{ "id": "moisture-and-water-activity", "url": "https://coffearium.org/entries/moisture-and-water-activity/", "license": "CC-BY-4.0", "title": "Moisture and water activity", "summary": "How much water a coffee seed holds (moisture content) and how freely that water can act (water activity) – the two numbers that decide when coffee is dry, safe from mould and stable in storage.", "domain": "post-harvest-processing", "kind": "Concept", "stage": "process", "topics": [ "science" ], "topicSections": {}, "lang": "en", "status": "verified", "reviewer": "Coffearium verification pass", "lastVerified": "2026-10-08T00:00:00.000Z", "updated": "2026-10-10T00:00:00.000Z", "brief": [ "Moisture content is how much water the coffee contains by weight; the Codex code and the ISO oven method express it on a wet basis (wb), as a share of the wet sample.1,2", "Water activity (aw, 0 to 1) measures how available that water is to microorganisms and chemical reactions; it is not the same as moisture content.5,6", "The Codex code of practice considers parchment coffee protected from fungi at below 12.5% moisture (wb) and a maximum water activity of 0.67–0.70.1", "Moisture meters should be calibrated against the ISO 6673 oven method, which dries about 10 g of green coffee at 105 °C for 16 hours.1,2" ], "quickFacts": [ { "label": "Safe for storage (Codex)", "value": "< 12.5% moisture (wb), aw 0.67–0.70 maximum" }, { "label": "Freshly harvested coffee", "value": "About 50–60% (wb); fruits range 30–65% by ripeness" }, { "label": "Parchment after demucilaging", "value": "About 45–55% moisture" }, { "label": "Traded green coffee", "value": "About 12% moisture (wb)" }, { "label": "OTA-producing Aspergillus cannot grow", "value": "Below aw 0.76–0.78 (no toxin below 0.80)" }, { "label": "EU OTA limit, roasted / soluble", "value": "3.0 / 5.0 µg/kg" }, { "label": "Reference method", "value": "ISO 6673 – 105 °C, 16 h (2003 edition)" } ], "aka": [ "Moisture content", "Water activity" ], "linkTerms": [ "wet basis", "dry basis", "sorption isotherm", "equilibrium moisture content" ], "related": [ "drying", "washed-process", "natural-process", "coffee-cherry-anatomy", "microorganisms", "green-coffee-storage", "ochratoxin-a" ], "contested": [ "Target water activity for green coffee in storage: a 2023 study states a recommended range of 0.4–0.6, citing earlier work (Harris and Miller, 2008), and treats 0.6 as the critical level in its own trial15, while the Codex code of practice treats 0.67–0.70 as the maximum that still protects parchment coffee from fungi1 and a 2020 study equates safe storage at 10–12% moisture with about 0.65–0.70.14 The lower range is given for preserving quality, the higher ones for protection from fungi; the sources do not reconcile the two. In the 2023 trial, green coffee at 10.76% (wb) had a water activity of 0.57.15", "Acceptable moisture range for graded green coffee: the older SCAA grading reproduced by FAO allows 9–13% for its Speciality, Premium and Exchange grades8, whereas the Codex code sets 12.5% as the maximum anywhere in the chain (wet basis where the code states one)1. The narrower 10–12% (wb) appears in peer-reviewed work as a safe or recommended storage range9,14,15, not as a grading limit. The SCA's archived (heritage) green coffee standards set no moisture range; for specialty grade they use a water-activity limit instead: the beans must measure lower than 0.70 aw.20 A current SCA document stating a 10–12% grading range could not be found in openly published form, so the 9–13% figure remains the only sourced grading range here.", "Moisture of fresh fruit: a 2026 review gives 50–60% (wb) for freshly harvested coffee9, while a drying study states that fruits are harvested at 30–65 kg of water per 100 kg of material, depending on maturation stage.17 The two agree once ripeness is taken into account: the wider range covers fruit from ripe to partly dried on the tree.", "Oven methods do not give identical numbers: ISO 6673 (105 °C, 16 h) gives results about 1.0 percentage point lower than the ISO 1446 reference method for