Green Coffee Grades Explained

What a grade actually describes

Grading happens at the dry mill, after hulling. The coffee passes through a sequence of separations, and the grade names the fraction that comes out of a particular step. Three physical properties do almost all of the work.

1. 1Size. Beans are passed over perforated screens whose hole diameters are measured in 64ths of an inch. A parcel "retained on screen 18" is one whose beans will not fall through an 18/64 inch hole. This produces AA, A, AB, B and C in Kenya and Tanzania, and the numbered screens used for Ugandan Robusta.
2. 2Density. A gravity table or pneumatic separator lifts lighter beans out of a size fraction. The lighter stream becomes its own grade — TT in Kenya and Tanzania — while the heavy stream stays in the main run.
3. 3Shape. Peaberries are round rather than flat-faced and are separated on slotted screens, becoming PB. This is a shape separation and says nothing about quality.

Defect counting sits alongside these mechanical separations. A defined sample — commonly 300 g of green coffee — is picked over by hand, and each physical defect is weighted before being converted into "full defects". Several partial black beans make one full defect; one full black bean makes one on its own. Origins differ in their weighting tables, which is one reason defect counts are not directly comparable across countries.

What a grade does not describe

This matters commercially. Buyers who specify "Kenya AA" and nothing else are specifying bean size and paying the premium that the AA designation attracts, without having specified the thing they actually want. In many Kenyan offer lists the AB from the same factory is the better purchase, because the price differential between AA and AB is larger than the cup difference.

The practical rule: use the grade to define the physical deliverable, and use the approved sample to define the cup. Both belong in the contract.

The five grading systems, side by side

Preparation: the term that sits underneath the grade

"Preparation" describes how thoroughly a parcel has been cleaned and sorted, independent of its size class. European Preparation and American Preparation are the two conventions most often cited, and they set different defect ceilings. In practice East African contracts more often state the defect tolerance explicitly than rely on the shorthand, which is the safer approach — the shorthand means slightly different things to different mills.

• State the sample size the defect count is drawn on (300 g is the usual basis).
• State whether the count is of full defects or of physical pieces — they are not the same number.
• State whether a colour-sorter pass is included, and how many passes.
• State the maximum foreign matter, and whether stones are counted separately.

Writing a specification that works

A specification that relies on a grade name alone leaves too much undefined. A workable East African green coffee specification names at least the following, and each one is a lever you can move independently.