{ "id": "espresso-machine-care", "url": "https://coffearium.org/entries/espresso-machine-care/", "license": "CC-BY-4.0", "title": "Espresso machine care", "summary": "What builds up inside an espresso machine – scale, milk residues and bacteria – what tests found about metals released after descaling, and what the evidence does and does not say about cleaning.", "domain": "equipment", "kind": "Practice", "stage": "brew", "topics": [ "equipment" ], "topicSections": {}, "lang": "en", "status": "verified", "reviewer": "Coffearium verification pass", "lastVerified": "2026-10-27T00:00:00.000Z", "literatureSearched": "2026-10-27T00:00:00.000Z", "updated": "2026-10-27T00:00:00.000Z", "added": "2026-10-27T00:00:00.000Z", "brief": [ "Scale forms when heat drives calcium carbonate out of hard water; it lowers heat transfer and clogs valves and jets, especially in the hot-water sections.1,2", "In tests of eight new coffee machines, portafilter machines released the most metals, and release rose sharply after descaling.5", "The German Federal Institute for Risk Assessment (BfR) recommends rinsing machines thoroughly after descaling and, in daily use, a water-only rinse before preparing espresso or coffee.4", "In 2023 and 2024, two Swiss cantonal laboratories found milk from fully automatic coffee machines above hygiene guideline values in 4 of 19 and at least 4 of 15 samples.9,10", "No study consulted for this entry tested backflushing, group cleaning detergents or the replacement of gaskets and shower screens." ], "quickFacts": [ { "label": "Scale", "value": "Mostly calcium carbonate; magnesium hydroxide at pH above 10" }, { "label": "Scale limits (SCA core zone)", "value": "12 mg/L in a brew boiler; 30 mg/L in a steam boiler at 130 °C" }, { "label": "Corrosion risk (SCA)", "value": "Alkalinity below 40 ppm as CaCO₃; pH below 6 or above 8" }, { "label": "Metals after descaling", "value": "Lead and nickel release rose sharply in portafilter machines (one 2015 study)" }, { "label": "Lead release limit", "value": "10 µg/kg food (Council of Europe recommendation; no legal EU limit)" }, { "label": "Milk systems", "value": "4 of 19 samples above guideline values (Basel-Stadt 2023); 4 of 15 above the Enterobacteriaceae guideline value (Zürich 2024)" } ], "aka": [ "Espresso machine maintenance", "Espresso machine cleaning", "Cleaning an espresso machine" ], "linkTerms": [ "descaling", "descale", "descaler", "backflushing", "backflush", "three-way valve", "gicleur" ], "related": [ "espresso-machine", "water-treatment", "hardness-and-alkalinity", "water-for-coffee", "contaminants-and-food-safety", "coffee-oil", "milk-and-milk-drinks", "grinder-care", "history-of-the-espresso-machine", "reading-a-shot" ], "contested": [ "Nickel and lead from coffee machines: a 2000 Danish study found that coffee machines tested like kettles released no aluminium, lead, chromium or nickel in quantities of any significance7; a 2015 German study found releases of lead, nickel, manganese, chromium and zinc within and partly beyond the Council of Europe release limits in portafilter machines, with release rising sharply after descaling.5 The Danish abstract does not name the machine types tested." ], "corrections": [ { "date": "2026-10-27T00:00:00.000Z", "change": "The BfR rinse advice now reads as a water-only rinse before each preparation in daily use, not only before the first coffee of the day.", "reason": "The BfR FAQ of 7 May 2025 recommends a rinsing step before the actual preparation of espresso or coffee in daily use; it does not say first of the day." }, { "date": "2026-10-27T00:00:00.000Z", "change": "Zürich 2024 results now state 4 of 15 samples above the Enterobacteriaceae guideline value plus one further case above the aerobic mesophilic guideline; the 'one in four or five' summary was replaced by the counts.", "reason": "The Kantonales Labor Zürich release reports a further case beyond the four; the total above any guideline is not stated." }, { "date": "2026-10-27T00:00:00.000Z", "change": "Müller et al. 2015: portafilter releases now 'within and partly beyond' the Council of Europe limits (was 'reached or exceeded'); the low-release machine now shows release above the level of concern is 'technically avoidable' (was 'by design').", "reason": "Wording of the abstract (read in the science.gov record)." }, { "date": "2026-10-27T00:00:00.000Z", "change": "Konieczyński et al. 2025: the statement that the source could not be identified is limited to nickel; for lead the authors' discussed reasons are given.", "reason": "The paper states the source limitation for nickel only." }, { "date": "2026-10-27T00:00:00.000Z", "change": "Removed an inference that acid descaling acts much less on silica scale (drawn from a statement on rainwater weathering of silicate rock) and an uncited claim that the steam boiler gets more scale because it runs hotter; removed an unsupported citation for coffee residues in the group.", "reason": "The cited passages do not support these