Hygroscopic analysis of pre-dried madder roots (Rubia tinctorum L.) using a solar dryer: Implications for safe storage conditions

Current study investigated the sorption proprieties of madder roots. Sorption isotherms were measured through a gravimetric static technique using chemical salts, to control the surroundings water activity in a range of 0.05-0.9. Sorption measurements were conducted in three temperature 20, 30, and 40 degrees C. Integral proprieties were evaluated through Clausius-Clapeyron equation for a constant value of spreading pressure. In this study the optimal conditions for storage of madder roots were discussed in the indicated interval of temperature. The study findings showed that the madder roots isotherms were type II and the moisture content of product increased with the increase of water activity. Peleg and GAB models showed highly performance in fitting the sorption data. Integral entropy exhibited a minimum value in the region of 6-7% db, and 0.34-0.38 for moisture content and relative humidity, respectively. These conditions were considered as the best to preserve the madder roots shelflife.