{ "id": "espresso", "url": "https://coffearium.org/entries/espresso/", "license": "CC-BY-4.0", "title": "Espresso", "summary": "A small, concentrated coffee made by forcing hot water at high pressure through a compacted bed of ground coffee. It carries dissolved solids, oil droplets, fine particles and a foam, the crema.", "domain": "extraction", "kind": "Method", "stage": "brew", "topics": [ "science" ], "topicSections": {}, "lang": "en", "status": "verified", "reviewer": "Coffearium verification pass", "lastVerified": "2026-10-19T00:00:00.000Z", "updated": "2026-10-19T00:00:00.000Z", "added": "2026-10-19T00:00:00.000Z", "brief": [ "Espresso is brewed by forcing hot, high-pressure water through a compacted bed of ground roasted coffee.1,3", "The Italian Espresso National Institute (INEI) specifies 7 ± 0.5 g, 9 ± 1 bar, 25 ± 2.5 s and 25 ± 2.5 mL, as reported in 2013.2", "In one study, two espressos held 5.20% and 8.44% total dissolved solids (TDS); filter and immersion brews of the same coffee held 1.35–1.55%.8", "Espresso dissolves a similar share of the coffee as filter brewing does, around 20% in a typical shot against 19% for filter coffee in another study.7,9", "Crema is a foam of mainly carbon dioxide from roasting; a review and a textbook chapter state it should make up at least 10% of the volume.5,12" ], "quickFacts": [ { "label": "Principle", "value": "Pressure percolation through a compacted bed of ground coffee" }, { "label": "Certified Italian Espresso (as reported)", "value": "7 ± 0.5 g · 90 ± 2 °C · 9 ± 1 bar · 25 ± 2.5 s · 25 ± 2.5 mL" }, { "label": "Strength (TDS)", "value": "5.20–9.68% in three lab espressos; 1.35–1.55% for filter and immersion brews" }, { "label": "Extraction yield", "value": "Around 20% typical; 17–18% in shots under 10–15 s" }, { "label": "Flow into the cup", "value": "Close to 1 mL/s" }, { "label": "Crema", "value": "At least 10% of the volume; lasts up to 40 min" }, { "label": "Caffeine per serving", "value": "51–322 mg (20 cafés, Glasgow); 152–174 mg (lab double espresso)" } ], "aka": [ "Caffè espresso", "Espresso coffee" ], "linkTerms": [ "espresso", "crema", "espressos", "espresso shot", "espresso coffee" ], "related": [ "espresso-machine", "espresso-recipes-and-ratios", "puck-preparation", "extraction-yield-and-tds", "body-and-mouthfeel", "degassing", "particle-size-and-fines", "grinding", "brew-methods", "moka-pot", "caffeine", "water-for-coffee", "co2-and-the-bloom", "water-minerals-and-flavour", "coffee-drinks-around-the-world" ], "contested": [ "Brew water temperature in definitions: INEI as reported in a 2013 study, 90 ± 2 °C2; the same definition as reported in a textbook chapter, 88 ± 2 °C3; a definition cited in a 2016 study, 92–95 °C4; a 2011 review, 92–94 °C5; the COMITCAF specification, 90–96 °C.6", "Brew pressure in definitions: 9 ± 1 bar (INEI, as reported)2, 9–10 atmospheres (definition cited in 2016)4, 9 ± 2 bar (review)5 and at least 8 bar (COMITCAF).6 A textbook chapter lists 8–19 bar for espresso and notes that good espressos are also made at higher pressures.3", "Diterpenes in espresso: four espressos sampled in Gothenburg had a median of 1,059.3 mg/L cafestol (range 35.6–2,446.7 mg/L), against about 12 mg/L for paper-filtered home brews15; a 2020 Brazilian study found similar cafestol and kahweol per 50 mL serving in espresso and paper-filtered coffee.16 Coffees, machines and sampling differ.", "Robusta and foam: a textbook chapter suggests that robusta, richer in caffeic and chlorogenic acids, promotes the foam-active compounds of crema12; a 2011 review states that the claim that robusta is indispensable for crema has been substantially contradicted by recent data, and reports no significant difference in foam volume between arabica and robusta.5" ], "corrections": [ { "date": "2026-10-19T00:00:00.000Z", "change": "The supersaturation hypothesis of crema formation is now cited to the crema chapter only, and the claim that pressure dissolves carbon dioxide into the water to the brewing chapter; the 2011 review is cited for saying that the bubble mechanism has not been investigated in detail.", "reason": "Verification read Illy and Navarini 2011, which does not state the dissolution or supersaturation hypothesis." }, { "date": "2026-10-19T00:00:00.000Z", "change": "The robusta view of the 2011 review now reads that the claim that robusta is indispensable for crema has been substantially contradicted, with no significant