{ "id": "seed-physiology", "url": "https://coffearium.org/entries/seed-physiology/", "license": "CC-BY-4.0", "title": "The living seed during processing", "summary": "The coffee seed is still alive when it is picked. Processing switches on germination and drought-stress metabolism inside it, and heat or storage can kill it – changes that leave traces in green coffee chemistry and quality.", "domain": "post-harvest-processing", "kind": "Concept", "stage": "process", "topics": [ "science" ], "topicSections": {}, "lang": "en", "status": "verified", "reviewer": "Coffearium verification pass", "lastVerified": "2026-10-21T00:00:00.000Z", "updated": "2026-10-21T00:00:00.000Z", "added": "2026-10-21T00:00:00.000Z", "brief": [ "The coffee seed is alive during processing; germination markers peaked about two days after the start of wet processing and about one week after the start of dry processing.1", "Drying puts the living seed under drought stress; the stress metabolite GABA (γ-aminobutyric acid) was always markedly higher in natural than in washed arabica.2,3", "Hot-air drying can kill the embryo; natural coffee dried at 60 °C, or at 60 °C then 40 °C, did not germinate at all, while sun-dried natural coffee reached 80%.12", "Washed green coffee has less glucose and fructose than natural coffee; seed metabolism explains at least part of the gap, and leaching during soaking may add to it.4,5,6", "Viability is tested with the tetrazolium and germination tests, and membrane damage with electrical conductivity and potassium leaching of soaked beans.10,11" ], "quickFacts": [ { "label": "Germination peak", "value": "About 2 days into wet processing; about 1 week into dry processing" }, { "label": "Stress marker", "value": "GABA, markedly higher in natural coffee" }, { "label": "Embryo", "value": "3–4 mm long, near the convex surface of the seed" }, { "label": "Seed storage class", "value": "Intermediate" }, { "label": "Metabolism stops", "value": "Assumed below about 20% water in the cells" }, { "label": "Viability tests", "value": "Tetrazolium (0.5%, 30 °C, 2 h); germination (30 °C, 30 days)" }, { "label": "Membrane test", "value": "Electrical conductivity of 50 beans in 75 mL water, 25 °C, 5 h" } ], "aka": [ "Seed physiology during processing", "Seed metabolism during processing" ], "linkTerms": [ "living seed", "seed metabolism", "seed viability", "germination processes", "germination metabolism", "drought stress", "desiccation tolerance" ], "related": [ "coffee-cherry-anatomy", "washed-process", "natural-process", "pulped-natural", "fermentation", "drying", "resting", "moisture-and-water-activity", "green-coffee-composition", "green-coffee-storage", "mechanical-dryers", "processing-defects" ], "contested": [ "Why washed green coffee has less glucose and fructose: in a drying study, the two sugars fell within the first day of drying with no process water present, and the authors attribute the gap at least in part to seed metabolism, not leaching4; an earlier comparison found that they fall during wet processing but stay the same or rise during dry processing.5 A field study links the drop during fermentation to anaerobic sugar consumption by the seed, but adds that during a later soak osmotic pressure helped monosaccharides leave the beans6, and a trial in Yunnan found fructose among the most abundant compounds in the soaking water.7 No study opened here measures how much each mechanism contributes.", "Fast or slow drying for the living seed: in a seed-science study, seeds dried rapidly in silica gel tolerated lower final moisture than seeds dried slowly over salt solutions16, while in processing trials with heated air a higher drying rate lowered the cup quality of washed (pulped and fermented) coffee.14 The studies differ in temperature, purpose and the coffee material dried." ], "corrections": [ { "date": "2026-10-21T00:00:00.000Z", "change": "Natural coffee is no longer said to dry more slowly because the fruit surrounds the seed; the 2017 trial is cited only for the longer drying time it measured.", "reason": "The cited study measured longer drying times but did not give that cause." }, { "date": "2026-10-21T00:00:00.000Z", "change": "The 2017 heated-air study now compares natural with washed (pulped, fermented and washed) coffee, and its cup-quality finding refers to that washed coffee, not to pulped coffee in general.", "reason": "In the study, \"pulped coffee\" was fermented for 20 h and washed." }, { "date": "2026-10-21T00:00:00.000Z", "change": "The order of the two drought-stress GABA peaks (embryo, then endosperm) was removed; the entry now notes that the paper gives the order differently in its abstract and main text.", "reason": "The source is inconsistent on the order." }, { "date": "2026-10-21T00:00:00.000Z", "change": "Glucose values in the 2010 drying study now include the fall within day 1 and the rise from under 15 to nearly 40 mg/100 g in pause-dried beans before the final value of about 20.", "reason": "The earlier text implied a steady fall." }, { "date": "2026-10-21T00:00:00.000Z", "change": "Wording softened to match the sources: sodium hypochlorite lowered (not distorted) tetrazolium results on low-moisture seeds; the viability loss is suggested (not shown) to be relevant to cup flattening; cup scores at 60 °C were lower across both processes, not for natural coffee alone; washed coffee holds less (not much less) glucose and fructose; the metabolism-not-leaching conclusion is attributed to the 2010 study.", "reason": "The previous wording was stronger than the sources." }, { "date": "2026-10-21T00:00:00.000Z", "change": "The claim that the seed is alive at picking now cites the germination and stress-metabolism studies instead of the seed-physiology review; the clause on the glyoxylate cycle turning fat into sugar was removed; the 2015 trial is no longer placed in Lavras.", "reason": "The previously cited sources do not state these points." }, { "date": "2026-10-21T00:00:00.000Z", "change": "Storage conditions for the viability row now note the figure-caption values (20 °C, 60% RH) and that the moisture basis is not stated; the loss of more than half within 3 months was added.", "reason": "Completeness against the source." } ], "sources": [ { "id": "s1", "title": "Transient occurrence of seed germination processes during coffee post-harvest treatment", "authors": "Bytof, G.; Knopp, S.-E.; Kramer, D.; Breitenstein, B.; Bergervoet, J. H. W.; Groot, S. P. C.; Selmar, D.", "year": 2007, "publisher": "Annals of Botany 100(1), 61–66", "url": "https://academic.oup.com/aob/article/100/1/61/2763338", "doi": "10.1093/aob/mcm068", "tier": "A", "quote": "The highest germination activity occurred 2 d after the onset of wet processing, whereas the corresponding maximum in the course of dry processing appeared about 1 week after the start of post harvest treatment.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s2", "title": "Influence of processing on the generation of γ-aminobutyric acid in green coffee beans", "authors": "Bytof, G.; Knopp, S.-E.; Schieberle, P.; Teutsch, I.; Selmar, D.", "year": 2005, "publisher": "European Food Research and Technology 220(3–4), 245–250", "url": "https://portal.fis.tum.de/en/publications/influence-of-processing-on-the-generation-of-%CE%B3-aminobutyric-acid-/", "doi": "10.1007/s00217-004-1033-z", "tier": "A", "quote": "This result underlined the assumption that during postharvest treatment a significant metabolism occurs within coffee seeds.", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s3", "title": "Stress Metabolism in Green Coffee Beans (Coffea arabica L.): Expression of Dehydrins and Accumulation of GABA during Drying", "authors": "Kramer, D.; Breitenstein, B.; Kleinwächter, M.; Selmar, D.", "year": 2010, "publisher": "Plant and Cell Physiology 51(4), 546–553", "url": "https://academic.oup.com/pcp/article-abstract/51/4/546/1826697", "doi": "10.1093/pcp/pcq019", "tier": "A", "quote": "A decrease in water potential of the still living coffee seeds induces massive drought stress responses.