CoffeariumBeta
  1. Plant
  2. Farm
  3. Process
  4. Trade
  5. Roast
  6. Brew
  7. Cup
Post-harvest processing · Science

Mucilage

The sticky, pectin-rich inner layer of the coffee fruit that clings to the parchment – removed in washed coffee, kept in honey and pulped-natural coffee, and the main food for fermentation.

Across the chain: Sustainability · Science

Source-checkedby Coffearium verification pass· Last source-checked 8 Oct 2026· Prepared with AI assistance· 15 sources (13 tier A)· 4 contested· Updated 8 Oct 2026
Reading depth
Contents
  • Mucilage is the innermost part of the fruit flesh (inner mesocarp), a thin hydrated layer attached to the parchment and made mostly of pectic substances and sugars.4
  • It is mostly water – 85–91% in figures compiled by one study12 – and makes up about 22% of the weight of pulped coffee.1
  • Washed coffee removes it by fermentation, enzymes or machines; the semi-dry (pulped natural) method leaves it on the seed to dry.5,10
  • Its sugars and pectins are the substrate that yeasts and bacteria work on during fermentation.1

What mucilage is#

Mucilage is the innermost part of the fruit flesh – the inner mesocarp. It is a thin, hydrated layer of elongated cells, attached to the parchment.4 After the pulper has removed the skin and the rest of the pulp, this layer stays on the seed.4

It is not evenly thick. Cenicafé measured about 0.4 mm on the flat face of the seed and up to 2.0 mm on the curved side.1 It makes up about 22% of the weight of pulped coffee and about 13% of the weight of the whole cherry.1 A figure cited in a 2018 study likewise gives about 22% of the “grain wet weight”, without saying exactly what the grain includes.12

What it is made of#

Mucilage is mostly water. Figures compiled in a 2018 study give 85–91% water and 6.2–7.4% sugars (w/w), with small amounts of protein, non-reducing sugar and pectin.12 Cenicafé gives 15% solids, in the form of a water-insoluble colloidal gel.1 It also lists cellulose and ash among the components.1

The solids are mainly pectic substances and sugars.4 How much of each is disputed: published figures range from a solid fraction that is mostly pectin to one that is mostly sugar.1,2,12 The comparison is under Contested and in the Reference values table.

Removing it or keeping it#

Mucilage is what separates the processing families. The skin and pulp come off in a pulper; the mucilage does not, because it is bound to the parchment.4

  • Fermentation – pulped coffee is left in a tank, usually under water, and microbes and enzymes break the mucilage down so it can be washed off.5,1
  • Enzymes – added pectinases speed up the breakdown.5 In one study, adding pectinase and lactic acid accelerated the breakdown to less than 8 hours.6
  • Machines – a mechanical demucilager rubs the seeds against each other and against a rotor.5 Cenicafé reports that coffee can be demucilaged and cleaned this way with 0.6 L of water per kg of dry parchment coffee.1
  • Alkali – strong alkali such as caustic soda dissolves mucilage quickly. At the Hawaii Agricultural Experiment Station, a 6% sodium hydroxide solution broke it down in about one minute.9
  • Keeping it – in the semi-dry method, also called pulped natural, the pulp is removed but there is no fermentation; the mucilage stays on the seed and dries with it.10 Honey processing uses the same idea with a chosen amount of mucilage, from about 20% to 80% kept.15 The colour names are producer conventions, and the sources that define them do not refer to any standard. A 2016 review gives about 25% mucilage for yellow and about 50% for red, with black usually fully covered, and links the colour to drying as well: yellow dries fast in the sun in about 8 days, red under shade or cloud in about 12, black in about 30.15 A 2023 review instead gives white to black as 20% to 80% of the pulp material left on the seed, with drying over 12 to 30 days.5 In a 2025 Arabica study, the treatment that kept 75–80% of the mucilage gave the highest relative content of volatile compounds.11

Food for fermentation#

Mucilage is the substrate of wet fermentation. Bacteria, yeasts and enzymes turn its pectic compounds and sugars into alcohols and acids – acetic, lactic, propionic and butyric among them.1 Some of these products do not stay outside: in a trial with Arabica in Ecuador, lactic acid, acetic acid and mannitol formed in the mucilage were also found inside the seed.14

The more mucilage stays on, the more there is to ferment. In a Yunnan trial with Arabica, the water of fermentations with the mucilage left on held far more fructose and glucose than that of mechanically demucilaged coffee.13