difference in foam volume between arabica and robusta.", "reason": "The review does not say that robusta's foamability \"cannot be generalised\"." }, { "date": "2026-10-19T00:00:00.000Z", "change": "The COMITCAF dry residue is now described as oven-dried with or without filtering (TDS given as an alternative unit), not as total solids only; the issuer is described as named on the document only by its contact details.", "reason": "Verification read the specification, which has filtered and unfiltered methods and names no issuer in its text." } ], "sources": [ { "id": "s1", "title": "Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment", "authors": "Cameron, 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.", "year": 2020, "publisher": "Matter 2(3), 631–648", "url": "https://pure.hud.ac.uk/en/publications/systematically-improving-espresso-insights-from-mathematical-mode/", "doi": "10.1016/j.matt.2019.12.019", "tier": "A", "quote": "Espresso is a beverage brewed using hot, high-pressure water forced through a bed of roasted coffee.", "abstractOnly": true, "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s2", "title": "Importance of Espresso Coffee Machine Parameters on the Extraction of Chlorogenic Acids in a Certified Italian Espresso by Using SPE-HPLC-DAD", "authors": "Caprioli, G.; Cortese, M.; Odello, L.; Ricciutelli, M.; Sagratini, G.; Tomassoni, G.; Torregiani, E.; Vittori, S.", "year": 2013, "publisher": "Journal of Food Research 2(3), 55–64", "url": "https://www.openaccessrepository.it/record/70454/files/fulltext.pdf", "doi": "10.5539/jfr.v2n3p55", "tier": "A", "quote": "coffee temperature in the cup of 67°C ± 3°C, water pressure of 9 bar ± 1, percolation time of 25 seconds ± 2.5,", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s3", "title": "The Brew – Extracting for Excellence", "authors": "Mestdagh, F.; Glabasnia, A.; Giuliano, P.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 15, pp. 355–380", "url": "https://doi.org/10.1016/B978-0-12-803520-7.00015-3", "doi": "10.1016/B978-0-12-803520-7.00015-3", "tier": "B", "quote": "The character of an espresso is connected to this pressure and a low water-to-coffee ratio.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s4", "title": "Extraction kinetics of coffee aroma compounds using a semi-automatic machine: On-line analysis by PTR-ToF-MS", "authors": "Sánchez López, J. A.; Wellinger, M.; Gloess, A. N.; Zimmermann, R.; Yeretzian, C.", "year": 2016, "publisher": "International Journal of Mass Spectrometry 401, 22–30", "url": "https://doi.org/10.1016/j.ijms.2016.02.015", "doi": "10.1016/j.ijms.2016.02.015", "tier": "A", "quote": "An espresso is defined as a 25–35 ml beverage prepared from 7 to 9 g of coffee, through which clean, 92–95 °C water has been forced at 9–10 atmospheres of pressure, and where the grind of the coffee is such that the brewing ‘flow’ time is approximately 20–30 s.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s5", "title": "Neglected Food Bubbles: The Espresso Coffee Foam", "authors": "Illy, E.; Navarini, L.", "year": 2011, "publisher": "Food Biophysics 6(3), 335–348", "url": "https://link.springer.com/article/10.1007/s11483-011-9220-5", "doi": "10.1007/s11483-011-9220-5", "tier": "A", "quote": "As reported in Table 1, by increasing water pressure, the foam volume steadily increases from 5.1 to 6.9 mL.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s6", "title": "Disciplinare Caffè Espresso Italiano Tradizionale", "authors": "Issuer not named on the document; every page carries the contact details of COMITCAF (Comitato Italiano del Caffè), Corso di Porta Nuova 34, Milan", "year": 2018, "publisher": "Specification document, no version date stated (file dated 2018)", "url": "https://www.ilcaffeespressoitaliano.com/wp-content/uploads/Disciplinare-Caffè-Espresso-Italiano-Tradizionale-2018.pdf", "tier": "A/B", "tierNote": "A for the parameters and test methods the specification itself sets (owner ruling of 19 Oct 2026 on trade and standards-body specifications); B otherwise. Cited only as a specification, never as evidence of effects.", "quote": "Preparato con macchina professionale da bar e miscela di caffè in grani.