Characterization of ionically crosslinked alginate films: Effect of different anion-based metal cations on the improvement of water-resistant properties

In general, calcium ions are widely used to crosslink alginate films for improved mechanical properties. But for the poor water resistance of the alginate films due to their hydrophilicity, various ion-crosslinking agents (CaCl2, FeCl2, FeCl3, and FeSO4) were used to prepare alginate films, with an emphasis on cations (Ca2+, Fe2+, and Fe3+) and anions (Cl- and SO42-) with different valences. Chemical, morphological, optical, and mechanical properties were evaluated to demonstrate their influences on ion-crosslinking. All crosslinked films exhibited higher tensile strength and lower elongation at break than the control alginate film. Optically, the films crosslinked with Fe ions showed a yellow appearance, while the CaCl2-crosslinked film showed no significant color difference from the alginate film. As for the water-resistant properties [moisture content, water solubility, water vapor permeability (WVP), and swelling index (SI)], the FeSO4-crosslinked film presented the lowest WVP value with the highest SI value than others, suggesting the possibility to WVP increase depending on the reaction time. Thus, we studied the effect of crosslinking time on the water resistance of films using CaCl2 and FeSO4. The FeSO4-crosslinked film exhibited a significant decrease in WVP value with increased crosslinking time, an opposite trend from that of the CaCl2-crosslinked one. Because water vapor permeability is an important property of food packaging materials, FeSO4, which is generally recognized as safe (GRAS) but has rarely been studied as an ionic crosslinking agent for alginate, has a high potential to be used as such in alginate films.

Automated summary

Calcium ions are commonly used to crosslink alginate films for better mechanical properties. However, due to the poor water resistance of alginate films, various ion-crosslinking agents have been used to prepare alginate films with different valences. The properties of these crosslinked films were evaluated, and FeSO4 was found to have the potential to improve water resistance. The water vapor permeability and swelling index of FeSO4-crosslinked film were significantly reduced with increased crosslinking time.