statements." } ], "sources": [ { "id": "s1", "title": "The SCA Water Quality Handbook, edition 2 (ISBN 978-0-9995807-3-8)", "authors": "Wellinger, M.; Smrke, S.; Yeretzian, C.", "year": 2018, "publisher": "Specialty Coffee Association", "url": "https://digitalcollection.zhaw.ch/handle/11475/12365", "tier": "A/B", "tierNote": "A for the SCA's own targets and limits (core-zone scale limits, corrosion thresholds); B for explanations of scale chemistry and the yearly scale estimate", "quote": "Figure 5 illustrates the maximum amount of scale that can form at 95 °C and 130 °C at 1.7 bar steam pressure.", "quotes": [ { "text": "Scale formation: Caused by high levels of hardness and alkalinity that lead to the scaling of boilers and machines, which in turn lead to:", "for": "what causes scale" }, { "text": "Clogging of valves and jets especially in the hot water sections.", "for": "scale clogs valves and jets" }, { "text": "Corrosion: Caused by low levels of alkalinity below 40 ppm CaCO3 (also indicated by low electrical conductivity), pH < 6 or pH > 8, and high concentrations of chlorides, sulfates or nitrates", "for": "corrosion thresholds" }, { "text": "The lower left and upper border area of the core zone is shaped by the technical thresholds for corrosion prevention, and to the right, by the maximum scale formation of 30 mg/L in a steam boiler at 130 °C, or 12 mg/L in a brew boiler of an espresso machine.", "for": "core-zone scale limits" }, { "text": "calcium carbonate (CaCO3), and also at high pH (>10) by magnesium hydroxide (Mg(OH)2)", "for": "what scale is made of" }, { "text": "The acidic rainwater that comes into contact with carbonate rock (e.g. CaCO3, MgCO3, or a combination of both), dissolves part of it and acquires magnesium, calcium and hydrogen carbonate ions (HCO3-).", "for": "acid dissolves carbonate" }, { "text": "it is worth bearing in mind that the lower the pH the less scale forms", "for": "lower pH, less scale" } ], "scope": { "claim": "guidance", "context": "Water handbook of the Specialty Coffee Association; scale amounts are calculated with a modified Langelier index, not measured in machines.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" }, { "id": "s2", "title": "Water for Extraction – Composition, Recommendations, and Treatment", "authors": "Wellinger, M.; Smrke, S.; Yeretzian, C.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 16, pp. 381–398", "url": "https://doi.org/10.1016/B978-0-12-803520-7.00016-5", "doi": "10.1016/B978-0-12-803520-7.00016-5", "tier": "B", "quote": "As an additional measure, coffee machine companies recommend regular descaling using acid dissolved in water (preferably one with little taste), and also offer commercial solutions.", "quotes": [ { "text": "The primary technical concerns in terms of scale formation when using hard water are the decrease in efficiency of the heating system (when the layer of scale acts as an insulator) and the blockage of valves and flow restrictors (usually called gicleurs in coffee machines).", "for": "scale insulates and blocks flow restrictors" }, { "text": "The latter occurs in steam boilers, especially with sodium-softened water.", "for": "magnesium hydroxide scale in steam boilers" } ], "scope": { "claim": "description", "context": "Textbook chapter on water for coffee extraction; general account of scale and corrosion in coffee machines, no measurements.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" }, { "id": "s3", "title": "The Water Quality Handbook (ISBN 978-1-882552-08-5)", "authors": "Beeman, D.; Songer, P.; Lingle, T. (ed. Everage, L.)", "year": 2011, "publisher": "Specialty Coffee Association of America", "tier": "A/B", "tierNote": "A for the SCAA's own standard values; B for treatment advice, field observations and explanations (all uses in this entry)", "quote": "Lab analysis of corrosion material showed high concentration of chlorides, iron and nickel.", "quotes": [ { "text": "The culprit is usually acid flux used in the soldering of copper pipe joints.", "for": "corrosion from soldering flux" }, { "text": "silica, which is typically present in a much lower concentration than hardness, will scale to the same extent as calcium hardness regardless of pH and alkalinity.", "for": "silica scale" }, { "text": "Even after an appliance has been descaled, the microscopic particles, or irregular surfaces and seeds, left behind accelerate the re-development of scale; it may be best to replace the part.", "for": "scale returns faster after descaling" }, { "text": "If these compounds are not thoroughly rinsed off, they will dissolve into the coffee beverage as it is brewed and be found in the beverage that is served.", "for": "cleaning residues reach the cup" }, { "text": "Since the oxidizing ability of chlorine is removed, bacteria can build up within the filter.", "for": "bacteria in carbon filters" }, { "text": "Brass pumps will often corrode under these conditions, and a switch to more expensive stainless steel pumps will be necessary.", "for": "brass pumps corrode" } ], "scope": { "claim": "description", "context": "Trade handbook of the SCAA; field observations from brewer service in the United States, not a controlled study.