Mechanical dryers

Reference values#

The table collects the operating values reported in the sources. They describe practice and trials, not a single correct setting.

One arroba is 12.5 kg: Cenicafé equates 60 arrobas with 750 kg of dry parchment.¹

How each dryer moves air and coffee#

Static beds and the moisture front. In a static bed, the coffee nearest the air inlet dries first, and moisture gradients form along the airflow path.^1,4 Reversing the air direction evens out the final moisture.² In the two-level dryer, only the lower chamber has air reversal.² A three-level dryer can also run without reversal, moving coffee from chamber to chamber instead. This version costs least to build but needs the most labour.² Too little airflow lengthens drying; too much raises the cost of fan and heat.¹

Drums. In a guardiola the air path is radial: from the central tube, through the turning coffee, out through the drum wall.² The air that leaves still carries much drying capacity, especially late in drying, and that capacity is lost.² The honey process entry explains why tumbling suits sticky coffees.

Columns and concurrent flow. In concurrent flow, coffee and air both enter the drying chamber at the top and move down together, so the hottest air meets the wettest coffee.² In a Brazilian concurrent and counter-current dryer with a rest chamber, the grain temperature stayed within recommended values even with air at 70 °C.⁹

Silos with near-ambient air. In a Brazilian combined system, pulped and mechanically demucilaged coffee (cereja descascado e desmucilado) was pre-dried with heated air to about 25% (wb). It was loaded into a 51.7 m³ silo in successive lots of 4.8 m³: the bottom lot entered at 25% and the top lot at 18% (wb).¹⁰ The fan delivered 58 m³/min; per cubic metre of coffee this fell from 10.82 to 1.14 m³/min as the silo filled.¹⁰ The coffee ended at 11.3% (wb) at the bottom and 12.2% at the top.¹⁰ According to a university guide, silo drying uses electricity over a long period, and in humid regions a low-cost supplementary heat source is needed.^{11<Dehumidified air. In the dew-point study, parchment was dried in two steps at several controlled dew points. The authors report that the drying conditions had a significant effect on cup quality and recommend the warmer, moderately dry setting they tested.14 The study appeared in a journal rated Tier B here, so its values are not given.>}

Smoke and combustion taints#

Combustion gases are kept away from the coffee in Colombian and Brazilian guidance.^1,6 According to a 2026 review, smoke recirculated from biomass furnaces can give smoky or phenolic notes, so furnaces must burn completely and keep air and gas apart.⁴

The only study opened for this entry that measured contamination tested LPG. Washed coffee was dried at 50 ± 1 °C in a static-bed dryer without a heat exchanger, to 10–12% (wb).⁵ Most samples had no detectable polycyclic aromatic hydrocarbons (PAHs). Where PAHs appeared, they measured 0.08–4.1 µg/kg, and none of the 17 PAHs considered carcinogenic was found.⁵ Cups scored 80.09–82.69 points without defects. Drying took somewhat longer than with the electric control.⁵ With a deliberately incomplete (yellow) flame, parchment and beans turned black, which the authors consider not appropriate.⁵ Solid fuels were not tested.

Tempering and rest between passes#

Cenicafé defines rest time as the time the grains are not exposed to the hot air stream; moisture then diffuses from inside to outside.² Some dryers build this in. In the Brazilian concurrent and counter-current dryer, treatment at about 44 °C needed about 24 h of drying plus 12 h of rest; at 70 °C with continuous mixing, about 12 h of drying.⁹

Longer rests have been tested in boxes between passes:

• Natural coffee. Drying in fixed-bed dryers (10 cm layers, mass at 40 ± 1 °C) was stopped at 20, 17 or 14% (wb), and the coffee rested 5, 15 or 30 days.¹² Cup quality improved as the rest lengthened. Stopping at 17 or 20% did not change quality compared with complete drying on the patio.¹²
• Parchment coffee. After two days on the patio, coffee was dried at 40 °C mass temperature to 16, 20 or 24% (wb), rested 2, 6 or 12 days, then dried to 11% (wb).¹³ Drying could be interrupted at up to 24% and rested up to 12 days without fermentation. Rests of 2 and 6 days gave lower electrical conductivity than continuous drying.¹³

Both trials were small-scale; the 2014 authors call for full-scale studies.¹³ Short interruptions of mechanical drying and their microbial and ochratoxin A results are described in the drying entry.