water content2, and a university extension guide describes 105 °C for 24 h instead.7" ], "corrections": [], "sources": [ { "id": "s1", "title": "Code of Practice for the Prevention and Reduction of Ochratoxin A Contamination in Coffee (CAC/RCP 69-2009)", "authors": "Codex Alimentarius Commission (FAO/WHO)", "year": 2009, "publisher": "Codex Alimentarius", "url": "https://www.fao.org/input/download/standards/11250/CXP_069e.pdf", "tier": "A", "quote": "a maximum aw of 0.67 to 0.70 and moisture content < 12.5% (wet basis) is sufficient for protecting parchment coffee from damage by fungi.", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s2", "title": "ISO 6673:2003 Green coffee — Determination of loss in mass at 105 °C", "authors": "International Organization for Standardization", "year": 2003, "publisher": "ISO", "url": "https://cdn.standards.iteh.ai/samples/38375/65dea88d3d9e4129abb9b7fe1b84a080/ISO-6673-2003.pdf", "tier": "A", "quote": "This method of determining the loss in mass can be considered, by convention, as a method for determining the water content and can be used as such by agreement between the interested parties, but it gives results which are lower, by about 1,0 %, than those obtained with the method described in ISO 1446", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s3", "title": "ISO 6673:2025 Green coffee — Determination of loss in mass at 105 °C", "authors": "International Organization for Standardization", "year": 2025, "publisher": "ISO", "url": "https://www.iso.org/standard/87697.html", "tier": "A", "quote": "This document specifies a method for the determination of the loss in mass at 105 °C of green coffee.", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s4", "title": "Grain storage techniques: Evolution and trends in developing countries (FAO Agricultural Services Bulletin 109), Drying principles and general considerations", "authors": "Proctor, D. L. (ed.)", "year": 1994, "publisher": "FAO", "url": "https://www.fao.org/4/t1838e/t1838e0u.htm", "tier": "A", "quote": "Under no circumstances is it possible to dry to a moisture content lower than the EMC associated with the temperature and humidity of the drying air.", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s5", "title": "Handling and preservation of fruits and vegetables by combined methods for rural areas (FAO Agricultural Services Bulletin 149), Chapter 3", "authors": "Barbosa-Cánovas, G. V.; Fernández-Molina, J. J.; Alzamora, S. M.; Tapia, M. S.; López-Malo, A.; Welti Chanes, J.", "year": 2003, "publisher": "FAO", "url": "https://www.fao.org/4/y4358e/y4358e06.htm", "tier": "A", "quote": "yeasts and moulds are more tolerant and usually no growth occurs at aw <0.62", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s6", "title": "Measuring Moisture Content & Water Activity", "authors": "Mermelstein, N. H.", "year": 2009, "publisher": "Food Technology (Institute of Food Technologists)", "url": "https://www.ift.org/food-technology-magazine/measuring-moisture-content-water-activity", "tier": "B", "quote": "Water activity, aW, is a measure of how much of that water is free, i.e., unbound, and thus available to microorganisms to use for growth.", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s7", "title": "Measuring Coffee Bean Moisture Content (Engineer's Notebook EN-3)", "authors": "Gautz, L. D.; Smith, V. E.; Bittenbender, H. C.", "year": 2008, "publisher": "College of Tropical Agriculture and Human Resources, University of Hawai'i at Mānoa", "url": "https://www.ctahr.hawaii.edu/oc/freepubs/pdf/EN-3.pdf", "tier": "A", "quote": "A bean fresh from demucilaging, whether through fermentation or mechanical means, will be at 45–55 percent moisture.", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s8", "title": "Post Harvest Handling and Processing of Coffee in African Countries, Annex 7: Green Coffee Classification and Grading", "authors": "Mutua, J.", "year": 2000, "publisher": "FAO", "url": "https://www.fao.org/4/x6939e/x6939e13.htm", "tier": "A", "quote": "Moisture content is between 