Most Vietnamese coffee farmers face difficulties during the drying process. While some have invested in drying yards, they remain dependent on weather conditions. Except for a few farmers in Arabica-growing regions who process coffee using the wet method, the majority still rely on dry processing.
Currently, two main dry-processing methods are commonly practiced:
Drying Whole Coffee Cherries
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Advantages:
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Requires less labor since the coffee cherries are dried as-is without pulping.
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If dried on a well-drained yard, the beans are less likely to turn black when exposed to short rains.
Disadvantages:
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The drying period is long, requiring constant monitoring and frequent turning.
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The longer the coffee stays on the drying yard, the higher the risk of rain damage. Each rain event adds several days of additional drying and labor.
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High risk of loss due to theft or runoff in sloped areas.
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When market prices rise, the coffee is often not dry enough for sale; by the time it dries, prices have usually dropped.
Pulping Before Drying
Advantages:
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Dries much faster, usually in about three sunny days, helping overcome the disadvantages of whole-cherry drying.
Disadvantages:
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If rain occurs even once during drying, the beans can turn black easily.
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After pulping, if there is no sunlight, the coffee quickly becomes moldy.
Drying Whole Cherries Using Dryers
To overcome the drawbacks and preserve the benefits of both traditional methods, we propose two mechanical drying methods for whole cherries.
If farmers must choose between investing in a drying yard or a drying machine, it is better to invest in a dryer, as it is not dependent on the weather.
The drying time for whole cherries is about 18–20 hours per batch, using coffee husks or coal as fuel.
1. Static Dryer (Suitable for Most Farmers)
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Capacity: 150–200 kg of fresh cherries per m².
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Fuel: coffee husks — the husks from processing can be used to dry an equivalent amount of cherries.
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The blower must be a centrifugal fan, capable of pushing hot air (100–120°C) evenly through the drying grid.
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Power source: electric motor or engine.
Drawback: manual turning is required every 2–3 hours.
However, after 18–20 hours, the coffee reaches the desired dryness and is no longer susceptible to mold or darkening.
Notes:
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The drying rack can be easily made by any local workshop.
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Only use coffee husks for drying whole cherries.
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If drying green beans, use coal instead to avoid smoky odor contamination.
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2. Drum Dryer
Disadvantage: higher investment cost than the static dryer.
Advantages:
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No manual turning required.
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Can dry whole cherries, parchment coffee, or green beans.
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Produces very uniform drying results, independent of operator skill.
Drying process: simply load the coffee into the drum, ignite the furnace, and run the dryer for about 18–20 hours.
Available sizes (developed by Viết Hiền company):
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5 m³, 15 m³, and 20 m³ models, corresponding to 2.5, 7.5, and 10 tons of fresh cherries per batch.
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The larger models (7.5–10 tons/batch) are ideal for commercial processors rather than small farmers.
3. Rack Drying Method
Advantages:
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Faster drying due to air circulation on both sides.
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When it rains, there’s no need to move the coffee; just roll up or cover the rack.
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Prevents moisture reabsorption during nighttime dew.
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Often cheaper to build than a traditional drying yard.
4. Rack Drying Combined with Greenhouse Effect
Method:
Use transparent materials such as nylon or polyester sheets, available in the market.
When covered, sunlight passes through and creates a greenhouse effect, raising the internal temperature to 40–50°C.
When the sun is strong, open or vent the cover to allow moisture to escape.
When it rains, close it quickly — a very convenient system.
This can be built using local materials such as wood or bamboo frames.
5. Partial Greenhouse-Type Solar Dryer
Description:
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A dome-shaped transparent structure allows sunlight in but prevents rain.
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Ideal for medium-to-large-scale farms.
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The inside temperature can reach 60°C, while the outside remains around 30°C.
Some models include ventilation systems that allow hot air to circulate and release moisture, ensuring efficient drying.
Principle of the Greenhouse Effect in Drying
When sunlight passes through transparent materials (glass, polyester, etc.) and hits dark surfaces, the light energy is converted into heat.
The dark surface acts as a heat trap, absorbing radiation and turning it into thermal energy.
This principle is applied in:
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Solar water heaters
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Solar-powered dryers for agricultural and seafood products
These methods are clean, eco-friendly, and cost-effective, reducing fossil fuel use.
A simple solar dryer works as follows:
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Sunlight passes through the transparent cover.
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The black metal plate inside the collector absorbs heat.
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Hot air rises and flows into the drying chamber.
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Moisture evaporates from the coffee and escapes through vents.
Under good sunlight, the drying chamber can reach 50–60°C.
Farmers can choose between mechanical drying or solar-assisted drying depending on scale and resources.
Solar dryers are environmentally friendly, while drum or static dryers offer faster and more controlled results, helping ensure consistent coffee quality for export.