Brewing the Perfect Espresso, With Chemistry!

coffee beans and grounds and cup of espresso ftloscience post

“My God, that’s the best coffee I’ve ever tasted; why the hell are we making meth?” Coffee is widely consumed and loved (even by drug kingpins), yet no one can agree on what makes an ideal cup. The problem lies in the variation between espresso pulls, resulting in poor taste consistency. Luckily, chemistry and math come to our rescue with a strategy to help you brew the perfect espresso every time.

Consistently Inconsistent Coffee

Coffee beans contain various chemicals, from the all-important caffeine to obscure-but-delicious Maillard reaction products (also found in baked goods). The extraction process is the key to getting all this good stuff out of the roasted bean, which involves grinding it into grounds and then passing water through it. An espresso is arguably the most popular extraction method, where all the good stuff is ‘pulled’ from the grounds using a small volume of water under high pressure. This allows the most chemicals to be extracted from the coffee beans and into your lovely morning brew.

The problem with coffee extraction lies in the variation between espresso pulls, even when the same espresso machine and the same amount of coffee are used (i.e., in a café setting). They include differences in ground particle size, pressure and flow rate, tamping (mechanical compression of the grounds before extraction) and even the time elapsed since the bean was roasted. Combined with individual preferences in coffee taste, these ensure it is almost impossible to brew a perfect espresso; until now.



What’s in a Perfect Espresso?

In 2020, researchers at the University of Oregon and the University of Portsmouth applied mathematical models to the process of espresso-making. They found that only two variables significantly altered the extraction yields (and hence, taste) of the final product: the particle size and the number of coffee beans used. So, they set about searching for the perfect balance.

Optimum Particle Size

Using a grinder, coffee beans must be ground before pulling an espresso shot through. Electric grinders often have dials to adjust for a finer or coarser grind. A low grind setting (producing smaller, finer particles) should increase the surface area of the coffee grounds. In theory, the smaller, the better; this would increase contact time with water and solubilize more chemicals, resulting in a better extraction.

However, the mathematical models used by the researchers showed that clogging occurs when the particles become too small, leading to poor extraction yields. Clogging is also responsible for much of the variation between cups of coffee since water flowing through the grounds is impeded ununiformly each time.

extraction yield vs grind setting

Therefore, there needs to be a balance in the grind setting to ensure maximum extraction while preventing clogging, as seen in the graph above. However, it is essential to note that maximum extraction doesn’t always equate to the best taste. According to experts in the coffee industry, extraction yields of close to 20% result in an optimal cup of espresso; otherwise, it will taste too sour (<17%) or too bitter (>23%).

Number of Coffee Beans

Naturally, the amount of coffee beans used will influence the final product’s taste. Again, this is to do with the extraction yield, as more beans = more extracted contents and vice versa. However, the mathematical models predicted something counterintuitive instead. The researchers studied five different doses (as seen below) across several grind settings, finding a trend of decreased extraction yield overall as the dose of coffee grounds used for the extraction increased.  

extraction yields of different doses of coffee grounds

This data also showed that increasing the grind setting of a smaller coffee dose would result in a similar extraction yield to a larger quantity of a finer grind. For example, if a 24g dose at a 1.1 grind setting produces the perfect espresso, we might achieve the same taste using a 22g dose at a 1.7 grind setting, saving us some coffee beans!

Ultimate Strategy for Reducing Variation

With these two parameters in mind, the research team designed an experiment with the help of a professional barista. First, they established the ‘tasty point’, or the grind setting that would produce the best-tasting cup from 20g of coffee beans. Then, they played around with the grind settings to find the point of maximum extraction. Once the grind setting that would produce maximum extraction was found, they decreased the volume of water used to correspond to the extraction yield of the ‘tasty point’ grind setting. This is seen in the top half of the flowchart below.

strategy for consistent espresso flowchart
There seems to be a typo in the caption of the bottom right graph. It should be ‘arrive at similar tasty point EY by grinding much coarser

Next, the researchers set about testing their theory of reducing the dose of coffee grounds. They reduced the amount to 15g and increased the coarseness of the grinds until it gave a similar extraction yield to the ‘tasty point’ of the initial 20g dose. This is seen in the bottom half of the flowchart below.

Repeating each step makes it possible to systematically improve reproducibility while reducing the overall mass of coffee beans used in each espresso pull. We now have a viable strategy to obtain the optimal grind setting and coffee bean amount—the two most important parameters influencing taste—in virtually any setup. The perfect espresso is finally within our grasp; just be careful not to consume too much at once!

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