Why 9 Bars Are The Ideal Pressure For Espresso

Pressure is what makes espresso. Espresso allows us to brew coffee in seconds rather than minutes and produces the concentrated liquid gold shots that have come to define coffee worldwide.

So isn’t it a little strange that we tend to set and forget the pressure on our espresso machines? What if there is a perfect pressure that we’ve been missing this whole time?

If you were anything like me when I started working as a barista, I was always taught that coffee machine pressure should be set at the sacred nine bars of pressure. This is how it’s always been done, okay?

So this is normally the point where I say, “it’s time to shatter the mystic traditions with some cold hard facts,” but this time, our tests do suggest that the old nine-bar tradition still holds up.

So if you’re in a hurry, there’s your answer; based on the testing, we recommend a brew pressure of 9 bars.

Of course, there’s a whole lot more to it than that, but you’re probably not interested in that.

So, before we dive into the results, let me set some things up with a quick note on the pressure.

In most modern espresso machines, an electric pump forces water throughout the ground coffee at a set pressure.

On older lever machines, this pressure is created by a lever and the massive biceps of the barista. Thus, a lever machine is a very simple example of what baristas sometimes call pressure profiling.

Pressure profiling refers to manipulating the pressure of the water throughout the shot, unlike an electric pump that maintains constant pressure.

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The pressure starts high with a lever machine and gradually drops throughout the shot as the spring returns to its original position.

Another example of this is what we call pre-infusion. This is where the line pressure of water runs for the first few seconds to allow the ground coffee to get wet and saturated before getting smashed by the full pressurized force of the water.

The goal here is to prevent channels from occurring in the bed of coffee during extraction.

For these tests, we didn’t focus on pre-infusion lever machines, massive biceps, or any other ways in which water pressure can be varied throughout the shot.

No, we tried to keep things focused on our mission of finding out if there is an ideal pressure to set your espresso machine too. And with that out of the way, here’s the tests that our engineers cooked up.

We measured espresso shots brewed at 6 bars, 9 bars, and 12 bars of pressure using 14 grams, 17 grams, and 21 grams of ground coffee.

We then measured the extraction time. That is how long the espresso took to brew, and we measured the TDS, or the total dissolved solids, using a refractometer. This tells us the overall strength of the espresso.

We also monitored the flow rate using our shiny new flow telemetry system. Oh yes, and we tasted the coffee too, you know the old school methods.

Now, it is worth noting that the pressure was measured using the dial on the front of the machine, which, in this case, was the La Marzocco Linea PB and KB90.

Pressure measured at the dial will always be higher than the pressure at the group head. The pressure at the group head will also vary depending on the flow rate of the coffee.

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So for us, we decided that the most practical way to measure pressure is using the dial, which is what most users will see from day to day.

Finally, we use the same coffee for all of these tests. We used a light-medium roast made up of natural processed coffees which we rested for a week after roasting. Well, except for one of the tests, but I’ll get to that in a moment.

The first thing that we found was that as pressure increased, the extraction times also increased; in other words, the more pressure we used, the more slowly the shots poured.

Logically you would expect the opposite to happen, so what is the deal here? We guessed that the carbon dioxide gas produced by the fresh coffee was producing more and more resistance to the water as the pressure increased.

So to test this theory, we tried the same experiment with the same blend of coffee, except this time it was stale.

We chose stale coffee because as coffee gets older, the trapped gases slowly escape the roasted coffee beans, and that’s why stale coffee produces less crema.

Sure enough, with stale coffee, we found that the extractions behave the way you would expect. So it seems likely that the gas theory is why the high pressures result in slower pouring shots.

This also seems to be supported by looking at the rate of water flowing through the bed of ground coffee.

We used a flow telemetry device that graphs the pulses from the machine’s flow meter, it gives a visual guide to each espresso’s flow rate.

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The shots brewed at 12 bars quickly saturate the bed of ground coffee but then immediately drop down to a slower flow rate than the lower pressure shots. This indicates that this resistance is occurring really early on in the process.

The next thing we found was that the TDS, that is, the strength of the espresso, had more to do with the extraction time than the pressure.

When extracting times were low so was the TDS; when extraction times were higher, so was the TDS, regardless of the pump pressure.

Now, that’s not to say that the pump pressure doesn’t have an impact. But, as we noted earlier, when using fresh coffee, it increases the pressure.

It helps to increase the extraction time, which might be a good thing if you’re using a lighter roasted coffee that’s harder to extract from.

Of course, grinding finer will also cause the extraction times to increase, but the grind size will change. Not only the strength but also the overall balance of the flavor extracted from the coffee.

Speaking of flavor, let’s get to the point. Which one tastes the best? We started with our standard brew recipe. For this coffee, the pump was set to 9 bars of pressure, and we used that as our benchmark.

We then kept everything else the same and reduced the machine’s pressure to six bars; the results weren’t great. Our tasters describe these shots as watery and thinner texture than the control.

Of course, as we already discovered, lowering the pressure also made the shots run quicker, so we did try a few adjustments to compensate for this.

We tried grinding finer, then we tried increasing the dose, and then we tried a combination of both. The result was that they all came up short in flavor and texture compared to the shots brewed at 9 bars.

Then we tried the same process again at 12 bars of pressure. This time our results were mixed.

Some tasters preferred the thicker texture and bigger, bolder flavors of these shots. On the other hand, some felt they were starting to taste over-extracted and preferred the balance at 9 bars.

None of us had regularly made espresso at higher pressures in a cafe setting, so we thought we would push this little experiment a touch further.

Our espresso bar’s head barista agreed to run his machine at 12 bars for a day to see the impact on various coffees and get some feedback from a broader mix of people.

The feedback from both baristas and customers was less than enthusiastic. Baristas told us that they struggled to get the coffee’s tasting right, and customers told us that the coffee tasted different in milk and the result of all of this is that we’re going to stick with nine bars now.

For the home or pro barista, our suggestion is this if your roaster recommends a specific brewing pressure for their coffee, then go for it. Otherwise, your best bet is probably still going to be nine bars.

There’s more to be investigated on using lower pressure for darker roasts and higher pressures for lighter roasts. Perhaps there are some wins in there, but that will have to wait till next time.

So how do you adjust the pressure on your machine? Well, it does depend on the type of machine that you have. Commercial machines and some high-end home machines have a rotary pump.

Rotary pumps are larger and quieter than the vibrating pumps found on many smaller home machines. If your machine has a rotary pump, then the process is fairly simple.

Locate the pump and find the screw that controls the pressure. Load up a fresh shot of espresso hit go, and adjust the screw until the dial reaches the pressure you desire.

For machines with a vibrating pump, which is usually found in more affordable brands, then the pressure is generally not adjustable without getting into some DIY hacks.