9-13%.", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s9", "title": "Advances in coffee drying: A comprehensive review of traditional, solar, mechanical, hybrid, and emerging methods", "authors": "Duque-Dussán, E.; Figueroa-Varela, P. A.; Cruz-Ospina, V.; Banout, J.", "year": 2026, "publisher": "Foods 15(10), 1737", "url": "https://www.mdpi.com/2304-8158/15/10/1737", "doi": "10.3390/foods15101737", "tier": "A", "quote": "Freshly harvested beans typically contain 50–60% moisture on a wet basis, depending on the processing method and local climatic conditions", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s10", "title": "Water sorption properties of coffee fruits, pulped and green coffee", "authors": "Goneli, A. L. D.; Corrêa, P. C.; Oliveira, G. H. H.; Afonso Júnior, P. C.", "year": 2013, "publisher": "LWT – Food Science and Technology 50(2), 386–391", "url": "https://www.sciencedirect.com/science/article/pii/S0023643812003866", "doi": "10.1016/j.lwt.2012.09.006", "tier": "A", "quote": "Equilibrium moisture content data were correlated by the Guggenheim–Anderson–de Boer model, which presented good fit to the data, according to statistical procedures.", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s11", "title": "Effect of coffee cherry maturity on the performance of the drying process of the bean: Sorption isotherms and dielectric spectroscopy", "authors": "Velásquez, S.; Franco, A. P.; Peña, N.; Bohórquez, J. C.; Gutierrez, N.", "year": 2021, "publisher": "Food Control 123, 107692", "url": "https://www.sciencedirect.com/science/article/abs/pii/S0956713520306083", "doi": "10.1016/j.foodcont.2020.107692", "tier": "A", "quote": "The dielectric constant was utilized to predict both the moisture content and the water activity of the beans.", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s12", "title": "Adsorption isotherms and isosteric heat in Coffea arabica beans", "authors": "Santos, S. G. F. dos; Silva, D. P. da; Sarti, J. K.; Almeida, V. G.; Rodovalho, R. S.", "year": 2020, "publisher": "Pesquisa Agropecuária Tropical 50", "url": "https://www.redalyc.org/journal/2530/253068033006/html/", "doi": "10.1590/1983-40632020v5058128", "tier": "A", "quote": "For a constant water activity, an increase in the temperature promotes a decrease in the equilibrium moisture content of Arabica coffee beans.", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s13", "title": "Glass transition of green and roasted coffee investigated by calorimetric and dielectric techniques", "authors": "Iaccheri, E.; Ragni, L.; Cevoli, C.; Romani, S.; Dalla Rosa, M.; Rocculi, P.", "year": 2019, "publisher": "Food Chemistry 301, 125187", "url": "https://www.sciencedirect.com/science/article/abs/pii/S0308814619312932", "doi": "10.1016/j.foodchem.2019.125187", "tier": "A", "quote": "Green and roasted coffee beans are principally characterized by a glassy structure that slowly evolves during storage.", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s14", "title": "Resilience of Aspergillus westerdijkiae Strains to Interacting Climate-Related Abiotic Factors: Effects on Growth and Ochratoxin A Production on Coffee-Based Medium and in Stored Coffee", "authors": "Akbar, A.; Medina, A.; Magan, N.", "year": 2020, "publisher": "Microorganisms 8(9), 1268", "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC7569885/", "doi": "10.3390/microorganisms8091268", "tier": "A", "quote": "final safe storage m.c. for green coffee beans should be in the range of 10–12% (approx. 