Why ‘pressure’ is so important for espresso

Pressure matters more than you think; espresso has always been extracted using pressure, but it’s not that simple. This article will dive into the importance of espresso machine pump pressure differences for quality and how technology has changed over the years to produce the espresso we are served today.

What is pressure?

Pressure is the measure of a force applied over an area, calculated with the formula Pressure = Force/Area; it describes how concentrated a force is. Pressure is measured in the coffee industry using ‘bars’, 1 bar of pressure is approximately equivalent to the atmosphere, or 14.5 psi (pounds per square inch).

Espresso is pressurised hot water forced through finely ground coffee. The coffee creates resistance and this resistance is varied based on the amount of coffee and how fine it is ground. Without pressure we can’t achieve a highly concentrated dose of coffee. To put things into perspective, a car tyre is 30-40 psi; hold that thought as we’ll come back to this later.

The first espresso machines from the early 1900s used huge vertical-standing cylindrical steam boilers that beautifully dominated the bar. The boilers heated water to high temperatures – past the boiling point of water and in a pressurised system of 1-2 bars (14.5 – 29 psi). Water under pressure has a higher boiling point than low-pressure environments. Water boils at 100 degrees Celsius at ‘sea level’, but have you ever boiled a kettle at altitude, where atmospheric pressure is lower? At 1000m, the boiling point drops to 96.7°C; at 2000m, it's 93.4°C. You get the picture!

So, the pressurised environment of the boiler caused water to have a much higher boiling point of 120°C, creating steam pressure of up to 2 bar. This pressure forced boiling water through a piping network to allow the water to cool slightly, which was then able to pass through the coffee grounds to create ‘espresso’. Note: this style of espresso was a lot weaker than modern-day espresso with little to no crema and much less body and texture. The pressure couldn’t extract those sought-after characteristics of a classic espresso. With only 1-2 bars of pressure, you had to grind quite coarsely, and the temperature was very hot, so the result was quite an unbalanced flavour profile. However, this was the first of its kind!

The dawn of the lever machine

In 1947 this was replaced by lever technology, where the pump was a manual lever. The barista would pull downwards to compress a heavy spring, and this would drive a piston downwards, forcing hot water into the ground coffee. This was able to achieve much higher pressures than a steam pressure boiler – in some cases up to 16 bar or 232 psi (almost 7 car tyres' worth of pressure)!

The pressure profile of this espresso shot was unique, and it took a lot of experience to be able to create consistent shots.

The lever pulls hot water into the group, soaking the coffee puck. This fills the space between the coffee surface and the shower screen, like a pre-infusion. However, the effect changes depending on how quickly you pull the lever. That’s why it requires experience to get it right. Once the lever is fully pulled and released, pressure would peak and would slowly decline throughout the extraction.

Towards the tail end of any type of espresso extraction process, the puck integrity gradually degrades and the potential for channelling increases. The pressure profile of a lever-style espresso machine delivers a gradual and steady decline to approx. 3 bars. The result is a much smoother finish. Simply put, they do not force an aggressive amount of pressure on a degraded puck of coffee! Tasty, but unfortunately not very consistent unless you’re very experienced using it.

Modern-era espresso

The third era is the electric pump, which was first introduced in 1961 by Faema, with the E61. The first kind to be able to have a fixed pressure throughout the extraction. The goal was consistency and ease of use compared to the lever style, which was physically demanding if you worked in a high-volume café.

The E61 machine created the benchmark for the modern machines that we use today by introducing the ‘9 bar’ fixed pressure style of espresso (130.5 psi). But why 9 bar? While no one has the exact answer, it's widely agreed that it is optimal for forcing hot water through finely ground coffee, creating a rich, concentrated brew without under- or over-extraction. Some say it was the average pressure of a lever-style pressure profile.

We won’t know for sure, but there is a theory it might have allowed the highest flow rate of espresso over a certain period. A high flow rate gives the barista an opportunity to grind finer and therefore extract more coffee into the espresso, but this also depends on the coffee you’re using and what you want to get out of it.

Something to try at home

A great experiment to try if you want to dive deeper is to establish ‘What pressure will achieve the highest flow rate of espresso?’. If you’re able to easily change the pump pressure of your machine, this is something to try! You’ll need a consistent grinder like the Niche Zero to pair with a single-origin coffee. A blend isn’t recommended due to the unpredictable percentages of different coffees in the grind.

1. Select a dose of coffee based on the size of your basket, e.g. 18 g.
2. Choose 3 pump pressures to compare, e.g., 9, 7 & 4 bar.
3. Before step 4 it’s vital that we eliminate as many variables as possible, so be as consistent as possible with every step of making your espresso. Consider the same distribution technique, and use a force tamper if you have access to one.
4. Choose the pressure you want to start with, e.g. 9 bar. Prepare and extract the shot for an exact amount of time and immediately stop at 25 sec.
5. Weigh your espresso yield. The pressure that has the largest yield in 25 sec has the greatest flow rate.
6. Once you’ve established the pressure that delivers the greatest yield, repeat the process again but with a smaller pressure distribution. i.e. if 7 delivered the highest yield, maybe try 6, 7 & 8 bar or even 6.5, 7 & 7.5 bar.

Once you’ve fine-tuned your experiment, you should have a good idea of the exact pressure that works best for your machine. Taste along the way, and you’ll learn a lot!

The graph below demonstrates the pressure differences between 3 different eras of espresso boiler/pump-style technology. The outlier as explained above, is the lever style of profile with its manual pre-infusion/soaking of water with a peak of pressure and a gradual decline. Whereas a classic steam boiler pressure wouldn’t peak above 2 bars of pressure, needing a much coarser grind size and delivering little to no crema – essentially a much weaker shot of espresso than your standard espresso shot today. How do you think that style of espresso would have tasted with milk?

Let's compare

To compare the 3 espresso profiles above, I prepared 3 espresso shots per profile to create a more consistent test and attempted to keep within the recipe parameters. To do this for the Lever profile, I manually changed the pressure on the electric pump during the extraction. For the other profiles I simply pre-altered the pump pressure before the extraction started. The equipment used in this test was the Niche Zero Grinder and a La Marzocco GS3 using a natural Ugandan coffee. My sensory results are as follows:

And in conclusion...

My overall preference was the lever – very sweet and balanced with a silky texture, though it was noticeably weaker and slightly less extracted compared to the 9-bar fixed espresso, which was more intense but still balanced. If paired with milk for a flat white, the 9-bar espresso would come out on top.

9 bars averaged 9.66% TDS (concentration) & 21.3% extraction (% of dose in yield). Whereas the lever espresso averaged 9.19% TDS & 20.36% extraction. Both tasty, but different styles, even if they are the same recipes.

To conclude, pressure is one of the defining factors in espresso; without it, it wouldn’t be espresso. Steam boilers introduced the concept, lever machines added flavour and nuance, and electric pumps standardised quality & consistency.

Don’t get me started on the Strada!