What is drying?

The grain moisture content that avoids breakage at harvest and threshing may be too high for effective storage or milling.  Drying is usually necessary before cereals can be safely stored or processed. Natural drying or sun drying is a low-cost method used widely by smallholder farmers. Mechanical drying reduces the drying time, allows improved moisture control, and avoids losses associated with drying in open air, but many remote farm producers lack access to mechanical drying technology. A safe moisture content for most crops is below 13 percent. Higher moisture levels will allow mold, fungal or aflatoxin growth resulting in In addition to reducing grain quality, aflatoxin is associated with up to 35 percent of child stunting.

What causes postharvest loss at this stage?

Sun drying can expose grain to loss from birds, insects, and other animals, damage from rain or other weather events, and contamination from dirt, dust and insects that can diminish the value of the crop. Losses associated with weather may be growing more severe due to changes in climate. Properly used machine dryers can eliminate drying losses, but improper temperature control during mechanical drying can reduce grain quality and germination rates.

How large are drying losses?

Drying losses vary widely but one example places the range from an average of 3.1% with open sun drying of paddy to an average of .39% with machine drying. (Alam 2019)

What does research focus on?

Finding the best alternative drying tools for different crops under different circumstances is the primary focus of much of the research. The cost of alternatives to natural or sun drying often place them out of reach for many smallholder farmers, so finding economical solutions is also of interest.

To read research about drying, refer to the table below.


Small Scale Batch Grain Dryers

Small scale dryers allow farmers or groups of farmers to dry grain before selling to a trader or consumers, or storing for household use. Grain dried to certain specifications is less likely to develop aflatoxins or molds in storage. Using hermetic storage options in combination with grain dryers can help farmers avoid postharvest losses.

Download ADMI’s complete assessment of Small Scale Batch Grain Dryers

  • Open-air sun drying

    Open-air sun drying
    Open sun drying generally means spreading grain on the ground in a yard or leaving cut grain in the field. In some cases, grain is spread on a tarp or mat, which can reduce losses. Open sun drying is the least resource-and-cost-intensive method of drying and is suitable for small batches. However, weather conditions can sharply limit the effectiveness and safety of open sun drying.

    Capacity: limited by space
    Drying time: depends on weather conditions, 2 to 5 days
    Crops: any
    Fuel: sun
    Portable? not applicable
    Performance considerations: grain needs to be shifted periodically to improve drying.

    Learn more about sun drying
  • Bubble dryer

    Bubble dryer

    The solar bubble dryer uses solar energy for heating and electric energy for forcing air through the grains. It is a tunnel-type dryer, designed as an alternative to open field drying. Grain is placed in the bubble and a fan circulates the warm, dry air through the tunnel to dry the grain.

    Capacity: 500 or 1,000 kg

    Drying time: Reducing moisture by .5 percent every hour for rice or corn in good conditions. With optimal conditions, a crop can be brought from 22 to 14 percent moisture in 6 to 8 hours, according to the manufacturer.

    Crops: tested for rice and maize

    Fuel: solar or electric options available

    Portable? yes, can be rolled up and transported easily

    Ease of construction: professional

    Product available for purchase? Yes, from GrainPro

    Performance considerations: grain should be mixed regularly for optimum drying.

    An instructional setup video is available from GrainPro.

    FAQ from the International Rice Research Institute (IRRI)

  • BAU-STR bin dryer

    BAU-STR bin dryer

    Circular bin dryers ventilate the grain outwards from a center column. They are compact, save floor space, and can be expanded or contracted to accommodate different batch sizes. Based on the Vietnamese SRR dryer design, modifications by ADMI partner Bangladesh Agricultural University include a mesh cage, improved blower, and an option for LP gas as a heat source.

    Capacity: 500 kg and up, bin can be expanded to more than 500 kg

    Drying time: 500 kg of paddy dried from 24.75% to 8.5% moisture content in 7.5 hours. Time can depend on air temperature and blower speed.

    Crops: rice, maize

    Fuel: Electricity needed for blower, heat generated by burning rice briquettes or LP gas

    Portable? yes, can be disassembled and transported

    Ease of construction: some technical expertise required

    Performance considerations: grain should be mixed regularly for optimum drying

    Detailed instructions and specifications from Bangladesh Agricultural University

  • Vietnamese flatbed dryer

    Vietnamese flatbed dryer

    This classic simple flatbed dryer was developed in Vietnam. It can be constructed in 4-ton (SGH4) or 8-ton (SGH8) configurations.


