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Equipment · Espresso

How an espresso machine works

An espresso machine heats water, pressurises it to about 9 bar, spreads it over a compacted bed of fine coffee and ends the shot. The entry covers how pumps, boilers, groups and levers do these jobs and how the coffee bed resists the water.

Across the chain: History · Science · Equipment

Source-checkedby Coffearium verification pass· Last source-checked 10 Oct 2026· Prepared with AI assistance· 20 sources (10 tier A)· 6 contested· Updated 13 Oct 2026
Reading depth
Contents
  • Espresso is made by rapidly percolating a small amount of hot, pressurised water through a compressed cake of ground coffee.1,3,8
  • The machine has four jobs: heat the water and hold it steady, build pressure (about 9 bar by convention), spread the water evenly over the bed and end the shot.1,2,12,16
  • According to a coffee science textbook, the pressure on the bed is the balance between the push of the pump or lever and the resistance of the bed.1
  • Early steam machines brewed at low pressure, about 1–1.5 bar according to a patent and 1.5–3 bar according to a historian, until spring levers and pumps separated pressure from temperature.10,14
  • Crema is a foam, mainly of carbon dioxide formed during roasting, stabilised by coffee compounds.3,9

What espresso is#

Espresso is brewed by percolation: a small amount of pressurised, heated water passes rapidly through a compressed cake of ground roasted coffee.1,3,8 Each serving is made on request, “expressly” for the customer.1,8,10 That was the first purpose of the machine: to make coffee on demand, cup by cup.1,12

Short contact time and pressure set espresso apart. A definition cited in a 2016 study describes a 25–35 mL beverage made from 7–9 g of coffee.4 In this definition, clean water at 92–95 °C is forced through at 9–10 atmospheres, with a flow time of about 20–30 s.4 According to a coffee science textbook, water in filter coffee passes through the bed mainly by gravity, with almost no pressure, and contact lasts minutes.1 The result is a concentrated liquid with dissolved solids, fragments of the cell structure and tiny oil droplets spread through it (an emulsion).1,3,8,9 How much of the coffee ends up dissolved is covered in the extraction yield entry.

Definitions differ. The Italian Espresso National Institute specifies a Certified Italian Espresso by a set of parameters. As reported in a 2013 study, they include 7 ± 0.5 g of ground coffee and water at 90 ± 2 °C as it leaves the machine.2 The other values are 9 ± 1 bar, 25 ± 2.5 s and 25 ± 2.5 mL in the cup, including foam.2 A textbook chapter reports the same definition with 88 ± 2 °C, 25 ± 5 s and a 25–40 mL cup (see Contested).1 A 2011 review gives 92–94 °C, 9 ± 2 bar and a 30 mL cup percolated in 30 s.3 A 2002 magazine article gives 92–94 °C, 9 atmospheres, about 30 s and about 30 mL.3,8 The textbook chapter also notes a parallel trend towards more coffee for the same amount of water.1 The parameters are a reference, not a guarantee of quality; the 2013 authors note that meeting them alone is not enough.2

The four jobs of the machine#

1. Heat the water and hold it steady. Brew water must reach the coffee at a set temperature, shot after shot.2,8 In one comparison, machines built differently gave different temperature curves.2 A pump machine with a heat exchanger (a tube that heats brew water inside the boiler) held a fairly constant temperature. A spring-lever machine heated by a boiler lost about 10 degrees during the shot.2

2. Make pressure. Espresso is conventionally brewed at about 9 bar.2,3,8 The force comes from a pump in most machines, from a spring in lever machines and from steam in moka pots.1,2 According to a coffee science textbook, pressure is not set independently: it balances the force on top of the bed against the bed’s resistance.1 The same textbook notes that a pump delivers its highest pressure at its lowest flow, and the reverse.1 The bed’s permeability, meaning how easily water passes through it, decides where on that curve the shot runs.1

3. Spread the water over the bed. In Bezzera’s 1902 patent, boiler water and steam passed through a bell-shaped conduit and a perforated diaphragm to reach the entire layer of coffee.12 In one patent, the brewing chamber (the group) consists of a water diffusion shower head, a filter holding the coffee and its filter holder.14 According to a coffee science textbook, even distribution matters because the bed must be homogeneous to avoid channelling (uneven flow through the bed).1

