This is how espresso is made
Espresso brewing begins with grinding the beans to a fine consistency. Then, around 18-20 grams of this ground coffee is compacted into the iconic puck-shaped filter and inserted into the espresso machine. The machine then forces hot water (approximately 90-96°C) at high pressure (around 9 bars) through this compacted coffee. This process, called extraction, lasts between 25-30 seconds for a single shot of espresso.
During extraction, the hot water pulls out the coffee’s flavours, caffeine, oils, and other compounds, resulting in a concentrated beverage with a strong, rich taste. A perfect shot of espresso should yield a layer of golden-brown crema on top, signifying quality extraction.
Extraction is impacted by how finely the beans are ground
In 2020, researchers found that more finely ground coffee beans brew a weaker espresso. This is counterintuitive. It would make sense to expect that the finer the grind, the easier it is for the hot water to extract all the goodness from the beans. It turns out that very finely ground coffee forms more regions where extraction is poor. Researchers from the University of Huddersfield decided to explore this phenomenon in a 2023 study.
Uneven extraction is the problem
The researchers created a mathematical model to test uneven coffee extraction. They split the coffee into two regions to examine whether uneven flow does in fact make weaker espresso. One of the regions had more tightly packed coffee than the other.
Water flows more quickly through more tightly packed grains. This means that water encountered different resistance when flowing through each region. And the extraction of coffee itself decreases the flow resistance further, because coffee grains lose about 20% to 25% of their mass during the process.
“Our model shows that flow and extraction widened the initial disparity in flow between the two regions due to a positive feedback loop, in which more flow leads to more extraction, which in turn reduces resistance and leads to more flow. This effect appears to always be active, and it isn’t until one of the regions has all of its soluble coffee extracted that we see the experimentally observed decrease in extraction with decreasing grind size,” explained co-author William Lee.
The researchers were surprised to find that their model always predicts uneven flow across different parts of the coffee bed.
“This is important because the taste of the coffee depends on the level of extraction. Too little extraction and the taste of the coffee is what experts call underdeveloped, or as I describe it: smoky water. Too much extraction and the coffee tastes very bitter. These results suggest that even if it looks like the overall extraction is at the right level, it might be due to a mixture of underdeveloped and bitter coffee,” said Lee.
How do we prevent uneven extraction?
Understanding the origin of uneven extraction and avoiding or preventing it could enable better brews and substantial financial savings by using coffee more efficiently. Unfortunately, more research needs to be done in order to get us there.
“Our next step is to make the model more realistic to see if we can obtain more detailed insights into this confusing phenomenon. Once this is achieved, we can start to think about whether it is possible to make changes to the way espresso coffee is brewed to reduce the amount of uneven extraction,” added Lee.