Modeling for thin layer drying of black pepper (Piper nigrum) in a reverse air flow dryer

Drying of freshly harvested green pepper (Piper nigrum) was performed in a reverse air flow mechanical dryer (RRLT-NC - 101). Fresh and blanched pepper of varieties, Sreekara and Panniyur-1 were dried at 50 degrees C, 55 degrees C, and 60 degrees C. Blanching was done by dipping in boiling water for 1 min. The experimental data for moisture loss was converted to moisture ratios and fitted to five thin layer drying models to describe the drying process mathematically. The results were compared for their goodness of fit in terms of coefficient of determination (r(2)), root mean square error (RMSE) and mean square of deviation (chi(2)). Logarithmic model was found most suitable to describe the drying process of black pepper. The unblanched Sreekara took 42, 34, and 26 h and blanched took 36, 30, and 24 h to dry from moisture content of 180.89-9.4% d.b. at air temperatures of 50 degrees C, 55 degrees C, and 60 degrees C, respectively. In case of Panniyur the unblanched took 44, 36, and 28 h and blanched took 37, 32, and 26 h to dry from moisture content of 197.62-9.2% d.b. for the same temperatures. The effective moisture diffusivity varied from 3.28 x 10(-7) to 6.44 x 10(-7) m(2)/s. The activation energy was higher for unblanched than for blanched black pepper varied between 39.45 and 44.05 kJ/mol. Practical applications Drying of harvested pepper is one of the most important unit operations during processing of green pepper. Sun drying is the conventional method followed for drying. The despiked pepper berries are dried under sun for 4-5 days to bring the moisture content below 11%. The process of blanching green pepper in boiling water for 1 min followed by mechanical drying could produce better quality dried product. A simple reverse air flow, natural convection mechanical dryer suitable for drying various agricultural produce at farm level was used for drying green pepper. The results of the study could be applied for gathering information related to the drying process in a mechanical dryer like the drying kinetics, effective moisture diffusivity and activation energy as well as the effect of blanching on these parameters.