4. End the shot. The shot can be stopped by time, by the volume of water pumped or by the weight of beverage in the cup. Some machines meter the dose with a flow sensor on the input side.16 One patent notes that this assumes the water pumped in equals the coffee that comes out.16 In its account, water held back in the grounds and the system breaks that assumption.16 In one patented gravimetric (weight-based) machine, a load cell, an electronic weighing sensor, weighs the cup.17 The control unit stops dispensing when the coffee in the cup meets the brew ratio (infusion ratio) chosen by the operator.17 According to a coffee science textbook, advanced espresso equipment has weight-sensitive platforms below the spouts that measure the extracted liquid exactly.1

The parts#

Pump. According to the background of one patent, pump-driven machines became the standard design.14 Two families are found:

  • Vibratory (oscillating-piston) pumps. A solenoid (an electric coil) driven by mains alternating current moves a piston back and forth against a spring. Two check valves (one-way valves), one carried by the piston and one at the outlet, turn this into flow.15 Coffee machines are among their listed uses.15
  • Motor-driven pumps. An electric motor drives a pump. A 1961 patent specifies a centrifugal pump.13 A later patent describes machines in which the pressure is generated by a rotary pump.14

Boiler and heat exchanger. In a heat-exchanger machine, brew water does not come from the boiler directly: it passes through a tubular heat exchanger inside the boiler.13 In the 1961 patent, the pump injects fresh cold water into the exchanger during the shot.13 Between shots, the water circulates on its own between exchanger and group by thermosiphon (circulation driven by heat), keeping the group hot.13

Separate brew boilers and thermal blocks. Other machines keep a main boiler for hot water and steam.14 Each group gets its own small secondary boiler, so that brew temperature is set independently.14 One patent states that domestic machines may heat the water in a boiler or a thermal block, a heated block that water is pumped through.16

Group, portafilter and basket. The group is where the brew water meets the coffee: a water diffusion shower head, the coffee filter (basket) and the filter holder (portafilter) that carries it.14 The coffee is placed in the perforated basket and firmly tamped (pressed) into a compact bed.3,8

Steam wand. Steam from the boiler is led to a wand at the side of the machine. According to a coffee historian, early bars used it to warm alcoholic punches as well as milk, and the cappuccino grew out of this practice.10 The steam boiler runs hotter than brew water.18 The water handbook of the Specialty Coffee Association (SCA) therefore treats it separately when it calculates scale (mineral deposits).18 The milk entry covers steaming.

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  1. Water supply. From the mains or a tank.13,16
  2. Flowmeter. Measures the water volume, for flowmeter dosing.14,16
  3. Pump. Builds the pressure, about 9 bar at the group1,2; vibratory or motor-driven.13,15
  4. Heat exchanger. Brew water is heated in a tube on its way through the boiler and does not mix with the boiler water.13
  5. Boiler. The steam boiler that holds the heat exchanger.13
  6. Steam wand. Steam from the boiler, for milk. Its pipe runs behind the cut, marked ⊙.10,18
  7. Thermosiphon loop. Between shots the water circulates on its own between exchanger and group, keeping the group hot.13
  8. Group and shower screen. The brew water meets the coffee14; the screen spreads it over the bed.12,14
  9. Portafilter, basket and puck. The puck is the tamped coffee bed; a textbook and a magazine article describe its resistance as setting the flow.1,8
  10. Cup. Espresso with crema, about 1 mL/s through the bed.3
A heat-exchanger espresso machine, cut along its centre line. The dashed line follows the water from the supply to the cup; point at a part, its number or its line to see them together. Schematic, not to scale.

Lever machines#

According to a coffee science textbook, the first espresso machines worked with steam rather than high pressure.1 According to one patent, the pressure in a steam-boiler machine depends on the boiler temperature.14 The same patent puts the pressure of a saturated-steam boiler, where steam sits over boiling water, at about 1–1.5 bar.14 It calls the extraction pressure a compromise.14 A coffee historian gives about 1.5–3 bar for the first production machine (see Contested).10 According to the textbook, steam-powered machines tended to extract too hot and gave a burnt flavour.1

The spring lever broke this link. In a lever machine, a manually operated, spring-loaded piston forces the water through the coffee.2,10 According to a coffee historian, Achille Gaggia’s lever machine of 1948 brewed at around 9 bar.10 Its extract carried a head of coffee oils, which Gaggia promoted as a new beverage, “Crema Caffè”.10 A museum exhibition gives 10 bar for the same machine (see Contested).11 In one study, pressure was not constant during a lever shot: the machine peaked at 8 bar and fell to 2 bar after 26 s.2 A pump machine in the same study held 9 bar.2 According to one patent, lever machines also wet the coffee at low pressure before full pressure arrives, as a built-in step.14