0.65–0.70 aw)", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s15", "title": "The use of desiccants for proper moisture preservation in green coffee during storage and transportation", "authors": "Anokye-Bempah, L.; Han, J.; Kornbluth, K.; Ristenpart, W.; Donis-González, I. R.", "year": 2023, "publisher": "Journal of Agriculture and Food Research 11, 100478", "url": "https://www.sciencedirect.com/science/article/pii/S2666154322002113", "doi": "10.1016/j.jafr.2022.100478", "tier": "A", "quote": "The recommended wet-basis moisture content (MCwb) and water activity (Aw) of green coffee during storage range from 10 to 12% and 0.4 to 0.6, respectively (Harris and Miller, 2008).", "accessed": "2026-10-08T00:00:00.000Z" }, { "id": "s16", "title": "Comparison of chemical compounds and their influence on the taste of coffee depending on green beans storage conditions", "authors": "Zarebska, M.; Stanek, N.; Barabosz, K.; Jaszkiewicz, A.; Kulesza, R.; Matejuk, R.; Andrzejewski, D.; Biłos, Ł.; Porada, A.", "year": 2022, "publisher": "Scientific Reports 12, 2674", "url": "https://www.nature.com/articles/s41598-022-06676-9", "doi": "10.1038/s41598-022-06676-9", "tier": "A", "quote": "High temperatures and a high water content have been reported as the main factors that cause sensory changes in coffee during storage", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s17", "title": "Vacuum drying of peeled coffee cherry beans: drying kinetics and physiological effects", "authors": "Rocha, H. A.; Correa, J. L. G.; Borém, F. M.", "year": 2021, "publisher": "Coffee Science 16, e161921", "url": "https://repositorio.ufla.br/server/api/core/bitstreams/caaea665-7a25-4a1c-b8c8-0adce24edbd1/content", "doi": "10.25186/.v16i.1921", "tier": "A", "quote": "In general, coffee fruits are harvested with varying moisture content between 30 kg of water 100 kg of material⁻¹ and 65 kg of water 100 kg of material⁻¹, depending on its maturation stage", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s18", "title": "Producers and Important Dietary Sources of Ochratoxin A and Citrinin", "authors": "Ostry, V.; Malir, F.; Ruprich, J.", "year": 2013, "publisher": "Toxins 5(9), 1574–1586", "url": "https://www.mdpi.com/2072-6651/5/9/1574", "doi": "10.3390/toxins5091574", "tier": "A", "quote": "OTA is produced worldwide in foodstuffs by microfungi of the genera, Aspergillus, mainly in subtropical and tropical areas, and Penicillium, especially in temperate and colder zones", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s19", "title": "Commission Regulation (EU) 2023/915 on maximum levels for certain contaminants in food (consolidated text, 01.01.2025), Annex I, section 1.2", "authors": "European Commission", "year": 2025, "publisher": "EUR-Lex", "url": "https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:02023R0915-20250101", "tier": "A", "quote": "Roasted coffee beans and ground roasted coffee except products listed in 1.2.15", "accessed": "2026-09-30T00:00:00.000Z" }, { "id": "s20", "title": "Heritage Coffee Standards (archived), Green Coffee Standards", "authors": "Specialty Coffee Association", "publisher": "Specialty Coffee Association", "url": "https://www.sca.coffee/heritage-coffee-standards", "tier": "A", "quote": "Specialty grade green coffee beans shall have a water activity measurement lower than 0.70aw.", "accessed": "2026-10-10T00:00:00.000Z" } ], "markdown": "
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Water activity thresholds that matter for coffee. The Codex storage ceiling sits below the level at which ochratoxin-producing fungi can grow; the general mould limit is for foods in general, not coffee. The 0.4–0.6 storage range is cited by one study from earlier work.1,5,15
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\n\n## Two different numbers\n\n**Moisture content** tells you how much water the coffee contains. **Water activity** tells you how much of that water is free – unbound and available to microorganisms for growth.6 Formally, water activity is the ratio of the partial pressure of water in a food to the vapour pressure of pure water at the same temperature.5 It runs from 0 to 1 and is directly related to the equilibrium relative humidity (ERH) of the air around the sample.5\n\nThe two are linked, but not one-to-one. The curve that connects them at a given temperature is called the **sorption isotherm**, and it is specific to each food – its \"fingerprint\".5 It can even differ between lots of the same coffee: in parchment coffee, the ripeness at which the cherries were picked changed how available the water in the bean was.11 Temperature matters too: at a constant