The modelling of adsorption isotherms is an important method for analysing the stability of spray-dried powders. These properties are intimately related with the inflection points of the isotherm curve. The accurate calculation of the inflection points provides a method to determine the critical water activity (RHc) of powders. Though the inflection points can be estimated from standard models such as BET or GAB, in the present work we employ Least Squares Fitting-Polynomials for representing the adsorption isotherms, which in turn leads to a minimum error in the calculation of the inflection points. Then we investigate the physicochemical sense of the inflection points thus calculated for spray-dried acai juice (Euterpe oleracea Mart.) and soy sauce powders and compare with the results reported in literature. Here we observe that from the LSF-polynomials, the inflection points may give an approximation of the value of the monolayer moisture content, and the critical water activity. (C) 2018 Elsevier B.V. All rights reserved.