The moka pot
The moka pot is a small aluminum boiler that pushes water upward, through a bed of ground coffee, by the sole pressure of the air and steam trapped inside it. Set on the heat, it gives without a machine a coffee more concentrated than a filter.
What happens in a moka pot?
Start the scene or choose a stage. The animation simplifies the mechanism; what follows gives the details and the available references.
The water fills the bottom of the boiler, which has a valve; a pocket of air stays above it, and the finely ground coffee waits in the funnel. The physicist Warren King showed that the volume of coffee that can rise is proportional to the volume of air left: the step that changes the cup most is done before turning on the heat.
Read the stages without animation
- Fill the boilerThe water fills the bottom of the boiler, which has a valve; a pocket of air stays above it, and the finely ground coffee waits in the funnel. The physicist Warren King showed that the volume of coffee that can rise is proportional to the volume of air left: the step that changes the cup most is done before turning on the heat.
- HeatOn the heat, the trapped air and steam rise in pressure well before the water boils. King showed that the pressure rises much more slowly than the vapor pressure of water: it is first the air that pushes on the surface of the water.
- The water passes through the coffeeWhen the pressure overcomes the water column and the resistance of the bed, the water rises through the funnel tube, passes through the grind and gushes up the column into the upper chamber. With water that started cold, half the coffee passes below 70 °C; King advises preheating the water to around 70 °C and leaving at least 100 mL of air, to extract closer to 90 to 95 °C.
- The boiler emptiesWhen the water drops below the bottom of the tube, the last drops leave with the steam and the jet sputters. Plate XV advises cutting the heat as soon as this gurgling starts: the last jets are steam, which burns the bed.
- In the cupThe drink has no crema: under 1.5 atmospheres at most, according to the historian Jonathan Morris, the pressure does not form it. It is more concentrated than a filter and less than an espresso: in Gloess's trial, 7.5 g of coffee for 110 mL, about 1:15, give 2.13% dry matter in 224 seconds, against 1.03% for the filter; plate XV gives 3 to 4% for a ratio close to 1:10.
To remember: The moka pot pushes water upward with trapped air and steam, at 1.5 atmospheres at most: enough to pass through a bed of fine coffee, too little for crema.
In history: Louis Bernard Rabaut patents in 1822 an apparatus in which steam forces water upward through the coffee; Alfonso Bialetti designs his aluminum coffee maker in 1933.
Read the full article and its sources ↓History
The idea of making the water rise instead of letting it descend precedes the object by a century. In 1822, Louis Bernard Rabaut obtained an English patent for an apparatus in which steam forces boiling water upward through the coffee, the reverse of the French filter coffee pot; Casseneuve, a Parisian tinsmith, filed the same idea in France in 1824 (source 1).
The object that made the principle domestic comes from Crusinallo, a hamlet of Omegna. According to the Lombardy heritage inventory sheet, Alfonso Bialetti founded there in 1919 an aluminum foundry workshop that worked for others, transferred it in 1927, then reopened an activity to make the coffee maker he designed in 1933 (source 2). A 2016 article attributes the design to Luigi di Ponti, who is said to have sold his patent to Bialetti (source 10); neither the Lombard sheet nor Il Post mentions it, and the 1933 patent was not found.
Success was slow. Il Post puts it at about a thousand pieces a year at the end of the thirties and the start of the forties (source 9); the 2016 article speaks of 70,000 coffee makers in all when the son, Renato Bialetti, took over in 1947 (source 10). The Lombard sheet dates the fame from 1947, when Renato launched exports, and places in 1952 the birth of the little man with a mustache drawn by Paul Campani, star of the company's advertisements on the Carosello program from 1958 (source 2). Il Post estimates at about 300 million the coffee makers sold since, and a 2010 study cited by the Lombard sheet found a Bialetti moka pot in 90% of Italian households (sources 9 and 2).
According to the historian Jonathan Morris, it replaced in kitchens the Neapolitan flip coffee pot, the coffee maker you turn over. Bialetti advertising promised an espresso better than at the bar; Morris judges the comparison false, the pressure not exceeding 1.5 atmospheres, too little for crema, even though aroma and body exceeded those of the Neapolitan (source 3).
The motion
The moka pot is assembled in three parts: the boiler, which is filled with water and has a valve (source 2), the funnel filled with ground coffee, and the upper chamber that is screwed on above. On the heat, the air trapped above the water and the steam rise in pressure; when the pressure overcomes the water column and the resistance of the coffee bed, the water rises through the funnel, passes through the coffee and comes out in the upper chamber. Gianino put it into equations with Darcy's law, that of flow through a porous medium (source 6).
The step that changes the cup most is done before turning on the heat. The physicist Warren King showed that the pressure rises much more slowly than the vapor pressure of water, because it is first the air that pushes: the volume of coffee that can be extracted is proportional to the volume of air left in the boiler, and, starting from cold water, half the coffee passes while the water is still below 70 °C (source 4). He advises preheating the water to around 70 °C before closing and leaving at least 100 mL of air, to extract closer to 90 to 95 °C (source 4). The team of Luciano Navarini, at Illy, likewise concludes that extraction begins at a fairly low temperature and depends on the air present at the start (source 5).
The end must be watched: plate XV advises cutting the heat at the first gurgle, because the last jets are steam that burns the bed (source 12).
The settings
- GrindFine: in Gloess's trial, the same grinder setting as for machine espresso (source 7).
- Dose7.5 g for a three-cup Moka Express in Gloess's trial (source 7).
