Water sorption isotherms of different sodium alginates: Thermodynamic evaluation and influence of mannuronate‐guluronate copolymers

Abstract

Water desorption isotherms of three alginates with different structural features were determined at 25, 37, and 50°C. The Halsey model was selected to fit the equilibrium water sorption data. Differential and integral enthalpy and entropy were estimated for tested alginates. Optimal storage conditions of tested alginates (moisture content from 0.15 to 0.20 kg water/kg dry solid and relative humidity from 35% to 50%) were determined from the maximum and minimum integral enthalpy and entropy values, respectively. A model was proposed to estimate the water sorption isotherms of alginates based on the alginate monomers (mannuronate, M, and guluronate, G) at low water activity (<0.4). M fraction was mainly responsible for the hygroscopicity of alginates. Alginates with similar G fraction showed different hygroscopic features by the presence of more homopolymeric G blocks that could form helical structures at low moisture content, decreasing the water affinity. Practical applications Determination of water sorption isotherms is fundamental to determine the optimal storage conditions at different temperatures. Their knowledge is essential for designing drying equipment, selecting adequately drying conditions (temperature and relative humidity of air) and drying time. Mathematical models are useful to estimate equilibrium moisture content in wide ranges of water activity and temperature. The thermodynamic study also provides valuable information about energy consumption and consequently operational costs. In this case, high content of mannuronate, M, increases the hygroscopic character of alginates, but the optimal moisture content of dried alginates to achieve maximum stability during storing varies in a narrow interval (0.15–0.20 kg water/kg dry solid).