What is humidity in coffee processing and storage?

Humidity refers to the moisture content of the air - specifically the relative humidity (RH), expressed as a percentage of the maximum moisture the air can hold at a given temperature. In coffee, humidity is a critical environmental variable at two stages: during drying (where it affects how quickly and evenly cherry or parchment loses moisture) and during green coffee storage (where it determines whether stored beans absorb moisture and deteriorate).

During drying, high ambient humidity slows the rate at which coffee loses moisture, extending drying time and increasing the risk of mould or over-fermentation if not carefully managed. Low humidity accelerates drying but can cause case hardening - where the outer surface of the bean dries too rapidly, trapping moisture inside and creating an uneven moisture profile. Black honey and natural processing require the deliberate management of humidity and shade to control drying speed.

In green coffee storage, relative humidity above 60-65% creates conditions where beans begin absorbing ambient moisture, raising their own moisture content towards the critical threshold above which mould can develop. Hermetic storage (GrainPro bags, vacuum-sealed containers) and climate-controlled warehouses protect against humidity fluctuation. For roasters storing green coffee, keeping storage below 60% RH and away from moisture sources significantly extends freshness and quality life.