Sorption characteristics of amaranthus stems under storage conditions and water activity prediction

Stems of amaranthus are considered as prospective biofuel in the Czech Republic. The study presents results of water sorption tests of this biomass in the range of 10–30 °C and water activity (aw) ranging from 0.4 to 0.99. The experimental procedure used was a gravimetric dynamic method. Four sorption models (Chung–Pfost, Halsey, Henderson, Oswin) were evaluated. The modified Henderson’s equation was the best model for moisture adsorption and desorption of amaranthus stems. Critical values of equilibrium moisture content, corresponding to the aw = 0.6, were 11.45% and 13.28% (wb) for water adsorption and desorption respectively, at the temperature of 20 °C. Heat of sorption (qst) was calculated using the chosen sorption model and Clausius–Clapeyron equation. An exponential function was found to fit the experimental qst values of amaranthus stems at moisture contents ranging from 7% to 40% (wb) for adsorption and desorption.