Espresso Science: Debunking 6 Coffee Extraction Myths with Dr. Samo Smrke

A common sight in specialty cafes is the barista leaning their entire body weight into a tamper to save a fast-running shot. However, Samo Smrke explains that human strength is no match for the hydraulic forces of an espresso machine. When you engage a standard pump, it exerts roughly nine bars of pressure—equivalent to a staggering 250 kg of force—onto the coffee bed. Most manual tamping falls between 15 and 25 kg. Once you reach full physical compression of the grounds, additional effort is wasted. The coffee particles are not perfectly rigid, but they hit a point of maximum density long before the machine takes over. If you notice a change in shot time when pressing harder, it is likely due to an unintentional change in your tamping angle, creating uneven density and channeling, rather than the pressure itself.

Rethinking Freshness and the Gas Problem

The "roasted yesterday" mantra is one of the most persistent myths in the industry. While stale coffee is certainly a problem, ultra-fresh coffee brings its own set of chemical hurdles. Freshly roasted beans are packed with carbon dioxide, which creates turbulent blooming in pour-overs and excessive crema in espresso, physically blocking water from effectively reaching the coffee solubles. More importantly, Samo Smrke identifies a specific volatile aroma compound called metan tile. This compound, also added to natural gas to give it a detectable scent, is highly concentrated immediately after roasting. It produces a harsh, "gassy" flavor that masks delicate notes. Because metan tile is oxygen-sensitive and degrades quickly, allowing coffee to rest for several days—or even weeks for light roasts—results in a more balanced and palatable cup.

Boiling Point: Why You Aren't Burning Your Beans

Many enthusiasts fear that using water straight off the boil will "burn" the coffee, leading to bitter, ashy flavors. In reality, you cannot burn coffee with water at 100 degrees Celsius. Burning requires temperatures closer to 180 or 200 degrees Celsius to trigger the chemical changes associated with smoky notes. What people perceive as a "burnt" taste is actually the efficient extraction of bitter compounds that are harder to dissolve at lower temperatures. High heat also increases the volatility of delicate aroma compounds, meaning they evaporate into the air before you can taste them. While 92 to 96 degrees might be a sweet spot for many beans, using boiling water is a tool for increasing extraction, not a recipe for scorching the grounds.

The Acidity Paradox in High Extraction

There is a common misconception that pushing for a high extraction yield (EY) will mute a coffee’s acidity. Chemically, this is impossible. Acids are among the most soluble compounds in coffee and are extracted early. As you continue to run water through the puck, the concentration of acids in your cup actually continues to rise. However, the concentration of bitter compounds increases at a much faster rate during the later stages of brewing. On the palate, these bitter notes dominate and overwhelm the acidity, leading to the perception that the vibrance has vanished. In reality, the acid is still there; it’s just buried under the weight of more stubborn, bitter material.

The Myth of the Perfect 9-Bar Shot

For decades, nine bars has been the industry standard for espresso. Yet, the relationship between pressure and flow is not linear because the coffee puck is compressible. As you increase pressure from six bars toward nine, you are simultaneously squishing the particles closer together, which increases resistance. In many cases, the extraction at six bars is indistinguishable from nine bars because the added pressure simply compacts the bed further rather than forcing more flavor out. Once you exceed 11 or 12 bars, the puck can undergo a secondary compression that causes the permeability to collapse, often leading to massive channeling and erratic shots. Instead of obsessing over a specific pressure, focus on the fundamentals: grind size, brew ratio, and consistent puck preparation.