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s4", "title": "Influence of drying on the content of sugars in wet processed green Arabica coffees", "authors": "Kleinwächter, M.; Selmar, D.", "year": 2010, "publisher": "Food Chemistry 119, 500–504", "url": "https://doi.org/10.1016/j.foodchem.2009.06.048", "doi": "10.1016/j.foodchem.2009.06.048", "tier": "A", "quote": "Within the first day of drying, fructose concentration decreased down to about 40 mg/100 g d.w. in the case of continuous drying, and to about 30 mg/100 g d.w. in beans dried with pauses (Fig. 1).", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s5", "title": "Influence of processing on the content of sugars in green Arabica coffee beans", "authors": "Knopp, S.; Bytof, G.; Selmar, D.", "year": 2006, "publisher": "European Food Research and Technology 223, 195–201", "url": "https://link.springer.com/article/10.1007/s00217-005-0172-1", "doi": "10.1007/s00217-005-0172-1", "tier": "A", "quote": "While in washed coffee beans (wet processing) only low amounts of these both hexoses were present, those in unwashed coffees (dry processing) were significantly higher.", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s6", "title": "Exploring the impacts of postharvest processing on the microbiota and metabolite profiles during green coffee bean production", "authors": "De Bruyn, F.; Zhang, S. J.; Pothakos, V.; Torres, J.; Lambot, C.; Moroni, A. V.; Callanan, M.; Sybesma, W.; Weckx, S.; De Vuyst, L.", "year": 2017, "publisher": "Applied and Environmental Microbiology 83(1), e02398-16", "url": "https://journals.asm.org/doi/10.1128/aem.02398-16", "doi": "10.1128/AEM.02398-16", "tier": "A", "quote": "Several of the metabolites associated with lactic acid bacterial metabolism (e.g., lactic acid, acetic acid, and mannitol) produced in the mucilage were also found in the endosperm.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s7", "title": "Influence of various processing parameters on the microbial community dynamics, metabolomic profiles, and cup quality during wet coffee processing", "authors": "Zhang, S. J.; De Bruyn, F.; Pothakos, V.; Contreras, G. F.; Cai, Z.; Moccand, C.; Weckx, S.; De Vuyst, L.", "year": 2019, "publisher": "Frontiers in Microbiology 10, 2621", "url": "https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.02621/full", "doi": "10.3389/fmicb.2019.02621", "tier": "A", "quote": "At the same time, the endogenous coffee bean metabolism remained active at different stages along the processing, as could be seen through changes in the concentrations of carbohydrates, organic acids, and free amino acids.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s8", "title": "The Storage of Green Coffee (Coffea arabica): Decrease of Viability and Changes of Potential Aroma Precursors", "authors": "Selmar, D.; Bytof, G.; Knopp, S.-E.", "year": 2008, "publisher": "Annals of Botany 101(1), 31–38", "url": "https://academic.oup.com/aob/article/101/1/31/185681", "doi": "10.1093/aob/mcm277", "tier": "A", "quote": "Whereas all other coffees lost viability within the first 6 months of storage, coffee beans stored within the parchment remained viable for >1 year.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s9", "title": "Coffee seed physiology", "authors": "Eira, M. T. S.; Amaral da Silva, E. A.; De Castro, R. D.; Dussert, S.; Walters, C.; Bewley, J. D.; Hilhorst, H. W. M.", "year": 2006, "publisher": "Brazilian Journal of Plant Physiology 18(1), 149–163", "url": "https://www.scielo.br/j/bjpp/a/zZgRS3Mbwg8KjW8R9bn9X3L/?lang=en", "doi": "10.1590/S1677-04202006000100011", "tier": "A", "quote": "Coffee seeds are now considered to have storage behavior defined as intermediate.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s10", "title": "Suitability of the tetrazolium test methodology for recently harvested and stored coffee seeds", "authors": "Clemente, A. C. S.; Carvalho, M. L. M.; Guimarães, R. M.", "year": 2012, "publisher": "Ciência e Agrotecnologia 36(4), 415–423", "url": "https://scielo.br/j/cagro/a/Pmt8vVrWkmCjYccRfFtNyHm/?lang=en", "doi": "10.1590/S1413-70542012000400005", "tier": "A", "quote": "An imbibition time of 48 hours facilitates extraction of embryos and does not affect test results.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s11", "title": "Aspectos histoquímicos e morfológicos de grãos de café de diferentes qualidades", "authors": "Goulart, P. F. P.; Alves, J. D.; Castro, E. M.; Fries, D. D.; Magalhães, M. M.; Melo, H. C.", "year": 2007, "publisher": "Ciência Rural 37(3), 662–666", "url": "https://www.redalyc.org/pdf/331/33137310.pdf", "tier": "A", "quote": "A higher potassium lixiviation with the consequent increase in electric conductivity showed to be a strong indicator of membrane and cell wall damage.