Milling, Sorting & Grading

What Happens After Drying

Once coffee leaves the drying beds or mechanical dryers, it is not yet ready for export. Whether processed by the washed method, the natural (dry) method, or a honey or pulped-natural route, dried coffee still carries a protective outer shell—either the hardened parchment of washed lots or the dried fruit husk of naturals—along with residual silver skin. The collective term for removing these layers and preparing the bean for market is dry milling, also called post-harvest processing or simply milling.

Dry milling is typically carried out at a dedicated dry mill (also called a hulling or threshing station), which may be located on a large estate, at a cooperative, or at an exporter's central facility. The sequence of operations generally follows this order:

1. Hulling — mechanical removal of parchment or husk
2. Density / gravity sorting — separation by mass and density
3. Screen grading — separation by physical size
4. Color sorting (optical) — detection and ejection of discolored beans
5. Hand sorting and defect removal — final visual inspection
6. Grading and classification — assignment of a grade designation for trade

Each stage can be bypassed on lower-budget operations or layered with additional passes on high-end specialty lines.

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Hulling: Removing the Parchment and Husk

Hulling (sometimes called threshing for naturals) is the first mechanical operation. Its goal differs slightly depending on the processing method upstream:

• For washed/wet-processed coffee, hulling removes the parchment (endocarp), the papery shell that formed around the bean after the mucilage was washed away. This step is straightforward because the parchment is relatively thin and uniform.
• For natural/dry-processed coffee, hulling must strip away the entire dried fruit—skin, pulp, mucilage, and parchment—in a single operation, requiring more aggressive machinery.
• For honey-processed coffee, the dried mucilage layer varies in thickness, placing the difficulty somewhere between the two.

The most common machine is the disc huller or drum huller, which uses friction and impact to crack and abrade the outer shell without damaging the green bean beneath. Machine settings—gap width, rotor speed, and throughput rate—must be calibrated carefully: too aggressive and the equipment chips or cracks beans (increasing defect counts); too gentle and parchment fragments remain.

After hulling, a polisher (an optional secondary machine using friction rollers) removes the remaining silver skin (silverskin or chaff). Polishing is more common for milds and high-end arabicas intended for particular export markets, though it is not universal and some argue it marginally increases surface oxidation.

At this stage the coffee is referred to as green coffee or green bean—the raw, unroasted seed ready for further sorting.

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Density and Gravity Sorting

Beans that are physically similar in size may still differ dramatically in density. Underdeveloped beans, insect-damaged seeds, and beans affected by drought or disease are typically less dense than healthy, fully developed seeds. Density sorting exploits this difference.

The primary machine is the gravity table (or gravity separator / destoner)—a vibrating, slightly inclined deck with a perforated surface through which air is blown upward. Heavier beans migrate toward the high side of the deck and are discharged separately; lighter or hollow beans travel to the low side. A single pass can resolve a lot into three to five density fractions.

A related but simpler device, the destoner, uses an air column and vibration specifically to remove rocks, soil clods, and metal fragments that share similar screen size with coffee beans but are denser. Destoning typically occurs before or concurrently with gravity separation.

Density is closely associated with altitude: beans grown at higher elevations mature more slowly, accumulate more sugars and complex cell structures, and tend to be denser—which is part of the rationale behind altitude-based grades such as Strictly Hard Bean (SHB) and Hard Bean (HB) used in Central America (discussed further below).

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Screen-Size Grading

After density sorting, coffee passes through a series of vibrating screens (also called sieves) to separate beans by physical size. Screens are perforated metal plates; each hole is sized in units of 1/64 inch. A Screen 18 hole measures 18/64 inch (approximately 7.1 mm); a Screen 15 measures 15/64 inch (~6.0 mm).

Beans are loaded onto the top screen; those too large to fall through are retained and collected as the largest-size fraction. Progressively finer screens below catch progressively smaller beans. Fractions are named by the smallest screen size that retained them.

Why screen size matters:

• Uniform bean size ensures even heat transfer during roasting. A lot with a wide size distribution will have small beans roasting faster than large beans in the same drum, leading to inconsistency in the cup.
• Screen size correlates loosely—but imperfectly—with bean development and altitude, so many grading systems use it as a primary designator.
• Large-screen fractions command price premiums in most producing countries.