- Brew ratioAbout 1:15 in Gloess's trial (7.5 g for 110 mL, calculated) (source 7); plate XV gives a ratio close to 1:10 (source 12).
- TemperatureBelow boiling for a good part of the extraction (source 4).
- Time224 seconds on average, to within 12 seconds, in Gloess's trial (source 7).
- PressureNo more than 1.5 atmospheres according to Morris (source 3); no published measurement using its protocol was read.
What it has going for it
- It requires no electricity or machine: a heat source is enough to make the pressure (source 3).
- It extracts a lot: in Gloess's trial, the moka pot drew 31.2% of the coffee's mass, the highest yield of the nine methods studied (source 7).
- It gives a cup more concentrated than a filter, 2.13% dry matter against 1.03% for the filter in the same trial (source 7).
- With no pump or electronics, it lasts: the Lombard sheet describes a unit from the 1960s or 1970s still in good condition in 2024 (source 2).
What is held against it
- The extraction temperature escapes the user: with water that starts cold, half the coffee passes below 70 °C (source 4).
- It does not make espresso despite its advertising: under 1.5 atmospheres at most, it forms no crema (sources 3 and 8).
- The 31.2% yield measured by Gloess far exceeds the range of 17 to 23% that the American specialty coffee association associates with successful cups, beyond which coffees are considered bitter (sources 7 and 11).
What it gives in the cup
The most complete measurement found comes from the trial by Gloess and his colleagues (2013), who prepared nine coffees with the same Guatemalan coffee. Their three-cup Moka Express, loaded with 7.5 g for 110 mL, gave a drink at 2.13% dry matter, too little for an espresso in the strict sense, with a water-to-coffee ratio closer to a long coffee (source 7). It carries no crema, its tasting scores are average, and its aroma profile resembles that of the capsule tested, though that was made of a different coffee (source 7).
This figure depends on the ratio chosen: at 1:10, the usual ratio that plate XV adopts (source 12), the cup should be more concentrated, but nothing was measured. Lomolino and colleagues, who compare the moka pot with two capsule espressos, describe it as a low-pressure extraction, without crema, with rather weak sensory descriptors (source 8). The moka pot is strong next to gravity methods, light next to espresso.
Anecdotes
- According to the most widespread version, which Il Post judges the most credible, Alfonso Bialetti got the idea around the twenties watching washerwomen use a tub fitted with a central tube; no document from the company confirms it in the sources read (source 9).
- Renato Bialetti, who died in February 2016 at 93, rests in an urn shaped like a giant moka pot: his ashes were placed there at the funeral in Montebuglio (source 10).
The sources for this sheet
- William H. Ukers, All About Coffee, New York, The Tea and Coffee Trade Journal Company, 1922, chapters XXXIV and XXXVI and chronology (UTF-8 transcription from the local library): Rabaut 1822, Casseneuve 1824 · online · primary
- Regione Lombardia, SIRBeC, sheet OA SWDI1-00269, “Caffettiera, produzione piemontese,” Moka Express Bialetti, historical notice, FAI, 2024 · online · institutional
- Jonathan Morris, “Making Italian Espresso, Making Espresso Italian,” Food and History, vol. 8, no. 2, 2010, p. 155-183, DOI 10.1484/J.FOOD.1.102222 (author's preprint, University of Hertfordshire) · online · academic
- Warren D. King, “The physics of a stove-top espresso machine,” American Journal of Physics, vol. 76, no. 6, 2008, pp. 558-565, DOI 10.1119/1.2870524 (abstract) · online · academic · reading: abstract only
- Luciano Navarini, Enrico Nobile, F. Pinto, A. Scheri, F. Suggi Liverani, “Experimental investigation of steam pressure coffee extraction in a stove-top coffee maker,” Applied Thermal Engineering, vol. 29, 2009, pp. 998-1004, DOI 10.1016/j.applthermaleng.2008.05.014 · online · academic · reading: no: abstract relayed by the University of Trieste notice; text blocked (HAL anti-robot, ScienceDirect 403)
- Concetto Gianino, “Experimental analysis of the Italian coffee pot ‘moka,’” American Journal of Physics, vol. 75, no. 1, 2007, pp. 43-47, DOI 10.1119/1.2358157 (abstract) · online · academic · reading: abstract only
- Alexia N. Gloess et al., “Comparison of nine common coffee extraction methods: instrumental and sensory analysis,” European Food Research and Technology, vol. 236, 2013, p. 607-627, DOI 10.1007/s00217-013-1917-x · online · academic
- Giovanna Lomolino et al., “Foam Characteristics and Sensory Analysis of Arabica Coffee, Extracted by Espresso Capsule and Moka Methods,” Beverages, vol. 8, no. 2, 2022, article 28, DOI 10.3390/beverages8020028 (abstract read) · online · academic · reading: abstract only
- Il Post, “Gli 80 anni della moka,” August 21, 2013 · online · press
- Fox News, “Ashes of Italian coffee king put in giant espresso pot,” February 20, 2016 (AP photograph) · online · press
- Michael I. Cameron, Christopher H. Hendon, Jamie M. Foster et al., “Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment,” Matter, vol. 2, no. 3, 2020, pp. 631-648, DOI 10.1016/j.matt.2019.12.019: 17 to 23% range recalled in the introduction · online · academic
- PXL Roasters, plate XV “Preparations,” section 09, The pressure of steam (preparations-09.html), revision of September 24, 2026 · internal