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s12", "title": "Post-harvest effects on beverage quality and physiological performance of coffee beans", "authors": "Taveira, J. H. S.; Borém, F. M.; Rosa, S. D. V. F.; Oliveira, P. D.; Giomo, G. S.; Isquierdo, E. P.; Fortunato, V. A.", "year": 2015, "publisher": "African Journal of Agricultural Research 10(12), 1457–1466", "url": "https://www.alice.cnptia.embrapa.br/alice/handle/doc/1039291", "doi": "10.5897/AJAR2014.9263", "tier": "A", "quote": "The sun-drying provided coffee beans with the best physiological performance and the best drink quality.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s13", "title": "Ultrastructural analysis of drying damage in parchment Arabica coffee endosperm cells", "authors": "Borém, F. M.; Marques, E. R.; Alves, E.", "year": 2008, "publisher": "Biosystems Engineering 99(1), 62–66", "url": "https://www.sciencedirect.com/science/article/abs/pii/S153751100700267X", "doi": "10.1016/j.biosystemseng.2007.09.027", "tier": "A", "quote": "The histochemical results showed that, for the coffee parchment beans dried at 40 °C, there was no change in the cellular integrity of the plasma membrane and vesicles.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s14", "title": "Physiological and sensorial quality of Arabica coffee subjected to different temperatures and drying airflows", "authors": "Alves, G. E.; Borém, F. M.; Isquierdo, E. P.; Siqueira, V. C.; Cirillo, M. A.; Pinto, A. C. F.", "year": 2017, "publisher": "Acta Scientiarum. Agronomy 39(2), 225–233", "url": "https://www.redalyc.org/journal/3030/303050431012/html/", "doi": "10.4025/actasciagron.v39i2.31065", "tier": "A", "quote": "coffee that is processed via a dry method is more sensitive to mechanical drying with heated air than coffee processed via a wet method, resulting in poor physiological performance", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s15", "title": "Antioxidant enzyme activity and hydrogen peroxide content during the drying of Arabica coffee beans", "authors": "Rendón, M. Y.; Gratão, P. L.; Salva, T. J. G.; Azevedo, R. A.; Bragagnolo, N.", "year": 2013, "publisher": "European Food Research and Technology 236(5), 753–758", "url": "https://www.doi.org/10.1007/s00217-013-1933-x", "doi": "10.1007/s00217-013-1933-x", "tier": "A", "quote": "Moreover, the hydrogen peroxide content of the natural coffee was greater than that of the pulped natural coffee.", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s16", "title": "Physiological and biochemical changes in coffee seed dried in silica gel and saturated saline solutions", "authors": "Coelho, S. V. B.; Figueiredo, M. A.; Clemente, A. C. S.; Coelho, L. F. S.; Rosa, S. D. V. F.", "year": 2015, "publisher": "Pesquisa Agropecuária Brasileira 50(6), 483–491", "url": "https://scielo.br/j/pab/a/khQZSkWv7gq5p47JMxWdhYG/abstract/?lang=en", "doi": "10.1590/S1678-3921.pab2015.v50.20937", "tier": "A", "quote": "Rapid drying shows greater damage potential to the endosperm than to the embryos.", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" } ], "markdown": "## A seed, not only a bean\n\nA coffee \"bean\" is the seed of the coffee fruit. When the cherry is picked, the seed inside it is alive.1,3 It holds a tiny plant, the embryo, only 3–4 mm long, which sits near the convex surface of the seed.9 Around it lies the endosperm, which is living tissue too.9\n\nProcessing changes the seed's surroundings. The pulp is removed, the seed may sit under water, and then it dries. The living seed responds with its own metabolism.1,3 That metabolism changes the chemistry of green coffee, and it differs between processing methods.1,2\n\n## Germination starts at the mill\n\nCoffee seeds start to germinate during processing. A 2007 study followed markers of germination in arabica seeds of the cultivar Acaiá from Brazil.1 The markers rose, peaked and fell again while the coffee was processed and dried.1\n\nThe timing depended on the method. In washed coffee (also called wet processing), germination activity peaked about two days after processing began. In natural coffee it peaked only about one week after the start.1 The activity is transient: it fades again as the seed dries.1\n\n
