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Post-harvest processing · Step 2

Pulping

The first step of wet processing – a machine squeezes the seeds out of the fresh cherry and strips off the skin and pulp, leaving the seeds in their mucilage and parchment.

Across the chain: Sustainability · Equipment

Source-checkedby Coffearium verification pass· Last source-checked 8 Oct 2026· Prepared with AI assistance· 16 sources (14 tier A)· 4 contested· Updated 8 Oct 2026
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Contents
  • Pulping removes the skin and pulp of the fresh cherry mechanically; the seeds leave the machine still covered in mucilage and parchment.1,2
  • Drum and disc pulpers press the cherry against a fixed plate; the seeds are squeezed out and the pulp is torn away.5,6,8
  • The gap must be set to the size of the cherry – a badly adjusted pulper nips beans, which count as defects and have been linked to stinkers.3,6,13
  • A conventional Colombian wet mill uses 40–50 L of water per kg of dry parchment, about half of it for pulping and for carrying the pulp and the pulped coffee in water; pulping without water removes the use at the pulping stage.11
  • Pulp is about 44% of the fresh cherry’s weight in Cenicafé’s figures and the largest source of pollution from a wet mill.11

What pulping removes#

Pulping is the mechanical step that opens the wet process. It strips the skin and pulp from the fresh cherry.1 The pulp leaves as a by-product; the seeds leave as the main product, still wrapped in parchment.1 A layer of mucilage stays on them: in Cenicafé’s figures it makes up about 22% of the weight of pulped coffee.2 What happens to that mucilage – fermentation, a machine, or drying it on the seed – decides the process that follows.

Pulping should happen soon after picking. Delay allows uncontrolled fermentation and affects the final flavour.5 Cenicafé advises not leaving harvested cherry more than 6 hours unpulped;16 a Kenyan field report sets a limit of 10 hours from harvest;3 an FAO manual for Lao PDR asks for same-day processing, without mixing in older cherries.13

How a pulper works#

All common pulpers do the same thing: they squeeze and tear.5 The cherry is pressed between a moving, roughened surface and a fixed plate. The skin splits, the seeds pop out, and the pulp is dragged away.5,8

  • Drum pulper – a rotating cylinder with flutes turns inside a fixed plate that carries pulping channels and ribs.6 In the Colombian design the cherry is compressed in the space between the breast plate (pechero) and the teeth of the drum sleeve (camisa); the pulp is carried off through the space between the teeth and the rib (vena) of the breast plate.8 One small drum pulper, built in Benin and tested on Arabica from Rwanda and Robusta from Benin, used a roughened sleeve with punched bulbs 20 mm long, 6 mm wide and 5 mm high.7
  • Disc pulper – one or more roughened discs turn against fixed surfaces; the pulp is rubbed off by the disc bulbs and chop rails.6 The tested prototype used two roughened discs 300 mm across, one fixed and one rotating.7
  • Pulpers that work without water – Cenicafé found that several commercial horizontal-cylinder pulpers work without added water, fed from a dry hopper, with little difference in capacity.9,2 Its ecological-processing work reports the same for horizontal- and vertical-shaft pulpers.2
A: Drum pulper1324657B: Disc pulper891011

A: Drum pulper

  1. Hopper.
  2. Breast plate (pechero). A fixed plate with channels and a rib.6,8
  3. Cherry. Squeezed in the gap; the skin splits.8
  4. Gap. Narrows gradually; adjustable, the key setting.6
  5. Drum. Sleeve with teeth or bulbs (camisa).7
  6. Seeds out. Seeds in parchment, with their mucilage.
  7. Pulp out. Carried round by the teeth.8

B: Disc pulper

  1. Cherries between the discs. The pulp is rubbed off between bulbs and chop rails.6
  2. Fixed disc with chop rails. Drawn dashed: its rails face the rotating disc.6
  3. Rotating disc with bulbs. 6,7
  4. Casing. Pulp and seeds leave by separate outlets.
A drum pulper (A) and a disc pulper (B). Both squeeze the fruit between a moving rough surface and a fixed one.5 Schematic, not to scale.

