A. John Mawson

A Mathematical Model Of Solar Drying Of Rice

Abstract A mathematical model describing the heat and moisture transfer within a solar drying system of rice was formulated. A numerical solution using MATLAB was implemented due to the many coupled PDEs and nonlinear algebraic equations. The model was checked for a range of the space steps and by comparison to analytical solutions for completed situations and was shown to contain no significant numerical errors. After estimating the best values and uncertainties of the system inputs the model was validated by comparison with experimental data for solar drying of rice. It was shown to be a very good mechanistic tool with advantages of simplicity and practical accuracy. The model accurately predicted the drying time and the temperature, and moisture content (MC) within the bed during drying except when a polystyrene drying pad was used. However, the model did not predict the experimental bed water activity (relative humidity) consistently well.

Temperature kinetics of texture changes in Actinidia chinensis ‘Hort16A’ during storage

Summary Kinetic models describing softening during storage of ‘Hort16A’ kiwifruit at different temperatures are useful tools for the development of pre-conditioning and ripening protocols. Magness-Taylor firmness, compression firmness, and stiffness were assessed for their usefulness as tools to monitor and predict softening of ‘Hort16A’ kiwifruit during storage at six storage temperatures ranging from 1.5°C to 25°C. Softening showed the same general biphasic pattern for all three measures, similar to that for green ‘Hayward’ kiwifruit, and could be described using simple exponential decay models. Magness-Taylor firmness reached an asymptotic minimum rapidly, whereas compression firmness and stiffness still decreased measurably at the end of storage. The rate of softening increased with temperature, and the biphasic pattern of the softening curves became more pronounced with later harvest dates. The effects of orchard, cane, and fruit position on parameter estimates such as rate constant, activation energy, and asymptotic firmness value, were negligible.

Effects of Different Solar Drying Methods on Drying Time and Rice Grain Quality

Paddy rice was sun dried in Cambodia in 2004 using a range of methods practiced by local rice farmers. For each treatment in the experiment, a grain sample at about 22% moisture (typical harvest moisture content) was sun dried between 8 a.m. and 4 p.m. During experiments, the grain moisture content was measured at regular intervals. The grain varieties used, bed depths, stirring of the grain, bulk tempering after drying and the drying pads had significant effects on the drying time. Drying was faster when bed depth was reduced, regularly stirred but not shaded or covered and when the drying was carried out on a porous pad. Damage to the dried grain was reduced when the bed was thin, stirred and shaded and when the drying was slow on pads with less air circulation.

Measurement of bed grain and air conditions during solar drying of rice.

Experiments on solar drying of rice were conducted in Cambodia in December 2004 using two local rice varieties and traditional methods practiced by farmers. For the whole drying time, the climate conditions—including the wind speed, ambient air temperature and relative humidity, as well as the solar intensity—were regularly monitored, along with the temperature, moisture content and relative humidity of the grain and air at different depths within the drying beds. Two drying treatments per day were extensively monitored. The monitoring showed that all of the processes of solar radiation, convection and evaporation at the surface, and conduction, convection, evaporation and diffusion within the bed are important.