\n
\n
What the living seed does in washed and natural processing: an early germination peak in washed coffee, a later one in natural coffee, drought stress during drying, and a slow loss of viability in storage. Schematic, not to scale.1,2,3,8
\n
\n\n## Drying is stress for the seed\n\nAs the seed dries, it loses water and its water potential falls. A seed that is still alive reacts to this with strong drought-stress responses.3 One sign is GABA (γ-aminobutyric acid), an amino acid that plants build up under stress.2\n\nIn a 2017 drying trial, natural coffee, dried as whole fruit, took longer to dry than washed coffee.14 In a comparison of processed arabica coffees, GABA in natural coffee was always markedly higher than in washed coffee or in untreated seeds.2 The authors proposed GABA as a marker of how a coffee was processed.2 Drying natural coffee also left more hydrogen peroxide, a sign of oxidative stress, than drying pulped natural coffee.15\n\n## When the seed dies\n\n**Heat.** Hot air can kill the embryo. In a Brazilian trial, natural coffee dried at 60 °C did not germinate at all, while sun-dried natural coffee reached 80% germination.12 Washed coffee survived better: 97% germinated after sun-drying and 61% after drying at 60 °C.12 Under the microscope, cell membranes in the endosperm stayed intact at 40 °C but ruptured at 60 °C.13\n\n
\n
\n
Germination of Catuaí Vermelho seeds after three drying treatments. Natural coffee dried with heated air did not germinate; washed coffee kept more of its viability.12
\n
\n\n**Too little water.** Coffee seeds tolerate drying only down to a point. Seed scientists class their storage behaviour as \"intermediate\".9 Arabica seeds were damaged when dried below about 0.10 g of water per g of dry matter.9\n\n**Time.** Dried seeds die slowly in storage. At ambient temperature, arabica seed viability falls rapidly after 4–6 months.9 Seeds kept in their parchment stay alive longer than hulled seeds.8 The resting entry covers storage in parchment.\n\n## Why it matters for quality\n\nThe seed's own metabolism changes the chemistry of green coffee.1,7 Washed green coffee holds less glucose and fructose than natural coffee.5 A 2010 drying study concluded that at least part of this gap comes from the seed using the sugars, not from sugars washing out.4\n\nDamage to the living seed can also go with a poorer cup. In the Brazilian drying trial, sun-drying gave both the best physiological performance and the best cup.12 Damaged cells leak potassium and other ions when beans are soaked in water. Measuring this leakage is a check for membrane damage.11\n\n\n\n## Reference values\n\nThe table collects the values reported in the sources. Each value holds for the conditions of the study named; values from different laboratories are not directly comparable.\n\n| Parameter | Value reported | Source |\n|---|---|---|\n| Germination peak, wet processing | Isocitrate lyase expression and β-tubulin both maximal 2 d after the start (Acaiá; 36 h fermentation at 21 °C; dried at 35–40 °C to 12% wb) | Bytof et al.1 |\n| Germination peak, dry processing | Isocitrate lyase about 1 week; β-tubulin 6 d after the start | Bytof et al.1 |\n| GABA | Natural (unwashed) arabica always markedly higher than washed arabica and untreated seeds | Bytof et al.2 |\n| Fructose, demucilaged arabica, oven drying at 30 °C | Nearly 110 mg/100 g dry weight fresh; about 40 after day 1 (continuous drying), about 30 (drying with pauses) | Kleinwächter & Selmar4 |\n| Glucose, same trial | About 55 mg/100 g dry weight fresh; fell within day 1; with pauses, rose again from under 15 to nearly 40, then about 20 in the fully dried beans | Kleinwächter & Selmar4 |\n| Sucrose, same trial | About 6000 mg/100 g dry weight; roughly constant | Kleinwächter & Selmar4 |\n| Drying time, same trial | About 5 days continuous; about 8 days with pauses (from about 56% to about 12% water, basis not stated) | Kleinwächter & Selmar4 |\n| Metabolism switched off | General assumption: below about 20% residual water in the cells (basis not stated) | Selmar et al.8 |\n| Lowest water content without damage, *C. arabica* | ≈0.09 g H₂O/g dry weight (about 8% wb, our conversion) | Eira