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" }, { "id": "s4", "title": "Some portafilter espresso machines release too much lead (FAQ, updated 7 May 2025)", "authors": "German Federal Institute for Risk Assessment (BfR)", "year": 2025, "publisher": "BfR", "url": "https://www.bfr.bund.de/cm/349/some-portafilter-espresso-machines-release-too-much-lead.pdf", "tier": "A", "quote": "Excessive lead release was detected in portafilter espresso machines.", "quotes": [ { "text": "It is possible that the acidic descaling products release lead from metal components.", "for": "descalers may release lead" }, { "text": "sets a specific release limit of 10 micrograms (µg) per kilogram (kg) of food", "for": "Council of Europe lead limit" }, { "text": "In 2007, three out of 17 machines analysed showed increased lead release.", "for": "Stuttgart 2007 survey" }, { "text": "no lead release was detected (limit of determination for lead at 0.005 milligrams (mg) per litre (l) of test water)", "for": "Stuttgart 2011 survey" }, { "text": "the BfR recommends that consumers rinse the machines thoroughly after descaling.", "for": "rinse after descaling" }, { "text": "always carrying out a rinsing step (preparation with water only) before the actual preparation of espresso or coffee.", "for": "daily rinse before first use" }, { "text": "Due to the small number of samples, the data is not representative.", "for": "small sample" }, { "text": "The BfR assumes that components within the machines and/or any soldered joints may contribute to the release of lead.", "for": "source of lead not established" } ], "scope": { "claim": "guidance", "context": "National risk-assessment body; eight new household machines tested in 2013 (three portafilter, three pad, two capsule) plus cited state-laboratory surveys from Stuttgart, 2007 and 2011.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" }, { "id": "s5", "title": "Metal release from coffee machines and electric kettles", "authors": "Müller, F. D.; Hackethal, C.; Schmidt, R.; Kappenstein, O.; Pfaff, K.; Luch, A.", "year": 2015, "publisher": "Food Additives & Contaminants Part A 32(11), 1959–1964", "url": "https://doi.org/10.1080/19440049.2015.1086929", "doi": "10.1080/19440049.2015.1086929", "tier": "A", "quote": "By far the highest leaching, both quantitatively and qualitatively, was found for the portafilter machines.", "quotes": [ { "text": "After decalcification a large increase in the amounts of elements released was encountered.", "for": "release rose after descaling" }, { "text": "Metal release on consecutive days showed a decreasing trend as well.", "for": "release fell over consecutive days" }, { "text": "The electric kettles showed only a low release of the elements analysed.", "for": "kettles released little" }, { "text": "Therefore, a careful rinsing routine, especially after decalcification, is recommended for these machines.", "for": "rinsing recommendation" }, { "text": "beyond the release limits as proposed by the Council of Europe", "for": "within and beyond Council of Europe limits" }, { "text": "The comparably lower extent of release of one particular portafilter machine", "for": "one portafilter machine released less" }, { "text": "technically avoidable", "for": "high release avoidable" } ], "abstractOnly": true, "scope": { "claim": "finding", "context": "Laboratory study, Germany; 8 coffee machines and 11 electric kettles, release into food simulants before and after decalcification.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" }, { "id": "s6", "title": "Release of metals from coffee machines and electric kettles (BfR Communication No 041/2015, 9 November 2015)", "authors": "German Federal Institute for Risk Assessment (BfR)", "year": 2015, "publisher": "BfR", "url": "https://www.bfr.bund.de/cm/349/release-of-metals-from-coffee-machines-and-electric-kettles.pdf", "tier": "A", "quote": "No legal limits have so far been defined for the release of metals from metallic food contact materials.", "quotes": [ { "text": "In 2013, the European Council recommended limit values for 21 metals and metalloids (Resolution CM/Res(2013)9).", "for": "Council of Europe resolution" } ], "scope": { "claim": "description", "context": "Short communication announcing the 2015 journal article; states the legal position in the EU.