{ "id": "cooling-and-quenching", "url": "https://coffearium.org/entries/cooling-and-quenching/", "license": "CC-BY-4.0", "title": "Cooling and quenching", "summary": "The last step of the roast – how air coolers and water quenching stop the roast at the intended degree, why quench water stays in the bean, and what it changes for grinding, freshness and the legal moisture limit.", "domain": "roasting", "kind": "Process", "stage": "roast", "topics": [ "equipment" ], "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": [ "Cooling is the last stage of roasting; the beans are cooled quickly with air, sometimes after a measured spray of water called water quenching.11,6", "Late in the roast the beans release heat of their own; according to a roasting textbook, a roast not stopped and cooled can overshoot the intended degree or catch fire.1,11", "Quench water that does not evaporate stays in the bean; in one industrial study, quenched coffees held 2.3–8.8 g water per 100 g (wet basis).3", "Wetter quenched coffees degassed faster, ground coarser and lost some aroma compounds faster in storage than drier ones.2,3", "Switzerland and Austria cap the water content of roasted coffee at 5% by mass; cupping samples under SCA 102-2024 are air-cooled without quenching.9,10,8" ], "quickFacts": [ { "label": "When", "value": "Right after the drop, the last step of the roast" }, { "label": "Methods", "value": "Cooling air through a stirred bed; optional water quench first" }, { "label": "Water left after air cooling", "value": "About 1–2%, basis not stated (SCA handbook, textbook)" }, { "label": "Water after quenching", "value": "2.3–8.8 g per 100 g wet basis in one industrial study" }, { "label": "Legal maximum", "value": "5% by mass in Switzerland and Austria" }, { "label": "Cupping samples", "value": "Air-cooled, no water quench (SCA 102-2024)" } ], "aka": [ "Quench cooling", "Water quench", "Precooling" ], "linkTerms": [ "cooling and quenching", "water quenching", "water quench", "quench water", "quench cooling", "quenched coffee", "cooling tray", "round-bed cooler", "cooling sieve" ], "related": [ "second-crack", "development-time", "roasting", "roast-profile", "drum-roaster", "fluid-bed-roaster", "tangential-and-centrifugal-roasters", "roast-loss", "degassing", "grinding", "freshness-and-storage", "packaging", "cupping", "preheating-and-charge" ], "contested": [ "Whether water quenching itself harms the coffee: a 2008 doctoral thesis notes that water quenching is generally associated with a loss of quality, citing an earlier textbook that linked it to cracked cell walls and faster degassing, but that explicit data were lacking15; in its own trials, quenching without a rise in water content degassed like air cooling, and air-cooled coffee re-moistened to 5 g per 100 g degassed like water-quenched coffee, so the effects followed water content, not the method.2,15 A roasting textbook chapter likewise states that moderate quench water does not affect flavour or physical bean properties.1" ], "corrections": [ { "date": "2026-10-19T00:00:00.000Z", "change": "Cupping-sample storage and rest under SCA 102-2024 now given as recommendations, and the rest counted from roasting; storage \"at about 20 °C\" corrected to storage once the sample reaches room temperature.", "reason": "Verification pass; SCA 102-2024 says the sample shall be air-cooled, but should be stored airtight once at room temperature (approximately 20 °C) and should rest 8 to 24 hours after roasting." }, { "date": "2026-10-19T00:00:00.000Z", "change": "Chaff removed from what the cooling air carries to the cyclone (text and sequence diagram); destoner now removes \"stones, metal fragments and other waste\" instead of \"other heavy objects\".", "reason": "Verification pass; EPA AP-42 9.13.2 names particulate matter from coolers