A by-product#

Mucilage leaves the mill as part of the wastewater, or as a separate stream when it is removed mechanically. Together with the pulp from one kilogram of cherry, it can use up the dissolved oxygen of 7.4 m³ of clean water.1 Discarded coffee residues can cause environmental problems because of their high biochemical oxygen demand.12

It is also a resource. Cenicafé lists pulp and mucilage as raw materials for compost and for animal protein.1 Its sugars can be fermented to ethanol: a 2018 study reached 0.46–0.47 g of ethanol per gram of sugar, 90–94% of the theoretical yield.12

Pectin chemistry#

Structure. Pectin is built on galacturonic acid. About a hundred galacturonic acid units linked by α-1,4 bonds form the homogalacturonan chain.7 Part of the carboxyl groups carry a methyl ester; the share that does is the degree of methylesterification. Highly methylated pectins are more open to base-catalysed breakdown, while weakly methylated ones hydrolyse faster in strong acid.7

Coffee mucilage pectin. Avallone and colleagues isolated the cell-wall material of mucilage from hand-dissected and commercially pulped beans. Both had a similar make-up: about 30% pectic substances, about 8% cellulose and about 18% other neutral polysaccharides.3 The extracted pectins held about 60% uronic acids, were highly methyl-esterified (about 62%) and moderately acetylated (about 5%).3 They were small – about 12–29 kDa – and did not form a gel with sugar under acid conditions.3 A 2023 review confirms the high degree of methylesterification and the roughly 60% galacturonic acid.5

The enzymes. Pectin is taken apart by three main kinds of enzyme:

  • Pectin methylesterase removes the methyl ester groups and releases methanol.7
  • Polygalacturonases split the glycosidic bonds of the chain by hydrolysis, using water.7 Most work best at pH 3.5–5.5 and 30–50 °C.7
  • Lyases split the same bond without water, by β-elimination, leaving an unsaturated product.7 Pectin lyases (endo-polymethylgalacturonate lyases) act on esterified pectin and release unsaturated methyl-oligogalacturonates.7 Pectin and pectate lyases generally work at alkaline pH, 7.5–10.0.7

The same chemistry explains why alkali works so fast. Highly methylated pectins are especially open to base-catalysed breakdown next to an esterified galacturonic acid unit.7 Coffee mucilage pectin is highly methylated.3

Which enzymes act in coffee. Erwinia dissolvens, a major pectolytic bacterium of fermenting coffee, was reported in an earlier study, cited by Avallone and colleagues, to produce only an exo-pectate lyase with very limited action on the high-methoxyl coffee pectin.4 A 2023 study found the yeasts Hanseniaspora uvarum and Pichia kudriavzevii more pectinolytic than the bacteria Klebsiella pneumoniae and Erwinia soli.6

Sugars#

Most of the sugar in mucilage is reducing sugar – 63% of the total in a literature figure cited by a 2018 study.12 A 2023 review table gives about 30% reducing and 20% non-reducing sugars on a dry-weight basis.5 In the fermentation water, sucrose, glucose and fructose first rise and then fall as microbes use them.13 Glucose falls throughout most fermentations while ethanol or lactic acid rises near the end.8

How mucilage comes off#

Several competing explanations have been proposed for what actually loosens the mucilage; a 2023 review calls the mechanism still unclear and controversial.5,6

  • Osmotic fracture. Microscopy of Arabica from Mexico in the 1990s showed the mucilage cell walls apparently intact after fermentation; the pectins were either not split or split to an undetectable degree.4 The authors proposed that microbes eating the sugars create an osmotic gradient that breaks the cells where they attach to the parchment.4 A review notes that the same group later put the breakdown down mainly to acidification by organic acids.5
  • Enzymes in two phases. A 2023 study ran seven treatments over 36 hours, suppressing bacteria or yeasts, adding pectinase or lactic acid, or holding pH at 7.6 In the first 12 hours, endogenous pectinolytic enzymes partly broke the beans’ mucilage down even without microbes. Complete breakdown was reached when yeasts were present throughout; bacteria played no critical role. Adding pectinase and lactic acid accelerated the breakdown to less than 8 hours.6