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" }, { "id": "s7", "title": "The release of nickel and other trace elements from electric kettles and coffee machines", "authors": "Berg, T.; Petersen, A.; Pedersen, G. A.; Petersen, J.; Madsen, C.", "year": 2000, "publisher": "Food Additives and Contaminants 17(3), 189–196", "url": "https://orbit.dtu.dk/en/publications/the-release-of-nickel-and-other-trace-elements-from-electric-kett/", "tier": "A", "quote": "Coffee machines tested similarly did not release aluminium, lead, chromium or nickel in quantities of any significance.", "quotes": [ { "text": "Ten out of 26 kettles sold on the Danish market released more than 50 mu g/l nickel to water", "for": "kettles above 50 µg/L nickel" }, { "text": "Fifty mu g/l of nickel in water was chosen as the threshold of action", "for": "action threshold" } ], "abstractOnly": true, "scope": { "claim": "finding", "context": "Danish food administration study of kettles and coffee machines on the Danish market under simulated household use; machine types not given in the abstract.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" }, { "id": "s8", "title": "A Comparative Elemental Analysis of Espresso Coffee from Poland and Portugal", "authors": "Konieczyński, P.; Seroczyńska, K.; Wesołowski, M.; Pinto, E.; Couto, C.; Cunha, A.; Azevedo, R.; Almeida, A.", "year": 2025, "publisher": "Foods 14(3), 426", "url": "https://mdpi-res.com/d_attachment/foods/foods-14-00426/article_deploy/foods-14-00426.pdf", "doi": "10.3390/foods14030426", "tier": "A", "quote": "81.0 (45.3–133) µg/L in samples from Poland, and 86.8 (58.0–121) µg/L in samples from Portugal.", "quotes": [ { "text": "the median (P25–P75) Pb concentration in espresso samples was 14.3 (2.49–34.0) µg/L in samples from Poland", "for": "lead in espresso, Poland" }, { "text": "4.43 (1.99–15.6) µg/L in samples from Portugal.", "for": "lead in espresso, Portugal" }, { "text": "5.58 (4.08–9.62) µg/L in Polish samples and 4.61 (1.09–6.55) µg/L in Portuguese samples", "for": "nickel in water" }, { "text": "The median (P25–P75) Pb concentration in water samples from Poland and Portugal was 2.15 (1.58–3.89) µg/L", "for": "lead in water" }, { "text": "2.15 (1.58–3.89) µg/L and 0.574 (0.254–1.37) µg/L, respectively", "for": "lead in water, Portugal" }, { "text": "water samples (15 mL, into polyethylene tubes) were also collected in the same locations", "for": "water from the same places" }, { "text": "the methodology used in the present study does not allow the identification of the source of Ni", "for": "source of nickel unknown" }, { "text": "Copper does not seem to leach in significant amounts from coffee machines", "for": "copper" }, { "text": "Espresso samples (5–10 mL) were taken (with a plastic pipette, into polyethylene tubes), directly from the beverage served in cafes, restaurants, and gas stations.", "for": "sampling" } ], "scope": { "claim": "data", "context": "Survey, 60 espresso and water pairs from northern Poland and 44 from northern Portugal (cafés, restaurants, petrol stations); ICP-MS; machine source of metals not identified.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" }, { "id": "s9", "title": "Milch aus Kaffee-Vollautomaten in Restaurationsbetrieben: Mikrobiologische Qualität (2023)", "authors": "Kantonales Laboratorium Basel-Stadt", "year": 2024, "publisher": "Gesundheitsdepartement des Kantons Basel-Stadt (report, 16 February 2024)", "url": "https://media.bs.ch/original_file/a1316cecf5b97f05668c14b616e19c830d8ee846/2023-milch-aus-kaffeautomaten.pdf", "tier": "A", "quote": "Von den 19 erhobenen Proben mussten vier Proben (21%)", "quotes": [ { "text": "können sich besonders in den milchführenden Teilen Mikroorganismen festsetzen und vermehren", "for": "microorganisms settle in milk-carrying parts" }, { "text": "Die für Bacillus cereus gefundenen Keimzahlen lagen in beiden Fällen unterhalb von 5000 KbE/g.", "for": "Bacillus cereus counts" } ], "scope": { "claim": "finding", "context": "Official food control, canton of Basel-Stadt, Switzerland; 19 milk samples from fully automatic coffee machines in 19 restaurants, 2023; judged against guideline values of the Swiss hospitality guideline.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" }, { "id": "s10", "title": "Mikrobiologische Qualität von Milchgetränken aus Kaffeeautomaten", "authors": "Kantonales Labor Zürich", "year": 2025, "publisher": "Kanton Zürich, Gesundheitsdirektion (press release, 7 July 2025)", "url": "https://www.zh.ch/de/news-uebersicht/mitteilungen/2025/gesundheit/lebensmittel/-mikrobiologische-qualitaet-von-milchgetraenken-aus-kaffeeautoma.html", "tier": "A", "quote": "Bei 4 Proben wurden Richtwertüberschreitungen der Enterobacteriaceen festgestellt.", "quotes": [ { "text": "Im Jahr 2024 wurden 15 Proben von Milchgetränken aus Kaffeeautomaten erhoben", "for": "15 samples in 2024" }, { "text": "Wenn die Maschine aber nicht richtig gereinigt wird, können sich Keime in Milchrückständen im Inneren ansiedeln.", "for": "germs in milk residues" }, { "text": "In einem weiteren Fall wurden zusätzlich aerobe, mesophile Keime über dem Richtwert gemessen.", "for": "one further case, aerobic mesophilic count" }, { "text": "Bei stichprobenmässigen Nachkontrollen wurden keine Richtwertüberschreitungen mehr festgestellt.", "for": "follow-up checks" } ], "scope": { "claim": "finding", "context": "Official food control, canton of Zürich, Switzerland; 15 samples of milk and milk drinks from fully automatic coffee machines with milk function, 2024; judged against guideline values of the Swiss hospitality guideline.