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s7", "title": "Comparison of nine common coffee extraction methods: instrumental and sensory analysis", "authors": "Gloess, A. N.; Schönbächler, B.; Klopprogge, B.; D’Ambrosio, L.; Chatelain, K.; Bongartz, A.; Strittmatter, A.; Rast, M.; Yeretzian, C.", "year": 2013, "publisher": "European Food Research and Technology 236, 607–627", "url": "https://digitalcollection.zhaw.ch/bitstream/11475/4251/2/2013_Gloess-etal_Comparison-of-nine-common-coffee-extraction-methods.pdf", "doi": "10.1007/s00217-013-1917-x", "tier": "A", "quote": "no significant difference was found in total solids between an espresso and a lungo.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s8", "title": "What kind of coffee do you drink? An investigation on effects of eight different extraction methods", "authors": "Angeloni, G.; Guerrini, L.; Masella, P.; Bellumori, M.; Daluiso, S.; Parenti, A.; Innocenti, M.", "year": 2019, "publisher": "Food Research International, pp. 1327–1335 (volume not confirmed in the copies opened)", "url": "https://flore.unifi.it/bitstream/2158/1142622/5/Angeloni%20et%20al_Food%20Research%20International_2018-in%20Press.pdf", "doi": "10.1016/j.foodres.2018.10.022", "tier": "A", "quote": "Maximum caffeine and CGA concentrations were found in Espresso coffees", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s9", "title": "The role of fines in espresso extraction dynamics", "authors": "Smrke, S.; Eiermann, A.; Yeretzian, C.", "year": 2024, "publisher": "Scientific Reports 14, 5612", "url": "https://www.nature.com/articles/s41598-024-55831-x", "doi": "10.1038/s41598-024-55831-x", "tier": "A", "quote": "During a typical espresso extraction around 20% of the solid coffee material is solubilized and extracted into the beverage (extraction yield).", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s10", "title": "Influence of Flow Rate, Particle Size, and Temperature on Espresso Extraction Kinetics", "authors": "Schmieder, B. K. L.; Pannusch, V. B.; Vannieuwenhuyse, L.; Briesen, H.; Minceva, M.", "year": 2023, "publisher": "Foods 12(15), 2871", "url": "https://mdpi-res.com/d_attachment/foods/foods-12-02871/article_deploy/foods-12-02871.pdf", "doi": "10.3390/foods12152871", "tier": "A", "quote": "The analysis of the studied parameters revealed that flow rate had the strongest effect on the component mass in the cup.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s11", "title": "A new methodology to estimate the steady-state permeability of roast and ground coffee in packed beds", "authors": "Corrochano, B. R.; Melrose, J. R.; Bentley, A. C.; Fryer, P. J.; Bakalis, S.", "year": 2015, "publisher": "Journal of Food Engineering 150, 106–116", "url": "https://research.birmingham.ac.uk/files/18167003/Corrochano_et_al_New_methodology_estimate_Journal_Food_Engineering_2015.pdf", "doi": "10.1016/j.jfoodeng.2014.11.006", "tier": "A", "quote": "It was observed that, upon application of flow, the beds consolidated.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s12", "title": "Crema – Formation, Stabilization, and Sensation", "authors": "Folmer, B.; Blank, I.; Hofmann, T.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 17, pp. 399–417", "url": "https://doi.org/10.1016/B978-0-12-803520-7.00017-7", "doi": "10.1016/B978-0-12-803520-7.00017-7", "tier": "B", "quote": "Unfortunately, systematic chemical and physical studies are currently too scarce to completely understand this complex system.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s13", "title": "Variations in caffeine and chlorogenic acid contents of coffees: what are we drinking?", "authors": "Ludwig, I. A.; Mena, P.; Calani, L.; Cid, C.; Del Rio, D.; Lean, M. E. J.; Crozier, A.", "year": 2014, "publisher": "Food & Function 5, 1718–1726", "url": "https://pubs.rsc.org/en/content/articlehtml/2014/fo/c4fo00290c", "doi": "10.1039/c4fo00290c", "tier": "A", "quote": "Regular extraction resulted in ∼67% recoveries, which increased to over 85% with over-extraction.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s14", "title": "Espresso coffees, caffeine and chlorogenic acid intake: potential health implications", "authors": "Crozier, T. W. M.; Stalmach, A.; Lean, M. E. J.; Crozier, A.", "year": 2012, "publisher": "Food & Function 3, 30–33", "url": "https://pubs.rsc.org/en/content/articlehtml/2012/fo/c1fo10240k", "doi": "10.1039/C1FO10240K", "tier": "A", "quote": "The amount of caffeine that a consumer would ingest per serving ranged from 51 to 322 mg", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s15", "title": "Cafestol and kahweol concentrations in workplace machine coffee compared with conventional brewing methods", "authors": "Orrje, E.; Fristedt, R.; Rosqvist, F.; Landberg, R.; Iggman, D.", "year": 2025, "publisher": "Nutrition, Metabolism and Cardiovascular Diseases 35, 103933", "url": "https://research.chalmers.se/publication/545621/file/545621_Fulltext.pdf", "doi": "10.1016/j.numecd.2025.103933", "tier": "A", "quote": "For espresso, there was a considerable and unexplained variation in diterpene concentration between our four samples tested.