What is the right pressure for making espresso

Espresso as a result of good coffee, water and pressure

In short, yes. Pressure is what differentiates espresso and filter coffee. The only pressure that is exerted on the coffee during the preparation of V60 or Chemex is the gravity of the water. This is also why it takes about 3-4 minutes to prepare. In addition, in order for the water to flow through the coffee, the beans have to be ground a little coarser.

Espresso is the opposite. Finely ground coffee, higher pressure and a short preparation time. The AeroPress is the intermediate between espresso and drip coffee . In its case, you need to use finer ground coffee. But because we only use our body pressure when making coffee with the AeroPress, the preparation is not as fast as with espresso, taking about a minute and coming a little closer to it. Even in taste it is only close to espresso and doesn't taste quite the same.

In order to create proper pressure and speed up the preparation of coffee, people started creating espresso machines more than 100 years ago.

The evolution of pressure in coffee machines over the years

For a better understanding, I'll briefly describe the evolution of coffee machines and how they got to the 9 bar pressure they are today.

The first espresso machines operated at a relatively low pressure. They used steam pressure to push boiling water through the coffee. They worked with about one to two bars of pressure, and even that meant you could make coffee faster than ever before.

The big innovation in modern espresso came in 1949 with theGaggia Achille. It made a huge leap in pressure. The coffee machine had a classic hand lever. These days, you're more likely to find that in theVictoria Andurino brand, for example .

After you pulled the lever down, the lever compressed a large spring, letting water into the brewing chamber, and when you released the lever, the spring expanded and pushed the water into the coffee. Forthe first time, this machine was able to get enough pressure to create crema.

Thenext leap forward in coffee brewing pressure came in 1961 with one of the most innovative coffee machines, the Faema E61. Its greatest advance was the use of an electric pump that provided constant pressure. So suddenly there was 9 bar of pressure as a kind of new constant in espresso preparation.

The next big leap was the introduction of pressure profiling. This was mainly due to the La Marzocco Strada, which came on the market at the turn of 2009/2010. With this machine, it was possible to create several different types of pressure thanks to the gear pump.

As a result, you did get a different taste of espresso, but not always a better one. Today, we know much better what happens when making espresso and it is possible to control all the factors more carefully.

The effect of pressure on espresso preparation

So what about the pressure during extraction? Why is 9 bar the norm, why is it the standard, and why are most espresso machines set to this value from the factory?

If you do a simple test, you take a certain batch of coffee, at a constant grind setting, and the espresso brewing time is, for example, 30 seconds, and you're brewing at seven bars, eight bars, nine bars, ten bars, eleven bars. You'll see that as the pressure increases, more liquid will pass through the coffee bud in a certain amount of time.

Pressures above 9 bars produce espresso that is too strong and thick, and anything below 9 bars makes the espresso weaker. That's why 9 bar pressure is standard on most coffee machines today. You could say it's a kind of tipping point where the consistency of the espresso is in balance. Moreover, the whole process is demonstrated by the aforementioned evolution of coffee machines.

Of course, you can also set 8.5 bar, for example, but then the water temperature and the coarseness of the grind also need to be adjusted to this, which is the same for the other pressure values.

The pressure of 9 bar (or any other set pressure) is only in the section behind the pump for the entire espresso brewing time. Gradually the pressure in the tubes increases, but the full 9 bar is never in the coffee head. During the preinfusion, a pressure of about 2 bar is applied to the coffee, so that the water does not press on the coffee and is able to wet it evenly. The pressure builds up and at the beginning of the extraction the pressure in the head is 7 bar and at the end the value is about 8.6 bar.

Why not skip preinfusion when making espresso?

What does preinfusion mean? When talking about filter coffee you will probably come across the term blooming, in espresso preparation this stage is called preinfusion. Which means the process of gently soaking the ground coffee bud in a portafilter before applying the full required pressure for brewing.

During preinfusion, the pressure is lower and the aim is to ensure that the water soaks the coffee evenly and then an equal amount of water flows through the entire bed. This is because if this does not happen, so-called channeling occurs, where the water flows through only some parts of the bed, so that the coffee is extracted more in some places and less in others. Channeling can be prevented by proper tamping and evenly ground coffee.

How does preinfusion work?

The way preinfusion works in different coffee machines is quite interesting. There are automated or mechanical solutions. The most common solution in commercial coffee machines is called flow restriction, you won't find it in every coffee machine, but if you do it is located closer to the pump than the head.

There's a hole that's normally 0.5 millimetres wide and what happens there is that the pump starts and all the pressure gets to this restrictor. This will allow the water flow to pass through, but it will restrict the flow. And only when the water fills from the restrictor up to the bud can all the pressure that has built up in the pump be transferred to the coffee bud.

The time during which this takes is usually six or seven seconds. After you press the start button, you'll see the pressure on the gauge go up and then right at the end when it's at number 9 it goes up some more. This little pressure rise is the final moment when you know the whole system has fully pressurized and the pump pressure is now hitting the puck.

The water pressure bursts on the bud, are not the same in every coffee machine, it depends on the diameter of the opening, but in any case it will limit the potential total flow to some extent.