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" }, { "id": "s11", "title": "The coffee-machine bacteriome: biodiversity and colonisation of the wasted coffee tray leach", "authors": "Vilanova, C.; Iglesias, A.; Porcar, M.", "year": 2015, "publisher": "Scientific Reports 5, 17163", "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC4655483", "doi": "10.1038/srep17163", "tier": "A", "quote": "Our results reveal the existence of a varied bacterial community in all the machines sampled, and a rapid colonisation process of the coffee leach.", "quotes": [ { "text": "which were substituted by a more balanced bacterial composition after one month.", "for": "community settled after one month" }, { "text": "enterobacteria (genus Pantoea; peaking 4–11 days after the beginning of the experiment)", "for": "early enterobacteria" } ], "scope": { "claim": "finding", "context": "Capsule machines (Nespresso system) in Valencia, Spain; waste reservoirs of nine machines used for at least one year, plus colonisation of one new machine over time.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" }, { "id": "s12", "title": "Device and process for controlling the extraction pressure of coffee in an espresso coffee machine (EP 3 653 086 A1)", "authors": "Cappellini, D. (applicant Carimali S.p.A.)", "year": 2020, "publisher": "European Patent Office; priority 18 July 2017, published 20 May 2020", "url": "https://data.epo.org/publication-server/rest/v1.2/patents/EP3653086NWA1/document.html", "tier": "A/B", "tierNote": "A for how the machines are built (valve and group construction, the uses in this entry); B for its background on prior designs", "quote": "The dispensing valve 34 is typically a 3-way solenoid valve that, in the deactivation step, puts the brewing chamber 36 in communication with a duct at atmospheric pressure, so as to discharge the residual overpressure of the brewing chamber 36 that is generated at the end of the dispensing of the beverage.", "quotes": [ { "text": "The brewing chamber 36 of each dispensing group 10 comprises, in a known way, a water diffusion shower head", "for": "group with shower head" } ], "scope": { "claim": "description", "context": "European patent application for a pump espresso machine; construction only.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" }, { "id": "s13", "title": "Steam wand treating device (US Patent 11,992,151 B2)", "authors": "Tanami, O.", "year": 2024, "publisher": "United States Patent and Trademark Office; priority 29 October 2019, granted 28 May 2024", "url": "https://patents.google.com/patent/US11992151", "tier": "A/B", "tierNote": "A for how the device is built; B for its background on steam-wand hygiene (the use in this entry)", "quote": "A steam wand is a device used to heat and froth milk that can be added on top of hot beverages.", "quotes": [ { "text": "since milk residues often stick to them serving as a platform for the growth of bacteria.", "for": "milk residues on wands" } ], "scope": { "claim": "description", "context": "US patent for a steam-wand cleaning device; the hygiene statement is background, not a measurement.", "checked": "2026-10-27T00:00:00.000Z" }, "accessed": "2026-10-27T00:00:00.000Z" } ], "markdown": "## What builds up inside the machine\n\nAn espresso machine keeps water hot, often for hours, and passes it through metal parts, coffee and milk.1,5 Several things build up or wear as a result. Hard water leaves scale, a mineral crust, in boilers and narrow passages.1,2 Water that is too soft or acidic attacks metal instead, and metal can then pass into the water.1,3 Milk leaves residues in steam wands and milk systems.10,13 Bacteria can grow wherever residues and water stay together.9,10,11\n\nHow the machine itself works is covered in How an espresso machine works. Water composition, hardness and treatment have their own entries.\n\n
\n
\n
    \n
  1. Water filter. Treats the supply; carbon filters can host bacteria once the chlorine is gone.3
    2. \n
  2. Boiler and heating element. Where heat makes scale; a scale layer insulates the heater.1,2
    3. \n
  3. Flow restrictor (gicleur). A narrow jet in the water line; scale can block it.2
    4. \n
  4. Three-way valve and drain line. Opens the group to a drain at atmospheric pressure at the end of the shot.12
    5. \n
  5. Group and shower screen. The shower head spreads water over the coffee.12
    6. \n
  6. Portafilter and basket. Portafilter machines released the most metals in one study.5
    7. \n
  7. Steam wand. Heats and froths milk; milk residues stick to it.13
    8. \n
  8. Drip tray. Waste coffee in capsule machines was colonised by bacteria within days.11
    9. \n
\n
A single-boiler espresso machine cut along its centre line, with the parts discussed in this entry. The accent marks scale in the boiler, the narrow restrictor and the wand tip. Schematic, not to scale.