Pre-infusion and profiling#

Pre-infusion is described in one patent as flooding the bed with water at low pressure before full pressure is applied.14 The 1961 heat-exchanger patent describes a soaking phase of the coffee before dispensing begins.13 In pump machines, pre-infusion needs a separate valve or control step.14 In one research machine, pre-infusion ran at 7 mL/s until the basket was full and pressure exceeded 2.5 bar.7

Profiling. According to a coffee science textbook, newer machines let the barista control water flow or vary pressure during the shot, to reproduce chosen extraction profiles.1 Patents describe storing sets of predetermined pressure curves and replaying them automatically.14 According to the textbook, the bed ties pressure and flow together.1 A machine that sets the pressure lets the flow follow the bed; one that sets the flow lets the pressure follow.1 In one flow-controlled study, flow rate had the strongest effect on how much of each compound reached the cup.7

TimePressure1234567
  1. Pre-infusion. The bed is flooded with water at low pressure before full pressure is applied.14 In one research machine: 7 mL/s until the basket was full and pressure exceeded 2.5 bar.7
  2. Pressure ramp. Pressure builds as the bed resists the water. A textbook describes it as the balance between the force on top of the bed and the bed’s resistance.1
  3. First coffee in the cup. 5–6 s after the pump started, in one study.4
  4. Main extraction. Conventionally about 9 bar2,3,8; 9 ± 1 bar for a Certified Italian Espresso.2
  5. End of the shot. By time: in one study the pump stopped at 25 s and the flow 1 s later.4 By volume, with a flowmeter16, or by weight, with a load cell under the cup.17
  6. The whole shot. 25 ± 2.5 s for a Certified Italian Espresso, as reported2; about 30 s in a review and a magazine article.3,8
  7. Spring lever. Pressure was not constant in one study: a lever machine peaked at 8 bar and fell to 2 bar after 26 s.2 A pump machine held 9 bar.2
One shot from pump start to end, pressure over time; the height of the shape is qualitative. Schematic, not to scale.

Crema#

A coffee science textbook describes crema as the dense brown foam on top of an espresso.9 Its gas is mainly carbon dioxide formed during roasting and partly trapped in the cell structure.3,9 According to one explanation in the textbook, carbon dioxide dissolves in the brew water under pressure and heat.9 When the brew leaves the bed for the cup at ambient pressure, the liquid holds more gas than it can keep dissolved: it is supersaturated.9 Tiny bubbles then form on fine particles and cell-wall fragments.9 In the same account, the short path in a small cup keeps the bubbles small.9

The machine affects crema through pressure. In a study on arabica espresso described in a review, foam volume rose steadily, from 5.1 to 6.9 mL, as water pressure increased.3 Water temperature between 88 and 98 °C made no statistical difference to foam volume.3 According to a review and a textbook chapter, crema should make up at least 10% of the volume and can last up to 40 minutes.3,9 The textbook chapter describes colour and bubble size as only secondary signs of a good extraction, after flavour.9 According to the same chapter, coffee with more carbon dioxide gives more foam.9 Foam volume and weight rose in line with the carbon dioxide content of the ground coffee.9 The freshness entry covers how this changes with age.

The puck: permeability and flow#

A packed bed of ground coffee behaves like a porous filter. At low flow, the flow rate is proportional to the pressure drop, the cross-section and the permeability of the bed, and inversely proportional to the viscosity of the water and the height of the bed (Darcy’s law).5 In one study on arabica beds at 80 °C, measured permeabilities lay between about 2.6 × 10⁻¹⁴ and 4.4 × 10⁻¹³ m², depending on grind and packing density.5

The bed is not a fixed filter. In the same study, the bed consolidated as soon as water started to flow: its height fell and the grounds became an agglomeration of particles.5 According to a coffee science textbook, the particles swell as they wet, rearrange under flow and pressure, and the bed’s porosity falls; flow drops and the pressure drop across the bed rises.1 The textbook adds that the finer the grind, the more the bed consolidates.1 Earlier experiments, cited in the permeability study, found that raising the pressure above about 5–7 bar lowered the average flow rate.5