Peaberries (single-seed cherries, comprising roughly 10–15% of any harvest according to botanical surveys of Coffea species) are round rather than flat-sided and fall through oblong slots on standard screens. They are collected separately and often sold as a distinct product because their shape causes them to roll uniformly in a roasting drum.

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Optical Color Sorting

Modern dry mills employ optical sorters (also called color sorters or electronic sorters) to detect and remove beans whose color deviates from the expected green. The machine passes a single-layer stream of beans past high-resolution cameras and, in advanced units, near-infrared (NIR) sensors. When a bean is identified as an outlier—black, brown, pale, or bleached—a precisely timed pneumatic air jet ejects it from the stream.

Optical sorters are highly effective at removing:

• Black beans (over-fermented, diseased, or severely damaged)
• Brown and partial-black beans
• Sour/yellow beans (beans that underwent uncontrolled fermentation)
• Immature/quaker-prone beans (pale green or whitish, indicating underdevelopment)

The trade-off is throughput vs. sensitivity: a tightly calibrated sorter running slowly will catch more defects but process fewer kilograms per hour. High-volume commodity mills often run multiple passes at different sensitivity settings.

Color sorting has largely replaced the earlier practice of running all coffee past teams of hand-pickers for initial removal of visually obvious defects, though hand sorting remains the final check for the highest-grade specialty lots.

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Defect Removal and Hand Sorting

Even after mechanical density separation, screen grading, and optical sorting, a final hand-sorting (or table-sorting) pass is standard practice for specialty-grade lots. Workers spread green coffee on illuminated tables and remove any remaining primary defects (full blacks, full sours, pods, large stones, sticks) and secondary defects (partial blacks, partial sours, broken/chipped beans, shells, small stones, insect-damaged beans).

The SCA Green Coffee Defect Handbook (drawn from the widely used SCAA classification framework) provides the reference most specialty buyers and exporters rely on. Under this system:

• Category 1 (Specialty Grade): no primary defects allowed; a maximum of 5 full defects per 350 g sample; maximum 5% above or below screen size; no quakers allowed (zero quakers in the roasted sample).
• Category 2 (Premium Grade): allows up to 8 full defects per 350 g sample.

Defect counts are computed using a defect equivalency table: for example, one full black bean equals one full defect, while five broken/chipped beans together equal one full defect. This weighting reflects the relative impact each defect type has on cup quality.

The physical 350 g sample used for SCA defect counting is distinct from the larger export samples (typically 300–500 g) used in some national systems, so buyers must be careful not to conflate methods when comparing grades.

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How Grading Systems Differ by Country

No single global standard governs green coffee grades. Each major producing country developed its own classification system, often reflecting the physical characteristics most relevant to its dominant cultivars, altitudes, and export markets. The result is a patchwork of overlapping but incompatible nomenclatures.

Kenya: AA, AB, PB, and Beyond

Kenya grades by screen size and applies strict defect limits. The main commercial grades are:

• AA: retained on Screen 18 or larger
• AB: a blend of A (Screen 17) and B (Screen 15–16)
• C: Screen 14 and below
• PB (Peaberry): round beans separated from the above
• T and TT: light density beans separated during grading
• MH/ML: machine-husked/machine-lugged lower grades

Kenya AA is among the most recognised grade designations in the specialty world, primarily because Kenyan arabica—grown on volcanic soils at altitude—tends to be large-screened, dense, and complex in cup. It is important to note, however, that AA designates screen size, not cup quality: a poorly processed AA lot can still cup poorly, and rigorous cup scoring at the Kenya Coffee Auction adds a further quality dimension beyond the physical grade.

Colombia: Supremo and Excelso

Colombia grades washed arabica principally by screen size:

• Supremo: retained on Screen 17 or larger (the top commercial export grade)
• Excelso: a blend passing through Screen 17 but retained on Screen 14; includes EP (Europa) and UGQ (Usual Good Quality) subgrades

Colombia's two annual harvest cycles—main crop (April–June in many regions) and fly crop (November–December)—mean the grading system must accommodate continuous supply. The Federación Nacional de Cafeteros (FNC) oversees standards, though the rise of Colombian specialty micro-lots has created a parallel quality tier that operates above and alongside the traditional grade framework.