\n
\n\n## Scale and descaling\n\n**Why scale forms.** Tap water carries dissolved calcium and hydrogen carbonate. Heat drives off carbon dioxide, and calcium carbonate settles out as scale.1 In steam boilers, at a pH above 10, magnesium hydroxide can form as well, especially with sodium-softened water.1,2 Silica scales too, whatever the pH and alkalinity.3 The chemistry is explained in Hardness and alkalinity.\n\n**What scale does.** A layer of scale insulates the heating element, so heating becomes less efficient.2 Scale also clogs valves, jets and flow restrictors, especially in the hot-water sections.1,2 According to a water handbook of the former Specialty Coffee Association of America (SCAA), scale grows back faster once a first deposit has formed. Even after descaling, the rough surface left behind speeds up new scale, and the handbook suggests replacing the part.3\n\n**Descaling.** Descaling dissolves scale with an acid. A coffee science textbook chapter states that machine companies recommend regular descaling with an acid dissolved in water, preferably one with little taste.2 Prevention starts with the water: lower alkalinity and a lower pH mean less scale.1 Water treatment covers filters and softeners.\n\n## Metals released from machines\n\nMetal parts of a machine meet hot water, coffee and descaling acid.4,5 In a 2015 German laboratory study, 8 coffee machines and 11 electric kettles were filled with food simulants, and the released metals were measured.5 Portafilter machines released by far the most, in amount and in the number of elements.5 Release fell during each day and over consecutive days. After descaling it rose sharply.5\n\nThe BfR considers it possible that acidic descalers release lead from metal components.4 It recommends rinsing the machine thoroughly after descaling. For daily use, it also recommends a rinse with water only before each preparation of espresso or coffee, as manufacturers also prescribe.4 The BfR states that its sample was small and not representative.4 Food-safety limits for metals are set out in Contaminants and food safety.\n\n**In the cup.** A 2025 survey sampled espresso from cafés, restaurants and petrol stations in Poland and Portugal.8 Nickel and lead were higher in the espresso than in the tap water at the same places.8 The authors could not say whether the nickel came from the coffee or from the machine.8\n\n## Coffee and milk residues\n\n**Group, shower screen and valve.** In the group, the shower head spreads water over the coffee bed.12 According to one patent, the valve that ends the shot is typically a three-way valve. It opens the group to a drain line at atmospheric pressure, so the leftover pressure escapes.12 Cleaning compounds that are not rinsed off can dissolve into the next brew, according to the SCAA handbook.3\n\nNo study consulted for this entry tested backflushing, group detergents or how often gaskets and shower screens should be replaced. These routines are set by machine makers; their instructions were not consulted. How coffee oil goes rancid is covered in Coffee oil.\n\n**Steam wands and milk systems.** A steam wand heats and froths milk with steam from the boiler, and milk residues tend to stick to it.13 Swiss food inspectors tested milk and milk drinks from fully automatic coffee machines with milk functions. In Basel-Stadt in 2023, 4 of 19 samples were above guideline values.9 In Zürich in 2024, 4 of 15 samples were above the guideline value for Enterobacteriaceae.10 Both laboratories named poorly cleaned milk-carrying parts as one cause.9,10\n\n**Drip trays and waste.** In a 2015 study of capsule machines, bacteria colonised the waste coffee within days.11 The community settled into a more stable mix after about a month.11\n\n## Water filters\n\nA filter changes what reaches the boiler, so it changes how much scale forms. The options and their limits are covered in Water treatment. Filters need care of their own. Activated carbon removes the chlorine that kept bacteria in check, so bacteria can build up inside the filter.3 Treatment must also not go too far: the SCA links corrosion to alkalinity below 40 ppm as CaCO₃.1\n\n\n\n## Scale in brew and steam boilers\n\nThe Specialty Coffee Association (SCA) water handbook calculates the most scale that can form at 95 °C and at 130 °C, which it pairs with 1.7 bar of steam pressure.1 It uses a modified Langelier index with an equilibrium pH based on alkalinity.1 Its core zone for espresso machines allows at most 12 mg/L of scale in a brew boiler and 30 mg/L in a steam boiler at 130 °C.1 How these values turn into grams of scale per year is worked through in Water for coffee.\n\nThe calculation gives a maximum. At most the lower of total hardness and alkalinity can leave the water as calcium carbonate.1 According to the SCA handbook, lower pH means less scale.1 Magnesium hydroxide scale needs a pH above 10.1 A textbook chapter places it in steam boilers, especially with sodium-softened water.2\n\n## Corrosion and metal release\n\n**Corrosion from water.