Water adsorption properties of microalgae powders: Thermodynamic analysis and structural characteristics

Moisture adsorption isotherms and thermodynamic properties of dried Chlorella pyrenoidesa and Spirulina platensis powders were investigated by using a static gravimetric at temperatures of 20, 30, and 40 °C and within aw range of 0.112–0.976. Structural characteristics involving morphology, functional group, surface element composition, pore size distribution, and surface area were analyzed to explore the water adsorption mechanism. FTIR and XPS analysis indicated that spirulina powder had a higher proportion of hydrophilic sites to bind water molecules in comparison with chlorella powder. BET surface area for chlorella and spirulina powders were 25.10 and 25.19 m2/g, respectively. Water adsorption data were adjusted using a variety of sorption models, and the GAB equation was the most appropriate for representing the experimental data in the studied aw and temperature range. The monolayer moisture content (Xm) values at 20, 30 and 40 °C were 0.0526, 0.0483, and 0.0456 g/g dry basis for chlorella powders, and 0.0581, 0.0553, and 0.0420 g/g dry basis for spirulina powders, respectively. The net isosteric heat (Qst) and differential entropy (δSdif) for both microalgae powders decreased with an increase in water content. Using the compensation theory, we determined that the processes were governed by the enthalpy and that they were non-spontaneous (δG>0) for the samples. Both microalgae powers were classified as micro and mesoporous materials, and the pore radius (Rp) varied from 1.481 to 23.694 nm for chlorella powders and from 1.193 to 39.392 nm for spirulina powders, respectively.