et al., citing Ellis and colleagues9 |\n| Embryo | 3–4 mm long, axis and two cotyledons, near the convex surface | Eira et al.9 |\n| Germination time course (30 °C, dark) | Radicle protrusion from day 5–6; about 50% by day 10; most by day 15 | Eira et al.9 |\n| Germination test | 4 × 50 seeds without parchment, paper rolls, 30 °C; final count after 30 days | Clemente et al.10, Taveira et al.12 |\n| Tetrazolium test | Imbibe 36–48 h at 30 °C, remove parchment by hand, excise embryos; 0.5% tetrazolium, 30 °C, dark, 2 h | Clemente et al.10 |\n| Electrical conductivity test | 50 beans in 75 mL deionised water, 25 °C, 5 h | Goulart et al.11, Taveira et al.12 |\n| Electrical conductivity by cup class | Mole 136.03, dura 137.98, rio 195.23 µS/g | Goulart et al.11 |\n| Electrical conductivity by drying | Patio: 59.59 natural, 48.22 washed; 60 °C: 117.5 natural, 95.67 washed (µS cm⁻¹ g⁻¹) | Taveira et al.12 |\n| Germination by drying | Patio: 80% natural, 97% washed; 60 → 40 °C: 0% natural, 85% washed; 60 °C: 0% natural, 61% washed | Taveira et al.12 |\n| Viability in storage, hulled washed arabica (22 °C, 63% RH in the methods; 20 °C, 60% RH in a figure caption; 11.5% moisture, basis not stated) | More than half lost within 3 months; under 10% viable after 6 months; none after 1 year | Selmar et al.8 |\n| Viability in storage, parchment (same conditions) | More than half viable after more than 1 year | Selmar et al.8 |\n\n## Germination metabolism\n\n**Markers.** The 2007 study used three markers.1 Isocitrate lyase (ICL) is the key enzyme of the glyoxylate cycle; coffee seeds are rich in fat, and ICL marks the transition from late embryogenesis to germination.1 ICL transcripts were measured in whole seeds by competitive RT-PCR.1 β-Tubulin, a marker of cell division or elongation, was measured in excised embryos by Western blot, and the ratio of 4C to 2C nuclei by flow cytometry as a measure of cell-cycle activity.1\n\n**Timing.** In wet processing, ICL expression and β-tubulin both peaked 2 days after the start.1 In dry processing, ICL peaked about one week after the start and β-tubulin after 6 days.1 In both methods β-tubulin declined during further drying, and the maximum frequency of 4C nuclei was markedly lower in dry-processed seeds.1 The authors attribute the chemical differences between wet- and dry-processed coffee to this differential germination activity.1\n\n**Under water.** A 2019 trial in Yunnan, China, saw the bean's own metabolism stay active at several stages of wet processing, through changes in carbohydrates, organic acids and free amino acids.7 A 2017 field study in Ecuador linked the fall of endosperm sugars during fermentation to the lack of oxygen under water, which triggered germination and anaerobic sugar consumption by the seed.6\n\n## Stress metabolism during drying\n\nFalling water potential in the still-living seed induces massive drought-stress responses.3 A 2010 study followed GABA and the expression of dehydrin genes during drying. The first GABA peak coincided with germination activity; two later peaks matched maxima of dehydrin expression and are thought to be induced by drought stress in the embryo and in the endosperm.3 The paper's abstract and main text give the order of the two tissues differently.3\n\nGABA itself is a well-known plant stress metabolite. Its content in natural arabica was always markedly higher than in washed arabica and in untreated seeds, which the authors took as evidence of significant metabolism in the seed during postharvest treatment.2 They proposed GABA as a tool to characterise the processing method.2\n\nDrought stress also produces reactive oxygen species. In a 2013 study, natural coffee showed lower activity of the antioxidant enzymes superoxide dismutase, guaiacol peroxidase and glutathione reductase during drying, and more hydrogen peroxide, than pulped natural coffee.15 The authors read this as oxidative stress, controlled more efficiently in pulped natural coffee.15\n\n## Sugars: metabolism or leaching\n\nWashed arabica held only low amounts of glucose and fructose, while natural arabica processed in parallel held significantly more.5 In washed coffee the two sugars fall during processing; in natural coffee they stay the same or rise.5\n\n**Evidence for metabolism.