Experimental determination and mathematical fitting of sorption isotherms for Lemon Balm (Melissa officinalis L.)

Abstract

Knowledge of the equilibrium moisture content (EMC) is important for modelling and planning of drying and, to select the conditions for storage of biological products.  Equations for modeling water sorption isotherms have special importance in many aspects of product preservation by dehydration.  In this work, desorption and absorption isotherms for Melissa officinalis L. (Lemon Balm) were determined at four different temperatures (25, 40, 50 and 60℃) by using the method recommended by the project COST 90.  This method is based on maintaining a known mass of sample to an atmosphere generated by the use of saturated saline solutions until achieving the balance.  The saline solutions were prepared according to DIN 50008 for a range of relative humidity between 10 and 85%.  Samples of 0.5 grams of whole leaves of Melissa officinalis L. variety Citronella were placed in perforated stainless steel bins, positioned inside of tightly sealed glass containers.  The weight of the samples was registered until the samples achieved balance.  Experimental data were fitted using the following mathematical models: BET, GAB, Halsey, Lagmuir, Oswin, Peleg, Henderson and Chung & Pfost.  Using nonlinear regression, the experimental data was fitted to the considered models.  The chi-square test and the coefficient of determination were used to evaluate the fit.  Halsey’s model was found to be the most suitable for describing the desorption curves at the temperatures of 25℃ and 40℃ and BET’s model was most suitable for describing the data at the temperatures of 50℃ and 60℃.

Keywords: Melissa officinalis L., desorption isotherms, water activity, Equilibrium moisture content EMC