If the machine has no restriction, and so lacks a pre-infusion stage, the coffee stream will start incredibly quickly. You will encounter this especially with cheap automatic coffee machines.

There's also a reason why people sometimes don't like flow restrictors, and that's because these small holes can get clogged with scale very quickly. Which causes the whole coffee machine to be broken. That's why it's important to have water filtration, it's a win-win situation.

Some machines have this flow restriction in the cold water circuit before heating the water, which also makes sense because if the water is hot, it is more likely to precipitate scale. But you can't say it's better for cold or hot.

And why is preinfusion important? The reason is simple. Even if you grind your coffee more finely or have a café full of customers and prepare one espresso after another, you can rely on the same quality of drinks

One other option in the absence of a limiter is to start the machine and stop after a while. This gets the bud wet, but it's already applying finite pressure, but it's actually a pretty disruptive process overall, so I don't recommend it entirely.

Why is the coffee maker sometimes listed at 9 and sometimes 15 bar?

You may be wondering why some semi-professional and home coffee machines state that they produce 15 bar or more when 9 bar is the ideal value. Is a 15 bar coffee machine better?

If you've ever seen machines advertising 15 or 18 bars, you've probably believed that the higher the number, the better the espresso. In reality,these types of coffee machines are usually cheaper and also lower quality than professional coffee machines.

These cheap coffee machines lose most of the pressure between the pump and the head. So even though the coffee maker states that it produces 15 bar of pressure at the beginning, by the time it gets to the steaming head, the pressure is much lower.

It is likely that by the time the espresso is extracted, the pressure is still less than 9 bar, it could drop as low as 3 bar. One factor is what pump is built inside the machine. I'll tell you more about that in a moment. The fact that the pressure is not what it should be during extraction is something you will encounter especially with automatic coffee machines.

A pressure of 14, 15 or 18 bar is the maximum pressure, but what is more important for us is the constant pressure, which is usually 9 bar.

In some coffee machines you will find two pressure gauges, one showing the pressure in the head and the other in the nozzle, the other option is a gauge that shows both pressures. The lower needle usually shows the pressure in the nozzle and the upper in the head of the machine. Source.

Pressure in home and professional coffee machines

The mechanism that determines the maximum pressure in the system is called the pressure relief valve (OPV). The OPV reaches the set pressure value and diverts excess pump flow to maintain that pressure.

This means that the pump is no longer delivering maximum flow to the coffee bud. In commercial pumps, the OPV is located between the pump inlet and outlet, while in home coffee machines the OPV is located just after the pump outlet. The measured constant pressure of the system varies depending on where the gauge is located.

This small difference can make it so that if someone with a home coffee machine and someone with a professional coffee machine are talking about pressure, they may not be talking about the same thing. This is because with a professional coffee machine you can adjust the pump pressure, but this is not the same as a home coffee machine. Its reading on the pump gauge is before the OPV and will therefore read 9 bar, even if the pressure at the group head is lower.

The difference between a rotary and a vibrating pump

Previously levers and human power were needed to create pressure, today coffee machines work in a more modern way so that a pump is needed to create pressure. When choosing a coffee maker you may come across two types of pumps, rotary and vibratory. So what is the difference between them and is either of them better?

Coffee makers with rotary pumps are the top of the line among coffee makers. Rotary pumps are quiet, sturdy and thus more spatially distinctive. In most cases, if the coffee machine is equipped with a rotary pump, it can beconnected directly to the water supply line.

In the case of a rotary pump, this is a complex mechanism. The motor rotates a disc that is moved inside a large round chamber. The rotating disc is divided into parts by blades. As the disc rotates, the vanes push against the wall of the outer chamber, reducing the size of the section and creating pressure. Water enters during the large phase and is forced out as the section shrinks.

In a vibrating pump, the piston is attached to a magnet and is inserted into a metal coil. An electric current passes through the coil, which causes the magnet to move the piston back and forth rapidly, pushing water into the machine. The average vibrating pump has a speed of sixty squeezes per second.

Effect of the pump on the final espresso

To give you an even better description of the difference between the pumps I'll use Marc's test from Whole Latte Love. The test used Rocket coffee machines that were virtually identical except for the pump used. They also used the same coffee with the same coarseness.

The coffee machine with the rotary pump reached a pressure of over 8 bar almost immediately, whereas the vibrating pump took a little longer. Even though it was switched on later at the moment its pressure gauge shows 2 bar. With the first drops of coffee, the coffee machine with the vibration pump moves up to 5 bar.

During extraction ,the pressure on the rotary pump's gauge is steady, while on the vibrating pump it jumps between 8 and 9 bar. The pressure fluctuation also seems to be related to the switching on of the heater. While the coffee machine with rotary pump delivers a constant pressure, even when the PID briefly switched on the boiler.

As the extraction continues the shots of espresso don't look quite the same. The crema in the espresso from the rotary pump is darker, has a more consistent color, a finer texture of bubbles, and looks much better overall.

Both shots finish with a volume of 60 milliliters and 29 seconds from the first drop. Once the shots settle, the crema layer of the espresso from the rotary pump shots is much thicker and more even, while the crema of the vibrating pump shots fades quickly and has a very distinct color difference and a coarser bubble structure. The difference between the pumps is therefore more than clear.