Processing

Coffee Fundamentals

Course 1 · Chapter 3 · Processing: From Fresh Fruit to Dry Green Seed

The Layers Around the Seed

A picked coffee cherry is a wet, sugary fruit with a seed inside. Processing is everything done between picking and the dry green coffee that gets shipped to a roaster. Wikipedia's Coffee processing article labels the layers of the fruit, from the outside in:

Layer	Other name	Fate in processing
1. Outer skin	Exocarp	Removed or dried and stripped off
2. Pulp	Mesocarp	Removed or dried and stripped off
3. Mucilage	Pectin layer	Washed or fermented off, or left on to dry
4. Parchment	Hull, endocarp	Removed by hulling
5. Silver skin	Testa	Mostly lost later, in roasting (check); can be polished off
6. The bean	Endosperm	What the roaster wants

The methods differ in how much of the fruit is dried together with the seed.

Dry (Natural) Process

The simplest method: the whole fruit is cleaned and then dried in the sun, on patios or raised trestle beds, and turned by hand or rake. According to the article, drying can take about four weeks to reach a maximum moisture content of 12.5 percent, depending on the weather. The dried fruit is stored in bulk, then hulled, sorted, graded and bagged.

The article says drying "governs the final quality of the green bean more than any other stage". Over-drying makes beans brittle; under-drying invites fungal and bacterial spoilage. It also states that around 90 percent of Brazilian arabica uses this method.

Wet (Washed) Process

Cherries are cleaned and graded (the article lists winnowing, screens, flotation and hydraulic separation), then pulped. The remaining mucilage is removed either by ferment-and-wash or by mechanical demucilaging. Published fermentation times run from about 12 to 36 hours, with millers judging the end point by touch. The beans are then washed in clean water and dried.

Washed coffee leaves the mill at about 53 percent moisture and must be dried to 10 to 12 percent (wet basis). The article warns that sun drying above about 40 °C (parchment) or 45 °C (cherry) damages quality.

On flavour, the article cites the International Trade Centre: washed coffee is the most common of the three main methods and often the highest quality. Fermenting underwater is said to emphasise acidity and aroma and suppress astringency, and mechanically demucilaged beans have sharper acidity while fermented beans are juicier and finer. These are the article's reported claims, not my own tasting (check).

Honey (Pulped Natural) Process

The skin is removed but the mucilage is left on the parchment and the beans dry with it in place, with no washing step. How much mucilage stays gives the colour names:

Name	Mucilage left	Drying time
Yellow honey	Partly removed	About 8 to 10 days
Red honey	Roughly half to three-quarters kept	About 12 to 15 days
Black honey	All kept	Around 30 days, with frequent turning

The article describes the cup as mellow and sweet, with mild acidity and fruit or honey notes, and says the method uses considerably less water than wet processing. "Honey" refers to the sticky mucilage, not to anything added.

Wet-Hulled (Semi-Washed)

In the Indonesian method called giling basah, the parchment is removed while the bean is still wet. Beans are hulled at 40 to 45 percent moisture, sun-dried for 3 to 5 days to 14 to 17 percent, and dried further by exporters to about 13 percent. The article describes low acidity, heavy body and spicy, earthy and fruity notes, and warns that wet storage can allow mould: a 2005 study found ochratoxin A in 5 of 42 samples, below the EU limit.

Comparing the Methods

Method	What is dried	Reported flavour	Moisture target
Dry (natural)	Whole fruit	Not described in the part I read (check)	12.5% maximum
Washed	Seed in parchment, mucilage removed	Often highest quality; acidity and aroma	10 to 12%
Honey	Seed in parchment with mucilage	Mellow, sweet, mild acidity	Not stated in the part I read (check)
Wet-hulled	Seed after early hulling	Low acidity, heavy body, earthy	About 13% at export

After Drying: Hulling, Sorting, Grading

Hulling removes the remaining parchment or dried fruit covering; below 12 percent moisture, chipping and breakage rise sharply. Polishing, which removes silver skin by friction, is optional: the ITC does not recommend it for arabica, and unpolished coffee weighs 0.5 to 0.7 percent more. The beans are then cleaned by air and sorted by size, density and colour. The article says peaberries (chapter 1) are roughly a tenth of a typical crop, which is slightly above the 5 to 10 percent given there for fruits (check).