A review and an earlier magazine article describe the same balance from the barista’s side: the energy of the water pressure is spent within the coffee cake, and the beverage should flow into the cup at close to 1 mL/s.3,8 According to the textbook, too low a permeability gives too high a pressure, low flow and a long shot, which can over-extract.1 In one flow-controlled study, brew pressure was higher for finer grinding.7

Channelling. If water does not flow evenly through the bed, parts of it are extracted more than others.1,6 In one study combining a mathematical model with experiments, the model assumed even flow and predicted that extraction yield falls steadily as the grind gets coarser; the experiments instead showed a peak, with lower yields at very fine settings too.6 The authors attribute this to inhomogeneous flow at fine grind settings, which also explains poor reproducibility and wasted coffee, and they outline a procedure to address it.6 According to the textbook, tamping, the homogeneity and shape of the bed, and the force of the water on top of it all bear on channelling.1

Temperature stability#

Temperature changes what is extracted. In an on-line study of aroma, brewing at 82, 92 and 96 °C (at 9 bar) showed that higher temperature increased the extraction of volatile compounds, especially in the second half of the shot, and most for the less polar compounds.4 At 96 °C, some less polar compounds were still rising when the 25 s shot ended.4

In one comparison, the heating system shaped the temperature curve. A pump machine with a heat exchanger and a spring-lever machine with a boiler were compared: the pump machine peaked at 93 °C and stayed fairly constant, while the lever machine peaked at 98 °C and dropped about 10 degrees during the shot.2 The authors attributed the pump machine’s higher extraction of chlorogenic acids mainly to temperature, and found pressure less influential.2

These effects depend on context. In a flow-controlled study at 80, 89 and 98 °C, higher temperature mainly strengthened the effect of flow rate, and all these effects were minor compared with how much beverage was collected.7 How hotter water shifts the balance of taste compounds is covered in the extraction yield entry.

Pressure and aroma over the shot#

In one study, on-line mass spectrometry followed 46 aroma compounds in the stream of an espresso, second by second, with 18 g of a Brazilian Yellow Bourbon arabica in a double basket, a 25 s pump time and about 30 g in the cup.4 Coffee started to flow 5–6 s after the pump started; the pump was stopped at 25 s, and the flow stopped 1 s later.4 Of these compounds, 44 fell into 5 families by their time profiles, and the authors identified polarity as the main force driving how fast they came out:4

  • Family A (most polar, such as acetaldehyde and acetic acid): peaked at 10 s and fell fastest, to below 40% of the maximum by the end.4
  • Family B: rose like A, held a plateau from 10 to 16 s, then fell to 60–70% of the maximum.4
  • Family C (very volatile, such as dimethyl sulfide): a shoulder at 4 s, before any coffee flowed, then a profile like B.4
  • Family D (such as methylpyrazine and furfural): peaked at 14–16 s, ending at 70–80% of the maximum.4
  • Family E (least polar, such as guaiacol and pyrazines): rose most slowly to a peak at 20 s and stayed at 80–95% at the end.4
02468101214161820222426s after pump startMachine: first coffeeflows5–6 s4 Family A(most polar) – < 40% of peak atendpeak 10 s4 Family B – 60–70% of peakat endplateau 10–16 s4 Family C(very volatile) – profile like B4 s · then plateau 10–16 s4 Family D – 70–80% of peakat endpeak 14–16 s4 Family E(least polar) – 80–95% of peak atendpeak 20 s4 Machine: pump stops25 s4
Data table
Points after pump startSource
Machine: first coffee flows5–6 s5–64
Family A (most polar) – < 40% of peak at endpeak 10 s104
Family B – 60–70% of peak at endplateau 10–16 s10–164
Family C (very volatile) – profile like B4 s44
Family C (very volatile) – profile like Bthen plateau 10–16 s10–164
Family D – 70–80% of peak at endpeak 14–16 s14–164
Family E (least polar) – 80–95% of peak at endpeak 20 s204
Machine: pump stops25 s254
When each family of aroma compounds peaked in the coffee stream during a 25 s shot at 9 bar and 92 °C, and how much of the peak was left at the end; time zero is pump start.4

In this study, the shoulder at 4 s showed that hot water released very volatile compounds from the wetted coffee before the brew reached the cup.4 The most polar compounds were extracted within the first seconds; an important fraction of the least polar ones was still being extracted at 25 s.4 The authors conclude that where the shot is stopped changes the aroma balance in the cup.4