    Capacity: 4 (SGH4) or 8 tons (SGH8) per batch depending on configuration

    Crops: paddy, maize

    Drying time: reduce moisture by about 1.5% per hour; 4 ton batch of paddy dried from 25% to 15% in 9-11 hours

    Fuel: generator or electricity connection for fan and additional biomass furnace for heat

    Performance considerations: grain needs to be mixed by hand; cover over the dryer allows use in more weather conditions.

    Portable? no

    Ease of construction: no technical expertise required

    Technical specifications and further information available from IRRI

  • Reversible Air Flatbed Dryer

    Reversible Air Flatbed Dryer

    The reversible air dryer adds a cover to a standard flatbed dryer and an air channel to allow the operator to force air from the top down or from the bottom up. By reversing the airflow, these dryers reduce the moisture differential between the topmost and bottommost layers of grain, thereby increasing the consistency in drying.

    Capacity: 500 kg up to 12 tons

    Drying time: reduce moisture by about 1.15% per hour; an 8-ton batch of paddy can be dried from 24% to 12.5% moisture content in 9-11 hours.

    Crops: paddy, whole coffee, coffee bean, pulped coffee, peanuts

    Fuel: generator or electricity connection for fan and additional biomass furnace for heat

    Portable? no

    Performance considerations: Due to reverse flow drying, grain does not need to be mixed.

    Read more about flat-bed dryers from the IRRI Knowledge Bank.

  • Collapsible Flatbed Dryer

    Collapsible Flatbed Dryer

    The Easydry M500 dryer is a small collapsible flatbed dryer designed to be portable, unlike the conventional flatbed. In addition to portability, it differs from the conventional flatbed dryer by using a heat exchanger after the blower, so that clean air is blown through the grains to be dried.

    Capacity: 500 or 1,000 kg

    Drying time: for 500 kg, the dryer can reduce moisture by about 1.6% per hour; a batch of maize can be dried from 17% to 13% moisture content in 2.5 hours.

    Crops: paddy, whole coffee, coffee bean, pulped coffee, peanuts

    Fuel: generator or electricity connection for fan and additional biomass furnace for heat

    Portable? yes

    Performance considerations: Grain needs to be mixed frequently.

    Ease of construction: technical expertise required

    Download the EasyDry user manual from ACDI-VOCA

  • Flatbed Wagon Dryer

    Flatbed Wagon Dryer

    The Flatbed Wagon Dryer is a completely self-contained mobile flatbed dryer, which is commercially available. This dryer can be shipped into a standard 20-foot shipping container anywhere in the world. It is designed for precision drying and contains a thermo-humidistat, which regulates the heater to bring the product to the desired moisture content.

    Capacity: 215 bushels and 315 bushels

    Drying time: reduce moisture by about 1.67% per hour; the 215 bushel dryer takes about 3 hours per 5% moisture reduction.

    Crops: maize, but steel mesh can be customized to accommodate finer grains including flaxseed, rice, and wheat

    Fuel: electricity

    Portable? yes

    Ease of construction: professional production required

    Performance considerations: No grain mixing necessary. Difficult to use in poor electricity supply situations. Sides are 9 feet high, which could make loading difficult.

    Learn more about the drying wagon, including a demo video

Drying Resources

TitlesAuthorsTagsPublication DateCategory

Scope of BAU-STR in Existing Paddy Drying System of Bangladesh

Saha, Chayan Kumer, Md. Ashraful Alam, Md. Monjurul Alam,et al.

Drying, Paddy, PHL Technology


Journal Article

Evaluation of S4S Solar Grain Dryer for Drying Paddy Seeds

Roy, Dipika, Chayan Kumer Saha, Md. Ashraful Alam, et al.

Drying, Paddy, PHL Technology


Journal Article

Mechanical Drying of Paddy Using BAU-STR Dryer for Reducing Drying Losses in Bangladesh

Alam, Md. Ashraful, Chayan Kumer Saha, and Md. Monjurul Alam

Drying, Paddy, PHL Technology


Journal Article

Effect of Hot Air Velocity on Drying Kinetics Undergoing STR Drying of Paddy

Kumar, Ashok, Satish Kumar, Sanoj Kumar, et al.

Drying, Paddy, PHL Technology


Journal Article