water activity, warmer Arabica coffee holds less water.12\n\n## Wet basis and dry basis\n\nMoisture content can be expressed in two ways:4\n\n- **Wet basis (wb)** – mass of water per unit mass of the *wet* sample. This is the basis of the Codex code and of the ISO 6673 oven method, and the one a university extension guide uses for traded green coffee.1,2,7\n- **Dry basis (db)** – mass of water per unit mass of *completely dry* matter. Sorption research often reports this, for example as g of water per 100 g of dry solids.10,12\n\nConversion, with both values in per cent:\n\n- Mdb = 100 × Mwb / (100 − Mwb)\n- Mwb = 100 × Mdb / (100 + Mdb)\n\nSo the Codex limit of 12.5% wb equals 14.3% db, and a freshly demucilaged bean at 50% wb holds 100% db – as much water as dry matter. The gap grows with moisture, which is why the basis must always be stated.\n\n## How water leaves the seed\n\nCoffee is hygroscopic: it loses or gains water until it reaches equilibrium with the surrounding air. It cannot be dried below the equilibrium moisture content (EMC) set by the temperature and humidity of the drying air.4 The Codex code gives the practical consequence for storage and transport: below 60% relative humidity coffee keeps drying, above 80% it starts to absorb water.1 Washed coffee in particular can reabsorb moisture in humid environments.9\n\nTypical moisture along the chain:\n\n1. **Harvest** – freshly harvested coffee holds about 50–60% moisture (wb), depending on the processing method and climate.9 Across maturation stages, fruits are harvested at between 30% and 65% (wb).17\n2. **Parchment after demucilaging** – about 45–55% moisture, whether the mucilage was fermented off or removed mechanically.7\n3. **Dried** – reduced to 10–12% (wb) for safe storage.9 Green coffee is generally dried to, and traded at, about 12%.7 The Codex code says moisture should not exceed 12.5% anywhere from the loading area to roasting; where it states the basis, it is wet basis.1\n\n## Why it matters: mould, toxins and quality\n\nAbove 12% (wb), green coffee can deteriorate through bacteria, mould or yeast, especially if the seed has died.7 The Codex code ties protection from fungi to two limits together: moisture below 12.5% (wb) and a water activity of no more than 0.67–0.70.1 Three different water-activity numbers are often confused. Below about 0.62, yeasts and moulds in foods in general usually do not grow.5 The 0.67–0.70 figure is the Codex storage ceiling for parchment coffee, not a growth limit.1 Below 0.76–0.78, the OTA-producing fungi of coffee cannot grow.1 Rewetting dried coffee is to be avoided, because it favours rapid fungal growth and ochratoxin A production.1\n\nEven well below the mould threshold, water still matters. High temperature and high water content are the main reported causes of sensory change during storage.16\n\n\n\n## Reference values\n\nAll moisture values are given with their basis as stated in the source. \"Not stated\" means the source gives no basis.\n\n| Parameter | Value reported | Basis | Source |\n|---|---|---|---|\n| Freshly harvested coffee | 50–60% | wb | Duque-Dussán et al.9 |\n| Coffee fruits at harvest, by maturation stage | 30–65% | wb (kg water per 100 kg material) | Rocha et al.17 |\n| Parchment after demucilaging | 45–55% | not stated | Gautz et al.7 |\n| Safe storage level | 10–12% | wb | Duque-Dussán et al.9 |\n| Trading moisture | About 12% | wb (source convention) | Gautz et al.7 |\n| Hawai'i grade standard | 9–12% ± 0.3 points | not stated | Gautz et al.7 |\n| SCAA grades (Specialty, Premium, Exchange) | 9–13% | not stated | Mutua8 |\n| Codex maximum, whole chain | Not to exceed 12.5% | wb where the code states a basis | Codex1 |\n| Codex water activity ceiling | 0.67–0.70 | – | Codex1 |\n| SCA specialty grade, archived (heritage) standard | Water activity lower than 0.70; no moisture range | – | SCA20 |\n| Target kept in a storage and transport trial | 10–12% | wb | Anokye-Bempah et al.15 |\n| Recommended in storage (cited from Harris and Miller, 2008) | 10–12% and aw 0.4–0.6 | wb | Anokye-Bempah et al.15 |\n| Green coffee at the start of that trial | 10.76% ± 0.2 at aw 0.57 | wb | Anokye-Bempah et al.15 |\n| Safe storage equivalence | 10–12% ≈ aw 0.65–0.70 | not stated | Akbar et al.14 |\n| Storage air | < 60% RH: keeps drying; > 80% RH: absorbs water | – | Codex1 |\n| Yeasts and moulds in foods generally (not coffee-specific) | Usually no growth below aw 0.62 | – | FAO5 |\n| OTA-producing *Aspergillus* in coffee: growth floor | No growth below aw 0.76–0.78 | – | Codex1 |\n| OTA-producing *Aspergillus* in coffee: toxin floor | Present but no toxin below aw 0.80 | – | Codex1 |\n| OTA risk window during drying | aw 0.80–0.95; 5 days or less in the drying yard prevented OTA accumulation in trials | – | Codex1 |\n| Optimum OTA, *A. westerdijkiae* on stored beans | aw 0.95–0.97 at 30 °C | – | Akbar et al.14 |\n| EU maximum OTA, roasted beans and ground roasted coffee | 3.0 µg/kg | – | Reg. (EU) 2023/91519 |\n| EU maximum OTA, soluble (instant) coffee | 5.0 µg/kg | – | Reg. (EU) 2023/91519 |\n| Glass transition, green coffee | 48.76 °C at aw 0.155 → 34.89 °C at aw 0.512 | – | Iaccheri et al.13 |\n| Arabica adsorption, 30–60 °C | 0.0191–0.2534 (decimal; 1.9–25.3%) over aw 0.10–0.83 | db | dos Santos et al.12 |\n\n
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Moisture content reported at each stage, from cherry to green coffee. Ranges are as published; check the basis of each value in the table.1,7,8,9,17
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\n\n## Sorption isotherms and hysteresis\n\nAn isotherm plots equilibrium moisture content against water activity at one temperature. For Arabica beans between 30 and 60 °C, the adsorption isotherms had a sigmoidal type II shape.12 Raising the temperature at constant water activity lowered the equilibrium moisture content.12\n\n**Models.** The Guggenheim–Anderson–de Boer (GAB) model fitted desorption data well for green coffee, pulped coffee and whole fruits at 10–50 °C.10 It describes the isotherm with three parameters: the monolayer moisture content Xm, a constant C for how strongly water binds to the monolayer, and a constant K for the multilayer.11 In parchment coffee, K values close to one pointed to multilayer water behaving close to free water.11 The GAB model is not always the best fit: for Arabica adsorption at 30–60 °C, the Peleg model performed best.12\n\n**Hysteresis.** Drying follows a desorption isotherm, rewetting in storage an adsorption isotherm. The two studies opened here measured one direction each – desorption in coffee fruits, pulped and green coffee,10 adsorption in Arabica beans12 – so the gap between the two curves is not quantified in this entry. In the desorption study, the differential enthalpy, entropy and Gibbs free energy rose as equilibrium moisture fell, and whole coffee fruits showed the highest values.10\n\n**Ripeness.** In parchment coffee, cherry maturity changed water availability in the bean. Unripe stages lost water activity faster during drying, down to values (aw < 0.4) that the authors link to possible stalling after roasting.11\n\n## Glass transition\n\nGreen coffee is mainly a glassy structure that changes slowly during storage.13 Water lowers its glass transition temperature (Tg): as water activity rose from 0.155 to 0.512, Tg of green coffee fell from 48.76 °C to 34.89 °C. Roasted coffee was less sensitive, falling from 45.73 °C to 40.15 °C over the same range.13 Whether green coffee stored warm enough to pass above its Tg ages faster has not been shown in the sources for this entry.\n\n## Mould and ochratoxin A\n\nIn coffee, only *Aspergillus* species produce ochratoxin A (OTA): *A. ochraceus* and its relatives *A. westerdijkiae* and *A. steynii*, *A. niger* and relatives, and *A. carbonarius*.1 These belong to *Aspergillus* sections *Circumdati* and *Nigri*.18 *Penicillium* also makes OTA, mainly in temperate and colder zones,18 but the Codex code names only *Aspergillus* species for coffee.1 The water-activity bands below apply to these coffee *Aspergillus* species; they are not general limits for all moulds.