Uniformity and over-drying#

Layers in a static bed. In multi-layer dryers, the outer layers can fall to 6–7% moisture (basis not stated) while the centre layers still dry.¹ Operators stop when samples show no under-dried beans, which over-dries the outer layers. Cenicafé suggests switching to ambient air near the target moisture to limit this.¹ In a test of the silo-secador, coffee in the drying chamber reached 7.9% (wb) while the pre-drying chamber held 10.4%.² Loading batches one after another gave a more even final moisture than loading them at the same time.²

Cost of over-drying. The Codex code names over-drying as the most common problem with mechanical dryers, causing weight and income loss.⁸ An Embrapa manual adds that coffee below 11% (basis not stated) loses weight and breaks in hulling.⁶ According to a 2026 review, over-dried or heat-damaged coffees often show lower complexity.⁴

Energy use#

Typical values. A 2026 review gives about 4,000–5,500 kJ per kilogram of water removed as typical.⁴ It does not name a primary study for this range. It puts overall energy efficiency at 20–40% for static and cross-flow dryers and 40–60% for optimised rotary and drum dryers.⁴ Losses come from poor insulation, excess airflow and incomplete combustion. Fans, blowers and rotating parts add a significant share.⁴ Cenicafé estimates 22% overall efficiency for its static silo dryers at one operating point.²

Measured values. Measured specific consumption is higher than the review’s range. A Brazilian concurrent and counter-current dryer used 10,544–14,713 kJ/kg including fan electricity; mixing continuously at 70 °C used less than intermittent operation at about 44 °C.⁹ In the combined-drying trial, complete drying in a fixed-bed dryer with an indirect-fired furnace used 10,945 kJ/kg. In the combined system, heated-air pre-drying to about 25% (wb) used 19,138 kJ/kg, and finishing in the silo with near-ambient air took only 193 kJ/kg.¹⁰

Saving fuel. Combining sun and machine saves the most. Cenicafé reports that solar pre-drying, to about 40% moisture (basis not stated) in one or two days, shortens silo drying by 22.4–66.7%. This cuts fuel and electricity use by up to 50%.³ A university guide also reports fuel savings when a pre-dryer, a dryer and a storage silo are combined.¹¹ Husk alone does not cover the whole harvest: by our calculation from Cenicafé’s figures, 18 kg of husk per 100 kg dry parchment meets about half of the 4.4 kg per arroba (35 kg per 100 kg) burnt in its husk-fired exchanger.³

Risks and defects#

• Smoke taint – combustion gases in the drying air can give the coffee a smoke odour.⁶ Incomplete combustion of directly burnt LPG blackened parchment and beans.⁵
• Black beans – immature beans dried at excessive inlet temperature turn black.⁸
• Thermal damage – coffee-mass temperatures above 40 °C cause thermal damage, according to a Lavras study citing earlier work.¹⁵ Natural coffee is more sensitive to heated-air drying than washed coffee.¹⁵
• Uneven moisture – beans near the air inlet dry faster than those near the outlet.⁴
• Over-drying – weight and income loss, and breakage in hulling.^6,8
• Mechanical stress – high airflow may increase the risk of mechanical stress and breakage.⁴
• Corrosion – sulphur from burning coke forms acid that corrodes metal parts.¹

Open questions#

• Energy benchmarks. The measured values lie well above the review’s typical range.^4,9,10 No study opened for this entry measured energy use in a commercial guardiola.
• Taints from solid fuels. PAHs were measured only for direct LPG firing.⁵ No study opened for this entry measured PAHs or smoke compounds in coffee from husk- or wood-fired dryers with leaking heat exchangers.
• Rest length at scale. Multi-day rests between passes were tested in small lots in boxes; their effect in full-size dryers is not established.^12,13
• Dehumidified air. The dew-point study reports cup scores but no energy use, so the cost of drying with dehumidified air is not quantified here.¹⁴

Key studies#