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s16", "title": "Kahweol and cafestol in coffee brews: comparison of preparation methods", "authors": "Wuerges, K. L.; Dias, R. C. E.; Viegas, M. C.; Benassi, M. T.", "year": 2020, "publisher": "Revista Ciência Agronômica 51(1)", "url": "https://scielo.br/j/rca/a/hCQkqtKzbTVpXBXwmytns5y", "doi": "10.5935/1806-6690.20200005", "tier": "A", "quote": "Similar contents of cafestol and kahweol (in mg per dose of 50 mL) were observed when comparing filtered and espresso brews.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s17", "title": "We Consumers – Tastes, Rituals, and Waves", "authors": "Morris, J.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 19 (pp. 471–472 and 476–477 cited)", "url": "https://doi.org/10.1016/B978-0-12-803520-7.00019-0", "doi": "10.1016/B978-0-12-803520-7.00019-0", "tier": "B", "quote": "In America, however, the caffè latte was effectively developed as an espresso version of café au lait, using textured, steamed milk topped with a flat thin layer of froth, rather than the macrofoamed and domed cappuccino.", "accessed": "2026-10-19T00:00:00.000Z" } ], "markdown": "## What espresso is\n\nEspresso is brewed by forcing hot water at high pressure through a bed of roasted coffee.1 The ground coffee is pressed into a compact cake, the puck.3,5 Water passes through it in well under a minute, and a small, concentrated cup comes out.3,4 According to a coffee science textbook, the character of espresso comes from this pressure and from the low ratio of water to coffee.3\n\nSeveral definitions put numbers on this. A definition cited in a 2016 study describes a 25–35 mL beverage made from 7–9 g of coffee.4 In it, water at 92–95 °C is forced through at 9–10 atmospheres, and the flow lasts about 20–30 s.4 The Italian Espresso National Institute (INEI) sets narrower values for its Certified Italian Espresso. Examples are 7 ± 0.5 g of coffee and 9 ± 1 bar.2 A 2018 specification for traditional Italian espresso, issued with the contact details of COMITCAF (Comitato Italiano del Caffè), Milan, sets limits for espresso made on a professional bar machine from a blend of whole beans.6 The definitions do not agree on every value; the Deep dive compares them.\n\nPractice has moved beyond the single 25 mL cup. A textbook chapter notes a parallel trend towards 20 g of coffee for 40 mL of water.3 Research shots since 2023 use similar recipes, for example 20 g of coffee to 40 g of beverage.9,10 How dose, beverage mass, time, temperature and pressure are chosen is covered in espresso recipes and ratios.\n\n## How water passes through the puck\n\nA packed bed of ground coffee resists the flow of water, like a porous filter.11 According to a coffee science textbook, the pressure on the bed is a balance between the push of the pump and this resistance.3 A finer grind leaves smaller gaps between particles, so the pressure rises and the water flows more slowly.3 The textbook adds that too tight a bed gives a long shot, which can over-extract.3\n\nThe bed changes while water flows. In a 2015 study, coffee beds packed down (consolidated) as soon as flow began.11 According to the textbook, wetted particles swell and rearrange, and the bed's porosity falls during the shot.3 A 2024 study found that fine particles, called fines, act mainly by making the bed less permeable.9\n\nEven flow matters as much as total flow. In a 2020 study, measured extraction yield fell at very fine grind settings instead of rising.1 The authors attribute this to uneven flow through the bed at fine settings.1 According to the textbook, the bed must be even in density and shape to avoid channelling, in which water takes easy paths.3 Grinding into the basket, distribution and tamping shape the bed; they are covered in puck preparation. The machine side is in the espresso machine entry.\n\n
\n
\n
    \n
  1. Hot water. About 9 bar above the bed in the Italian definition and a review2,5, spread over the bed by the shower screen.
