exactly do those grind changes impact what’s happening within the portafilter? An international team of scientists decided to find out.
The study, published in the journal Royal Society Open Science, investigated how various grind sizes can impact puck permeability (or how water moves through ground and tamped espresso) and extraction. To do this, the researchers ground two different coffees at 11 grind settings, ranging from fine to coarse. They then packed the samples into tubes and used X-ray computed microtomography to create a 3D map of each sample.
The maps allowed the researchers to see the internal structure of each sample, including all the little hidden passageways and rivulets known as pore spaces—these create paths for water to move through an espresso puck. They then ran digital tests to see how grind size impacted water flow. With this data, they were able to create a mathematical equation that predicts how well a shot might pull based on grind size and tamping.
“We have proposed and validated a constitutive model for the permeability of beds/pucks of ground coffee,” the authors wrote in their conclusion. They also note that other factors, like oxidation and coffee staling as it ages, and how the grounds expand when in contact with water, can impact permeability and require further investigation.
Read more on the espresso equation from Sprudge here.
Photo by ThisisEngineering on Unsplash