The same study compared 7, 9 and 11 bar at 92 °C. Shots at 7 and 9 bar did not differ significantly, but 11 bar raised volatile concentrations over the whole shot without lowering the average flow.4 The authors cite earlier work in which pressures up to 11 bar gave coffees with higher odour intensity but lower consumer acceptance than 7 or 9 bar.4 According to a coffee science textbook, higher pressure has also been linked to higher viscosity and body.1

Water, scale and corrosion#

Machines meet water at high temperature, which makes scale. The SCA water handbook calculates the maximum scale that can form under brew-water and steam-boiler conditions.18 According to the handbook, scale reduces heat transfer and clogs valves and jets, especially in the hot-water sections; water with too little alkalinity corrodes.18 The handbook’s recommended “core zone” for espresso machines is bounded by corrosion thresholds and by a maximum scale formation of 30 mg/L in a steam boiler or 12 mg/L in the brew boiler of an espresso machine.18 The steam-boiler value is calculated for the hotter conditions of a steam boiler.18 Water composition, treatment and corrosion limits are covered in the water entry.

The sources opened for this entry give no tested procedures for backflushing, cleaning detergents or gasket replacement, so routine maintenance is not described here.

Reference values#

Parameter Value reported Context Source
Dose (single) 7 ± 0.5 g Certified Italian Espresso Caprioli et al.2, Mestdagh et al.1
Dose (double, research) 18 g; 20 g Aroma study; flow study Sánchez López et al.4, Schmieder et al.7
Brew pressure 9 ± 1 bar Certified Italian Espresso Caprioli et al.2, Mestdagh et al.1
Brew pressure 9 ± 2 bar; 9 atmospheres Traditional espresso Illy & Navarini, Illy3,8
Steam-machine pressure (historic) About 1–1.5 bar; about 1.5–3 bar Saturated-steam boilers; La Pavoni Ideale 1905 Carimali patent14, Morris10
Lever machine pressure Around 9 bar or 10 bar; peak 8 bar falling to 2 bar after 26 s Gaggia 1948 (two reports); one measured lever machine Morris10, MUMAC exhibition11, Caprioli et al.2
Water temperature 90 ± 2 °C; 88 ± 2 °C Certified Italian Espresso, as reported by two sources Caprioli et al.2, Mestdagh et al.1
Water temperature 92–94 °C Traditional espresso Illy & Navarini, Illy3,8
Temperature in the cup 67 ± 3 °C Certified Italian Espresso Caprioli et al.2
Shot time 25 ± 2.5 s; 25 ± 5 s; about 30 s Italian standard (two reports); Illy Caprioli et al.2, Mestdagh et al.1, Illy & Navarini, Illy3,8
Cup volume (single) 25 ± 2.5 mL; 25–40 mL; about 30 mL As above Caprioli et al.2, Mestdagh et al.1, Illy & Navarini, Illy3,8
Flow through the bed About 1 mL/s Traditional espresso Illy & Navarini, Illy3,8
First coffee in cup 5–6 s after pump start 18 g, 9 bar, 92 °C Sánchez López et al.4
Pre-infusion (research machine) 7 mL/s until pressure exceeds 2.5 bar Flow-controlled study Schmieder et al.7
Bed permeability About 2.6 × 10⁻¹⁴ to 4.4 × 10⁻¹³ m² Steady state, packed arabica beds, 80 °C (Table 2) Corrochano et al.5
Flow falls above About 5–7 bar Earlier experiments, cited Corrochano et al.5
Crema share At least 10% of volume Recommendation Illy & Navarini3, Folmer et al.9

Heating systems compared. The table lists what each design does, as described in the sources. No study opened for this entry measured the temperature stability of the designs side by side.

Heating system How brew water is heated Pressure source Source
Steam boiler (early 1900s) Boiler water and steam pushed through the coffee Steam pressure, tied to temperature Bezzera patent12, Carimali patent14
Lever with boiler Water heated in the boiler Spring-loaded piston Caprioli et al.2, Morris10
Heat exchanger in steam boiler Fresh water through a tube in the boiler; thermosiphon loop to the group Pump Valente patent13, Caprioli et al.2
Separate brew boiler(s) Small boiler per group; main boiler for steam and hot water Pump Carimali patent14
Thermal block Water pumped through a heated block Pump Breville patent16