The gap between the moving surface and the fixed plate is the main setting. A Philippine national standard requires it to be adjustable and to narrow gradually along the path of the cherry.6 Classifying the cherries by size before pulping, and setting the pulper to that size, reduces damage to the beans.5 Cenicafé’s newer design sets the breast-plate gap with steel feeler gauges.8

The pulping line#

A pulper rarely works alone. A typical East African wet mill shows the order: receiving tank, siphon, pulper, separating screen, repasser (a second pulper), then fermentation tanks.4

  • Before the pulper – cherries are washed and sorted to remove twigs, leaves and other foreign matter.13 Green immature cherries and dried fruits can be removed in a water separator.1 In the East African layout, the siphon sits between the receiving tank and the pulper.4
  • After the pulper – a separating screen follows, then a repasser, which is a second pulper.4 The Codex code asks each mill to decide what proportion of unpulped cherries and nipped beans it accepts.1
  • Green-cherry separation – where machines harvest the crop, mills can use a classifier-type pulper that separates hard, immature cherries from ripe ones.15

Water and pulp#

Conventional Colombian mills use 40–50 litres of water per kilogram of dry parchment coffee; about half of it goes on pulping, carrying the pulp to pits in water and carrying the pulped coffee to the fermentation tanks.11 An earlier Cenicafé report gives about 40 litres per kilogram for the whole conventional process.2 Pulping without water removes the use at this stage entirely, and the pulp can be moved to covered pits without water.11 In the Becolsub module, which combines dry pulping with mechanical demucilaging, the target for the whole process is 0.7–1.0 litres per kilogram.10

05101520253035404550L/kg dry parchmentDry pulping — pulpingstage0 L/kg at this stage11 Conventional pulping +hydraulic transport(half of whole process)about half of 40–50 L/kg11 Becolsub module — wholeprocess0.7–1.0 L/kg10 Conventional wet process —whole process40–50 L/kg11
Data table
L/kg dry parchmentSource
Dry pulping — pulping stage0 L/kg at this stage11
Conventional pulping + hydraulic transport (half of whole process)about half of 40–50 L/kg11
Becolsub module — whole process0.7–1.0 L/kg10
Conventional wet process — whole process40–50 L/kg11
Water use in litres per kilogram of dry parchment coffee, for the pulping stage and for whole processes, as reported by Cenicafé. The conventional pulping-stage bar is half of the 40–50 L/kg whole-process range and also includes carrying the pulped coffee to the tanks.10,11

Pulp is the largest by-product. Cenicafé puts it at about 43.58% of the fresh fruit’s weight, or on average 2.25 tonnes of fresh pulp per hectare per year.11 It carries 73.7% of the mill’s potential pollution; the mucilage carries the other 26.3%.11 Composted, the pulp is transformed in about four months; vermicomposting yields 35–40% (wet basis).11 Other uses include animal feed, biogas and cascara drinks.14

Mechanics of skin rupture#

Pulping works because ripe fruit gives way easily and the seed does not. In a compression model of Arabica fruit, built on fruit properties taken from earlier studies, ripe cherries deformed most (mean 2.20 mm/mm) and needed the least strain energy (0.07 mJ) and stress (0.25 MPa).12 Immature fruit needed about five times the strain energy (0.34 mJ) and stress (1.25 MPa).12 The authors conclude that the riper the fruit, the more it deforms and the less energy and stress it takes.12

In the machine, the cherry enters the narrowing gap, the skin splits under compression, and the seeds are squeezed out while the teeth drag the torn pulp through the space between the teeth and the breast-plate rib.8 The mucilage helps: it lets the seeds slip through the pulper grooves. Seeds without mucilage may be nicked, scratched or chipped by the rough drum.15

The seed itself tolerates only a small squeeze. Cenicafé reports that seeds deformed by more than 1 mm suffer structural damage that can reduce their commercial value.8 The gap therefore has to be wide enough for the seeds and narrow enough to split the skin – which is why cherry size matters.5,6

Unripe cherries and pulper performance#

Unripe cherries resist compression,12 so they tend to pass through unpulped or to be damaged. Classifier pulpers use this: they separate hard, immature cherries from ripe ones.15 In Kenya, the peak season makes hand sorting before pulping impossible, so all coffee is pulped as it arrives.3

Fruit type matters too. In one trial, the same small drum pulper reached about 98% efficiency on Arabica but 85% on Robusta; the disc pulper did better on Robusta (about 88%) than on Arabica (about 76%).7 The test used a smaller gap for Robusta (12 mm) than for Arabica (16 mm).7

Reference values#

The table brings together values reported in the sources. Machine settings are specific to the machine tested; they are a guide to the range, not a recipe.

