Abstract
Physical dimensions, 1000 pod weight, bulk density and specific heat of red chilli were measured at the moisture contents of 5.14, 13.94, 26.47, 40.69, 52.64, 62.80 and 73.03% (wb) and found to be linearly dependent on moisture content. Adsorption and desorption equilibrium moisture contents of red and green chillies were determined experimentally at the temperatures of 20, 30, 40 and 50°C and relative humidity range of 10 to 97% by the dynamic method. Twelve sorption isotherm equations were used to fit to the experimental data and the Smith equation was found to be the best fitted equation for both red and green chillies. Red chilli was treated with water blanching, sulphiting, sodium hydroxide and dipsol whereas green chilli was blanched, sulphited and treated with sodium hydroxide and sodium carbonate before solar drying. Chemical additives significantly reduced the pungency of both red and green chillies. Blanching was found to be effective for red and green chillies before solar drying to increase drying rate and for retention of colour, glossyness and pungency. Thin layer drying experiments were conducted under controlled conditions of temperature, relative humidity and air velocity for the overflow-underflow and the through flow methods for green chilli in Bangladesh and for through flow method for red chilli in the United Kingdom. Drying rate increased with the increase of air temperature and temperature above 65°C affected the colour of both red and green chillies. Drying rate decreased with the increase of relative humidity. In the overflow-underflow drying, drying rate increased with the increase of air velocity but in case of the through flow drying, drying rate became independent of air velocity above 0.50 m/s. The Page equation was found to describe the thin layer drying of green chilli better than the Newton equation and the Newton equation was found to describe the thin layer drying of red chilli better than the Page equation. Solar tunnel drier was tested for full load condition in Bangladesh in 1999 and 2000. In each drying batch in the solar tunnel drier, 20 kg of dried red chilli and 18 kg of dried green chilli were produced from 80 kg of fresh red or green chilli. Drying time in the solar tunnel drier was reduced to 40% and 50% in comparison to those in improved open sun drying method (blanched before drying) and traditional open sun drying method (unblanched before drying) respectively. The chilli dried in the solar tunnel drier was completely protected from dust, dirt, rain, insects, birds, rodents and microorganisms and it was a quality-dried product in terms of colour, texture, glossyness and pungency. A simulation model was developed for solar drying of red and green chillies using solar tunnel drier and the model was validated with the field data. Good agreement was found between experimental and simulated air temperature and moisture content. The simulation model was used for the optimization of the drier design. A technique had been developed to optimize the design of a PV operated forced convection solar tunnel drier for the drying of chilli. Two types of optimum designs were obtained. One was 14.0 m long and 1.9 m wide collector or drying unit and another was 13.0 m long and 2.0 m wide against the dimension of basic mode collector or drying unit of 10.0 m long and 1.8 m wide. This optimization technique may be applied for design and construction of solar tunnel drier for drying of chilli.