and describes destoners as air classifiers removing stones, metal fragments and other waste; it does not mention chaff." }, { "date": "2026-10-19T00:00:00.000Z", "change": "Causal claim that more water \"was still leaving\" after the fast roast replaced by the study's figures (10.2 g against 11.4 g evaporated in the roaster); \"stable only in protective packaging\" corrected to \"stable if stored in protective packaging\"; \"beans absorb\" softened to \"may absorb\".", "reason": "Verification pass; wording went beyond Geiger et al. 2005 (Table 2), the SCA freshness handbook and the roasting textbook chapter." } ], "sources": [ { "id": "s1", "title": "The Roast – Creating the Beans' Signature", "authors": "Schenker, S.; Rothgeb, T.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 11, pp. 245–271", "url": "https://doi.org/10.1016/B978-0-12-803520-7.00011-6", "doi": "10.1016/B978-0-12-803520-7.00011-6", "tier": "B", "quote": "Roasting needs to be stopped abruptly at the desired degree of roast with an efficient precooling or cooling step.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s2", "title": "Influence of Water Quench Cooling on Degassing and Aroma Stability of Roasted Coffee", "authors": "Baggenstoss, J.; Poisson, L.; Luethi, R.; Perren, R.; Escher, F.", "year": 2007, "publisher": "Journal of Agricultural and Food Chemistry 55(16), 6685–6691", "url": "https://agris.fao.org/search/fr/records/65e011e00f3e94b9e5deef67", "doi": "10.1021/jf070338d", "tier": "A", "tierNote": "Quote from the abstract; methods and results read in full in the first author's 2008 ETH thesis (s15), chapter 6, where the study is reprinted in adapted form.", "quote": "Degassing is faster in water-quenched coffees with higher moisture content, but pore size distribution in the different coffee samples did not correlate with degassing behavior.", "abstractOnly": true, "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s3", "title": "Water content of roasted coffee: impact on grinding behaviour, extraction, and aroma retention", "authors": "Baggenstoss, J.; Perren, R.; Escher, F.", "year": 2008, "publisher": "European Food Research and Technology 227(5), 1357–1365", "url": "https://agris.fao.org/search/ru/records/65df955b7c7033e84bee3b18", "doi": "10.1007/s00217-008-0852-8", "tier": "A", "tierNote": "Quote from the abstract; methods and results read in full in the first author's 2008 ETH thesis (s15), chapter 8, where the study is reprinted in adapted form.", "quote": "Different amounts of water were applied during quenching, resulting in water contents in the range of 2.3-8.8 g/100 g wb.", "abstractOnly": true, "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s4", "title": "The Coffee Freshness Handbook (ISBN 978-0-9995807-2-1)", "authors": "Smrke, S.; Sage, E.; Wellinger, M.; Yeretzian, C.", "year": 2018, "publisher": "Specialty Coffee Association, Santa Ana", "url": "https://sca.coffee/store/p/the-coffee-freshness-handbook-ld8kh", "tier": "B", "quote": "The residual water content in freshly roasted coffee is approximately 1-2% when no quenching is used and is stable if the coffee is stored in a protective packaging.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s5", "title": "Carbon Dioxide Evolution and Moisture Evaporation During Roasting of Coffee Beans", "authors": "Geiger, R.; Perren, R.; Kuenzli, R.; Escher, F.", "year": 2005, "publisher": "Journal of Food Science 70(2), E124–E130", "url": "https://ift.onlinelibrary.wiley.com/doi/10.1111/j.1365-2621.2005.tb07084.x", "doi": "10.1111/j.1365-2621.2005.tb07084.x", "tier": "A", "quote": "An estimation of the loss of gases during the cooling step was done by linear extension of the cumulative carbon dioxide and moisture evaporation values for another 20 s after the end of roasting.