Green coffee language for home drinkers

Moisture content sets the storage clock

Green coffee is traded at a moisture content of about 10–12%, read as oven loss at 105 °C; these are figures used across the trade rather than clauses of a published standard. Below the band the bean is brittle and browns too fast at the surface; above roughly 12.5% the risk of mould during storage rises sharply.

A companion figure is water activity, which measures how much of that water is available to microorganisms and is kept below about 0.6 for safe storage. Two lots can report the same moisture percentage and still differ in water activity, depending on how tightly the water is bound inside the bean.

• Moisture content — around 10–12% at the point of sale, reported as oven loss at 105 °C.
• Water activity — free water available for microbial growth, held below roughly 0.6 in storage.
• Sampling — moisture migrates between beans in a bag, so lots are tested at several points.
• Consequence — this figure sets both the storage window and the early part of the roast.

Screen size and density: the physical grade

Green coffee is sorted for size across slotted screens graded in 64ths of an inch. A screen 15 retains beans of about 5.95 mm, a screen 18 retains about 7.14 mm, and lots are named for the screen they sit above: "screen 17/18" means most of the coffee stayed on the 17 and 18 screens. Size sorting is not cosmetic. Beans of similar size heat at similar rates, so a uniform lot develops as one roast instead of two.

• Screen 15 — an opening of 15/64 inch, about 5.95 mm.
• Screen 18 — an opening of 18/64 inch, about 7.14 mm.
• Sorting purpose — uniform size gives a batch that browns on a single schedule.
• Screen names on labels — the numbers describe retention on a screen, not a quality rank by themselves.

Density is the second physical criterion. Green coffee bulk density commonly falls between 0.6 and 0.75 g/mL, and denser beans usually come from higher altitudes and slower maturation, where the seed develops a tighter structure. Density is measured by filling a fixed volume and weighing it, which makes it simple to compare two samples on the same day.

Defect counting: how a 300 g sample becomes a grade

Green grading converts a pile of beans into a number by counting defects in a fixed sample, traditionally 300 g, and converting smaller faults into full-defect equivalents. In the classification used across the specialty trade, specialty coffee is defined as zero Category 1 defects and no more than five Category 2 full defects in that sample.

The categories exist because faults differ in what they do to a roast. Category 1 covers defects that ruin a batch or damage equipment: full black and full sour beans, pods and cherries, large stones and sticks. Category 2 covers partial versions of those faults and structural damage such as broken, chipped, insect-damaged and shelled beans, which are counted in groups — several broken beans make up one full defect instead of counting individually.

• Sample size — 300 g of green coffee, drawn and counted under the classification rules.
• Specialty threshold — zero Category 1 defects and at most five Category 2 full defects per 300 g.
• Category 1 defects — full black, full sour, pod or cherry, large stone and comparable faults.
• Category 2 defects — partial black or sour, broken, chipped, insect-damaged and shelled beans, counted by equivalence groups.
• National grades — several producing countries publish their own defect scales, and Ethiopian export grades 1 and 2 are defined by low counts within the same 300 g sample.

Processing words that arrive with a green sample

A green sample reaches a buyer with a processing name attached, and each name describes a step that changed moisture and chemistry before the coffee was ever roasted.

• Washed — fruit and mucilage are stripped before drying; fermentation runs about 12–36 hours, and washed lots are usually the most uniform in green colour.
• Natural — the cherry dries whole, so the seed spends one to three weeks in contact with fruit sugars.
• Honey — part of the mucilage stays on the parchment through drying, and the colour names describe how much remains.
• Sealed fermentation — the cherry or seed ferments in a closed tank, with temperature and duration set by the producer rather than by the weather.

All routes end at the same target: a bean dried back into the 10–12% band so it can be stored and shipped without spoiling.

What these figures predict before roasting

The four numbers do practical work once they are on a screen. Denser, larger beans accept more energy at the start of a roast and can carry a faster rise in bean temperature; a lower-density lot from the same origin browns sooner and reaches first crack earlier. High moisture lengthens the drying phase and delays first crack at a fixed gas setting. Poor screen uniformity shows up as a roast with pale and dark beans in the same batch. Defect counts predict what a buyer will find in the cup before any roasting is attempted.

Bean composition also tracks where the coffee grew and how it was processed, so the physical figures and the flavour notes on a roasted bag are related rather than opposed. A screen size and a moisture reading describe the material; a tasting note describes one outcome of roasting it.