How temperature & humidity affect coffee during drying — Natural vs Washed
Kenya Coffee School — practical guidance for producers, processors and quality teams
Drying is one of the most critical post-harvest stages: it fixes the bean’s final moisture (target ~10–12%), stops fermentation, and strongly shapes cup quality. Temperature and relative humidity (RH) control the speed and uniformity of drying, the microbial activity on the mucilage/fruit, and therefore defects and final sensory character. Below is a focused, practical comparison with actionable recommendations for Kenyan producers.
Quick headline recommendations
- Aim for final green-bean moisture 10–12% (measure with a moisture meter).
- For sun/solar drying in East Africa expect daytime surface temps ~20–35°C; protect beans from rain and high night RH.
- If using mechanical drying, keep drying air generally ≤45°C (higher temps shorten time but risk flavor loss / splits if not managed).
What temperature does (short version)
- Higher drying air temperature → faster moisture removal. This reduces time that beans spend in the hygroscopic, microbially active zone — good for preventing mould/over-fermentation when weather is poor. But too much heat too fast can produce surface hardening, split beans, baked or muted flavors and physical defects. Controlled mechanical heat (moderate, even flow) can shorten drying without harming quality if airflow and loading are correct.
What humidity does (short version)
- High relative humidity slows drying and increases microbial/fermentation risk. Mucilage and fruit flesh are hygroscopic—on humid nights or rainy days they re-absorb moisture and prolong fermentation pockets, which causes off-flavors, mould, or uneven drying (leading to inconsistent roast). Low RH speeds drying but if very low it can make beans dry too quickly at the surface (case-hardening).
Natural (dry) process — how T & RH matter
Natural-processed cherries are dried with the fruit on the seed, so the bean is in close contact with sugars, yeasts and bacteria for the entire drying time.
Main impacts:
- High humidity / slow drying: prolonged fermentation inside the cherry → heavy, sometimes ‘over-fermented’ fruit notes, risk of mould and sourness; sticky patches that dry irregularly. Frequent turning and thin layers are essential to avoid pockets.
- High temperature (but controlled): can shorten drying time and reduce microbial risk, but naturals are sensitive to heat timing — aggressive heat when the fruit is still wet can cause scorching of the sugar matrix and bitter/caramelized off-notes or split beans.
- Practical measures for naturals: dry in thin (~2–3 cm) layers on raised beds or patios, turn often (several times per day), shade during hottest hours if sun is intense, use mesh/roof to protect from rain, and consider partial mechanical drying only at the end if humidity/rain threatens quality.
Washed (wet) process — how T & RH matter
Washed coffees have the pulp/mucilage removed early; beans are dried with much less fruit residue (sometimes with fermentation tanks prior to washing).
Main impacts:
- Lower risk of sticky pockets and mould because most sugars are removed. This lets you be more aggressive with heat and speed (if done carefully), which shortens total processing time and stabilizes quality in humid windows.
- High humidity still problematic — rewetting and slow drying can still promote off-flavors or prolonged, uncontrolled fermentation (especially if drying floors/beds are thick).
- Practical measures for washed: use slightly thicker layers than naturals is OK, but monitor moisture closely; mechanical drying at moderate temps (e.g., up to ~40–45°C) is often used to finish drying safely when weather is poor.
Sensory outcomes linked to T & RH differences
- Natural (slow, low-temp drying in stable sun): more fruity, winey, heavy body — can be excellent if fermentation is controlled.
- Natural (slow + high RH / intermittent rain): risk of off-fermented, earthy, or musty notes.
- Washed (controlled drying): cleaner, brighter acidity, clearer cup definition; more forgiving of finishing with moderate mechanical heat.
Practical drying protocol — stepwise (Kenya-friendly)
- Harvest & sort (remove underripe/overripe).
- Choose process (natural vs washed) based on cherries, weather window, and target cup.
- Initial sun drying (both): spread on raised beds; keep layer thin for naturals, moderate for washed. Turn frequently (naturals: many turns/day). Protect from rain.
- Monitor moisture with a meter — move from sun to covered/roofed area if nights are humid or when RH rises. Aim for 10–12% final moisture.
- Mechanical finish (if needed): use moderate air temp (≤45°C) and good airflow; avoid overheating—use staged drying (lower temps initially, slightly higher at the end) to reduce splits.
- Resting: after drying, allow coffee to rest (stabilize) in clean, dry storage (RH ~50–60%) before milling/export.
Quick comparison table
| Aspect | Natural | Washed |
|---|---|---|
| Contact with fruit during drying | Yes — whole cherry (high sugar, yeast load) | No/less (pulp/mucilage removed) |
| Vulnerability to high RH | High — sticky pockets, mould risk | Lower but still vulnerable if drying slow |
| Tolerance for mechanical heat | Lower (heat can scorch sugars) | Higher — can be more aggressive if controlled |
| Typical cup result (if well-managed) | Fruity, full body, complex | Clean, bright acidity, clarity |
| Key control measures | Thin layers, very frequent turning, rain protection | Even layers, monitor moisture, controlled finish heat |
| (Principles summarized from practical and scientific literature.) |
Common mistakes and how temperature/humidity cause them
- Case hardening / brittle beans: too-fast surface drying (very high heat) → uneven moisture inside → cracking during milling/roast.
- Mould / sour pockets: prolonged drying under high RH → microbial growth. Happens more in naturals and honey processes.
- Over-fermentation (off flavors): slow drying + warm temps that favour uncontrolled yeast/bacteria activity.
Monitoring & equipment (basic list)
- Moisture meter (target 10–12%).
- Hygrometer to watch RH during nights.
- Raised drying beds or covered patios (keep coffee off the ground).
- Tarpaulins/roof for sudden rain and shading nets for heat control.
Final note from Kenya Coffee School
Temperature and humidity are your two levers during drying: temperature controls speed, humidity controls the ceiling of how quickly moisture can leave the bean and whether microbes will grow. Washed coffees give you more operational flexibility with heat because most sugars are removed early; naturals demand gentler, more attentive sun management (frequent turning, thin layers, rain protection) to achieve consistent, high-quality results. When the weather window is uncertain, consider combining careful sun drying with a short, controlled mechanical finish rather than risking prolonged slow drying in high RH.