A field study in Nicaragua tracked seven batches on four farms. The pH started at 5.5–5.7 and fell sharply to about 4.6 as fermentation neared completion.8 A review, citing that study, reports that once the fermenting mass reached pH 5, fermentation was complete within about two hours, whatever the starting pH or duration.5 In a Yunnan trial, pH fell from about 6.0–6.5 to about 4.0.13

Reference values#

Parameter Value reported Source
Anatomical position Inner mesocarp; elongated cells attached to the parchment Avallone et al.4
Thickness About 0.4 mm (flat face) to 2.0 mm (curved side) Cenicafé1
Share of pulped coffee About 22% by weight Cenicafé1
Share of the grain wet weight About 22% (literature figure) Orrego et al.12
Share of the cherry About 13% by weight Cenicafé1
Water 85–91% (w/w) and 84.2% (two compilations); 91.77% Orrego et al.12, González-Freire & Martínez-Hernández2
Solids 15%, as an insoluble colloidal gel Cenicafé1
Total sugars 6.2–7.4% (w/w) and 4.1% (two compilations); 6.43% (basis not stated) Orrego et al.12, González-Freire & Martínez-Hernández2
Pectic substances 80% of the solids (“pectic acids”); 0.91% (compilation); 0.81% (basis not stated); about 35% of dry weight (compiled table) Cenicafé1, Orrego et al.12, González-Freire & Martínez-Hernández2, Elhalis et al.5
Sugars (dry weight) About 30% reducing, 20% non-reducing (compiled table) Elhalis et al.5
Protein (dry weight) 8.9% (compiled table) Elhalis et al.5
Cellulose (dry weight) 17% (compiled table) Elhalis et al.5
Cell-wall polysaccharides Pectic substances about 30%, cellulose about 8%, neutral non-cellulosic about 18% of the alcohol-insoluble residue Avallone et al.3
Pectin uronic acids About 60% Avallone et al.3, Elhalis et al.5
Degree of methylesterification About 62% (high-methoxyl) Avallone et al.3
Degree of acetylation About 5% Avallone et al.3
Pectin molecular weight About 12–29 kDa; does not gel with sugar at acid pH Avallone et al.3
pH during fermentation 5.5–5.7 falling to about 4.6; 6.0–6.5 falling to about 4.0 Jackels & Jackels8, Zhang et al.13
Breakdown with pectinase + lactic acid Accelerated to less than 8 h Elhalis et al.6
Removal with 6% NaOH About 1 min Fukunaga9
Mechanical demucilaging water use 0.6 L per kg dry parchment coffee Cenicafé1
Honey retention levels (no standard) Yellow about 25%, red about 50%, black usually 100% of the mucilage (2016 review); white to black 20% to 80% of the pulp material (2023 review) Poltronieri & Rossi15, Elhalis et al.5
Honey drying time (one review) Yellow about 8 days, red about 12 days, black about 30 days Poltronieri & Rossi15
Retention with most volatiles (one study) 75–80% of the mucilage Hu et al.11
Ethanol yield from mucilage sugars 0.46–0.47 g/g (90–94% of theoretical) Orrego et al.12

The composition figures come from different methods and bases (w/w, dry weight, cell-wall residue, or not stated), and several are compilations of older studies. They cannot be compared line by line.

Measuring the end point#

Traditionally, producers rub the seeds between their fingers: the texture changes from slimy to gritty when the mucilage has gone.5 pH is a simple chemical alternative. The Nicaraguan field study suggests the pH curve could predict when fermentation is complete and help prevent over-fermentation.8

Risks#

  • Over-fermentation – leaving coffee in the tank after the mucilage has broken down is a known risk; tracking pH is one proposed control.8
  • Water pollution – mucilage and pulp carry a high oxygen demand into rivers if wastewater is not treated.1,12
  • Alkali handling – strong alkali is corrosive and needs careful handling; its user also has to buy it and dose it precisely.9 In the Hawaii trials the parchment turned dark yellow, though the green bean looked normal and seed viability was not affected.9

