Determination and Modeling Desorption Isotherms of Okra (Abelmoschus esculentus L. Moench) and Sweet Green Pepper (Capsicum annum L. Moench)

Abstract

The aim of this study was to determine the experimental moisture sorption isotherm of green pepper and okra, assess the performance of literature sorption models describing their behavior, and specify bound water properties. Isotherms were determined through the static gravimetric method at 30, 40, 50, and 60 °C. Twenty models were studied and fittings were done through nonlinear regression using MATLAB R2015b. GAB and Caurie models were used to determine energy constants and bound water properties respectively. Structural parameters were determined according to Kelvin and Halsey equations. All the experimental isotherm were of sigmoid shape. For both samples, Peleg, Enderby, and Guggenheim Anderson de Boer (GAB) models gave the best validation statistic criteria. R-squared (R2) values were all greater than 0.926 and root-mean-square error (RMSE) values less than 0.04 for both substrates. The GAB energy constants varied with temperature between 6.00 to 51.75 and 34.23 to 176.78 for Cg constants, and 0.70–0.99 and 0.75–0.93 for Kg constants, respectively, for okra and green pepper. The Cg energy constant globally decreased with increasing temperature. The monolayer moisture content Wm, varied with temperature between (0.0740–0.1044) kg kg−1 and (0.0760–0.1522) kg kg−1 respectively for okra and green pepper. Green pepper bound water property values were globally higher than those of okra and both were successfully modeled according to the equilibrium moisture content and according to the temperature. Analysis based on the IUPAC classification revealed a macroporous structure of the two materials. The hygroscopic equilibriums, physicochemical and microbiological stability conditions of green pepper and okra, were determined, for given environmental temperature values. The results obtained from the present study would serve in understanding the water state and mechanism of water sorption, defining drying and storage conditions of the tested vegetables, as well as designing corresponding dryers and packaging.