    2. \n
  2. Puck. Compacted ground coffee; their resistance sets the flow.3,11
    3. \n
  3. Pressure drop. Pressure is lost across the bed. At low flow, flow rate is proportional to the pressure drop and inversely proportional to bed height (Darcy’s law).11 The bed packs down as water flows.3,11
    4. \n
  4. Channel. An easy path through the bed: flow and extraction become uneven.1,3
    5. \n
  5. Ambient pressure. The brew returns to ambient pressure as it leaves the bed.12
    6. \n
  6. Cup. The beverage should flow at close to 1 mL/s5; crema forms as carbon dioxide comes out of solution.3,12
    7. \n
\n
Water through the puck, from the shower screen to the cup. Schematic, not to scale.
\n
\n\n## What ends up in the cup\n\nEspresso is far stronger than other brews. In a 2019 study of eight methods, two espressos held 5.20% and 8.44% total dissolved solids (TDS).8 Filter and immersion brews of the same coffee held 1.35–1.55%.8 In a 2023 study, 40 g espressos brewed from 20 g of coffee held 9.68 g of dissolved solids per 100 g, averaged over all its experiments.10\n\n
\n
\n
Strength of espressos and other brews in four sources. TDS was measured by refractometer; total solids by weighing after drying in an oven, which also counts oil and particles. The specification's dry residue is weighed after oven drying, with or without filtering first. Hollow circles are limits, not measurements.6,7,8,10 Compare within a source.
\n
\n\nThe share of the coffee that dissolves is not higher, however. In a 2024 study, a typical espresso dissolved around 20% of the ground coffee.9 In a 2013 comparison of nine methods, filter coffee reached 19%.7 Espresso is strong mainly because so little water is used per gram of coffee.3 The extraction yield and TDS entry explains the arithmetic.\n\nEspresso also carries more than dissolved solids. A 2011 review describes an emulsion of microscopic oil droplets in the liquid, with fragments of cell walls.5 According to the textbook, the pressure pushes oil droplets out of the cells into the cup; paper filters, by contrast, absorb oil efficiently.3 How these phases are felt in the mouth is covered in body and mouthfeel.\n\n## Crema\n\nCrema is the dense brown foam on top of an espresso.12 Its gas is mainly carbon dioxide formed during roasting and trapped in the cell structure.5,12 According to the brewing textbook, pressure is decisive: brew methods without it form no crema.3 One explanation is that the pressure forces carbon dioxide from the grounds into the water.3,12 When the brew reaches the cup at ambient pressure, the gas comes out of solution as tiny bubbles.12\n\nA review and a textbook chapter state that crema should make up at least 10% of the volume.5,12 It can last up to 40 minutes.5,12 The COMITCAF specification requires crema that covers the whole surface for at least 120 s.6 The textbook chapter calls colour and bubble size only secondary signs of a good extraction, after flavour.12 How crema changes as roasted coffee loses its gas is covered in degassing.\n\n## Origins and milk drinks\n\nAccording to a coffee historian, the first espresso machines brewed at low pressure, and the coffee tasted like a concentrated drip brew.17 A head of crema came with Achille Gaggia's spring-lever machine, and Gaggia promoted the result as a new beverage.17 The espresso machine entry follows this history.\n\nEspresso is also the base of many milk drinks. In early Italian bars, steam wands on the machines warmed milk as well as alcoholic punches, according to the same historian.17 The cappuccino grew from this practice into an espresso topped with steamed milk.17 In the United States, the caffè latte developed as an espresso version of café au lait, with textured, steamed milk.17 How much espresso goes into a milk drink varies. In one survey, cappuccinos from three coffee shops held about double the caffeine and caffeoylquinic acids of their espressos.13\n\n\n\n## Definitions compared\n\nThe definitions below are reference points, not quality guarantees. The 2013 study that reports the INEI parameters notes that meeting them alone would not fully satisfy quality requirements.2\n\n| Parameter | INEI, as reported 20132 | Same definition, textbook 20173 | Definition cited 20164 | Review 20115 | COMITCAF specification6 |\n|---|---|---|---|---|---|\n| Dose | 7 ± 0.5 g | 7 ± 0.5 g | 7–9 g | 6.0–8.0 g | 7–9 g |\n| Water temperature | 90 ± 2 °C, leaving the machine | 88 ± 2 °C | 92–95 °C | 92–94 °C | 90–96 °C |\n| Pressure | 9 ± 1 bar | 9 ± 1 bar | 9–10 atmospheres | 9 ± 2 bar | ≥ 8 bar |\n| Time | 25 ± 2.5 s | 25 ± 5 s | About 20–30 s | 30 s | 20–27 s from the first drop |\n| In the cup | 25 ± 2.5 mL including foam | 25–40 mL | 25–35 mL | 25–30 mL | 13–26 g |\n| Other limits | 67 ± 3 °C in the cup; viscosity at 45 °C > 1.5 mPa·s; total fat > 2 mg/mL; caffeine < 100 mg per cup | – | – | Flow close to 1 mL/s | Flow 0.48–1.3 g/s; dry residue ≥ 5%; crema covering the surface for ≥ 120 s |\n\n**Notes.