Quest for Coffee (3) | (Under) Pressure

Ever wish you could silence your coffee machine every day in the morning? 

Ah, the peace it would bring. No loud whirring or churning like a jet taking off from Top Gun. 

I am here to say that this noise, often dismissed as just “noise”, comes from a critical part of the coffee-making process – pressure. 

Pressure—specifically around 9 bars—is the heart of a good espresso shot. The machine forces hot water through finely ground coffee at this pressure to extract the oils, acids, sugars, and aromatic compounds from the coffee grounds in a tight 30-second window, give or take. After all, coffee-making is more art than science. 

Want that coveted golden layer of crema gently floating above your aromatic espresso shot? Well, can’t be done without pressure – the right amount of it at that.

 Here’s how varying that pressure impacts the outcome:

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Too Little Pressure (<9 bars):

Your coffee is under-extracted. This simply means that not all the lovely, amazing soluble compounds in the coffee grounds have been brewed into the espresso shot.

Wasteful isn’t it?

Once you glance at your espresso, a pale, thin and lacklustre looking layer of crema sits above it, dissipating almost as soon as you notice it. The coffee lets off a weak, grassy and unappealing scent, leaving you slightly baffled as to why your coffee is not like what the coffee bean packaging describes – “freshly roasted premium beans for the perfect espresso”.

Surely it couldn’t taste that bad…right? 

If you have ever tried acidic coffee, not by its origins and terroir, but as a result of poor extraction and low pressure, this is about as close as you can get to replicating it. 

Right, so let’s take it up a notch and crank up the pressure. 

Too Much Pressure (>9 bars):

You load up your coffee machine and brew a second cup with greater pressure. This time, you seem satisfied.

The espresso has a dark, thick and almost-brown crema, but yet looks burnt. You notice a lack of acidity in scent and taste, but a strong hint of bitterness lingers in your mouth. It reminds you of burnt toast or worse, rubber.

What happened? 

Well, this is what connoisseurs call an over-extracted shot. The coffee has been brewed with too much pressure. Think of this as “suffocating” the coffee beans due to excessive pressure, leading to “some good friends (the coffee beans, obviously) screaming: ‘Let me out’.” 

Ideal Pressure (~9 bars):

Ok, so you are wondering what a good shot of espresso looks like with the right amount of pressure.

It’s hard to describe (remember, coffee-making is more an art than a science), but as a rule of thumb, you’ll know it when you see it with sufficient practice. 

Your espresso would have undergone a balanced extraction, where 9 lovely bars of pressure blends the soluble compounds harmoniously to bring the bitters, acids and aromatics together. It’s almost like assembling the Avengers if you will (pressure would either be Thanos or Tony, I’d pick Tony). 

The art to pulling a great espresso shots lies in the mastery of several factors affecting espressos including pressure, grind size, dosage and even temperature.

Try one cup after another. The first time I pulled an espresso shot properly, it was worth all the attempts before.

One step at a time, let’s continue tackling pressure for now.

With a rich, thick golden crema with a hint of tiger-striped reddish hues, your espresso shot is accompanied by a bright, fruit-like acidity that highlights the terroir and origin character. It includes a touch of bitterness that rounds off the tartness. The vicinity is filled with a deep, nuanced and even floral aroma that lingers, depending on the type of bean used.

Here’s my chapter on beans from Quest for Coffee. 

To find out more about how to measure pressure, enjoy this side quest.

I know I said coffee-making is more art than science, but I can’t help but explore more of the science behind putting coffee grounds under pressure. What can I say?

Yes, it’s a nerdy thing.

But it’s a Quest for Coffee. (Fear not, this is not a textbook. No alien words are used. Please read on without caution.)

What’s really happening under pressure?

Remember that whirring, churning noise that pressure makes to force hot water (88–96°C) through a tightly packed puck (it’s that round metal tray in which your finely ground coffee is held and tamped)?