Central America: SHB / HB (Altitude-Based)

Guatemala, Honduras, Costa Rica, Mexico, and other Central American origins grade primarily by altitude of cultivation, which correlates with bean density:

• Strictly Hard Bean (SHB): grown above approximately 1,350 m (the exact threshold varies slightly by country)
• Hard Bean (HB): grown between approximately 1,200–1,350 m
• Semi-Hard Bean and lower designations continue down the altitude scale

The logic is that higher altitude → slower maturation → denser bean → more complex sugars and acids → better cup potential. The altitude thresholds are not internationally standardised and differ between Guatemala's Anacafé guidelines and the national standards of Honduras (IHCAFE) or Costa Rica (ICAFE), so buyers should verify which country's scale applies.

Ethiopia: Grade 1–9

Ethiopia classifies green coffee on a numerical scale from Grade 1 (best) to Grade 9 (lowest) based primarily on defect count and cup quality. Grades 1 and 2 are specialty-eligible; Grades 3–4 are specialty/commercial transition; Grades 5 and below are commodity. Ethiopia's system is notable for incorporating cup score into official grading—a hybrid of physical and sensory assessment.

Brazil: Strictly Soft, Soft, Hardish, Hard, Rio, Rioy

Brazil's grading is multidimensional, incorporating defect count (NY 2 through NY 8 on a New York-origin defect scale), cup quality descriptors (Strictly Soft through Rioy), and screen size (often listed alongside). The NY 2 designation (2 defects per 300 g sample) is the benchmark for high-quality Brazilian naturals. Brazil's large-scale strip-harvesting and mechanical drying create more variable lots, making the defect-count dimension particularly important.

"Screen 60" and Other Notations

Some producing countries and exporters use metric screen measurements (in millimetres) rather than the 1/64-inch convention. A Screen 60 in this system refers to 6.0 mm—equivalent to roughly Screen 15 in the 1/64-inch scale. Buyers working across multiple origins must be attentive to which convention is in use, as confusion between the two systems can lead to mismatched lot expectations.

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Green Grading vs. SCA Defect Counts

A common source of confusion in green coffee trade is the difference between a producing country's export grade (e.g., Kenya AA, Colombia Supremo) and an SCA-based defect count performed by an independent grader or roaster.

Export grades are assigned by the producing country's export authority or cooperative. They reflect that country's standards, machines, and inspection protocols, and they are the basis for contracts and pricing at origin.

SCA green grading is an independent, globally standardised assessment performed on a 350 g sample. It counts primary and secondary defects, measures moisture content (the SCA guideline is 10–12% moisture for specialty-grade green coffee), checks screen uniformity, and evaluates the roasted sample for quakers. The SCA system was designed to provide a common language for international specialty trade regardless of origin.

These two systems are complementary but not interchangeable:

• A lot can carry a prestigious export grade (e.g., Kenya AA) and still fail SCA specialty standards if defect counts exceed the threshold or quakers are present.
• Conversely, a small micro-lot from an origin without a formalised grading system (e.g., some emerging origins in Asia or the Pacific) can achieve SCA specialty grade without carrying any traditional grade designation.
• Moisture and water activity assessments, which are part of rigorous green grading, have no equivalent in traditional origin-grade systems.

For roasters and importers sourcing specialty coffee, performing an independent SCA defect count on arrival is considered best practice, as physical changes during shipping (temperature fluctuations, humidity exposure) can alter moisture content and occasionally introduce mold-related defects not present at origin.

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The Role of Milling in Cup Quality

Dry milling is sometimes treated as a purely mechanical, post-quality step—but decisions made on the mill floor directly influence what ends up in the roaster and, ultimately, in the cup.

• Hull damage from poorly calibrated machines increases the number of broken/chipped beans, which roast unevenly and can contribute astringency.
• Residual parchment fragments can scorch in the roaster drum and impart papery or smoky off-notes.
• Insufficient density separation leaves underdeveloped beans in the lot that become quakers on roasting—pale, peanutty, and disruptive to cup consistency.
• Moisture at hulling: hulling coffee that is too moist risks bean damage; hulling that is too dry (below roughly 10% moisture) risks cracking. The ideal moisture window for hulling parchment coffee is generally cited as 11–12%.

Understanding dry milling as an integral part of the quality chain—continuous with coffee processing upstream and directly affecting roasting outcomes downstream—is central to how specialty importers evaluate mill infrastructure when sourcing direct-trade or traceable lots.