Coffee: Espresso Extraction — Pressure, Time, and Ratio

Espresso is defined by pressure. Unlike every other common brewing method, which relies on gravity or manual pressure to move water through coffee, espresso forces near-boiling water through a tightly compressed puck of finely ground coffee at 9 bar — roughly nine times atmospheric pressure. This combination of fine grind, high pressure, and short time produces a concentrated beverage unlike any other.

Core Parameters

Parameter	SCA Specialty Standard	Notes
Pressure	9 bar	At group head; pump max ~15 bar reduced by OPV
Water temperature	90–96°C	At puck; boiler runs 5–10°C higher
Pre-infusion pressure	1–4 bar	Optional; 3–8 seconds before full pressure
Extraction time	25–30 seconds	From first drop to target yield
Dose (coffee in)	18–21 g	Standard double basket
Yield (liquid out)	36–42 g	At 1:2 ratio
Brew ratio	1:2	Coffee mass to liquid output mass
TDS	8–12%	Total dissolved solids
Extraction yield	18–22%	% of coffee mass dissolved into liquid

The 9 Bar Standard — Not 15 Bar

Consumer espresso machine marketing frequently advertises “15 bar” pumps. This figure refers to the vibratory pump’s maximum pressure capability — not the pressure at which coffee is actually extracted. Inside the machine, an over-pressure valve (OPV) or pressure regulator limits the brew circuit to approximately 9 bar.

The 9 bar standard emerged from the work of Italian researchers and engineers in the mid-20th century and was codified by the Specialty Coffee Association. Research showed that 9 bar consistently produced the best balance of extraction rate, emulsification of coffee oils, and crema formation. At pressures significantly below 9 bar, extraction slows and crema production drops. Above 9 bar, channeling risk increases and over-extraction compounds accumulate faster.

Temperature: Boiler vs. Brew

The temperature displayed on an espresso machine refers to the boiler setpoint, not the temperature of water reaching the coffee. Heat loss occurs as water travels through the group head components. Machines without PID controllers or dedicated brew boilers can have brew temperatures 5–10°C below the boiler setting, which is why many specialty machines use dual boilers (one for steam, one for brew) or PID temperature control.

For lighter roasts, many baristas push toward the higher end of the range (93–96°C) because lighter roasts have higher density and benefit from additional thermal energy to complete extraction. Darker roasts extract more readily and may use 88–92°C to avoid bitterness.

Pre-Infusion

Pre-infusion introduces water to the puck at 1–4 bar before ramping to full extraction pressure. The purpose is even saturation. Dry coffee grounds repel water initially; a short low-pressure phase (3–8 seconds) allows water to wet the entire puck surface before pressure increases. This reduces channeling — localized water paths through the puck that lead to uneven extraction, producing both over-extracted (bitter) and under-extracted (sour) compounds in the same shot.

Extraction Time and Flavor

Extraction time	Likely result	Reason
Under 20 seconds	Sour, underdeveloped	Acidic compounds extract first; sweetness/body hasn’t developed
20–25 seconds	Slightly underdeveloped	May lack sweetness and complexity
25–30 seconds	Target range	Balanced extraction of acids, sugars, and bitter compounds
30–35 seconds	Slightly over-extracted	Bitter notes begin to dominate
Over 35 seconds	Bitter, harsh	Excessive extraction of tannins and bitter phenolic compounds

Grind size is the primary variable used to adjust extraction time for a given dose and yield target. A finer grind increases resistance, slowing flow rate and extending extraction time. A coarser grind reduces resistance, speeding flow.

Ratio Variations

Style	Ratio	Example	Character
Ristretto	1:1 to 1:1.5	18g in / 18–27g out	Concentrated, syrupy, sweetness-forward
Standard espresso	1:2	18g in / 36g out	Balanced, full-bodied
Lungo	1:3 to 1:4	18g in / 54–72g out	Extended, more bitter, higher extraction yield
Americano	1:2 espresso + hot water	36g espresso + 120ml water	Dilute, cleaner, lower TDS

Ristretto and lungo are not simply more or less concentrated versions of the same espresso. Because the ratio affects which compounds have time to extract, these are qualitatively different beverages. A ristretto emphasizes early-extracting sweet and acidic compounds; a lungo extracts more of the bitter, later-extracting compounds.

Crema

Crema — the reddish-brown foam on top of a well-pulled espresso — forms from the emulsification of coffee oils and CO₂ dissolved in the liquid under pressure. At 9 bar, CO₂ that was trapped in the coffee grounds enters supersaturation in the brew liquid; when it exits the group head at atmospheric pressure, it nucleates rapidly, forming fine bubbles stabilized by surfactant coffee lipids. Fresh coffee (within a few weeks of roast) produces more crema because it retains more CO₂. Stale coffee or low-pressure extraction produces thin or absent crema.