There is a number that no one says out loud in any serious espresso café in Milan. The bar counter is made of worn marble, and the machine hisses with quiet authority. Nine. Nineteen pounds of force. It’s inside the rotary pump, every E61 group head’s history of calibration, and a hundred years of Italian coffee making. It wasn’t put up on a wall. It wasn’t necessary. That’s just how espresso worked.
But it looks like that might not be quite right. Modern research in fluid dynamics and a growing number of specialty coffee labs are starting to wonder if nine bars is really the best number or if it’s just the number that has been used for a long time and no one has thought to question it seriously enough. It looks like the answer is trickier than the business world wants to admit.
The story of how the nine-bar standard came to be is both interesting and disappointing. Achille Gaggia patented a spring-piston lever machine in 1948, which took the place of the old steam-pressure method of making coffee. For a long time, espresso was made with steam at about 1.5 to 2 bars. It was dark and bitter, and didn’t have the crema that we now associate with the drink. His lever system used real human force, and engineers were able to measure the highest pressure that the springs could produce—around nine bars. When commercial machines switched from using levers to electric rotary pumps, the new pumps were calibrated to the same number. Not because the research said to. Because that’s what the famous machines did. Nine bars was an old way to measure that was used as a standard.
| Detail | Information |
|---|---|
| Topic | The 9-bar espresso pressure standard and modern alternatives |
| Traditional Standard | 9 bars of pressure |
| Origin of 9-Bar Standard | Achille Gaggia’s 1948 spring-piston lever machine; peak spring pressure measured at approximately 9 bars |
| Why 9 Bars Was Adopted | Historical convention from lever machines; electric rotary pumps calibrated to match, not from scientific research |
| Traditional Extraction Parameters | 9 bars pressure, 88–93°C water temperature, 25–30 second extraction, 7–9g coffee dose |
| Key Problem with 9 Bars | Channeling — water exploits puck inconsistencies, over-extracting some zones while under-extracting others |
| Secondary Problem | Fines migration — high pressure pushes fine particles downward, clogging basket and causing erratic extraction |
| Low-Pressure Range Being Explored | 3–6 bars |
| Main Benefit of Lower Pressure | More even extraction, more sweetness, cleaner acidity, reduced bitterness |
| Extraction Sequence | Salts/acids extract first, then sugars/caramels, then bitter tannins/fibers — lower pressure avoids crossing into over-extraction |
| Machines Enabling Experimentation | Slayer, Decent Espresso, E61 with flow control paddle, OPV-modified Gaggia Classic / Breville / DeLonghi models |
| Recommended Low-Pressure Profile | Pre-infuse at 1–2 bars (10–15 sec), ramp to 5–6 bars, taper at end of shot |
| OPV Adjustment | Adjustable on many home machines; can be modded to relieve pressure at 6 bars instead of 9 |
| Traditional Italian Blend | ~90% Arabica, ~10% Robusta |
| Italian Roast Profile | Medium-dark; bittersweet, low acidity, full body |
| Crema Quality Indicator | 3–4mm thick, hazelnut color with red hints, compact and smooth texture |
| Key Historical Figures | Angelo Moriondo (1884 patent), Luigi Bezzera (1901 commercial machine), Desiderio Pavoni (La Pavoni company), Achille Gaggia (1938 lever machine) |
| Where Lower Pressure Works Best | Light roast, single-origin coffees (Ethiopian, Colombian etc.) |
| Where 9 Bars Still Suits | Medium-dark Italian-style blends with Robusta component |
| Notable Manufacturers of Commercial Machines | Cimbali, La Pavoni |
The issue is that coffee has changed a lot since 1948. Roasting has changed over time. The technology behind grinders has improved so that they can make much more accurate particle distributions. Single-origin light roasts from Ethiopia or Colombia are brewed in portafilters that were made for the darker Italian blends that were popular in commercial cafés in the 1960s and 1970s. The pressure setting didn’t change, though. It’s possible that no one spoke out because the results were good enough—thick crema, familiar flavor, happy customers—and good enough doesn’t call for revolution.
As we now know from fluid dynamics research, nine bars can be a blunt instrument. When the pressure is that high, water hits the coffee puck with a lot of force. There are always tiny flaws in the puck, no matter how carefully the barista distributes and tamps it. These flaws become channels for the coffee to flow through. The water goes the easiest way by boring through weak spots in the bed and over-extracting those channels while leaving most of the puck alone. Harsh, astringent chemicals pour out of the areas that were over-extracted. Sugars and acids from fruit stay locked up in the grounds. There are some really hard to pin-point differences in how the cup tastes that are easy to pick up on.
Less pressure—between three and six bars, depending on the coffee and the method—makes a big difference in how things work. The water moves more slowly and evenly through the puck. Fines, which are the tiny dust-like particles that every grinder makes, stay put more often instead of moving down and clogging the basket. The extraction goes more slowly, which gives the water time to break down the sugars that show up in the middle of the process before getting to the bitter compounds that come later. From what baristas who use flow-profiling machines have said, the end result is a cup that is noticeably sweeter, has less acidity, and has what some call a clarity that the nine-bar version hides.
Seeing how this conversation grows in the coffee world makes me think that the specialty coffee world is working through a real problem instead of following a trend. The Slayer, the Decent Espresso, and modified E61 setups with flow control paddles are some of the machines that let you try lower-pressure experiments. These machines really change what home and professional brewers can control. Pre-infusing at one to two bars, slowly building up to five or six bars for the main extraction, and then tapering off at the end. It sounds complicated, but people who do it say that light roasts become much more expressive.
That’s what you should say about the Italian coffee culture: it’s not easy to change. The rules for making espresso, like extracting it for 25 to 30 seconds at 88 to 93 degrees Celsius with 7 to 9 grams of coffee and a crema that is measured in millimeters, weren’t made up on the spot. People who really cared about what was in the cup worked on them for decades. The nine-bar convention kept this tradition alive because it worked for Italian espresso, which is meant to be strong, bittersweet, and concentrated. It is made from medium-dark roasted Arabica and Robusta blends that work well with higher pressure extraction. It’s still not clear if lower-pressure methods improve traditional Italian-style espresso or if they’re just a new discovery for the lighter roast specialty market, which wasn’t something that Italian tradition put much emphasis on.
The question is now being asked seriously in labs and specialty coffee shop back rooms by people who grew up with the nine-bar standard and aren’t brushing it off. That is a meaningful change in and of itself. The number isn’t holy. It was never there. What’s important is what gets into the cup.