\n\nThe Codex code describes water activity bands for OTA risk:1\n\n- **aw > 0.95** – fast-growing hydrophilic fungi and yeasts grow first; OTA producers are unlikely to take hold.\n- **aw 0.80–0.95** – the range in which OTA-producing fungi can grow. The time coffee spends in this range on the drying yard is the key control point; in trials, 5 days or less prevented OTA accumulation.\n- **aw < 0.80** – OTA producers can be present but do not produce the toxin.\n- **aw < 0.76–0.78** – they cannot grow.\n\nLaboratory work on stored green coffee agrees on the upper band: *Aspergillus westerdijkiae* produced most OTA at aw 0.95–0.97 and 30 °C. With elevated CO2 (1000 ppm) and 35 °C, OTA production was stimulated at aw 0.90–0.95.14 The authors note that the aw range of 0.90–0.97 used on the beans corresponds to roughly 40% down to 20–22% moisture (basis not stated).14\n\n## Measurement\n\n**Oven reference method (ISO 6673).** A test portion of about 10 g is heated at 105 °C for 16 h ± 0.5 h at atmospheric pressure.2 Strictly, it measures loss in mass, which by convention and agreement between parties is used as water content. It reads about 1.0% lower than the ISO 1446 basic reference method.2 These parameters are from the 2003 edition. A third edition was published in 2025 and applies to both decaffeinated and non-decaffeinated green coffee;3 its full text was not openly available to check whether the parameters changed. The Codex code asks for moisture measurements to be calibrated to ISO 6673.1 The oven method is a convenient reference because it needs no calibration itself.7\n\n**Capacitance meters.** The most common coffee moisture meters are adapted grain meters that measure capacitance; modern units also measure temperature and bulk density.7 Any electronic device drifts with time, and the drift is corrected by calibration against a reference such as oven drying.7 Repeatability – the spread of readings when the same sample is presented several times – is the real accuracy of a meter and cannot be removed by calibration.7\n\n**Dielectric spectroscopy.** In research, the dielectric constant of parchment coffee predicted both moisture content and water activity (R² 0.82–0.88), with 0.3–0.9 GHz as the most useful range.11\n\n**Water activity meters.** Chilled-mirror dewpoint instruments combine a dewpoint sensor, an infrared thermometer and a fan in one sensor block.6 Capacitance aw sensors use a hygroscopic polymer between two plates whose ability to hold a charge rises as it adsorbs water.6 Because the isotherm shifts with temperature, an aw reading should be reported with the sample temperature.12\n\n### Reference protocol\n\n| Step | Parameter | Value reported | Source |\n|---|---|---|---|\n| Sample | Temperature before metering | Cool to ambient unless the meter is rated for hot samples | Gautz et al.7 |\n| Reference moisture | Method | ISO 6673: about 10 g, 105 °C, 16 h ± 0.5 h, atmospheric pressure | ISO 66732 |\n| Reference moisture | Alternative extension protocol | 105 °C (220 °F) ± 5 °F, natural airflow, 24 h | Gautz et al.7 |\n| Meter | Calibration | Against ISO 6673 | Codex1 |\n| Reporting | Basis | State wb or db; trade and Codex use wb | Codex1, FAO4 |\n| Water activity | Instrument | Chilled-mirror dewpoint or capacitance sensor | Mermelstein6 |\n| Water activity | Temperature | Report with measurement temperature; EMC falls as temperature rises | dos Santos et al.12 |\n| Acceptance | Storage safety | < 12.5% (wb) and aw ≤ 0.67–0.70 | Codex1 |\n\n## Risks and defects\n\n- **Too wet** – above 12% (wb): deterioration by bacteria, mould or yeast; in live seeds, enzyme activity changes cup quality.7\n- **Slow drying** – too long in the aw 0.80–0.95 window gives OTA producers time to grow.1\n- **Rewetting** – favours rapid fungal growth and OTA production.1 Washed coffee can pick up moisture in humid storage and become more brittle.9\n- **Humid storage and transport** – fluctuating temperature and relative humidity change moisture content, water activity, colour and sensory quality.15\n- **Over-drying** – very low water activity (aw < 0.4) in unripe beans has been linked to possible stalling after roasting.11\n- **OTA above legal limits** – in the EU, roasted beans and ground roasted coffee may contain at most 3.0 µg/kg OTA, and soluble coffee at most 5.0 µg/kg.19 The regulation sets no OTA limit for green coffee.19\n- **Wrong basis** – a dry-basis value read as wet basis overstates moisture; at 12.5% wb the difference is 1.8 percentage points.