Experimental evaluation of sorption isotherms of chili pepper: an Ethiopian variety, Mareko Fana (Capsicum annum L.)

Abstract

Moisture sorption isotherms of chili peppers of  Ethiopian variety, “Mareko Fana” ,were determined at three different temperatures (30℃, 50℃ and 70℃) and relative humidity between (10%-90%), using the standard static, gravimetric method.  The modified Halsey, GAB, Oswin, and Henderson sorption models were tested to fit the experimental data.  A nonlinear regression analysis method using SPSS 16.0, and Sigma Plot 11.0 software were used to evaluate the constants of sorption models, to select the best equation and to plot graphs.  The net isosteric heats of sorption isotherms were determined from the equilibrium data.  The models were evaluated statistically by calculating mean bias errors (MBE), mean relative deviations (MRD), root mean square errors (RMSE) and coefficient of determination (R²).  The ranking of statistical errors for each model were done by calculating the average grade. High coefficient of determination and the lowest average grade of statistical errors (1.3 for desorption and 1.0 for adsorption) were obtained from the GAB model indicating that it  fits the best to the experimental data followed by modified Halsey’s equation.  The worst results were obtained from Henderson’s equation.  The values of constant a, b and c of “Mareko Fana” were within the ranges recommended by other researchers (Bell and Labuza, 2000).  The results also showed hysteresis indicating that “Mareko Fana” cannot adsorb the same quantity of water vapour as it released during desorption process.  The net isosteric heat of sorption were determined from the experimental data where the energy requirements for drying processes as well as design of drying equipment be obtained.

Keywords: chili pepper, equilibrium moisture content, water activity, mathematical models, net isosteric heat of sorption