Nine Bar Pressure: Espresso’s Greatest Accident?
When it comes to the question of pressure, we’re often presented with a false dichotomy, where, on the one hand, we’re told that nine bar pressure has long been the industry standard for pulling espresso shots. Conversely, experimenting with pressure profiling or flow control is the only way to unlock the deep mysteries of any given espresso bean. In reality, what initially seems like a rigid binary is a more fluid and rich continuum. Well, we're going to say something controversial: nine bar pressure doesn't really matter...at all. This magical number arose out of happenstance and just stuck around. Is nine bar pressure still all it's cracked up to be? Let's dive deep into how pressure impacts extraction and the correlated history of espresso machines.
What are BARS?
To keep it simple, nine-bar pressure refers to a unit of measurement used to describe the amount of force used to extract espresso. One bar equals atmospheric sea-level pressure, approximately 14.5 pounds per square inch (psi) or 100 kilopascals (kPa).
A Short History Lesson
Before there were pump-driven and spring-lever machines, we relied upon machines that used steam pressure to extract espresso at 1-2 bar pressure at the group head. These steam-powered shots were too hot for extraction, watery, and bitter, which was the case at the 1906 Milan World’s Fair when “espresso” debuted. While these primitive espresso machines by Bezzera and Pavoni innovated the speed at which you can produce coffee, they did not resemble the crema-laden espresso we know and love today. Not enough pressure = no crema. The main goal of these early machines was to make espresso faster, not better, and it produced a beverage more akin to batch-brewed coffee than espresso as we know it. The following points are not an exhaustive list of innovations but are a few of the most important ones.
- Following the Industrial Revolution, Pavoni created the first portafilter. Yet espresso was still brewed at 1-2 bar pressure and with boiling water. Espresso machines during this time had a penchant for exploding.
- In 1933, Francesco Illy produced a system that separated heating and pressure. This system no longer relied on boiler water to brew coffee and gave espresso a considerable boost in flavor.
- The next major innovation was 1945 when Achille Gaggia introduced a spring-powered piston machine to quickly force hot water through coffee grounds and produce a concentrated beverage. This created a rich, creamy layer on top of the drink we now call crema. Off the heels of the popularity of this new crema cafe in 1946, Gaggia introduced the spring lever machine. The spring lever machine significantly improved the efficiency of the espresso-making process and helped popularize the drink in Italy. This marks the first time a barista can produce nine bar pressure.
- Lever machines were roughly a decade's standard configuration for espresso machines until Ernesto Valente introduced the Faema E61 espresso machine in 1961. The E61 mechanically pumps water through the coffee bed at 9 bars of pressure. These machines were driven by an electric pump, which made it much easier to achieve consistent pressure for each shot in the café setting. Almost all semi-automatic and automatic espresso machines today are pump-driven and can achieve 9 bar pressure.
What is This Doing to My Coffee?
We’d be stuck in the espresso doldrums without the pump-driven machine. The home espresso machine as we know it would be less accessible to everyday folks passionate about the craft of espresso-making at home. More importantly, coffee would taste worse. The key takeaways are that temperature and 9 bar pressure tend to produce better and more consistent espresso outcomes.
Wait, Why Nine Bar?
It isn't clear, but historical records suggest that Gaggia did not design the lever machine with precise pressure in mind. Instead, the piston, chamber, and lever size were determined intuitively, guided more by practical considerations than a specific technical goal.
The resulting pressure—approximately 9 bars—was not a deliberate choice but a natural outcome of the mechanics of 1940s engineering combined with human strength. A shorter lever would have produced lower pressure, while an excessively long one could have generated more force. However, the design that emerged worked, and when Gaggia introduced the first spring lever machines, he effectively automated the process his earlier models had relied on.
As a result of Gaggia's innovation, when Faema introduced their pump-driven espresso machine, they set the pressure to a now-standard 9 bars. Why? There’s no evidence that Faema’s engineers had a precise rationale or technical manifesto behind this decision. Instead, they followed what Gaggia had already established—and Gaggia had done it because it worked. It seems as though Gaggia inadvertently set the standard for espresso extraction pressure—a benchmark that has remained essentially unchanged for nearly a century.
Does 9 Bar Pressure Produce Better Espresso?
Not exactly. To understand its importance, it's crucial to remember the fundamental purpose of applying pressure: to extract a concentrated coffee quickly. Nine bars of pressure happen to be a sweet spot for this process, as it’s sufficient to extract a significant amount of flavor from finely ground coffee in a short period of time.
Using less pressure limits the ability to push water through, evenly saturate the puck, and extract flavor from finely ground coffee. On the other hand, pressures above 9 bar may seem appealing, but they come with potential downsides.
There are several exceptions to experimenting with lower pressures, such as:
- Pre-infusion: Typically, line pressure (2-3 bar) is used to gently soak the espresso puck to allow for more even extraction and less channeling before you apply full 9 bar pressure.
- Pressure profiling & flow control are techniques that allow for more experimentation and flexibility on the fly, giving you more direct control of extraction from start to finish. For instance, you can start with a low-pressure pre-infusion, ramp up pressure to 6 bar for several seconds, then 9 bar, and in the post-infusion stage, slowly ramp down pressure as the puck degrades. The possibilities are endless.
Beware of 15 and 18 Bar Promises
More bars make for a better product, right? Not really. Manufacturers advertise this on machines with low build quality and use inferior parts. Fifteen bars of pressure is way too high to brew for espresso and would result in some unsatisfactory extractions. Semi-automatic espresso machines, especially ones that employ vibratory pumps, will also use an expansion valve or OPV to regulate this pressure at the group head down to 9 bar. Consistency and control are critical.
Final Thoughts
One thing that has become exceedingly clear from just a cursory look at espresso's origin and the espresso machine's history is that pressure is the key factor in getting a proper espresso extraction. Although seemingly discovered by accident, nine bar pressure is the sweet spot for getting the most out of that delicious concentrated beverage. Ultimately, the 9-bar standard is not an arbitrary rule but the product of historical and mechanical factors that proved effective. What began as a quick coffee solution during the Industrial Revolution has evolved through temperature, flow, and pressure control innovations, ultimately shaping espresso into a refined beverage with its own dedicated art and craft.