\n\n## Open questions\n\n- There is no agreed single target water activity for green coffee in storage. The sources opened here give upper limits for safety (Codex 0.67–0.70; about 0.65–0.70 in a 2020 study)1,14 and, for specialty grade, the SCA's archived limit of below 0.70,20 while the only lower target, 0.4–0.6 for preserving quality, appears in a 2023 study as a citation of earlier work rather than as its own finding (see Contested).15\n- Adsorption and desorption isotherms measured on the same green coffee lot at storage temperatures are scarce in the sources used here, so the size of the hysteresis loop in practice is not quantified.\n- How water activity is distributed across traded green coffee lots has not, to our knowledge, been reported in peer-reviewed work; the large datasets that exist come from the trade.\n- The link between glass transition and flavour loss in storage is suggested by the physics but not yet demonstrated for coffee in these sources.\n\n## Key studies\n\n
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Goneli et al., 2013 – Water sorption of coffee fruits, pulped and green coffee. Measured desorption isotherms of three forms of coffee at five temperatures (10–50 °C) and 11–95% relative humidity. The GAB model fitted well, and whole fruits showed the highest thermodynamic values.

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LWT – Food Science and Technology 50(2), 386–391 · doi:10.1016/j.lwt.2012.09.006

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Velásquez et al., 2021 – Cherry maturity, sorption and dielectric sensing. Showed that ripeness stage changes the water availability of parchment coffee and its drying behaviour, with unripe beans reaching very low water activity. Dielectric measurements at 0.3–0.9 GHz predicted both moisture content and water activity.

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Food Control 123, 107692 · doi:10.1016/j.foodcont.2020.107692

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Iaccheri et al., 2019 – Glass transition of green and roasted coffee. Used calorimetry and dielectric methods to track the glass transition at different water activities. In green coffee, Tg fell by about 14 °C as aw rose from 0.155 to 0.512, a stronger effect than in roasted coffee.

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Food Chemistry 301, 125187 · doi:10.1016/j.foodchem.2019.125187

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Akbar et al., 2020 – Aspergillus westerdijkiae under climate stress. Tested growth and OTA production across water activity, temperature and CO2 on coffee medium and stored beans. OTA peaked at aw 0.95–0.97 and 30 °C, and warmer, CO2-rich conditions stimulated production at slightly drier levels.

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Microorganisms 8(9), 1268 · doi:10.3390/microorganisms8091268

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Anokye-Bempah et al., 2023 – Desiccants in storage and transport. Examined how temperature and humidity swings during storage and shipping change the moisture, water activity, colour and sensory quality of green coffee, and tested desiccants as a way to keep beans in the recommended range. Over a simulated 42-day shipment from Brazil to Italy, hermetic packages with desiccants kept green coffee below 12% (wb) and below aw 0.6, while beans in jute sacks rose to 17.34% (wb) and aw 0.76.

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Journal of Agriculture and Food Research 11, 100478 · doi:10.1016/j.jafr.2022.100478

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