Parameter Value reported Source
Time from harvest to pulping Within 6 h (Cenicafé); within 10 h (Kenya); same day (Lao PDR) Cenicafé16, FAO3,13
Gap setting Adjustable, narrowing gradually PNS/PAES 2526
Gap in one prototype test (1,200 rpm) 16 mm Arabica, 12 mm Robusta Bizimungu et al.7
Seed deformation limit Damage above about 1 mm Cenicafé8
Minimum pulping efficiency 95% PNS/PAES 2526
Maximum mechanically damaged parchment 3.5% PNS/PAES 2526
Maximum unpulped cherries 5% PNS/PAES 2526
Cenicafé–Ingesec 300 pulper 300 kg cherry/h; 232 rpm; drum 14 cm × 22 cm Cenicafé8
Cenicafé–Ingesec 300, criteria for correct working Pulp in the grain < 2%; damage < 1%; unpulped < 1% Cenicafé8
Small drum pulper (Arabica) 872 kg/h; 98.4% efficiency; 0.91% damage Bizimungu et al.7
Small disc pulper (Arabica) 684 kg/h; 75.6% efficiency; 0.15% damage Bizimungu et al.7
Medium-sized machine About 3,000 lb (about 1,360 kg) cherry/h World Bank5
Water, conventional pulping + hydraulic transport of pulp and pulped coffee About half of the whole-process use Cenicafé11
Water, whole conventional process 40–50 L/kg dry parchment; about 40 L/kg in the earlier report Cenicafé11,2
Water, dry pulping None at the pulping stage Cenicafé9,11
Water, Becolsub (whole process) 0.7–1.0 L/kg dry parchment Cenicafé10
Water, pulper/demucilager units About 0.5 L/kg fresh cherry FAO13
Pulp share About 43.58% of fresh fruit weight (wet basis) Cenicafé11
Pulp’s share of pollution 72%; 73.7% Cenicafé2,11
Pulp composting About 4 months; vermicompost yield 35–40% (wb) Cenicafé11
1Receiving tank2Siphonfloaters out3Pulpermotor-driven4Separating screen5Fermentation tanks6Repassersecond pulper;rejoins the streamPulpby-productcoffee streampulp
Order of equipment in a wet-mill pulping line, from receiving tank to fermentation tanks.4 The siphon tank classifies cherries by density;2 the pulp leaves as a by-product.1

Risks and defects#

  • Nipped and broken beans – poor adjustment of the pulper causes broken or nipped beans, which count as defects and lower quality.3 The FAO manual for Lao PDR lists nipped beans from poor or poorly adjusted equipment as a cause of stinker beans.13 Physical damage makes coffee more susceptible to contamination and deterioration.1
  • Unpulped cherries and pulp in the parchment – these carry fruit into the fermentation tanks; Cenicafé counts its pulper as working correctly with under 2% pulp in the grain and under 1% unpulped fruit.8
  • Recycled water – mills that recycle pulping water must keep it free of outside contaminants; the gradual build-up of heat and enzymes in it speeds up the fermentation that follows.5
  • Delays at peak season – frequent breakdowns and congestion at pulping stations delay pulping.3
  • Pollution – the pulp from one kilogram of cherry, with its mucilage, can use up the dissolved oxygen of 7.4 m³ of clean water.2

Measurement and maintenance#

Measuring performance. The Philippine standard defines three figures by weight, in per cent: pulping efficiency (parchment collected at all outlets against cherry fed in), unpulped loss (unpulped cherry against cherry fed in) and mechanically damaged parchment (against the weight of a sample).6 The Codex code suggests mills monitor the quantity of broken, nipped and naked beans and the water used.1

Maintenance. Before each season, processing equipment should be cleaned, reassembled, lubricated and checked.1 The Hawaiʻi guide asks for every operating part to be adjusted to avoid bean damage, and for the pulper to be cleaned and overhauled every year.15 Cenicafé built its newer drum teeth in reinforced nylon; they can take a radial force 24.8 times that of copper and 5.5 times that of stainless steel.8

Open questions#

  • Nipped beans and stinkers. The link appears in a practical manual,13 but no study opened for this entry measures how often a nipped bean becomes a stinker, or through which microbial route.
  • Gap versus ripeness. The mechanical data on fruit ripeness12 have not been tied, in the sources opened here, to pulper damage and unpulped rates at different gap settings.
  • Pulp share. Published shares of pulp differ and do not always state the basis (fresh or dry weight).11,14

Key studies#

Bizimungu et al., 2024 – Small drum and disc pulpers. Built and tested small drum and disc pulpers for Arabica (from Rwanda) and Robusta (from Benin). The drum pulper was more efficient on Arabica and left fewer unpulped Arabica cherries; damage stayed below 1% for both machines.

Journal of the Saudi Society of Agricultural Sciences 23, 404–414 · doi:10.1016/j.jssas.2024.04.001

Pereira et al., 2023 – Compression of coffee fruit. Modelled ripe, semi-ripe and immature cherries under compression. Ripe fruit deformed more at lower stress and energy – the mechanical reason unripe fruit is harder to pulp.

AgriEngineering 5(4), 2303–2313 · doi:10.3390/agriengineering5040141

Oliveros, Moya & Ramírez, 2001 – A new pulper (Cenicafé). Describes a small horizontal-drum pulper with nylon teeth and a gauge-set breast plate, rated at 300 kg of cherry per hour; it counts as working correctly with under 1% damage and under 1% unpulped fruit.

