Water adsorption on lyophilized Arbutus unedo L. fruit powder: Determination of thermodynamic parameters

Abstract The knowledge on the hygroscopic equilibrium of powders is an average to maintain the moisture content at the levels suitable for safe storage. Presently, the experimental sorption curves of lyophilized Arbutus unedo L. fruit powder (LP), determined at 20, 30 and 40 °C with the standard static-gravimetric method, were fitted to six isotherm models (Kuhn, Caurie, Smith, Halsey, Oswin and GAB). The Halsey model best fits the experimental data with the mean relative percentage deviation modulus (E%)  2  ≤ 1.68 10 −3 and a root mean square error ((RMSE) ≤ 1.081 10 −2 ). The Clausius-Clapeyron equation was used to evaluate the net isosteric heat (Qst) of water adsorption through the Halsey model in the studied temperature range. Qst and the differential entropy values decreases trongly as the moisture content increase, varying from 6.771 to 52.337 kJ mol −1 and from 18.942 to 151.4 k J mol −1 , respectively, and could be well adjusted by an empirical exponential relationship. The value of Gibbs free energy is found to be (−1.251 k J mol −1 ). Such data represent a useful tool for choosing appropriate storage conditions of LP.