** The INEI values are taken from a 2013 study, which cites a 2006 publication; the institute's own document was not opened for this entry.2 The COMITCAF specification sets a dry residue of at least 5%. It is weighed after drying the beverage in an oven at 103 ± 2 °C to constant weight, either after filtering or unfiltered; the specification gives TDS as an alternative unit.6 The document names no issuer; it carries COMITCAF's contact details.6 The COMITCAF limits apply to espresso made on a professional bar machine from a blend of whole beans.6 The 2016 study does not name the body behind the definition it cites.4\n\n## Reference values\n\n| Parameter | Value reported | Conditions | Source |\n|---|---|---|---|\n| TDS, espresso | 8.44% · 5.20% | 18 g → 17.4 g and 14 g → 29.6 g, 93 °C, 9 bar (the study's Table 1; its methods text gives 14.5 g and 92 °C for the smaller shot); refractometer | Angeloni et al.8 |\n| TDS, espresso | 9.68 g per 100 g | 20 g → 40 g; flow-controlled machine | Schmieder et al.10 |\n| Total solids, espresso | > 4% | Three machines, 30 mL shots; oven-dried | Gloess et al.7 |\n| TDS, filter and immersion | 1.35–1.55% | V60, AeroPress, French press, cold brew | Angeloni et al.8 |\n| Total solids, filter coffee | 1.03% | Filter machine; oven-dried | Gloess et al.7 |\n| Extraction yield, espresso | Around 20% typical; 17–18% at < 10–15 s; 19–20% at about 30 s | 20 g → 40 g, 9 bar | Smrke et al.9 |\n| Extraction yield, espresso (our arithmetic) | ≈14.9% · ≈19.4% · ≈20.8% | 20 g → 20, 40 and 60 g; mean dissolved solids 2.97, 3.87 and 4.15 g per cup | Schmieder et al.10 |\n| Extraction yield, filter coffee | 19% | Total solids | Gloess et al.7 |\n| Flow into the cup | Close to 1 mL/s | Recommendation | Illy & Navarini5 |\n| Crema | ≥ 10% of the volume; up to 40 min | Recommendation; lifetime | Illy & Navarini5, Folmer et al.12 |\n| Oil droplets | 90% < 10 µm | Emulsion in the liquid | Illy & Navarini5 |\n| Cell-wall fragments | 2–5 µm | Suspended in the beverage | Illy & Navarini5 |\n\nThe extraction yields from the 2023 study are our own arithmetic: mean dissolved solids per cup in its Table 2, divided by the 20 g dose.10 The 2019 eight-method study also reports yields, but they cannot be reproduced from its TDS and beverage values, so they are left out here.8\n\n## Concentration over the shot\n\nEspresso is not uniform from first drop to last. In the 2023 flow-controlled study, the concentration of each compound fell exponentially as more beverage was collected.10 TDS and trigonelline fell fastest, followed by the chlorogenic acid 5-CQA (5-caffeoylquinic acid) and caffeine.10 In the 2024 study, shots of under 10–15 s already reached 17–18% extraction, more than 80% of the highest yield measured.9 The best flavour for that coffee, judged by one panellist, came at about 30 s and 19–20%.9\n\nSo the beverage mass collected dominates. In the 2023 study, flow rate had the strongest effect on the mass of each compound in the cup, but all effects of flow, grind and temperature were minor compared with the effect of beverage mass.10 Per cup, an espresso and a lungo held no significantly different total solids in a 2013 comparison.7 What a lungo adds is mostly water.7,10\n\n## Physics of the bed\n\nAt low flow, a packed bed follows Darcy’s law: flow rate equals permeability × cross-section × pressure drop ÷ (viscosity × bed height), or Q = K·A·ΔP ÷ (μ·L).11 The 2015 study applied it in steady state at low particle Reynolds numbers (below 10).11 Permeability fitted a Kozeny–Carman equation corrected with a porosity-dependent tortuosity.11 Permeability values for packed coffee beds are listed in the espresso machine entry.