Water sorption isotherms are unique for individual food materials and can be used directly to predict shelf life and determine proper storage conditions. In this context, the aim of this study was to determine the moisture adsorption isotherms of amaranth flour at 15˚C, 25˚C and 35˚C in a range of water activity from 0.1 to 0.9. Ex- perimental data were modeled using five equations commonly applied in the foods field. The goodness of the fit for each isotherm model was evaluated through the coefficient of determination, the variance due to error and the confidence interval of the estimated parameters. All models can predict the adsorption isotherms of ama- ranth flour, but the GAB equation gives a better understanding of the observed sorption behavior. Estimated adsorption monolayer water contents ranged from 6.4 g to 7.2 g of water per 100 g of dry material. It was ob- served by a weak dependence of water activity with temperature. For ensuring microbiological stability, water content in amaranth flour should not be higher than 13 g of water per 100 g of dry material.
In this paper, moisture adsorption of amaranth flour has been measured at 15˚C, 25˚C and 35˚C, using an alternative experimental method. The experimental adsorption isotherms exhibited a sigmoidal trend and were satisfactorily fitted to well-known models. The GAB equation adjusted the data quite well and is recommended, due to its physical meaning, to describe the adsorption behavior of amaranth flour.