** In a 2010 drying study, arabica from Costa Rica was pulped, demucilaged mechanically and fermented without water, then dried in an oven at 30 °C.4 Fructose fell from nearly 110 to about 40 mg/100 g dry weight within the first day of continuous drying, and to about 30 in beans dried with pauses.4 Glucose started at about 55 mg/100 g dry weight, also fell within the first day and, in the pause-dried beans, rose again from under 15 to nearly 40 before ending at about 20, while sucrose (about 6000 mg/100 g) stayed roughly constant.4 With no process water present, the authors concluded that the lower hexose content of washed coffee is, at least in part, due to metabolic processes and not to leaching.4\n\n
\n
\n
Free fructose and glucose in demucilaged arabica before and during oven drying at 30 °C, with no process water present. Approximate values as given in the text of the study.4
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\n\n**Evidence for leaching.** In the Ecuador study, fructose and glucose in the beans fell further during a 24-hour soak, and the authors proposed that osmotic pressure helped monosaccharides leave the beans.6 In the Yunnan trial, fructose was among the most abundant compounds in the soaking water of depulped coffee at the end of soaking.7\n\nThe two mechanisms are not exclusive. The washed process entry discusses them in the context of the method; see Contested for both positions.\n\n## Desiccation tolerance and embryo death\n\n**Storage class.** Coffee seeds were first classed as recalcitrant but are now considered intermediate.9 Cited minimum water contents without damage are about 0.09 g H₂O/g dry weight for *C. arabica* and 0.10–0.12 for *C. canephora*.9 Low temperature is harmful: the critical water content rises as temperature falls.9\n\n**Drying rate.** Seeds dried rapidly in silica gel tolerated lower final moisture than seeds dried slowly over saturated salt solutions, and rapid drying damaged the endosperm more than the embryo.16 In the seed-storage literature, slow equilibration drying lowered the antioxidants glutathione and ascorbate and raised free fatty acids.9\n\n**Heat.** In the 2015 Brazilian trial, natural coffee dried with heated air (60 °C throughout, or 60 °C down to about 30% moisture, wet basis, and then 40 °C) showed a small amount of root protrusion but no germination; the authors concluded that the seeds were already dead or far into deterioration.12 Washed coffee from the same treatments still germinated at 85% and 61%.12 A 2017 study likewise found that natural coffee is more sensitive to heated-air drying than washed coffee (pulped, fermented and washed), with poorer physiological performance.14 Electron microscopy shows the damage: at 40 °C plasma membranes and oil-body vesicles stayed intact, at 60 °C they ruptured.13\n\n**Storage.** Hulled washed arabica lost viability within months at 22 °C and 63% relative humidity, while seeds in parchment stayed alive for more than a year.8 The authors suggest that the loss of viability is relevant to the flattening of cup quality in stored coffee.8\n\n## Measuring the living seed\n\n**Germination test.** Seeds are germinated without parchment, because the endocarp strongly inhibits germination.9 Two Brazilian studies used four replicates of 50 seeds on moist paper at 30 °C with a final count after 30 days.10,12 Germination is slow and uneven, which makes the test long.10\n\n**Tetrazolium test.** Staining with tetrazolium is a quicker alternative for assessing viability.10 In a method study, seeds were soaked for 36 or 48 hours at 30 °C, embryos were excised, stained in 0.5% tetrazolium at 30 °C in the dark for 2 hours, cut lengthwise and scored at 10× magnification.10 A 48-hour soak made the embryos easier to extract without changing the result. Parchment had to be removed by hand: sodium hypochlorite did not remove it fully and lowered the test results on low-moisture seeds.10\n\n**Electrical conductivity and potassium leaching.