Rapid drying of high-moisture foods is a crucial step in ensuring food quality and minimizing post-harvest losses. In Vietnam, rice must be dried to a safe moisture content of 14% for long-term storage. However, the hot and humid climate of Southeast Asia makes it challenging to achieve this using traditional drying methods, especially during the wet season when rice moisture levels can exceed 30%. Limited space, labor shortages, and unfavorable weather conditions often make manual drying impractical, forcing the use of mechanical drying systems to prevent spoilage and loss.
The rapid reduction of moisture from 30% to 14% requires significant energy, and during peak harvest seasons, existing dryers struggle to handle large volumes efficiently. To address these challenges, the Australian Centre for International Agricultural Research (ACIAR) launched a grain drying project in Vietnam, focusing on developing a two-stage drying method. This approach aims to improve efficiency while preserving the quality of the rice.
In the two-stage drying process, rice is first rapidly dried to 18% moisture in the initial stage, followed by a slower, controlled drying phase to reach the target 14%. The key is to remove free water quickly without damaging the rice’s color, texture, or appearance. This method was first introduced in Vietnam through the ACIAR90008 project (1994–1997), which tested warehouse dryers with capacities ranging from 80 to 400 tons. A new ACIAR initiative is now working on designing an economically viable first-stage dryer and modifying the system to accommodate a grain depth of 4 meters instead of the previous 2 meters.
While flat bed dryers are common in Vietnam, they are not suitable for the fast first-stage drying due to their low throughput. In contrast, fluidized bed dryers have been widely used in Thailand and exported to several countries. These dryers offer faster drying but require more energy. Another promising alternative is the nozzle drying system, developed under the ACIAR97065 project, which combines advanced technology with energy efficiency.
The nozzle bed dryer uses a triangular cross-section design with a drying path of 4 meters. At inlet air temperatures of 80°C, 90°C, and 100°C, rice moisture can be reduced from 34% to 18% within 5–6 hours. The system also allows for temperature adjustments, with initial high-temperature drying (140–160°C) followed by lower-temperature drying (80–100°C) to minimize grain breakage. Experiments showed that maintaining grain temperature below 50°C is essential to reduce damage.
A hexagonal nozzle dryer was later constructed locally, capable of processing 23.5% moisture rice at once. It consists of six separate units, each equipped with a centrifugal fan powered by diesel-heated air. The system can operate individually or in combination, with hot air either discharged or recycled as needed. After testing at the Vietnam Agriculture and Forestry University, the system was deployed on a farm in Can Tho Province for real-world evaluation.
The adoption of two-stage drying technology has the potential to greatly benefit Vietnam’s rice industry. As a major rice exporter, Vietnam needs to enhance its product quality to meet international standards. Secondary processing adds costs, but two-stage drying can improve rice quality by reducing breakage and increasing grade. It also helps prevent mold growth and biochemical deterioration, which are major concerns when moisture levels exceed 18% in high humidity environments. By improving drying techniques, Vietnam can boost its economic returns and support farmers’ livelihoods.
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