In Cambodia, sun drying of rice has always been of great importance for preserving rice. The main goal of this study was to find the conditions for sun drying that maximise the throughput while minimising quality loss. A whole-bed approach was taken to investigate the conditions of the grain and the air at different layers during the drying process. Seven sets of sun-drying experiments were conducted in Cambodia using a range of methods practiced by rice farmers. These methods included drying with different bed depths (2 to 6 cm), with the bed on different pads (water-proof tarpaulin, mat, net, polystyrene or rice husk), and with different bed tempering methods (stirring regularly or shading and/or covering the bed around midday) for four Cambodian rice varieties (Pka Knhey, CAR11, Masary and IR66). The grain temperature was found to be more affected by the solar intensity than the temperature of the ambient air. Fastest drying was achieved when the bed was thin, less compacted, stirred regularly but not shaded or covered around midday, dried on a pad which allows some air and moisture movement and with high or strong solar intensity. Only the mechanical impact (MI) and milling tests of the rice quality provided useful results. Higher quality was found for grain that was dried in thin beds, stirred regularly, shaded with or without covering around midday and dried on pads with less air circulation. Among the methods used to determine the glass transition temperature of the grain, only the Differential Scanning Calorimetry method gave meaningful results. The glass transition temperature data were highly variable but generally decreased with increasing moisture content and compared quite well with the published glass transition temperatures for other varieties of rice. To provide additional detail on the local conditions within the bed, to better understand the drying process and the interactions between variables and to predict alternative parameters that might be used to correlate with the head rice yields (HRYs), a mathematical model for heat and moisture transport within the bed was developed. The model covered all the drying methods/conditions studied experimentally. A lumped parameter approach to energy and mass transfer in individual kernels was used in the bed model. The model was validated against experimental data. The predicted drying time, temperatures, moisture contents and water activities (relative humidity of the air within the bed) were found to compare very well with the experimental data except when a polystyrene pad was used. The model proved to be a very good mechanistic tool with advantages of simplicity and practical accuracy in the design and management of the sun drying system. A number of parameters related to postulated grain damage mechanisms were derived from the predicted conditions within the bed during drying. The best predictors of the grain quality were found to be rewetting the kernels when the grain is bulked (especially when the kernels are partly below and partly above critical moisture content) grain temperature and distance from the glass transition temperature line. It was concluded that in order to get the fastest drying conditions rice should be sun dried with thin bed, stirring, not shaded or covered around midday and dried on a pad with air circulation. For the highest quality grain, that is grain which would have the least breakage during milling, rice should be sun dried with a thin bed, stirring, shaded or covered around midday and dried on a pad with less air circulation. The optimal drying conditions to get the best quality combined with the fastest practical drying rate, the drying conditions should be drying with 2 cm bed depth, stirring the grain bed every hour, shading or covering the bed around midday and using a tarpaulin or net pad placed directly on the ground.

The main objective of this study was the application of solar energy for drying of parboiled wheat, corn and pea. Drying experiments were performed under open sun and also in the solar-spouted bed drier in which air heated by solar energy was used. The effects of these drying methods on drying rate and quality parameters were investigated for drying of parboiled wheat, corn and pea. The quality parameters evaluated were color, shrinkage, bulk density, apparent density, bulk and internal porosity, microstructure, pore size distribution, sphericity and rehydration ratio. For peas, ascorbic acid content was also measured. In solar-spouted bed drying, drying rates and effective diffusivity values for all samples were determined to be higher and therefore drying time was significantly lower as compared to open sun drying. Effective diffusivities were in the range of 0.30x10-10 m2/s - 0.65x10-10 m2/s for open sun and 1.35x10-10 m2/s - 3.65x10-10 m2/s for solar-spouted bed drying of different samples. In general, better quality parameters for solar-spouted bed dried samples were observed such as less shrinkage, higher rehydration capacities, more homogenous pore size distribution and higher ascorbic acid retention.

<div>The overall goal of this thesis was to study the performance of two related portable multipurpose solar dryers, DehytrayTM and DehymeleonTM, in comparison to open-air sun drying by drying tomatoes, apples and mint under West Lafayette, Indiana weather conditions. Thin layer drying tests were conducted on tomato slices, apples slices and mint leaves, with three temperatures [24°C (75°F), 35°C (95°F) and 54 °C (130°F)], and an airflow velocity of 1 m/s to determine the drying kinetics of these products during diurnal drying cycles typical for solar and/or open-air sun drying. Subsequently, field drying tests were conducted for tomatoes slices, apples slices and mint leaves with the two solar drying technologies (DehymeleonTM and DehytrayTM) and open-air sun drying using uncovered Dehytrays as the control. The average temperatures achieved for these technologies were 45°C (113°F), 60°C (140 °F) and 27°C (80.6 °F) for the DehymeleonTM, DehytrayTM and open-air sun drying, respectively. Moisture diffusivity were in the order of 10-4 to 10-9 (m2/s) for the different methods, depending directly on the product, temperatures and air flow inside the drying chamber.</div><div><br></div><div>Quality attributes (color, vitamin C and microbial growth) were measured before and after the field drying tests. Color difference (ΔE) for DehymeleonTM solar dryer showed the least variation compared with the fresh products. However, for the DehytrayTM ΔE increased due to the impact of its higher temperature and direct sunlight exposure that led to Maillard reactions and caramelization in the case of tomatoes and apples slices. Additionally, vitamin C (Ascorbic acid) content for tomatoes and apples slices was affected for the high ranges of temperatures reached inside the Dehytray™. Denaturing of vitamin C was less observed for DehymeleonTM, maintaining values of 166 mg/100 g dm for tomatoes, and 104.2mg/100g for apples slices. There was no significant difference (α = 0.05) in the microbial growth for the DehytrayTM and open-air drying compared to the fresh product, however, there was significant difference for the DehymeleonTM when drying tomatoes and apples slices, without up one log reduction on the original microbial population. In the case of mint, DehymeleonTM had a 2.3 log reduction, which is similar to L-lactic acid sanitizer achieved by another study in the literature, compared with 0.4 log obtained by the DehytrayTM and 0.47 log obtained by open-air sun drying. The differences in microbial growth were observed because the temperatures inside the drying chamber of the DehymeleonTM was low and product moisture content was above the safe equilibrium moisture content (EMC) for both tomatoes and apples during the early critical hours at the onset of the drying process, which was favorable to mold growth. The lack of a fan to intermittently or constantly flush out humid air released from the crop dried in the DehytrayTM negatively affected its performance. The insufficient airflow in the drying chamber of the DehymeleonTM and its inability to achieve the high temperatures observed in the DehytrayTM negatively affected its performance. Both solar dryers, DehymeleonTM and DehytrayTM achieved high hygienic condition during drying due to their enclosed chambers than protected the crop from contaminant in the environments. Their portability and design for large-scale manufacturing and deployment are a positive development that would be helpful to small and mid-size growers, as well as households (home gardens). Areas for further research were highlighted.</div><div><br></div><div><br></div>