** Fifty beans soak in 75 mL of deionised water at 25 °C for 5 hours, and the conductivity of the water is read.11,12 Potassium in the water is measured by flame photometry.11,12 Higher potassium leaching with higher conductivity is a strong indicator of membrane and cell-wall damage.11 In a 2007 study, conductivity rose from 136.03 µS/g in \"mole\" (soft) coffee and 137.98 in \"dura\" coffee to 195.23 in \"rio\" coffee, the lowest of the three Brazilian cup classes.11 Histology matched: \"mole\" coffee had well-defined lipid bodies inside the cells, while in \"dura\" and \"rio\" coffee the lipids were scattered and sometimes leaked into the tissue.11\n\n## Risks and defects\n\n- **Dead embryos after hot drying** – natural coffee dried with heated air at 60 °C did not germinate, and across both processes drying at 60 °C gave lower cup scores than sun-drying.12\n- **Membrane rupture** – at 60 °C, membranes break and oil leaks between cells,13 which raises conductivity and potassium leaching.11,12\n- **Oxidative stress in slow-drying naturals** – more hydrogen peroxide and lower antioxidant enzyme activity than in pulped natural coffee.15\n- **Loss of viability in storage** – hulled coffee dies within months, and the authors suggest the loss is relevant to the flattening of the cup.8\n\n## Open questions\n\n- **Metabolism and flavour.** Germination and stress metabolism change sugars and amino acids,1,2,4 but no study opened here traces a specific flavour compound in the cup back to one metabolic pathway in the seed.\n- **Honey and pulped natural coffee.** No study opened here has timed germination markers in coffee that is pulped but dried with its mucilage.\n- **Robusta.** The germination and GABA studies opened here used arabica only.1,2,3\n- **Embryo or endosperm.** Germination tests score the embryo, while cup quality comes from the endosperm, and rapid drying damaged the endosperm more than the embryo.16 How well embryo viability predicts endosperm quality is not established.\n\n## Key studies\n\n
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Bytof et al., 2007 – Germination during processing. Tracked isocitrate lyase, β-tubulin and nuclear DNA content in Acaiá seeds through wet and dry processing. Germination activity peaked two days into wet processing and about a week into dry processing, then faded as the seeds dried.

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Annals of Botany 100(1), 61–66 · doi:10.1093/aob/mcm068

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Bytof et al., 2005 – GABA as a processing marker. Measured free amino acids in green coffees from different processing. GABA was always markedly higher in natural than in washed arabica, evidence of significant metabolism in the seed; the authors discuss a possible link to drought stress.

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European Food Research and Technology 220(3–4), 245–250 · doi:10.1007/s00217-004-1033-z

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Kleinwächter & Selmar, 2010 – Sugars during drying. Dried demucilaged arabica in an oven at 30 °C, with and without night pauses. Fructose and glucose fell within the first day while sucrose stayed constant, with no process water present – evidence that seed metabolism explains at least part of the sugar gap between washed and natural coffee.

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Food Chemistry 119, 500–504 · doi:10.1016/j.foodchem.2009.06.048

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Taveira et al., 2015 – Drying, germination and the cup. Compared natural and fully washed Catuaí Vermelho dried on a patio, at 60 °C, or at 60 °C then 40 °C. Heated air killed the natural seeds and lowered germination in washed coffee; sun-drying gave the best physiological performance and the best cup.

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African Journal of Agricultural Research 10(12), 1457–1466 · doi:10.5897/AJAR2014.9263

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Clemente et al., 2012 – Tetrazolium test for coffee. Compared ways of removing the parchment and soaking times before staining excised embryos. Manual parchment removal was needed for reliable results, and a 48-hour soak eased embryo extraction without changing the result.

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Ciência e Agrotecnologia 36(4), 415–423 · doi:10.1590/S1413-70542012000400005

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