Green coffee beans aren't always made up purely of intact coffee beans. On the journey from harvest to processing, drying, transport, and storage, many kinds of unwanted material can travel along with the raw-material batch. The FAO notes that green coffee can contain soil, dust, small stones, husk fragments, pulp, twigs, and leftover dried fruit parts.

This shows that foreign-matter control needs to start with the raw material — it can't wait until the roasting facility to be addressed.

From the field to the collection point

Right at the point of harvest, soil, small stones, twigs, or other foreign material can get mixed in if cherries are gathered together with debris on the ground. Harvesting tools, containers, and collection areas can also become sources of foreign matter if left uncontrolled.

A simple principle is to avoid unnecessary contact between the raw material and surfaces or objects capable of introducing contamination.

During processing

Processing is a stage involving a lot of equipment and mechanical action. Improper adjustments can increase the amount of husk, pulp fragments, or other unwanted material remaining in the beans.

The FAO describes several common types of foreign matter found in coffee after processing, including soil and dust from drying, small stones from harvest or drying on bare ground, pulp from poor depulping, parchment fragments, and twigs.

The drying stage

The drying patio, the drying surface, and the surrounding environment directly affect the risk of unwanted material mixing in. Wind can carry dust or light material onto the layer of drying coffee. If coffee is dried on an unsuitable surface, the risk of soil and small stones also increases.

Transport and containers

Foreign matter doesn't necessarily originate at the farm. Old sacks, damaged pallets, ties, wood fragments, or torn packaging material can become sources of contamination during transport. The FDA also notes that foreign matter can enter food from raw material or arise during transport, processing, and distribution.

What role do cleaning and sorting play?

After receiving raw material, a business can use appropriate cleaning, sieving, separating, or sorting steps to reduce foreign matter. But cleaning equipment shouldn't be treated as a reason to accept poorly controlled incoming raw material.

Every step has its own limits. A good system tries to prevent foreign matter from appearing in the first place, reduce it through suitable processing, and recheck at the points where it's needed.

Seeing foreign matter as a signal about the system

A stone in a batch of coffee isn't just an object to be picked out. It raises a question: which stage did that stone come from? Harvest, the drying patio, the sacks, or transport? If the stone is simply removed without finding the cause, the problem can recur in the next batch.

Conclusion

Foreign matter can enter green coffee very early on: from harvest, collection, processing, and drying through to containers, transport, and storage. So a clean-coffee program needs to view the raw material as a continuous chain.