Kinetics and Mathematical Modeling of Microwave Drying of Sri Lankan Black Pepper (Piper nigrum)

Abstract

Drying characteristics of black pepper (Piper nigrum) was investigated in a microwave drying system. Drying experiments were carried out at three different microwave power levels, 180, 360 and 540 W and the moisture content was measured at different time intervals. Experimental results were fitted to seven thin layer drying models; Newton, Page, Henderson and Pabis, Logarithmic, Midilli et. al., Weibull and Kaleta et. al.. Statistical indicators; Coefficient of determination (R2), Root mean square error (RMSE) and reduced chi square values (c2) showed Midilli et. al., Weibull and Kaleta et. al. models give better fit to the experimental values. Drying rate constants and the equation constants were compared and analyzed. Similarities of the drying models were observed and discussed with respect to the equation parameters. The absence of the constant drying rate period in drying curves show the drying of black pepper lies totally in the falling rate period where the drying rate is controlled by the moisture diffusion. Maximum drying rates observed were 0.02, 0.05 and 0.08 kg moisture/kg of dry material/ min at 180, 360 and 540 W power levels. Results revealed drying rate constant and the effective moisture diffusivity values increases with the microwave power level. Drying rate constants were 0.03, 0.09 and 0.16 min-1 and the effective moisture diffusivity values were 2.43 x 10-10, 4.87 x 10-10, 1.42 x 10-9 m2/s for power levels of 180, 360 and 540 W respectively. The Activation energy of black pepper calculated based on the Arrhenius equation is 86.7 W/g.