Open questions#

  • Mechanism. Whether mucilage mainly comes off through enzymatic breakdown of pectin, through acidification, or through osmotic fracture of its cells is still debated.4,5,6
  • Composition. There is no agreed primary analysis of mucilage composition; the published figures disagree strongly.1,2,12
  • Honey grades. No standard defines the honey colours. Published definitions disagree on how much mucilage each colour means, and one also ties colour to drying speed and shade.5,15
020406080100material retained, %Filled: Elhalis et al. 2023 · open circle: Poltronieri & Rossi 2016White honeyabout 205 Yellow honeyabout 2515 Red honeyabout 5015 Black honeyabout 805 · usually 10015
Data table
Pointmaterial retained, %Source
White honeyabout 20205
Yellow honeyabout 252515
Red honeyabout 505015
Black honeyabout 80805
Black honeyusually 10010015
Material left on the seed by honey grade in two reviews: Elhalis et al. give white to black as 20% to 80% of the pulp material, Poltronieri & Rossi give yellow, red and black as mucilage shares. The same colour names mean different amounts – there is no standard.5,15

Key studies#

Avallone et al., 2000 – Polysaccharides of coffee mucilage. Isolated the cell-wall material from hand-dissected and commercially pulped mucilage and analysed its polysaccharides. The pectins were highly methyl-esterified, of low molecular weight and did not gel with sugar.

Journal of Food Science 65(8), 1308–1311 · doi:10.1111/j.1365-2621.2000.tb10602.x

Elhalis et al., 2023 – Yeasts and mucilage degradation. Ran seven wet-fermentation treatments that selectively suppressed microbes, added pectinase or lactic acid, or held pH neutral. Endogenous enzymes started the breakdown; complete breakdown was reached when yeasts were present, and bacteria played no critical role.

Yeast 40(9), 425–436 · doi:10.1002/yea.3888

Jackels & Jackels, 2005 – Chemical indicators of fermentation. Measured temperature, pH, glucose, ethanol and lactic acid in seven farm-scale batches in Nicaragua. A sharp pH drop marked the end of mucilage fermentation and could help farmers avoid over-fermentation.

Journal of Food Science 70(5) · doi:10.1111/j.1365-2621.2005.tb09960.x

Hu et al., 2025 – Mucilage retention in honey processing. Compared Arabica honey-process treatments that kept different amounts of mucilage. The treatment keeping 75–80% gave the highest relative content of volatile compounds; bacteria were more active at low and fungi at high mucilage levels.

Food Chemistry: X 25, 102251 · doi:10.1016/j.fochx.2025.102251

Orrego et al., 2018 – Mucilage to ethanol. Optimised and scaled up the fermentation of coffee mucilage sugars to ethanol, reaching about 90–94% of the theoretical yield in flasks and 5 L bioreactors.

Energies 11(4), 786 · doi:10.3390/en11040786

Contested
  • Mucilage composition: Cenicafé gives 15% solids, of which 80% are pectic acids and 20% sugars1; compilations cited in a 2018 study give 85–91% water and 6.2–7.4% sugars, or 84.2% water, 4.1% sugar and 0.91% pectin12; a 2022 review gives 91.77% moisture, 6.43% total sugars and 0.81% pectins2 – very different ratios of pectin to sugar. A 2023 review table, compiled from older studies, gives about 35% pectic substances and 50% sugars on a dry-weight basis5, also more sugar than pectin.
  • Structure: Cenicafé describes the mucilage solids as a colloidal hydrogel "without cellular structure"1, while microscopy of Mexican Arabica showed mucilage made of elongated cells attached to the parchment.4
  • How mucilage comes off: histological work in the 1990s found the mucilage pectins apparently intact after fermentation and proposed that sugar consumption causes an osmotic fracture at the base of the mucilage cells4; a 2023 study instead found pectinolytic activity from endogenous enzymes early on and from yeasts later.6
  • Honey grades: a 2016 review gives about 25% mucilage for yellow and about 50% for red honey, with black usually fully covered, and ties the colour to drying speed and shade as well as to the amount kept15; a 2023 review gives white to black as 20% to 80% of the pulp material left on the seed.5 Neither refers to a standard.
Share or cite this entry