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s6", "title": "Investigations on the Hot Air Roasting of Coffee Beans (Diss. ETH No. 13620)", "authors": "Schenker, S.", "year": 2000, "publisher": "ETH Zürich (doctoral thesis)", "url": "https://library.sweetmarias.com/wp-content/uploads/2020/08/roasting_hot_air_doctorate.pdf", "tier": "B", "quote": "The roasting process must be stopped by rapid cooling of the beans", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s7", "title": "AP-42, Fifth Edition, Section 9.13.2: Coffee Roasting", "authors": "U.S. Environmental Protection Agency", "year": 1995, "publisher": "U.S. EPA, Compilation of Air Pollutant Emission Factors", "url": "https://www.epa.gov/sites/default/files/2020-10/documents/c9s13-2.pdf", "tier": "A", "quote": "The coffee roasting process consists essentially of cleaning, roasting, cooling, grinding, and packaging operations.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s8", "title": "SCA Standard 102-2024 Coffee Value Assessment: Sample Preparation and Tasting Mechanics", "authors": "Specialty Coffee Association", "year": 2024, "publisher": "Specialty Coffee Association", "url": "https://sca.coffee/s/AW_SCA-102_Sample-Preparation_281024_Secured.pdf", "tier": "A/B", "tierNote": "A for what the standard specifies; B for its explanations", "quote": "The coffee sample shall be immediately air-cooled (without water quenching).", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s9", "title": "Verordnung des EDI über Getränke vom 16. Dezember 2016 (SR 817.022.12), Art. 43", "authors": "Eidgenössisches Departement des Innern (EDI)", "year": 2016, "publisher": "Swiss Confederation (consolidated version of 1 July 2020, via LexFind)", "url": "https://www.lexfind.ch/tolv/211066/de", "tier": "A", "quote": "Röstkaffee darf nicht mehr als 1 Massenprozent verkohlte Bohnen und nicht mehr als 5 Massenprozent Wasser enthalten.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s10", "title": "Österreichisches Lebensmittelbuch, IV. Auflage, Codexkapitel B 12 Kaffee, Kaffeemittel", "authors": "Bundesministerium für Soziales, Gesundheit, Pflege und Konsumentenschutz (Codexkommission)", "year": 2015, "publisher": "Republic of Austria (version with amendments of 2017 and 2020, via WKO)", "url": "https://news.wko.at/oe/gewerbe-handwerk/lebensmittelgewerbe/codex-b12-kaffee-aenderungen.pdf", "tier": "A", "quote": "Sein Wassergehalt darf 5 Gew.-%, sein Aschegehalt 6,5 Gew.-% nicht übersteigen.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s11", "title": "Modeling and simulation of coffee bean heating during roasting: effect of heat generation", "authors": "Bustos-Vanegas, J. D.; Martins, M. A.; Corrêa, P. C.; Baptestini, F. M.; de Oliveira, G. H. H.", "year": 2025, "publisher": "Frontiers in Food Science and Technology 5, 1603783", "url": "https://www.frontiersin.org/journals/food-science-and-technology/articles/10.3389/frfst.2025.1603783/full", "doi": "10.3389/frfst.2025.1603783", "tier": "A", "quote": "The roasting of coffee can be divided into three consecutive stages: drying, roasting or pyrolysis and cooling.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s12", "title": "The Grind – Particles and Particularities", "authors": "von Blittersdorff, M.; Klatt, C.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 13, pp. 311–328", "url": "https://eld.bl.uk/catalog/018161895", "tier": "B", "quote": "But the quenching water does not instantaneously distribute homogeneously in the coffee bean.