Assessment of solar drying techniques for reduction of post-harvest losses in paddy rice

Introduction

Post-harvest losses in rice remain one of the most solvable yet neglected problems in South Asian agriculture. Bangladesh produces over 36 million tonnes of paddy annually, but an estimated 12-14% is lost between harvest and market, with improper drying accounting for roughly half of these losses [1] . The Boro season harvest coincides with pre-monsoon hot weather, giving farmers a narrow window to reduce grain moisture from 22-26% down to the safe storage level of 14% [2] . Open sun drying on roads and courtyards remains the dominant practice among smallholders, but it exposes grain to contamination, uneven drying, and sudden rain damage [3] . Mechanical dryers are effective but too expensive for most small-scale producers. Solar cabinet dryers offer a middle path they use the same energy source as open drying but in a controlled enclosure that raises air temperature by 10-15°C above ambient, speeds moisture removal, and shields grain from dust and animals [4,5] . Grain cracking during drying is a direct cause of reduced head rice yield at milling, which in turn lowers the market price farmers receive [6] . The rate and uniformity of moisture removal are the main factors controlling crack formation [7] . This research compared open sun drying, a locally fabricated solar cabinet dryer, and shade drying across key quality indicators to determine whether affordable solar drying technology can meaningfully reduce post-harvest losses for Bangladeshi rice farmers.

Research area description

All trials were carried out at the Grain Quality Laboratory and adjoining open field of Bangladesh Agricultural University (BAU), Mymensingh (24.7434°N, 90.4203°E). The site sits on the Old Brahmaputra Floodplain at 18 m elevation. The drying area was a concrete pad (10 × 6 m) adjacent to the laboratory building, free from overhead shading. The solar cabinet dryer a locally fabricated unit with polycarbonate glazing, painted black metal absorber plate, and natural convection venting was positioned on the same pad for identical solar exposure.

Climatic conditions during drying trials

Trials ran from 18 April to 12 May 2023. Ambient temperatures ranged from 29.4°C to 37.8°C during drying hours (08:00-16:00). Relative humidity was 48-72%. Mean solar radiation measured 612 W/m², with peak values reaching 870 W/m² around midday. Inside the solar cabinet, air temperature averaged 12.6°C above ambient. Wind speed was 2.1-4.8 km/h. Two days of intermittent cloud cover occurred during the trial period, which were documented and accounted for in drying rate calculations.

Material and Methods

Material

Freshly harvested BRRI dhan28 paddy with initial moisture of 23.4-25.1% was collected from the BAU research farm within 6 hours of cutting. The solar cabinet dryer (1.2 × 0.8 × 0.6 m) was fabricated at the BAU workshop using locally available materials: galvanized iron frame, 4 mm polycarbonate sheet, and matt black painted absorber. Jute mats and polyethylene sheets were used for open sun and shade drying setups. A Delmhorst G-7 grain moisture meter, digital balance (Shimadzu BL-320H), and Satake rice mill (THU-35A) were used for quality assessments [8] .

Methods

Three treatments were tested: open sun drying (5 cm layer on polyethylene), solar cabinet drying (single layer on mesh trays), and shade drying (under roofed open-sided shelter). Each treatment had four replicates of 10 kg paddy. Grain was stirred every 30 minutes in open sun and shade treatments. Moisture content was recorded hourly. Drying continued until moisture reached 13.5-14.0%. Head rice yield was determined by milling 500 g samples in a Satake mill and separating whole grains (≥3/4 kernel length) from broken [9] . Grain cracking was assessed by examining 200 grains per replicate under a light box. Moisture uniformity was evaluated as the standard deviation of 20 individual kernel moisture readings per replicate [10] . Drying rate was calculated as percentage moisture loss per hour. Data were analyzed with one-way ANOVA and Duncan's multiple range test (p<0.05) using Statistix 10. Moisture removal was fastest during peak solar hours (10:00-14:00) for both sun-exposed methods (Table 2). The solar cabinet consistently maintained temperatures 12-14°C above ambient, driving more rapid and even moisture extraction. Both drying rate and solar radiation showed a strong linear relationship (Figure 1), with the solar dryer achieving consistently higher rates at every radiation level. Box plot distributions (Figure 2) confirm tighter, more favorable quality outcomes for solar-dried paddy across all three indicators.

Results

Discussion

The 46% faster drying rate of the solar cabinet over open sun agrees with findings from similar trials in India and Southeast Asia. Sreekumar et al. [4] recorded drying rate improvements of 38-52% for a comparable solar dryer design in Kerala, while Bala and Mondol [11] documented a 41% reduction in drying time for paddy in Mymensingh using a tunnel-type solar dryer. The enclosed environment limits re-absorption of moisture during cloudy spells, which was a recurring issue during open sun trials. The higher head rice yield (64.7% vs 58.3%) is the most economically meaningful result. In Bangladesh, milled rice price drops by roughly 8-12% for each 5-percentage-point decrease in head rice recovery [12] . The lower cracking rate in solar-dried samples (8.3% vs 14.7%) directly explains this improvement more uniform heat distribution inside the cabinet reduces thermal stress that causes fissures [13] .

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