DEBUNKING 6 MOST COMMON COFFEE MYTHS: Feat. Chemist Dr. Samo Smrke

Temperature and pressure are not the primary variables requiring adjustment when dialing in espresso; more importantly, baristas should focus on puck preparation, grind size, and ratio as the main levers for achieving desired extraction.

“So when we're looking at all the variables of extraction, pressures included, times included, temperatures included, but we can kind of push those aside as maybe static things that we don't really need to worry about as much to focus on what does matter, which is, well, first off, puck preparation, but very important, as we can tell through a lot of these answers. But we need to look at, start with coarse, go finer, we need to look at the ratio, we need to look at grind size, right?”

Methanethiol (methyl mercaptan) is the most volatile compound in fresh coffee and is also the compound added to natural gas to make gas leaks detectable by smell.

“One of the compounds in coffee, the aroma compound called metan tile, is the most volatile compound in coffee, and it has a very unpleasant smell. So typically, metan tile is the compound that is added to natural gas for us to be able to smell it”

What people perceive as burning from hot water is actually two distinct effects: (1) higher temperatures extract bitter compounds faster because bitter compounds are hard to extract and high temperature accelerates extraction, and (2) high-temperature water causes volatile compounds to evaporate during brewing, stripping delicate aromatic notes.

“One is the temperature is helping you extract compounds, so the bitter compounds are not volatile; they are usually quite hard to extract. That's why extracting at a higher temperature, you'll get more bitterness because you're extracting the bitter compounds better. But the volatility works the other way around, so if you have high-temperature water, you get more evaporation of volatile compounds during the brewing process, so you might be losing some of the delicate notes from the coffee.”

When you tamp very hard and hold the tamper at an angle (not level), you are likely to create uneven tamping, which changes shot time through increased channeling—not because the pressure itself differs, but because the puck density becomes uneven.

“There is a big change whenever you tamp really hard because I'm sure a ton of you have anecdotal evidence of a change occurring when you tamp harder. My assumption is that if you tamp really hard and you're up like this, you're likely going to be more uneven in your tamping, which can absolutely change your shot time due to the difference in channeling.”

The common belief that the freshest coffee is the best is false; very fresh coffee contains high concentrations of carbon dioxide and volatile compounds (especially methantile, an unpleasant gaseous aroma compound similar to natural gas), which degrade over days, making the coffee more palatable and balanced after a few days off roast.

“It's not always the best. Okay, it depends on your preference. If you like very gassy shots, then perhaps it's the best for you. But most people will not describe fresh coffee as the best coffee because fresh coffee contains a lot of carbon dioxide, and when coffee has a lot of carbon dioxide, the blooming is very strong.”

The myth that over-extraction causes loss of acidity is false; chemically, acids are among the first compounds extracted (approximately 90% of acids extracted at typical 20% extraction yield), and over-extraction still increases acid concentration in the brew because acids remain in the liquid once extracted.

“From a chemical point of view, when we are extracting coffee, we are extracting material compounds from coffee. We have a certain curve as we go through our brew. We go more and more and extract further and further until we extract everything. Typically, when we are at our 20% extraction yield, which is the typical extraction yield for most average brews, we are extracting maybe 90% of all the acids. Maybe we are at 80% caffeine and like 70% of the bitter compounds. But if we go higher than this, if we over-extract the coffee, we will not lose the acids because they are already in the brew.”

The coffee puck does not follow linear pressure-flow relationships (Darcy's Law does not apply) because the puck is compressible; as pressure increases, the puck compresses further, increasing water resistance, which can result in no net change in extraction yield between lower and higher pressures.

“the reason behind this is that the coffee puck does not behave according to a linear flow-pressure relation. So the Darcy's law does not apply for the coffee puck, and this we explain with the fact that the coffee puck is slightly compressible.”

There is no way to tamp too hard because espresso extraction uses nine bars of pressure, which applies 200-250 kg of force on the coffee puck—far more than the 15-25 kg a human can generate by hand—so manual tamping cannot exceed the compression already achieved by the machine.

“No, there's no way to tamp too hard. The reason behind it is that when we extract espresso, we usually use nine bars of pressure, and this nine bars of pressure acts as a force on the top of the puck. This force is around 20 to 250 kg, typically on press. It's way more than you can tamp.”

Channeling (concentrated water flow through isolated regions of the coffee puck) has been modeled theoretically and studied, but has never been directly observed in real-time within the puck itself; its actual importance to overall extraction is unknown.

“There are studies that did some tests to try to prove channeling. There are modeling studies that try to prove channeling, but nobody actually ever saw channeling in real life happening, right? So we kind of know that it happens from the fact that you see sometimes coffee flowing out at a certain point, yeah, from the open porta-filter or from the top; you see holes. But to what degree is happening within the puck or how important it is to overall extraction, we don't really know.”

Brewing with boiling water (100°C) or water that is too hot does not burn the coffee in the true sense; actual burning requires temperatures of 180–200°C to create smoky, ashy notes, which brew water cannot reach.

“I would say not because burning it means there's like a really high temperature involved to create a burning flavor, like smoky, ashy notes, and flavors like this. You need way higher temperature than your brewing water can be. Okay, so you're looking at 180 to 200 degrees Celsius, where you could start burning something, but with the brewing water temperature, you are not burning it.”