\n\nThree effects break the simple picture. First, the bed consolidates under flow.11 Earlier experiments, cited in the 2015 study, found that raising the pressure above about 5–7 bar lowered the average flow.11 Second, the pump does not set the pressure alone: according to the textbook, a pump gives its highest pressure at its lowest flow, and bed permeability decides where on that curve the shot runs.3 Third, flow need not be uniform. In the 2020 study, a model that assumed even flow predicted extraction yield falling steadily with coarser grind; measured yields peaked instead, with lower values at both very coarse and very fine settings.1 The authors conclude that uneven flow operates at fine settings and wastes coffee.1\n\nPressure also changes what reaches the cup. According to the textbook, aroma compounds cannot evaporate from the bed under pressure and reach the cup in larger amounts; non-volatile compounds are less sensitive to pressure itself.3 In the 2016 on-line study, 11 bar raised the extraction of volatile compounds over the whole 25 s shot.4 The same study cites earlier work in which higher water temperature gave more total solids, caffeine and lipids in the cup.4\n\n## Crema: formation and stability\n\nA 2011 review describes crema as foam over an oil-in-water emulsion. In it, 90% of the oil droplets are smaller than 10 µm, and cell-wall fragments of 2–5 µm are suspended.5 In one study described in the review, foam volume rose from 5.1 to 6.9 mL as water pressure increased.5 Water temperature between 88 and 98 °C made no statistical difference to foam volume.5\n\n**Formation.** One hypothesis, cited in the crema chapter, holds that carbon dioxide dissolved in the hot water under pressure becomes supersaturated in the cup.12 The 2011 review states that the bubble formation mechanism has not been investigated in detail.5 According to the crema chapter, fine solid particles and cell-wall fragments then act as nucleation sites.12 The bubbles rise only a short way in a small cup, which the chapter links to their small size.12 The chapter also links crema formation to the bicarbonate in the brew water, whose equilibrium shifts as the pH falls during brewing.12\n\n**Stabilisation.** The review reports a high correlation between foamability and protein content.5 Fractions rich in high-molecular-weight polysaccharides were linked to foam stability, and a melanoidin-proteinaceous fraction to foamability.5 According to the crema chapter, unpublished activity-guided work identified 4-vinylcatechol oligomers as key foam-active compounds in roasted coffee.12 These form from chlorogenic and caffeic acids during roasting, and their level rose with roasting time.12 Lipids work the other way. The chapter states that pure arabica espresso contains more lipids than robusta espresso on average, so lipid-induced foam collapse is more likely.12 Foam then breaks down by bubble coalescence, gas diffusion from small to large bubbles (Ostwald ripening) and drainage.12\n\n**Crema and perception.** According to the chapter, crema at first raised the concentration of volatile compounds above the cup compared with the same coffee without crema.12 A stable crema, by contrast, held aroma back.12 In a consumer study described in the chapter, more crema went with higher expected and perceived smoothness.12 The chapter knows of no sensory data on how crema changes the aroma perceived above the cup.12\n\n## Chemistry per cup: espresso and filter\n\nEspresso is the most concentrated brew per millilitre, but a cup of filter coffee is many times larger. Concentrations and amounts per serving therefore tell different stories.8,14\n\n| Compound | Espresso | Filter or other brews | Basis | Source |\n|---|---|---|---|---|\n| Caffeine, concentration | 4.10 mg/mL (14 g); 4.20 mg/mL (18 g) | 0.74 mg/mL (V60); 0.52 mg/mL (French press) | Same coffee, eight methods | Angeloni et al.8 |\n| Caffeine, per cup | 122.4 mg (30 mL) | 89.04 mg (V60, 120 mL) | Volumes assumed by the authors | Angeloni et al.8 |\n| Caffeine, per 10 mL | 21.0 mg (semi-automatic machine) | 4.7 mg (filter machine) | Same coffee | Gloess et al.7 |\n| Caffeine, lab double espresso | 152–174 mg | – | 18.1–20.4 g arabica; ≈67% of the caffeine recovered | Ludwig et al.13 |\n| Caffeine, by beverage mass | 130 · 182.6 · 204 mg | – | 20 g arabica to 20, 40 and 60 g of beverage | Schmieder et al.10 |\n| Caffeine, cafés | 51–322 mg per serving | – | 20 outlets, Glasgow; cups 23–70 mL | Crozier et al.14 |\n| Chlorogenic acids, total | 38.1–55.6 mg per 25 mL cup | – | Certified Italian Espresso, pump and lever machines | Caprioli et al.2 |\n| Caffeoylquinic acids, cafés | 24–422 mg per serving | – | 20 outlets, Glasgow | Crozier et al.14 |\n| Caffeoylquinic acids, by roast | 244 · 119 · 74 mg | – | Lab double espresso; light, medium, dark roast | Ludwig et al.13 |\n| 5-CQA, concentration | 4.46 mg/mL (14 g) | 0.80 mg/mL (V60) | Same coffee | Angeloni et al.8 |\n| Fat | > 2 mg/mL required | – | INEI limit, as reported | Caprioli et al.2 |\n| Esterified fatty acids | Below 0.2% by weight in every brew | 0.0010 g per cup (filter) | Nine methods, same coffee | Gloess et al.7 |\n| Cafestol | Median 1,059.3 mg/L (35.6–2,446.7), n = 4 | ≈12 mg/L (paper filter, home) | Gothenburg cafeterias and one workplace machine | Orrje et al.15 |\n| Cafestol and kahweol | Similar to paper-filtered coffee per 50 mL | – | 7 g, 15 bar, 32 s, 50 mL | Wuerges et al.16 |\n\n**Caffeine.