** The SCA links corrosion to alkalinity below 40 ppm as CaCO₃, to a pH below 6 or above 8 and to high chloride, sulfate or nitrate.1 The SCAA handbook reports corroded stainless-steel brewers in which deposits held high chloride, iron and nickel.3 It traced the usual cause to acid flux from soldering copper pipe joints, not to the water.3 The same handbook notes that brass pumps often corrode with unsuitable water.3\n\n**Release into food simulants.** In the 2015 German study, releases from portafilter machines of lead, nickel, manganese, chromium and zinc lay within and partly beyond the Council of Europe release limits.5 One portafilter machine released much less than the others; the authors take this as evidence that release above the level of health concern is technically avoidable.5 The kettles in the same study released little.5 The authors recommend a careful rinsing routine, especially after descaling.5 The study used food simulants without coffee, so the release it measured came from the machines. The full text was not consulted; the values per machine are not given here.\n\n**Earlier surveys.** A Danish study of 2000 found that coffee machines released no aluminium, lead, chromium or nickel in quantities of any significance.7 The BfR reports two surveys by the state laboratory in Stuttgart. In 2007, 3 of 17 machines showed increased lead release.4 In 2011, no lead release was detected, with a limit of determination of 0.005 mg/L of test water.4\n\n**Legal position.** As of 2015 and 2025, the EU had no legal limit for metals released from metallic food contact materials.4,6 In 2013 the Council of Europe recommended release limits for 21 metals and metalloids (Resolution CM/Res(2013)9).6 For lead, it sets a specific release limit of 10 µg per kg of food, a recommendation rather than law.4\n\n**Metals in café espresso.** The 2025 survey measured 60 espresso samples in Poland and 44 in Portugal by inductively coupled plasma mass spectrometry (ICP-MS), with tap water from the same places.8 Median nickel was 81.0 µg/L in Polish and 86.8 µg/L in Portuguese espresso, against 5.58 and 4.61 µg/L in the water.8 Median lead was 14.3 and 4.43 µg/L in espresso, against 2.15 and 0.574 µg/L in the water.8 The authors point to other studies in which nickel and lead leached from machines, especially portafilter machines after descaling.8 For nickel, they state that their method could not identify whether it came from the coffee powder or the machine; for lead, they discuss coffee composition, extraction conditions and machine build and maintenance as possible reasons for the differences.8 They add that copper does not seem to leach from coffee machines in significant amounts.8\n\n
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Nickel and lead in espresso served in Poland (60 samples) and Portugal (44 samples), and in tap water from the same places; bars show the middle half of the samples, medians in the labels. The study did not identify whether the metals came from the coffee or the machine.8
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\n\n## Descaling agents\n\nThe sources consulted describe descaling in general terms only. A textbook chapter states that machine companies recommend an acid dissolved in water, preferably one with little taste, and sell their own solutions.2 The BfR considers it possible that acidic descalers release lead from metal parts.4 In the 2015 study, release rose sharply after descaling.5 The BfR and the study authors both recommend thorough rinsing afterwards.4,5\n\nNo study consulted for this entry compared citric, lactic and sulfamic acid descalers for scale removal, attack on metal parts or residues in the cup. The descaling routine of each machine is set by its maker; those instructions were not consulted for this entry.\n\n## Milk systems and hygiene\n\nThe two Swiss surveys judged milk against guideline values of the Swiss hospitality guideline, not against legal maximum values.9,10 In Basel-Stadt, 4 of 19 samples (21%) exceeded guideline values in one or two parameters.9 The guideline value for Enterobacteriaceae is 100 colony-forming units (CFU) per gram, and Bacillus cereus counts stayed below 5,000 CFU/g in both samples that exceeded its guideline value.9 The laboratory named poor cleaning of milk-carrying parts, poor cooling and long or unhygienic storage of milk as possible causes.9 In Zürich, 4 of 15 samples exceeded the guideline value for Enterobacteriaceae, and in one further case the aerobic mesophilic count was above its guideline value.10 Enterobacteriaceae in heated drinks pointed to contamination after heating, for example inside the machine.10 Follow-up checks found no further exceedances, on a spot-check basis.10\n\nBoth surveys concern machines with built-in milk functions, not steam wands operated by a barista. No study consulted for this entry measured bacteria on steam wands. A patent for a wand-cleaning device states as background that milk residues stick to wands and serve as a platform for bacterial