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s13", "title": "Roasting Conditions and Coffee Flavor: A Multi-Study Empirical Investigation", "authors": "Münchow, M.; Alstrup, J.; Steen, I.; Giacalone, D.", "year": 2020, "publisher": "Beverages 6(2), 29", "url": "https://www.mdpi.com/2306-5710/6/2/29", "doi": "10.3390/beverages6020029", "tier": "A", "quote": "During the roasting process, the beans lose between 15 and 22% of their weight, of which most is water as it ends up as a rather dry product with between 1 and 2.5% moisture", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s14", "title": "Time-Resolved Gravimetric Method To Assess Degassing of Roasted Coffee", "authors": "Smrke, S.; Wellinger, M.; Suzuki, T.; Balsiger, F.; Opitz, S. E. W.; Yeretzian, C.", "year": 2018, "publisher": "Journal of Agricultural and Food Chemistry 66(21), 5293–5300", "url": "https://digitalcollection.zhaw.ch/bitstream/11475/12175/2/2017_Smrke-etal_Roasted-coffe-degassing-time-resolved-gravimetry.pdf", "doi": "10.1021/acs.jafc.7b03310", "tier": "A", "quote": "In addition, it was found that the cooling mode after roasting (air cooling vs water spraying/quenching) has an impact on the degassing of whole coffee beans.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s15", "title": "Coffee roasting and quenching technology – formation and stability of aroma compounds (Diss. ETH No. 17696)", "authors": "Baggenstoss, J.", "year": 2008, "publisher": "ETH Zürich (doctoral thesis)", "url": "https://doi.org/10.3929/ethz-a-005666902", "doi": "10.3929/ethz-a-005666902", "tier": "B", "tierNote": "Doctoral thesis (Tier B). Its chapters 6 and 8 reprint the peer-reviewed studies s2 and s3 in adapted form; values from those chapters are cited to both.", "quote": "Spray quenching is judged as the most efficient method due to high evaporation rates and intensive recurring contact of water with the surface of coffee beans.", "accessed": "2026-10-19T00:00:00.000Z" } ], "markdown": "## What cooling does\n\nCooling is the last stage of a roast, after drying and the roast reactions.11 The moment the beans leave the roaster is called the drop.1 From then on, the beans must lose their heat quickly.1,6\n\nSpeed matters because the roast does not stop by itself. Late in the roast, the reactions release heat, and the beans start to heat themselves.1,11 According to a roasting textbook chapter, a few seconds then separate the intended roast degree from an over-roasted coffee.1 If the roast runs on uncontrolled, the beans may catch fire.1\n\nTwo methods are used, alone or together: cold air, and a measured spray of water called water quenching.6 The United States Environmental Protection Agency (EPA) lists cooling as one of the main operations of a roastery, between roasting and grinding.7\n\nThis is the seventh and last step in the roast. **Previous step:** second crack.\n\n## Air cooling\n\nIn small drum roasters, the beans drop into a cooling tray.1 According to the roasting textbook chapter, industrial drum roasters most often use a round-bed cooler, a round sieve with a slowly turning stirrer.1 Paddle roasters may instead use a rectangular cooling sieve without any stirrer.1\n\nCold air is drawn through the layer of beans. In drum roasters it may flow from below or from above; in paddle roasters it usually flows upwards.1 Fluid-bed roasters move the beans into a separate cooling section.1\n\nCoolers also release dust. The EPA lists them among the sources of particulate matter, which is typically led to cyclones, separators that spin the dust out of the air.7 After cooling, the beans run through a destoner, an air classifier that removes stones, metal fragments and other waste.7\n\nAir cooling leaves the coffee dry. According to a Specialty Coffee Association (SCA) handbook and a textbook chapter, unquenched roasted coffee holds about 1–2% water (basis not stated).4,12 A 2020 multi-study paper gives 1–2.5% moisture for roasted beans in general.13\n\n
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From the drop to the bag: optional water quench, air cooling in a stirred bed, destoning, resting and packaging, with by-products.1,7,8,12 Schematic, not to scale.