Avances Técnicos Cenicafé 294 · PDF

Álvarez, 1991 – Pulping without water (Cenicafé). Compared pulpers run with and without water and showed that commercial horizontal-cylinder machines can pulp dry. This finding underpins Colombia’s ecological wet milling.

Avances Técnicos Cenicafé 164 · Cenicafé repository

Contested
  • What pulping removes: the Codex code of practice defines pulping as removing the pulp and "as much as possible of the mucilage"1, while Cenicafé treats the mucilage as a layer that stays on the pulped seed (about 22% of the weight of pulped coffee) and is removed in a later step.2
  • How soon after picking: Cenicafé advises not to leave harvested cherry more than 6 hours without pulping and processing16; a Kenyan field report sets a limit of 10 hours3; the FAO manual for Lao PDR asks for processing on the day of harvest.13 A Hawaiʻi growers' guide (Tier B copy) allows a longer delay.15
  • Pulp share of the cherry: Cenicafé gives about 43.58% of the fresh fruit’s weight (wet basis)11; a 2019 by-products review gives a lower share of the cherry without stating the basis.14 The difference may partly be one of fresh versus dry weight (see coffee cherry anatomy for the dry-matter share).
  • Pulp’s share of the pollution load: 72% in the Cenicafé ecological-processing report2, 73.7% in the 2013 Cenicafé manual.11 The two are close; the second is the more recent figure.
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Sources

  1. ACodex Alimentarius Commission (FAO/WHO) (2009). Code of Practice for the Prevention and Reduction of Ochratoxin A Contamination in Coffee (CAC/RCP 69-2009). Codex Alimentarius.
  2. 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).
  3. AMutua, J. (2000). Post Harvest Handling and Processing of Coffee in African Countries, 3.0 Post Harvest Handling and Processing of Green Coffee in African Countries. FAO.
  4. AMutua, J. (2000). Post Harvest Handling and Processing of Coffee in African Countries, Annex 10: Pictures. FAO.
  5. AWorld Bank, Finance and Agro-Industry Unit (1985). Agro-Industry Profiles: Coffee. World Bank.
  6. ABureau of Product Standards (Philippines) (2011). Agricultural Machinery – Coffee Pulper – Specifications (PNS/PAES 252:2011). Philippine National Standard / AMTEC, University of the Philippines Los Baños.
  7. ABizimungu, G.; Ahouansou, R. H.; Semassou, G. C. (2024). Design, fabrication and evaluation of small-scale disc and drum pulpers for Arabica (Coffea arabica L.) and Robusta (Coffea canephora L.) coffee. Journal of the Saudi Society of Agricultural Sciences 23, 404–414. doi:10.1016/j.jssas.2024.04.001 · other copies
  8. AOliveros T., C. E.; Moya M., N.; Ramírez G., C. A. (2001). Nueva despulpadora para una caficultura competitiva (Avances Técnicos 294). Cenicafé. doi:10.38141/10779/0294 · other copies
  9. AÁlvarez G., J. (1991). Despulpado de café sin agua (Avances Técnicos 164). Cenicafé.
  10. ASanz Uribe, J. R.; Oliveros Tascón, C. E.; Ramírez Gómez, C. A.; López Posada, U.; Velásquez Henao, J. (2011). Controle los flujos de café y agua en el módulo Becolsub (Avances Técnicos 405). Cenicafé.
  11. ARodríguez Valencia, N.; Zambrano Franco, D. A.; Ramírez Gómez, C. A. (2013). Manejo y disposición de los subproductos y de las aguas residuales del beneficio del café. Cenicafé — Manual del cafetero colombiano, Vol. 3.
  12. APereira, J. S.; Magalhães, R. R.; Santos, F. L.; de Andrade, E. T.; Marques, L. S. (2023). Modeling of Coffee Fruit: An Approach to Simulate the Effects of Compression. AgriEngineering 5(4), 2303–2313. doi:10.3390/agriengineering5040141 · other copies
  13. AWinston, E.; Op de Laak, J.; Marsh, T.; Lempke, H.; Chapman, K. (2005). Arabica coffee manual for Lao-PDR. FAO Regional Office for Asia and the Pacific, Bangkok.
  14. BTorres-Valenzuela, L. S.; Serna-Jiménez, J. A.; Martínez, K. (2019). Coffee By-Products: Nowadays and Perspectives. Coffee – Production and Research, IntechOpen. doi:10.5772/intechopen.89508 · other copies
  15. BCollege of Tropical Agriculture and Human Resources, University of Hawaiʻi Processing (Hawaiʻi coffee growers' guide). CTAHR, University of Hawaiʻi (copy hosted by Sweet Maria's Coffee Library).
  16. APuerta-Quintero, G. I. (2001). Cómo garantizar la buena calidad de la bebida del café y evitar los defectos (Avances Técnicos 284). Cenicafé.

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”.

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