growth.13\n\nIn the capsule-machine study, enterobacteria of the genus *Pantoea* peaked 4–11 days after a new machine was first used, before a more balanced community formed after one month.11\n\n## Reference values\n\n| Parameter | Value reported | Context | Source |\n|---|---|---|---|\n| Scale calculation temperatures | 95 °C; 130 °C at 1.7 bar | Brew water; steam boiler | SCA 20181 |\n| Maximum scale, core zone | 12 mg/L (brew boiler); 30 mg/L (steam boiler, 130 °C) | SCA recommendation | SCA 20181 |\n| Corrosion risk | Alkalinity below 40 ppm as CaCO₃; pH below 6 or above 8 | SCA thresholds | SCA 20181 |\n| Magnesium hydroxide scale | pH above 10 | Steam boilers, softened water | SCA 20181, Wellinger et al.2 |\n| Lead release limit | 10 µg/kg food | Council of Europe recommendation; no legal EU limit | BfR4,6 |\n| Machines with increased lead release | 3 of 17 (2007); none detected, limit 0.005 mg/L (2011) | State laboratory Stuttgart, as reported | BfR4 |\n| Nickel in espresso, median (25th–75th percentile) | 81.0 (45.3–133) µg/L; 86.8 (58.0–121) µg/L | Poland; Portugal, cafés | Konieczyński et al.8 |\n| Lead in espresso, median (25th–75th percentile) | 14.3 (2.49–34.0) µg/L; 4.43 (1.99–15.6) µg/L | Poland; Portugal, cafés | Konieczyński et al.8 |\n| Nickel in tap water, median (25th–75th percentile) | 5.58 (4.08–9.62) µg/L; 4.61 (1.09–6.55) µg/L | Same places | Konieczyński et al.8 |\n| Lead in tap water, median (25th–75th percentile) | 2.15 (1.58–3.89) µg/L; 0.574 (0.254–1.37) µg/L | Same places | Konieczyński et al.8 |\n| Milk samples above guideline values | 4 of 19 (21%) | Basel-Stadt, 2023 | Kantonales Laboratorium Basel-Stadt9 |\n| Milk samples above the Enterobacteriaceae guideline | 4 of 15 (one further sample above the aerobic mesophilic guideline) | Zürich, 2024 | Kantonales Labor Zürich10 |\n| Enterobacteriaceae guideline value | 100 CFU/g | Swiss hospitality guideline, as applied | Kantonales Laboratorium Basel-Stadt9 |\n\n## Open questions\n\n- **Descaling agents compared.** No study consulted for this entry compared acids for scale removal, metal release or residues, or tested how well descalers remove silica scale.\n- **Where the metal comes from.** The BfR did not establish the source of the lead and assumes that internal components or soldered joints may contribute4; the café survey could not identify whether the nickel came from the coffee or the machine.8\n- **Commercial machines.** The BfR tests used new household machines.4 No study consulted for this entry measured release from café machines after descaling.\n- **Group cleaning.** No study consulted for this entry measured coffee residues in groups, the effect of backflushing on flavour or detergent residues in espresso.\n- **Steam wands.** The microbiology consulted covers automatic milk systems and capsule-machine waste; steam wands were not tested.9,10,11\n\n## Key studies\n\n
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Müller et al., 2015 – Metals from coffee machines and kettles. Filled 8 coffee machines and 11 kettles with food simulants and measured the metals released, before and after descaling. Portafilter machines released the most, and release rose sharply after descaling; one portafilter machine released much less, which the authors read as showing that high release can be avoided.

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Food Additives & Contaminants Part A 32(11), 1959–1964 · doi:10.1080/19440049.2015.1086929

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Konieczyński et al., 2025 – Elements in café espresso. Measured macro and trace elements by ICP-MS in 104 espressos from Poland and Portugal and in the tap water used. Nickel and lead were far higher in the espresso than in the water; for nickel, the authors state that their method could not tell whether it came from the coffee or the machine.

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Foods 14(3), 426 · doi:10.3390/foods14030426

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Berg et al., 2000 – Nickel from kettles and coffee machines. Tested electric kettles sold in Denmark under simulated household use, and coffee machines in the same way; the abstract does not name the machine types. Ten of 26 kettles released more than 50 µg/L of nickel, the action threshold, but the coffee machines released no aluminium, lead, chromium or nickel in significant amounts.

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Food Additives and Contaminants 17(3), 189–196 · DTU Orbit record

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Vilanova et al., 2015 – Bacteria in capsule-machine waste. Sampled the waste reservoirs of capsule machines used for at least a year and followed a new machine over time. All machines held a varied bacterial community; enterobacteria came first and a more stable community formed within a month.

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Scientific Reports 5, 17163 · doi:10.1038/srep17163

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