How does the espresso machine work when beans keep changing?

Espresso is pulled under pressure as high as 9 bars, and extraction happens in seconds, so every parameter sits near its limit. Keeping dose, water temperature, yield, and grind size identical does not guarantee the same flavor. Coffee bean degassing, humidity, and burr wear all shift the result, which is why shops dial in their espresso each day.

Beans release carbon dioxide after roasting, and darker roasts produce more of it. That gas inhibits the dissolution of flavor compounds and affects crema, so a fresh roast pours differently than a rested one. Rainy weather raises humidity and changes how the puck resists water, making a good shot harder to pull and grind size adjustments necessary.

Many coffee enthusiasts are puzzled by the practice of coffee shops adjusting their espresso every day, believing that as long as the dose, water temperature, yield, and grind size remain consistent, the flavor should be stable. However, the pressurized extraction method of espresso is extremely sensitive—coffee bean degassing, weather humidity, and burr wear can all cause flavor deviations. This article provides a detailed analysis of how carbon dioxide affects crema and extraction, why it's harder to pull a good espresso on rainy days, and the reasons behind grind size changes, along with Front Street Coffee's professional take on the necessity of daily espresso calibration.

Many friends find this puzzling, thinking it seems like an unnecessary step. After all, if espresso extraction worked like pour-over—with fixed dose, water temperature, yield, and grind size—then today's output should taste nearly identical to yesterday's.

That said, in reality, the odds of achieving a flavor close to yesterday's are not high. Espresso extraction involves numerous factors—not to mention a full day apart, even just a few hours can change the flavor extracted from the same set of parameters. Otherwise, why would dialing in espresso remain a daily task at every coffee shop to this day?

The reason espresso extraction demands such precision lies in its unique pressurized extraction method. Under pressures as high as 9 bars, compounds in the coffee are rapidly extracted. But because extraction time is extremely short, every parameter is pushed to extremes—very high water temperature, very fine grind… This means that even the slightest change in the coffee beans or the surrounding environment will cause the flavor to fluctuate.

Changes in Bean Condition

After roasting is complete, the condition of coffee beans is constantly changing, primarily due to the release of gases from within the beans.

Everyone knows that coffee beans generate a large amount of carbon dioxide during roasting—the darker the roast, the more carbon dioxide is produced. These gases continue to escape after roasting ends, and our extraction is precisely what gets disrupted by carbon dioxide. Because of its presence, the dissolution and release of flavor compounds are inhibited to a certain degree.

Given identical extraction parameters, the higher the carbon dioxide content in the coffee beans, the fewer compounds are extracted, because carbon dioxide hinders water from dissolving flavor compounds. If you've ever pulled espresso with freshly roasted beans, Front Street Coffee believes you'll easily notice:

When making espresso with just-roasted coffee beans, although the crema looks very thick, the flavor is often bland and thin, with insufficient aroma. Moreover, the thick crema dissipates quickly, clumps up, and hardens. This happens precisely because carbon dioxide release replaces the release of flavor compounds, thereby affecting the coffee's taste.

But after the beans have rested for a period, the extracted coffee not only has a full aroma but also smooth, delicate crema. If you want to extract a similar delicious result with fresh beans, you need to achieve it by adjusting parameters.

The amount of carbon dioxide also brings another effect—flow rate. From the moment coffee grounds contact water, carbon dioxide is instantly released. The bubbles formed during release overflow upward, and their kinetic energy creates a reactive force that impedes hot water from penetrating downward, requiring more time for the same amount of water to pass through the filter bed.

Since coffee beans are constantly degassing, we must continuously adjust extraction parameters based on the beans' current condition. Only in this way can we ensure that the coffee we produce every day maintains the same delicious taste. And this is precisely the most critical factor in daily espresso adjustment.

(There is also the situation of switching to different batches of coffee beans.)

Weather and Environmental Changes

Weather also affects espresso extraction—this is something Front Street Coffee often uses as water to share

content about. Espresso on rainy days is always harder to dial in than on sunny days, because rainy day humidity is higher, and the air is full of active water molecules. Not only are coffee beans more prone to absorbing moisture, but the ground coffee is also more likely to clump.

If there are大量 clumps when tamping the puck, channels are极易 formed during extraction. Channeling causes water to concentrate through one area, resulting in uneven extraction and coffee that becomes both bitter and sour, with conflicting and jarring flavors.

Not only that, but falling atmospheric pressure also affects the coffee machine's pressure, preventing the machine from extracting espresso at full capacity. So espresso on rainy days is always harder to pull well, because various parameters fall short. For this reason, we need to adjust parameters even more so that espresso on rainy days can also taste as good as on sunny days.

Changes in Grind Size

Finally, changes in grind size! Even when using the same grinder, the same setting, and the same coffee beans, the particle size produced each day will still differ to some extent.

The main cause of this difference is burr wear. You'll notice that the grind setting number on the grinder keeps getting smaller—the reason being that the burrs wear down after repeated use, the gap between burrs grows larger, and we naturally need to keep adjusting the setting finer to narrow the gap.

(Although this factor has a relatively much smaller impact.)

There's also the fact that when switching to different batches of coffee beans, the grind size needs to change accordingly. Because not every roast batch produces exactly identical coffee beans, the grind size naturally needs to be adjusted correspondingly.

Combining the above three points, in order to ensure guests always enjoy the same delicious taste, espresso adjustment has become a required daily morning task at coffee shops. In fact, it's not just in the morning—espresso almost needs to be adjusted every few hours, because the effects of bean condition and environmental changes are truly very noticeable~

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The above content is compiled by CoffeeHunters, a coffee news website.