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\n\n## Water quenching\n\nMost medium- to large-scale roasters have a water quenching device.1 When the beans reach their final temperature, it sprays a set amount of cold water onto them.1,7 The water evaporates on the hot bean surface, and the evaporation takes the heat away.1\n\nQuenching is a precooling step: air cooling still follows.1 According to the textbook chapter, it helps to reach the same roast degree batch after batch.1 In one industrial roaster described in a doctoral thesis, water was sprayed first in the roasting chamber and again in the cooling chamber.6\n\nA 2008 doctoral thesis, following earlier work, describes three ways of applying the water.15 In spray quenching, fine droplets evaporate on the hot surface. In film quenching, water is poured over the beans, and in immersion quenching the beans are dipped in water.15 The thesis judges spray quenching the most efficient and widely used method.15\n\nHigh-speed video in the same thesis shows how it works. The first droplets evaporate on impact; as the surface cools, later droplets condense on the bean instead.15\n\nIdeally, the water evaporates fully rather than soaking into the bean.6 Beans may absorb part of it, and the amount of quench water can be used to set the final moisture.1 In one study on a production roaster, different amounts of quench water gave coffees with 2.3–8.8 g water per 100 g (wet basis).3 On a 320 kg bowl roaster, 5 L of water per batch left 2.4 g and 38 L left 5.3 g water per 100 g (wet basis).2,15\n\n
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Quench water per 320 kg batch and the water content of the roasted coffee, rotating bowl roaster, 2007 study; the dashed level marks the Swiss and Austrian 5% maximum.2,9,10,15
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\n\n## What extra water changes\n\nAccording to a textbook chapter on grinding, quench water does not spread through the bean at once. It first wets the surface and then moves inwards by diffusion.12 That is why the chapter recommends a rest between roasting and grinding.12\n\n**Grinding.** In a 2008 study, coffees with more water gave coarser particles on grinding.3 They ground better after a period of equilibration, while dry coffees did not need one.3 Above 6 g water per 100 g (wet basis), grinding stayed unacceptable even after 24 hours.3,15\n\n**Brewing.** In the same study, coffees with less water were extracted more effectively on an espresso machine than wetter ones.3\n\n**Gas and freshness.** In a 2007 study, quenched coffees with more water lost their carbon dioxide faster.2 A fine water spray that left the coffee almost as dry as air cooling made no such difference.2,15 Most aroma compounds were about as stable as in air-cooled coffee.2 One sulphur compound behaved differently, which the authors link to a faster loss of freshness.2 According to the textbook chapter, a high water activity in roasted beans speeds up staling and can shorten shelf life.1\n\n
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Water in roasted coffee: about 1–2% unquenched (basis not stated), 2.3–8.8 g per 100 g (wet basis) quenched in one industrial study, and the Swiss and Austrian 5% maximum.3,4,9,10,12
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\n\n## Rules and standards\n\nLegal limits apply to the total water in roasted coffee, including any left by quenching. Swiss law allows roasted coffee to contain no more than 5% water by mass.9 The Austrian Food Codex sets the same maximum of 5% by weight.10\n\nFor quality grading, quenching is excluded. Under SCA 102-2024, the SCA's standard for cupping samples, the roasted sample is air-cooled at once, without water quenching.8 Once it reaches room temperature (about 20 °C), the standard recommends airtight storage and a rest of 8–24 hours after roasting.8\n\n\n\n## Reference values\n\nValues reported in the sources. They describe particular coffees, roasters and methods; they are not targets.