The pressure is 9 bars. That’s 9 times the atmospheric pressure in which you exist or around 130 psi. (That’s a lot of pressure withstood by coffee grounds.)

Nonetheless, this pressure is critical.

This pressure is needed.

1. Solubility Increases with Pressure and Temperature

Just like sugar dissolves faster in hot tea than cold water, the hot, pressurized water simply dissolves the soluble compounds in the coffee grounds faster. The acids (like citric and malic acid), sugars, caffeine and aromatic compounds (which contribute to smell) are all brought out (or assembled as referenced earlier) together more easily. These molecules are small and dissolve easily in water, so they’re extracted early in the shot—often in the first 10 seconds (remember how long an amazing espresso shot takes?). 

2. Oils and Emulsions

That shiny finish on your dark roasted coffee beans is natural oils that are hydrophobic (they dislike mingling with water like how normal oil floats on water). The high pressure helps emulsify (think of it combining oil with water) these oils into tiny droplets suspended in the espresso to create the thick, velvety body and crema on top. These oils are also carriers of volatile aromatics (volatile just means they escape easily into the air)—the fragrant notes you smell (floral, nutty, fruity).

3. Even Extraction Requires Force

Coffee grounds, when tamped well, are compact and offer resistance to water flow. Without pressure, the water would just flow through the easiest paths (called channeling, see below) and you’d get an uneven, weak shot. This translates to watery espresso spurting out from the machine into your cup, rather than a smooth, slow, honey-like flow downwards. Pressure ensures water gets pushed through the puck thoroughly, but only if the puck has been evenly tamped, giving the desired espresso shot. 

With Pressure Comes Crema 

If done right, crema is the golden foam of oils, carbon dioxide, and coffee solids that sits atop a fresh espresso. It’s considered a sign of a well-extracted shot (though not the only one).

Pressure in play:

Ideal pressure (around 9 bars):

This forces carbon dioxide gases out of the coffee grounds, which were trapped during roasting, and like previously mentioned, emulsifies the coffee oils into microscopic bubbles that are found inside the rich, stable foam on top as well as within the espresso. The pressurized water extracts proteins and melanoidins (browning compounds), which stabilize the crema and give it that deep, reddish-golden hue. Cool huh?

Too little pressure:

With insufficient pressure, not enough CO₂ is released or emulsified so little to no crema is produced and it dissipates fast and looks pale.

Too much pressure:

Overdoing it can over-emulsify oils or disrupt flow, leading to a bitter shot with dense, overly dark crema that may look nice but tastes harsh. Proper pressure not only ensures even extraction but also crafts the crema — a delicate blend of gas, oils, and fine particles that signals quality.

Channeling: Kill Shot, Literally

Coffee connoisseurs call the phenomenon channeling as an espresso killer.

Channeling comes when pressure, regardless of calibration, forces water through weak spots within the coffee grounds placed in the puck.

This occurs due to cracks, air pockets, or uneven tamping in the puck, allowing water to rush through instead of flowing evenly, as water always follows the path of least resistance. 

How it affects your espresso:

First, you’ll see a blonde, watery stream flow from the machine instead of a rich, syrupy consistency. Like espressos made with a lack of pressure, the crema may appear thin, patchy, or quickly dissipate.

Not to be a harbinger of bad news, but channeling might be worse than having incorrect pressures.

The 3 most common outcomes from channeling are espressos, which 1, fall in the under-extracted zones, where water rushed too fast, resulting in sour, acidic notes; 2, fall in the over-extracted zones, where water stalled, giving bitter, harsh flavors; or 3, have a mix of both – doubly terrible. Essentially, you get brown water. 

You might have other observations like diminished and inconsistent espressos which smell sharp and blatantly acidic because channeling causes water to push through the coffee beans unpredictably every time. You lose the vibrant, layered aromatics of a well-pulled shot.

Suffice it to say, channeling is not good. 

You might ask: Can I prevent channeling? 

Yes. Yes you can. 

It starts right here, where I explore 8 ways to better prepare the puck for espresso shots.

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