Determination of moisture adsorption isotherm of St. John's wort (Hypericum perforatum L.) leaves

St. John’s wort (Hypericum perforatum L.) is an important medicinal plant that high percentages of hypericin in its leaves and flowers are considered as the most important feature of the Iranian species. Isotherm curves show the relationship between the air relative humidity and moisture content of the plant at constant temperature. Knowledge about isotherms is essential for storage and drying process. In this research, moisture equilibrium data of St. John’s wort’s leaves by adsorption were determined at 40, 50, 60 and 70°C and water activity ranging from 0.11-0.84 by using the gravimetric static method. Five mathematical models (Henderson, Halsey, Oswin, Chung-Pfost, and GAB) were used to fit the experimental data. Based on the results, the Chung-Pfost model was the best one for estimating adsorption isotherms curves of St. John’s wort’s leaves.