\n\n| Parameter | Value reported | Source |\n|---|---|---|\n| Water content without quenching | About 1–2% (basis not stated) | SCA freshness handbook4; grinding chapter12 |\n| Water content of roasted beans, general | 1–2.5% (basis not stated) | Münchow et al. 202013 |\n| Water content after quenching, industrial trials | 2.3–8.8 g per 100 g (wet basis), set by the amount of quench water | Baggenstoss et al. 20083 |\n| Equilibration before grinding, tested | Ground at once, after 6 h and after 24 h; above 6 g per 100 g (wet basis) grinding unacceptable even after 24 h | Baggenstoss et al. 20083,15 |\n| Cooling methods, 200 g laboratory roasts (228 °C air, 12 min) | Air, 1.4 m³/min for 4 min: 1.9 g; spray, 8.7 g water in 20 s: 2.1 g; film, 35 g water in the first 12 s: 4.2 g; passive, 45 min in a steel container: 1.7 g water per 100 g (wet basis) | Baggenstoss et al. 20072,15 |\n| Quench water and water content, 320 kg bowl roaster | 5, 15, 25, 35, 38 L per batch: 2.4, 3.2, 3.8, 4.8, 5.3 g per 100 g (wet basis) | Baggenstoss et al. 20072,15 |\n| Water lost during cooling, estimated, per 100 g green coffee | 1.6 g after a fast roast (air 260 °C, 170 s); 0.1 g after a slow roast (air 228 °C, 720 s); fluid-bed laboratory roaster, 100 g batches | Geiger et al. 20055 |\n| Carbon dioxide lost during cooling, estimated, per 100 g green coffee | 0.1 g (fast roast); 0.0 g (slow roast) | Geiger et al. 20055 |\n| Particulate matter from a continuous cooler with cyclone | 0.028 lb per ton of green coffee fed (filterable; about 0.014 kg per tonne; emission factor rating D) | EPA AP-427 |\n| Legal maximum, water in roasted coffee | 5% by mass (Switzerland); 5% by weight (Austria) | EDI beverages ordinance, Art. 439; Austrian Food Codex B 1210 |\n| Cupping samples | Air-cooled at once, no water quench (required); stored airtight once at room temperature, about 20 °C, and rested 8–24 h after roasting (recommended) | SCA 102-20248 |\n\n## Physics of cooling\n\n**Why the roast runs on.** According to the roasting textbook chapter, the roast becomes self-accelerating late on, as the beans generate heat of their own; its final stages approach the conditions of a combustion process.1 The chapter therefore calls for an abrupt stop with an efficient precooling or cooling step.1 A doctoral thesis on hot-air roasting describes cooling as excess cold air, a precise amount of water sprayed onto the hot beans, or both.6\n\n**Two ways to take heat out.** Cold air carries heat away from the beans.6 According to the 2008 thesis, air cooling takes several minutes and is relatively slow, while water quenching is faster because evaporating water absorbs heat.15 In the laboratory trials, passive cooling in a container took 45 minutes and the coffee came out slightly darker than air-cooled coffee.2,15 Quench water removes heat as it evaporates on the bean surface.1 The thesis states the aim as full evaporation on the surface rather than a large rise in bean water content.6 According to the roasting textbook chapter, excessive quench water is absorbed and raises the final moisture.1\n\n**The bean keeps losing mass.** Cooling is not only a loss of heat. A fluid-bed study estimated what left the beans in the first 20 s after the roast, by extending the measured release curves.5 After a fast roast, 1.6 g water per 100 g green coffee left during cooling; after a slow roast to the same colour, only 0.1 g.5 In the fast roast, less water had evaporated in the roaster (10.2 g against 11.4 g per 100 g green coffee), and more left during cooling.5 The roast loss entry gives the full mass balance.\n\n
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Water estimated to leave the beans in the first 20 s of cooling, after a fast (260 °C air, 170 s) and a slow (228 °C air, 720 s) roast to the same colour; 100 g batches, fluid-bed laboratory roaster.5
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\n\n## Water, structure and storage\n\n**Redistribution.** Quench water first wets the surface and needs time to spread through the bean by diffusion.12 Coffee oil pushed to the surface in dark and fast roasts also needs time to move back inside, according to the grinding chapter.12 In the 2008 trials, coffees were ground right after cooling and again after 6 and 24 hours of equilibration.3 High-moisture coffees needed the equilibration; low-moisture coffees did not.3\n\n**Grinding and extraction.** Coffees with a high water content gave coarser particles.3 Coffees with a low water content were extracted more effectively, and percolation through them was slower.3 The grinding entry covers how moisture and bean temperature change the particle size distribution.\n\n**Degassing.** The 2007 study compared air cooling and water quench cooling on a 200 g laboratory fluid-bed roaster and on a 320 kg production roaster with a rotating bowl.2 For safety reasons, all production batches had to be water-cooled, so only the amount of water was varied.2,15 Coffees with more water degassed faster; their pore size distribution did not explain it.2 The difference showed mainly early on: after one week of storage, degassing rates no longer differed markedly.2,15 Air-cooled coffee re-moistened to 5 g per 100 g degassed like film-quenched coffee, so the water content, not the cooling method, made the difference.2,15 The authors suggest that bean firmness, which rises with moisture, may play a part.2 In the thesis, firmness rose almost linearly with water content.15 A 2018 degassing study cites this result: the cooling mode, air or water, affects how whole beans degas.14\n\n**Aroma.