Mold doesn't appear just because coffee has been sitting around

Mold is the result of a chain of conditions, in which available water and the storage environment play major roles. For green coffee, the issue can begin at the post-harvest stage if drying is insufficient, if beans get rewetted, or if the warehouse maintains humid conditions for a long time.

The FAO notes that coffee stored in hot, humid conditions can absorb moisture from the air and develop mold, along with musty or damp odors. So “sitting for a long time” is only part of the story; time becomes a risk once it's combined with conditions favorable for fungal growth.

Humidity is a condition that needs continuous monitoring

A batch of coffee can be in suitable condition right after drying but change during storage. If packaging or the warehouse lets beans reabsorb moisture, moisture content and water activity can rise.

Research on green-coffee storage shows temperature and relative humidity fluctuations can change moisture content, water activity, color, and sensory characteristics. Another study on fungi in green coffee stored for a year also found a relationship between fungal population dynamics, water activity, moisture content, and quality indicators.

So mold control can't rely solely on looking at the surface of the beans. Conditions experienced throughout the storage period matter just as much as any visible sign.

Mold can start before coffee even reaches the warehouse

A common mistake is treating the warehouse as the only starting point for the problem. In reality, beans may have already been exposed to conditions favorable for mold during drying or transport. The FAO identifies drying and storage as particularly critical stages for preventing fungal contamination and mycotoxins.

If coffee is dried unevenly, a higher-moisture pocket can become a weak point even if the rest of the batch looks normal. Similarly, rain or condensation during transport can rewet the beans.

So when mold is found, the useful question isn't only “is the warehouse humid?” but also “what conditions did this batch go through before arriving here?”

Mold control starts with prevention

Once signs of mold appear, the goal shouldn't be finding a way to mask the smell or treat the surface so the batch can still be used. The FAO recommends avoiding moldy coffee and emphasizes prevention through proper drying, avoiding rewetting, and storing in well-ventilated warehouses that keep water away.

The FDA also lists mold and fungal decomposition in green coffee among the issues to inspect for. This shows mold control is part of raw-material management — not just handling a sensory flaw.

The right approach is controlling things beforehand: moisture level after drying, sack condition, warehouse conditions, the risk of rewetting, and regular inspection. When these layers are well managed, the likelihood of mold developing drops significantly.

References

• FAO — Arabica Coffee Manual for Lao PDR
• FAO — Ochratoxin A in Coffee
• FDA — MPM V-1. Beverages and Beverage Materials
• Journal of Applied Microbiology — Impact of storage conditions on fungal community composition of green coffee beans (2018)