Abstract
Adsorption isotherms of jackfruit leather were determined experimentally at the temperatures of 30, 40 and 50°C and relative humidity range of 11 to 97% by the dynamic method. Ten sorption isotherm equations were used to fit the experimental data of equilibrium moisture content of jackfruit leather and the GAB equation was found to be the best fitted equation. [The heat of sorption of water was determined from isotherm data using the clausius-Clapeyron equation and was found to be increased with the decrease of sample moisture content. Thin layer drying experiments were conducted under controlled conditions of temperature, relative humidity and air velocity for over-flow drying method. Drying rate increased with the increase of temperature and the temperature above 50°C affected the colour of the dried jackfruit leather. Drying rate decreased with the increase of relative humidity and increased with the increase of air velocity and became independent of air velocity for air velocity above 1.5 m/s. Drying air at 30% relative humidity was found suitable for drying from the view point of energy saving. Among the nine tested thin layer drying equations the modified page equation was found best to describe the thin layer drying behaviour of jackfruit leather. Four solar tunnel drying experiments were conducted at full load conditions in the department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh, Bangladesh. During solar tunnel drying, drying air temperature inside the collector increased along its length from the air inlet to the dryer while the dryer temperature remained almost constant (about 50°C) throughout as was expected. In each drying experiment about 14kg of dried jackfruit leather was produced from 50kg of fresh jackfruit juice. Less drying time was required to reach the same moisture content in solar tunnel drying in comparison to open sun drying method. The jackfruit leather in the solar tunnel dryer was completely protected from rain, dust, insect and microorganism and it was a quality dried product. The energy efficiency of collector varies between 29-42% and that of dryer varies between 32- 65% for the variation of solar radiation between 100-600 W/m2. The overall energy efficiency of the solar dryer was 42.47%. The exergetic efficiency of collector varied between 32-69% and the mean value of exergetic efficiency of dryer was 41.42% with small amplitude of random variation during the variation of solar radiation between 100-600 W/m2. During storage in room temperature the jackfruit leather packaged in LAF (Laminated aluminium foil) was acceptable for all attributes for 6 weeks' storage and the leather in PL (Polyethylene) and GJ (Glass jar) was for 4 weeks' storage and for storage in refrigerated condition the leather was acceptable in terms of all attributes throughout 12 weeks of storage period. Also the score of all attributes of jackfruit leather stored in refrigerated condition was higher than the score of leather stored in room temperature for the same storage period.