** In storage at 25 °C, most aroma compounds were stable to a similar degree in air-cooled and quenched coffees.2 Dimethyl trisulfide evolved differently in the wetter coffees, which the authors read as faster oxidation of thiols and a faster loss of freshness.2 In the 2008 trials, hexanal and sulphides were highly sensitive to water content in both open and closed storage; other aroma compounds changed only in closed storage, where more water harmed stability.3 The thesis summarises it: with more water, dimethyl sulfide, 3-mercapto-3-methylbutyl formate and N-methylpyrrole were lost faster in open storage and dimethyl trisulfide built up faster; under protective atmosphere, Strecker aldehydes and α-diketones also degraded faster.15\n\n## Measuring moisture after cooling\n\nSwiss and Austrian rules set the 5% maximum on the roasted coffee itself.9,10 According to the SCA handbook, the value of about 1–2% for unquenched coffee stays stable if the coffee is stored in protective packaging.4\n\nTiming matters for any reading. Quench water sits at the surface first, so a sample taken straight after cooling is not yet uniform.12 No study opened for this entry compared moisture meters on freshly quenched coffee.\n\n## Risks and defects\n\n- **Overshoot.** A roast that is not stopped quickly runs past its intended degree; uncontrolled, the beans can catch fire.1\n- **Over-quenching.** Excessive quench water is absorbed, raises the final moisture and speeds up staling.1 It can also push coffee towards the 5% legal maximum in Switzerland and Austria.9,10\n- **Grinding too early.** Freshly quenched, wetter coffee grinds coarser until the water has equilibrated.3,12\n- **Dust and emissions.** Coolers emit particulate matter, which is usually collected in cyclones.7\n\n## Open questions\n\n- **How much water is moderate.** The industrial trials show how the amount of quench water sets the water content2,15, but no study opened for this entry gives a water content at which flavour starts to suffer in a sensory test.\n- **Cooling speed and flavour.** No peer-reviewed study comparing air-cooling speeds and their effect on flavour was found.\n- **Moisture meters.** How well rapid meters read freshly quenched coffee was not found in the sources opened.\n\n## Key studies\n\n
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Baggenstoss et al., 2007 – Water quench cooling, degassing and aroma. Compared air cooling with water quench cooling on a 200 g laboratory fluid-bed roaster and a 320 kg production bowl roaster, then followed carbon dioxide release and aroma compounds in storage at 25 °C. Wetter coffees degassed faster, and one sulphur compound pointed to faster staling.

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Journal of Agricultural and Food Chemistry 55(16), 6685–6691 · doi:10.1021/jf070338d

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Baggenstoss et al., 2008 – Water content after quenching. Quenched industrial roasts to 2.3–8.8 g water per 100 g and ground them at once, after 6 h and after 24 h. Wetter coffees ground coarser and extracted less effectively, and their aroma was less stable in storage.

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European Food Research and Technology 227(5), 1357–1365 · doi:10.1007/s00217-008-0852-8

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Baggenstoss, 2008 – Roasting and quenching technology (doctoral thesis). Brings together the quenching, moisture and grinding studies with laboratory and industrial roasts, and adds high-speed video of water droplets hitting hot beans. Concludes that the effects of quenching follow the water left in the bean, and recommends storing coffee at low moisture for a longer shelf life.

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ETH Zürich, Diss. ETH No. 17696 · doi:10.3929/ethz-a-005666902

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Geiger et al., 2005 – Gas and water during roasting and cooling. Measured carbon dioxide and water vapour online in a fluid-bed laboratory roaster for a fast and a slow roast to equal colour, and estimated what left during cooling. After the fast roast, much more water was estimated to leave the beans during cooling than after the slow roast.

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Journal of Food Science 70(2), E124–E130 · doi:10.1111/j.1365-2621.2005.tb07084.x

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