Most green coffee falls between screens 12 and 19, where screen 16 is 6.30 mm and screen 18 is 7.10 mm. Defects are classed as primary (a significant effect on the cup) or secondary (minor). Under Specialty Coffee Association standards a 350 g sample is graded, and any category-one defect excludes specialty grade.

Water Leaves, the Seed Stays

Drying removes water and nothing else, so the amount of dry matter stays the same. If moisture is quoted on a wet basis (water as a share of total weight), the new weight is dry mass divided by (1 minus the new moisture). The script below applies this to the figures above and was run to get the output shown. It treats everything as if only water is lost, so it ignores the weight of fruit or parchment that is stripped off, and the wet-hulled line is therefore only a rough illustration (check).

def final_mass(start_kg, start_moist, end_moist):
    dry = start_kg * (1 - start_moist)
    return dry / (1 - end_moist)

for label, start, end in [("Washed", 0.53, 0.11), ("Washed", 0.53, 0.12), ("Wet-hulled", 0.425, 0.13)]:
    m = final_mass(100, start, end)
    print(f"{label}: 100 kg at {start:.1%} -> {m:.1f} kg at {end:.0%} (water removed {100 - m:.1f} kg)")

for name, days in [("Yellow honey", (8, 10)), ("Red honey", (12, 15)), ("Black honey", (30, 30))]:
    print(f"{name}: about {days[0]}-{days[1]} days drying" if days[0] != days[1] else f"{name}: about {days[0]} days drying")

# Output
Washed: 100 kg at 53.0% -> 52.8 kg at 11% (water removed 47.2 kg)
Washed: 100 kg at 53.0% -> 53.4 kg at 12% (water removed 46.6 kg)
Wet-hulled: 100 kg at 42.5% -> 66.1 kg at 13% (water removed 33.9 kg)
Yellow honey: about 8-10 days drying
Red honey: about 12-15 days drying
Black honey: about 30 days drying

Reading the output

Drying washed coffee from 53 percent moisture to about 11 percent removes almost half the starting weight. The difference between targeting 11 and 12 percent is only about 0.6 kg per 100 kg, which is why the target matters more for storage safety than for weight.

Not Yet Verified, and Sources

Sources. Wikipedia article Coffee processing, plus the Coffee and Coffea articles from earlier chapters. The processing article is about 115,000 characters; I read the first 100,000, and the unread tail appeared to be navigation text.
Not in the part read. The flavour of the dry method, the moisture target for honey process, and what becomes of the silver skin (all marked check).
My inference. The mass calculation assumes only water is lost, which is a simplification.
Secondary claims. Flavour descriptions are the article's reports of other sources, not tested by me.

Exercises

Exercise 1

Write the six layers of the fruit in order from outside to inside. Then say which layers are still on the seed when it is drying in (a) the dry method and (b) the black honey method.

→ Solution

Exercise 2

Use the script to find how much 250 kg of washed coffee at 53 percent moisture weighs after drying to 10.5 percent. Then work out the weight at 12.5 percent and say what the difference tells you.

→ Solution

Exercise 3

Make a table comparing the four methods on three points: how long the seed spends drying with fruit material on it, how much water the method needs, and the reported flavour. Mark any cell the chapter does not support with "(check)".

→ Solution

Quick Reference

Layers: skin, pulp, mucilage, parchment, silver skin, bean
Dry: whole fruit sun-dried, up to about four weeks, 12.5% maximum moisture
Washed: pulp and mucilage removed, 12 to 36 h fermentation reported, dried to 10 to 12%
Honey: mucilage left on; yellow 8 to 10 days, red 12 to 15, black about 30
Wet-hulled: hulled at 40 to 45% moisture, dried to about 13%
Hulling, sorting, grading come after drying

Coming next

Coffee Fundamentals 4 takes the dry green seed to the roaster.