Updated October 2026 · pressure, crema, channelling and flow
Commercially, espresso is all about speed: about 30 seconds a cup, compared with 4 minutes for filter, which matters when 30 people are waiting. Pressure is what speeds up the extraction, and it is also one of the levers you can use to taste.
9 bar is about the peak flow for most espresso machines, not a rule for taste. Higher pressure extracts more: around 9 bar helps light roasts, around 6 bar helps dark roasts. Each of the 121 espresso recipes gives the pressure to use.
History
Angelo Moriondo, owner of the Hotel and Café Ligure in Turin, wanted to end the 4-minute wait for filter coffee. His machine used steam pressure to push boiling water through the coffee at 1–2 bar: ‘express’ coffee.
Achille Gaggia's spring lever made a massive leap in pressure and could now generate crema. As the spring expands it exerts less force, so the pressure declines through the shot.
The Faema E61 used an electric pump to deliver a constant 9 bar.
The La Marzocco Strata's electric gear pump could vary the pressure at different times through the shot: pressure profiling.
Source: Angelo Moriondo: The Myth of the Selfish Barista – Barista Hustle
Crema
The first time high pressure was used to make coffee, it produced a bitter foam on top that was scraped off and discarded. In 1949 Achille Gaggia turned the negative into a positive, calling it “Crema caffè naturale” (natural coffee cream).
Crema forms from the CO₂ trapped in freshly roasted beans. Hot water under pressure forces the CO₂ into solution in the puck, like a SodaStream. When the coffee leaves the basket and the pressure drops, it is supersaturated and the CO₂ comes out of solution, where surfactants such as melanoidins (roast by-products) trap it in bubbles.
Robusta produces about twice the crema of arabica: it has less oil, and oils weaken surfactants, like getting egg yolk into egg white when beating. Dark roasts produce more CO₂ than light roasts. Tiger striping is just pieces of coffee on top of the foam.
Crema shows that the coffee was fresh and brewed with enough pressure. More crema adds a roasted note, and removing it reduces bitterness. It doesn't improve the taste of coffee; it just improves the appearance.
Why 9 bar?
Why are espresso machines set to 9 bar? Try a fixed dose for a fixed 30 seconds at 7, 8, 9, 10 and 11 bar. As pressure rises, more liquid gets through, until you pass about 9 bar. Then the water starts to compact the puck, making it harder to get through, and the flow drops. So 9 bar is about the peak flow for most machines, not a magic number for taste.
Pressure gauge on a Flair 58 lever espresso machine
Higher pressure extracts more. Around 9 bar helps you extract more from light roasts, and around 6 bar helps you extract less from dark roasts. Each espresso recipe in the series gives the pressure to use for its roast level.
This flow peak is covered in Espresso Coffee: The Science of Quality (2004) by Rinantonio Viani and Andrea Illy, and in Scott Rao's Espresso Extraction: Measurement and Mastery.
Pre-infusion
Pre-infusion starts when water enters the basket and ends when the machine reaches peak brewing pressure. The aim is to gently wet all of the coffee at under 3 bar before the full 9 bar hits it, which gives a more even extraction and better-tasting coffee. Some machines use 0.6–0.8 mm flow restrictors, which take about 6–7 seconds to reach full pressure.
Pre-infusion also lets you grind finer. One theory is fines migration: the puck swells and traps the fines, stopping them moving through the shot and clogging the basket holes. The higher the pressure, the more likely you are to get channels and uneven extraction.
As more of the puck is washed out during the shot, it becomes more prone to channelling. A lever machine's pressure naturally decreases as the shot goes on, which reduces the chance of channelling. That's a big part of why lever machines are great.
Channelling
Channelling happens when water flows unevenly through the coffee. It finds a weak point in the bed and takes a shortcut, flowing faster through that channel. The coffee near the channel is over-extracted while the rest is under-extracted, so the cup is weak and sour yet bitter at the same time.
Poor distribution of grounds in the basket, grinding too fine, uneven wetting of the puck, or too little coffee in the basket.
The Ross Droplet Technique, introduced by David Ross in 2005, means adding a drop or two of water, or a fine mist, to your beans before grinding. Grinding builds up static through friction, which makes grounds clump and cling to everything. A little moisture lets the charge dissipate, giving a more even grind, less mess and better taste (see the research).
Flow profiling
Pressure profiling varies the pressure; flow profiling controls the rate at which water flows through the puck, as if there were an adjustable valve between the boiler and the coffee. Like pre-infusion, it prevents sudden, forceful streams of water and reduces channelling.
Start with a restricted flow and gradually open the valve over the first 10–15 seconds, so the puck saturates evenly before pressure builds.
Late in the shot the puck offers less resistance. Easing the flow lets you hit your target weight precisely without changing the grind.
Machines such as the Lelit Bianca and the La Marzocco GS3 offer flow profiling.
Keep reading
Brew temperature from 85 °C to 95 °C, and Flair 58 settings.
Read the guideA spritz of water before grinding for less static and better taste.
Read the guideMatch the basket to your dose for an even extraction.
Read the guide