History#

  • 1902–1903. Luigi Bezzera of Milan patents a coffee-making machine in the United States, assigned to Desiderio Pavoni. A boiler supplies water and steam, which a perforated diaphragm spreads over the coffee; each cup is made separately, “so that fresh coffee is always served”.12 A museum exhibition dates Bezzera’s patent to 1901.11
  • 1905. Pavoni, who had acquired the patent, produces the La Pavoni Ideale, the first espresso machine to enter production.10,11 According to a coffee historian, brewing was still slow, at low pressure, and the coffee tasted like a concentrated version of drip-brewed coffee.10
  • 1948. Achille Gaggia introduces the spring-piston lever machine.10,11 A coffee historian gives around 9 bar and a head of coffee oils that Gaggia promoted as Crema Caffè.10 A museum exhibition gives 10 bar.11
  • 1961. Faema of Milan launches the E61.11,19 A design museum states that Carlo Valente, founder of Faema, “revolutionized the brewing of espresso” with the machine; the museum notes that it used a motorised pump to provide the brewing pressure.19 According to a historian of the machine, it drew cold water from an external source and passed it through a heat exchanger inside the boiler, and it sold well in Italy, especially to small bars.20 Carlo Ernesto Valente patented a machine with priority from November 1961; the patent describes a heat exchanger in the boiler, thermosiphon circulation through the group, a motor-driven pump and a soaking phase before dispensing.13
  • Since. According to one patent, pump-driven machines became the standard.14 Electronic control has added automatic pre-infusion and stored pressure curves14, flowmeter dosing16 and gravimetric shot control.17

Open questions#

  • Optimal pressure. A pressure of 9 bar is a convention, not a measured optimum. Flow can fall at higher pressure as the bed consolidates1,5; 11 bar extracted more aroma in one study, but earlier work found lower consumer acceptance (see Contested).4
  • Optimal temperature. Recommendations span 88–94 °C, and in one study higher temperatures extracted more of the less polar aroma compounds.1,2,3,4,8 No sensory study opened for this entry fixed strength and yield while varying espresso brew temperature.
  • Pre-infusion and profiles. Patents describe pre-infusion and stored pressure curves, but patents are not evidence of their sensory effect.13,14 No controlled sensory study of pre-infusion was opened for this entry.
  • Heating systems. No independent study opened for this entry compared heat-exchanger, separate-boiler and thermal-block machines under the same conditions.
  • Crema chemistry. According to a textbook chapter, systematic studies are too scarce to explain crema formation and stabilisation completely.9

Key studies#

Sánchez López et al., 2016 – Aroma during the shot. Sampled the coffee stream of a semi-automatic machine with on-line mass spectrometry and sorted 46 aroma compounds into 5 families by when they came out. Polarity drove the timing; hotter water and 11 bar raised aroma extraction, mainly late in the shot.

International Journal of Mass Spectrometry 401, 22–30 · doi:10.1016/j.ijms.2016.02.015

Caprioli et al., 2013 – Pump versus lever. Logged pressure and temperature during shots from a pump machine with a heat exchanger and a spring-lever machine. The pump machine held its pressure and temperature; the lever machine’s pressure fell to 2 bar and its temperature dropped about 10 degrees, and it extracted fewer chlorogenic acids.

Journal of Food Research 2(3), 55–64 · doi:10.5539/jfr.v2n3p55

Cameron et al., 2020 – Uneven flow. Combined a model of espresso extraction with experiments. Measured extraction yield peaked at an intermediate grind, unlike the model’s prediction, which the authors attribute to uneven flow at fine settings; they propose a procedure to make shots more reproducible.

Matter 2(3), 631–648 · doi:10.1016/j.matt.2019.12.019

Corrochano et al., 2015 – Permeability of coffee beds. Developed a method to measure the steady-state permeability of packed beds of ground coffee. The beds consolidated as soon as water flowed, and their permeability fitted a corrected Kozeny–Carman equation.

Journal of Food Engineering 150, 106–116 · doi:10.1016/j.jfoodeng.2014.11.006

Schmieder et al., 2023 – Flow, grind and temperature. Brewed flow-controlled shots on a profiling machine and measured caffeine, trigonelline, the chlorogenic acid 5-CQA (5-caffeoylquinic acid) and TDS. Flow rate had the strongest effect on what reached the cup; grind and temperature mainly strengthened it, and the beverage mass collected mattered more than any of them.