** Not all of the caffeine leaves the bed. In a lab test of double espressos, about 67% of the caffeine reached the cup; over-extraction with more water raised this to over 85%.13 Café espressos vary far more than lab shots, because doses and cup sizes vary.13,14 The caffeine entry has a calculator and the health limits.\n\n**Chlorogenic acids.** Espressos had the highest concentrations of caffeine and chlorogenic acids of the eight methods in the 2019 study.8 Roast degree also matters: in the lab study, a dark-roast espresso held under a third of the caffeoylquinic acids of a light-roast one.13 A 2013 machine comparison found 38.1–55.6 mg of total chlorogenic acids in a 25 mL cup.2\n\n**Oil and diterpenes.** Espresso is not filtered through paper, so oil reaches the cup.3 Whether this brings many diterpenes is disputed (see Contested). In a 2025 study, four espressos varied from 35.6 to 2,446.7 mg/L cafestol, a variation the authors could not explain.15 A 2020 study found espresso and paper-filtered coffee similar per 50 mL serving and saw no effect of pressure on diterpene extraction.16 Small cups mean that concentrations per litre do not translate directly into intake per cup.15,16\n\n## Open questions\n\n- **One definition.** INEI, COMITCAF, a review and a cited definition give different temperatures, pressures, times and cup sizes, and research recipes have moved to larger doses.2,3,4,5,6,9 No single definition is established.\n- **Espresso and sensory maps.** No study opened for this entry linked espresso strength and extraction yield to sensory preference in the way the brewing control chart does for filter coffee.\n- **Uneven extraction.** Uneven flow at fine settings lowered yield and reproducibility in a 2020 study.1 No study opened for this entry gives a way to measure in a café how evenly a puck was extracted.\n- **Crema chemistry.** According to the crema chapter, systematic studies are too scarce to explain crema formation and stability completely.12 The key foam-active compounds it names come from unpublished work.12\n- **Diterpenes.** Espresso samples ranged widely in cafestol for unexplained reasons, and two studies disagree on how espresso compares with paper-filtered coffee.15,16\n\n## Key studies\n\n
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Cameron et al., 2020 – Uneven flow in espresso. Combined a mathematical model of espresso extraction with measured shots. The model, assuming even flow, predicted yield falling with coarser grind; measured yield peaked at an intermediate setting, which the authors attribute to uneven flow at fine settings.

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

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Smrke et al., 2024 – Fines and extraction dynamics. Followed extraction over espresso shots of 20 g to 40 g at several grind settings, with and without added fines. Short shots already reached most of the yield, and fines acted mainly on bed permeability.

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Scientific Reports 14, 5612 · doi:10.1038/s41598-024-55831-x

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Schmieder et al., 2023 – Flow, grind and temperature. Brewed 20 g shots on a flow-controlled machine to 20, 40 and 60 g and measured caffeine, trigonelline, 5-CQA and TDS in fractions. Concentrations fell exponentially through the shot, and beverage mass outweighed flow, grind and temperature.

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

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Angeloni et al., 2019 – Eight methods compared. Brewed one coffee by eight methods, including two espressos, and measured TDS, viscosity, caffeine and chlorogenic acids. The espressos were the strongest and most viscous and held the highest concentrations of caffeine and chlorogenic acids.

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Food Research International · doi:10.1016/j.foodres.2018.10.022

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Illy & Navarini, 2011 – The espresso foam. Reviewed what is known about crema: its carbon dioxide gas phase, the emulsion and particles below it, and the proteins and polysaccharides linked to its stability. Foam volume rose with brew pressure but not with water temperature in the range studied.

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Food Biophysics 6(3), 335–348 · doi:10.1007/s11483-011-9220-5

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