Modelling the Water Sorption Isotherms of Quinoa Seeds (Chenopodium quinoa Willd.) and Determination of Sorption Heats

Adsorption and desorption isotherms of quinoa seeds (Chenopodium quinoa Willd.) were measured using the static gravimetric method at three temperatures (20, 40 and 60 A degrees C). Water activity ranged from 0.118 to 0.937. The moisture sorption behaviour of quinoa was temperature dependent, as indicated by a decrease in equilibrium moisture content, at all levels of a (w), with increasing temperature. Eight mathematical equations available in the literature were used to model the experimental data, namely, GAB, BET, Caurie, Henderson, Oswin, Halsey, Smith and Iglesias-Chirife. All the equations showed generally a good fit; however, the Iglesias-Chirife and Oswin equations were considered the best to predict the experimental data for both isotherms. Effect of temperature on model parameters was analysed and studied through an Arrhenius-type equation. The net isosteric heats of desorption and adsorption were determined by applying the Clausius-Clapeyron equation resulting in 69.24 kJ mol(-1) for desorption and 61.26 kJ mol(-1) for adsorption. The experimental heat data were satisfactorily modelled by Tsami's equation.