Sources

  1. ARoa, G.; Oliveros, C. E.; Álvarez, J.; Sanz, J. R.; Ramírez, C. A.; Dávila, M. T.; Álvarez, J. R.; Zambrano, D. A.; Puerta, G. I.; Rodríguez, N. Ecological Processing of Coffee at the Farm Level: An Ecological and Profitable Case. Cenicafé (Colombian Coffee Research Centre).
  2. BGonzález-Freire, A.; Martínez-Hernández, C. M. (2022). Mejoramiento y uso de los efluentes para el beneficio del café [Improvement and Use of Effluents for the Benefit of Coffee]. Revista Ciencias Técnicas Agropecuarias 31(2).
  3. AAvallone, S.; Guiraud, J.-P.; Guyot, B.; Olguin, E.; Brillouet, J.-M. (2000). Polysaccharide Constituents of Coffee-Bean Mucilage. Journal of Food Science 65(8), 1308–1311. doi:10.1111/j.1365-2621.2000.tb10602.x · other copies
  4. BAvallone, S.; Guyot, B.; Michaux-Ferrière, N.; Guiraud, J.-P.; Olguin Palacios, E.; Brillouet, J.-M. (1999). Cell wall polysaccharides of coffee bean mucilage: histological characterization during fermentation. ASIC, 18th International Scientific Colloquium on Coffee, Helsinki, 463–470.
  5. AElhalis, H.; Cox, J.; Zhao, J. (2023). Coffee fermentation: Expedition from traditional to controlled process and perspectives for industrialization. Applied Food Research 3(1), 100253. doi:10.1016/j.afres.2022.100253 · other copies
  6. AElhalis, H.; Cox, J.; Zhao, J. (2023). Yeasts are essential for mucilage degradation of coffee beans during wet fermentation. Yeast 40(9), 425–436. doi:10.1002/yea.3888 · other copies
  7. ASatapathy, S.; Rout, J. R.; Kerry, R. G.; Thatoi, H.; Sahoo, S. L. (2020). Biochemical Prospects of Various Microbial Pectinase and Pectin: An Approachable Concept in Pharmaceutical Bioprocessing. Frontiers in Nutrition 7, 117. doi:10.3389/fnut.2020.00117 · other copies
  8. AJackels, S. C.; Jackels, C. F. (2005). Characterization of the Coffee Mucilage Fermentation Process Using Chemical Indicators: A Field Study in Nicaragua. Journal of Food Science 70(5). doi:10.1111/j.1365-2621.2005.tb09960.x · other copies
  9. AFukunaga, E. T. (1957). A New Mechanical Coffee Demucilaging Machine. Hawaii Agricultural Experiment Station, University of Hawaii, Bulletin 115.
  10. AEckhardt, S.; Franke, H.; Schwarz, S.; Lachenmeier, D. W. (2022). Risk Assessment of Coffee Cherry (Cascara) Fruit Products for Flour Replacement and Other Alternative Food Uses. Molecules 27(23), 8435. doi:10.3390/molecules27238435 · other copies
  11. AHu, F.; Yu, H.; Fu, X.; Li, Z.; Dong, W.; Li, G.; Li, Y.; Li, Y.; Qu, B.; Bi, X. (2025). Characterization of volatile compounds and microbial diversity of Arabica coffee in honey processing method based on different mucilage retention treatments. Food Chemistry: X 25, 102251. doi:10.1016/j.fochx.2025.102251 · other copies
  12. AOrrego, D.; Zapata-Zapata, A. D.; Kim, D. (2018). Optimization and Scale-Up of Coffee Mucilage Fermentation for Ethanol Production. Energies 11(4), 786. doi:10.3390/en11040786 · other copies
  13. AZhang, S. J.; De Bruyn, F.; Pothakos, V.; Contreras, G. F.; Cai, Z.; Moccand, C.; Weckx, S.; De Vuyst, L. (2019). Influence of various processing parameters on the microbial community dynamics, metabolomic profiles, and cup quality during wet coffee processing. Frontiers in Microbiology 10, 2621. doi:10.3389/fmicb.2019.02621 · other copies
  14. ADe Bruyn, F.; Zhang, S. J.; Pothakos, V.; Torres, J.; Lambot, C.; Moroni, A. V.; Callanan, M.; Sybesma, W.; Weckx, S.; De Vuyst, L. (2017). Exploring the impacts of postharvest processing on the microbiota and metabolite profiles during green coffee bean production. Applied and Environmental Microbiology 83(1), e02398-16. doi:10.1128/AEM.02398-16 · other copies
  15. APoltronieri, P.; Rossi, F. (2016). Challenges in Specialty Coffee Processing and Quality Assurance. Challenges 7(2), 19. doi:10.3390/challe7020019 · other copies

Tier A · authoritative · B · trade and expert · C · general. Supporting quotes for every source are in the data version of this entry. If a publisher’s site refuses access, try the doi link or “other copies”.

Suggest a change

On Mucilage. Tell us what you would change and why. A source that supports it helps most.