Rheological characterization of deacylated/acylated gellan films carrying l-(+)-ascorbic acid

Films developed for supporting l-(+)-ascorbic acid (AA), natural antioxidant for the protection of foods, were rheologically evaluated. A film was formulated by mixing gellan gum with its acylated form to attain a higher AA stability and lower non-enzymatic browning. The polymer mixture allowed obtaining a less rigid film, permitting the usage of a lower proportion of glycerol. Mechanical spectra of films evidenced solid-like materials provided the time scale of the observation was shorter than the lifetime of the physical cross-links involved. There was evidence for a shorter time of relaxation in the films stored at 33.3–57.7% of relative humidity, which influenced small deformation (linear range) measurements. Out of linear viscoelasticity a greater hardening effect was observed for A- and B-films and related to enhanced polymer interactions in gellan based films. Its increase with the moisture content may be a result of macromolecular association. Higher rigidity characterized gellan based films (A- and B-networks) at intermediate deformations, which may also lead to higher softening degree till rupture. A smaller hardening and softening degrees were observed for C- and D-systems. This can be ascribed to spatial hindering in the film network by acylated gellan chains, fact that worsened polymer interactions. Atomic force microscopy images reinforced rheological conclusions.