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Wet-hulled (giling basah)

Used in northern Sumatra: coffee is pulped, fermented and washed, then has its parchment hulled off while still wet, so that the bare green bean is what gets dried.

In short

In almost every other process, coffee is dried before its parchment is hulled off. In North Sumatra and Aceh the order is reversed: arabica is pulped, fermented and washed, then hulled while it is still wet, and the bare green bean is dried afterwards. The trade usually calls the method giling basah, a name the study described here does not use.

The fullest description read is a study carried out for the Food and Agriculture Organization in the 2004/05 season, which also ran the one trial found that tests the method's supposed effect on taste. This page follows it. It is one study, twenty years old.

The sequence, with the study's moisture figures

Sourced

  1. Pulp, ferment and wash on the farm

    As in the washed process. The farmer does not dry the coffee.

  2. Sell it wet

    The study gives about 50 per cent moisture for parchment passing from farmer to trader. It may be traded and stored several times before it is hulled.

  3. Hull it wet

    The parchment is stripped at 35 to 45 per cent moisture on the study's main description, and 40 to 50 per cent in its statement of rationale. A huller built for dry parchment of 11 to 14 per cent is modified to do it.

  4. Dry the bare bean

    The green bean, with no parchment round it, is dried in the sun.

  5. Hold part-dried, then finishnot always done

    Traders commonly store the hulled bean at about 17 per cent moisture for up to three weeks before an exporter dries it to 12 per cent.

The figures are the study's, for North Sumatra in 2004/05, and describe what it found traders doing. They are not targets, and the study itself raises two of these stages as possible sources of mould.

The machine

Sourced

Wet parchment is soft, and a huller made for dry coffee would crush it. The study describes the adaptation: a screw huller with its steel hulling bar replaced by a rubber strip, its motor raised from 15 to 25 horsepower and its speed from 600 to 750 revolutions a minute, putting through around 800 kilograms of wet green bean an hour.

The risk the study was set up to examine

Sourced

The study was part of a project on mould in coffee, and its interest in the method was the time coffee spends wet. It names two points for investigation: the delays between pulping and hulling, while wet parchment is traded and stored, and the weeks that hulled beans are held at 17 per cent moisture. General guidance on drying explains the concern. Parchment gives the seed some protection while it is wet, and beans stripped of it go mouldy quickly; and the level usually recommended for stored coffee is about 12 per cent, well below 17.

CoffeeHQ's entry on Indonesia sets out what the study's trial found about mould and ochratoxin A in the lots it stored wet.

Does it make the taste?

Accounts differ

North Sumatran arabica sold as Mandheling has a reputation that the study summarises as low acidity and complex body. The study thought it likely that the long trading chain and wet hulling play a key part in it, alongside variety, growing conditions and farm management. It then tested that, sending 42 samples from six sources blind to four companies experienced in Sumatran coffee. Each source was processed seven ways: six varied how long the wet parchment and the part-dried bean were held, and the seventh, a control, was dried in its parchment and never wet hulled.

The trial could not reproduce the character consistently from any treatment or any source, and the tasters remarked that the samples in general were too clean in the cup to be normal Mandheling. Three of the four companies picked the same sample as the best example, one that had been hulled a day after washing and dried straight to 12 per cent. Some tasters rated the control samples, which were not wet hulled, as cleaner and more acidic; others found no difference. The study's conclusion is hedged at each step: storing parchment wet for one or two weeks, or holding the beans part-dried, appears not to be the primary factor; hulling quickly after washing appears to give the best chance of the character and to reduce the risk of musty and sour flavours; and wet hulling would appear to be an integral part of the process, by a mechanism the study could not identify. Variety, growing conditions and farm management were not controlled.

So the honest position is that the link between this method and a taste is what the trade believes and what one trial failed to pin down. It is not established, and this page does not assert it.

What remains unestablished about wet hulling

CoffeeHQ explanation

Why the method developed. The usual explanation is a climate too humid to dry coffee in its parchment. The study does not say so, and no document read does.

That wet hulling produces a particular taste. One trial tried to show it and could not.

How much Indonesian coffee is wet hulled, whether the practice has changed since 2005, or how the method is carried out anywhere other than the part of Sumatra the study covered.

What to do next

  • Indonesia — The trial in full, including what it found about mould, and where on Sumatra the method is used.
  • Drying coffee — Why time spent between wet and dry is the risk, and what moisture stored coffee is brought to.
  • Washed process — The conventional order, in which the parchment stays on until the coffee is dry.
  • Become a coffee expert — A structured programme from the plant and its processing to roasting, water and tasting.

Steps

  1. Pulping, fermentation and washing
  2. Hulling while the coffee is still wet
  3. Drying the bare bean

Where this method is documented in the atlas

Only countries CoffeeHQ has written an entry for, and only where a document it has read says so. Each line is what that document says. It is not a share of the crop, and this is not a list of everywhere the method is used.

described as present-day practice

  • Indonesia. Northern Sumatran arabica is wet hulled: a study carried out for the Food and Agriculture Organization sets out how parchment is traded wet, hulled while still damp and dried as a bare bean. Source: Storage of wet Arabica parchment prior to wet hulling, including the storage of dry green bean coffee: 'Origins of Mandheling Coffee Character' (Annex E.6).

Related flavour attributes

Learn it as a course

The Coffee Expert lesson Processing covers this ground in order, with exercises you can do at home.