Thermodynamic properties of moisture sorption in pullulan-sodium alginate based edible films

- Hydrophilic group -COO−Na+ was more easily exposure than the hydrophilic group -OH at high aw.
- Microstructures of all films were stable and did not suffer from any changes during moisture sorption within the tested temperature range.
- Moisture sorption process in pure pullulan films was non-spontaneous, whereas that process occurred spontaneous in alginate and blend films.

Moisture sorption isotherms of pure pullulan, alginate and blend films were determined using the gravimetric method at 25, 35 and 45 °C. The five-parameter Guggenheim-Anderson-de Boer (GAB) model was used to fit the isotherm data from the experiments by nonlinear regression. Additionally, the experimental data were used to determine the thermodynamic parameters for pullulan-alginate based films. Net isosteric heat values for the films decreased exponentially with increasing moisture content, and approached to zero after around 0.25 g/H2O g dry basis. Pure pullulan films exhibited lowest differential entropy values at a high moisture content range, whereas pure alginate films showed the lowest differential entropy values with increasing moisture content from 0.05 to 0.17 g/H2O g dry basis. Net integral entropy values for pullulan, alginate and blend films were observed to decrease to each minimum value, and then increase with moisture content increases. Moreover, according to the enthalpy-entropy compensation theory, the sorption mechanism for all tested films was enthalpy-controlled. The thermodynamic information obtained from this study is essential for designing equipment and standardized drying process for pullulan-sodium alginate based edible films.