Foods 12(15), 2871 · doi:10.3390/foods12152871

Contested
  • Certified Italian Espresso parameters: a 2013 study gives brew water at 90 ± 2 °C, 25 ± 2.5 s and 25 ± 2.5 mL in the cup2, while a 2017 textbook chapter gives 88 ± 2 °C, 25 ± 5 s and 25–40 mL.1 Both give 7 ± 0.5 g and 9 ± 1 bar. The institute’s own document was not opened for this entry.
  • Pressure of the first steam machines: a patent background puts saturated-steam boiler pressure at about 1–1.5 bar14; a coffee historian gives about 1.5–3 bar for the La Pavoni Ideale of 1905.10
  • Pressure of Gaggia’s 1948 lever machine: around 9 bar according to a coffee historian10; 10 bar according to a museum exhibition.11
  • Date of Bezzera’s patent: the US patent was filed on 10 June 1902 and granted in 190312; a museum exhibition dates his patent to 190111, which may refer to an earlier filing elsewhere; this was not checked.
  • The 9 bar convention: older experiments found that raising the pressure head above about 5–7 bar lowered the average flow5; in a study at 7, 9 and 11 bar, 11 bar extracted the most volatiles while 7 and 9 bar did not differ4; a textbook chapter notes that good espressos are also made at higher pressures, and lists 8–19 bar for espresso.1
  • Brew temperature: recommendations range from 88–90 °C (Italian standard as reported)1,2 to 92–94 °C.3,8 In one study, raising water from 82 to 96 °C clearly increased aroma extraction4, but in a flow-controlled study temperature effects on extraction were minor compared with the amount of beverage collected.7
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Sources

  1. BMestdagh, F.; Glabasnia, A.; Giuliano, P. (2017). The Brew – Extracting for Excellence. In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 15, pp. 355–380. doi:10.1016/B978-0-12-803520-7.00015-3 · other copies
  2. ACaprioli, G.; Cortese, M.; Odello, L.; Ricciutelli, M.; Sagratini, G.; Tomassoni, G.; Torregiani, E.; Vittori, S. (2013). Importance of Espresso Coffee Machine Parameters on the Extraction of Chlorogenic Acids in a Certified Italian Espresso by Using SPE-HPLC-DAD. Journal of Food Research 2(3), 55–64. doi:10.5539/jfr.v2n3p55 · other copies
  3. AIlly, E.; Navarini, L. (2011). Neglected Food Bubbles: The Espresso Coffee Foam. Food Biophysics 6(3), 335–348. doi:10.1007/s11483-011-9220-5 · other copies
  4. ASánchez López, J. A.; Wellinger, M.; Gloess, A. N.; Zimmermann, R.; Yeretzian, C. (2016). Extraction kinetics of coffee aroma compounds using a semi-automatic machine: On-line analysis by PTR-ToF-MS. International Journal of Mass Spectrometry 401, 22–30. doi:10.1016/j.ijms.2016.02.015 · other copies
  5. ACorrochano, B. R.; Melrose, J. R.; Bentley, A. C.; Fryer, P. J.; Bakalis, S. (2015). A new methodology to estimate the steady-state permeability of roast and ground coffee in packed beds. Journal of Food Engineering 150, 106–116. doi:10.1016/j.jfoodeng.2014.11.006 · other copies
  6. ACameron, M. I.; Morisco, D.; Hofstetter, D.; Uman, E.; Wilkinson, J.; Kennedy, Z. C.; Fontenot, S. A.; Lee, W. T.; Hendon, C. H.; Foster, J. M. (2020). Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment. Matter 2(3), 631–648. doi:10.1016/j.matt.2019.12.019 · other copies
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  9. BFolmer, B.; Blank, I.; Hofmann, T. (2017). Crema – Formation, Stabilization, and Sensation. In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 17, pp. 399–417. doi:10.1016/B978-0-12-803520-7.00017-7 · other copies
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  17. AOttavi, N. (assignee Simonelli Group SpA) (2020). Coffee machine and relevant coffee extraction method (US Patent 10,595,671 B2). United States Patent and Trademark Office; priority 30 October 2015, granted 24 March 2020.
  18. A/BWellinger, M.; Smrke, S.; Yeretzian, C. (2018). The SCA Water Quality Handbook (ISBN 978-0-9995807-3-8). Specialty Coffee Association.
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Tier A · authoritative · B · trade and expert · C · general. Supporting quotes for every source are in the data version of this entry. If a publisher’s site refuses access, try the doi link or “other copies”.

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