Equilibrium moisture contents of a medicinal herb (Melissa officinalis) and a medicinal mushroom (Lentinula edodes) determined by dynamic vapour sorption

The measurement of moisture sorption isotherms is of great importance in the fields of food and agricultural engineering. Although the static gravimetric method is still commonly used, automated instruments for the determination of sorption isotherms have been designed to overcome some of the drawbacks associated with the standard method. The dynamic vapour sorption (DVS) is a modern technique which can produce sorption data with high accuracy under controlled conditions in a short period of time. The method was examined and validated by measuring the equilibrium moisture contents of lemon balm herb (Melissa officinalis L.) and Shiitake mushroom Lentinula edodes (Berk.) Pegl.] at a temperature of 25 °C in the range of 0-95% relative humidity. The experiments were performed in triplicate for a total of fifteen target values of relative humidity. A parallel exponential kinetics model (PEK-model) was used to evaluate the change in mass during the adsorption process of each relative humidity step. Additionally, the GAB model was tested for its effectiveness to describe the equilibrium MC/RH data of both products. The PEK-model fitted adequately to the experimental sorption kinetic data over the entire range of relative humidity. Sigmoid characteristic curves, type II pattern were obtained, as indicated by the C and K values of the GAB model. Differences in the hygroscopic characteristics of the two products tested were ascribed to differences in nature of the material.