The swelling properties of single component (gelatin) and electrostatically assembled (gelatin–gum tragacanth) edible composite films exposed to water and salt solution

The four-dimensional (4D) printing concept, defined as a targeted change in material properties under stimuli such as water, ultraviolet (UV) exposure, and heat, has been under the spotlight in recent years due to its promising functionalities and design freedom for food applications. However, there is little progress in food applicability and compatibility compared to materials science. The complexity of food, poor response, and lack of trigger mechanisms are the major problems for expanding the 4D printing concept with edible ingredients. Therefore, exploring the possible mechanisms using edible materials to implement engineering-driven predictive changes in food applications is of significant interest. This study investigated the use of model systems composed of food biopolymer solutions (gum tragacanth and gelatin at 0.5, 2.5, and 5% w/v) and their swelling behavior in distilled water and salt (0.5 M CaCl2) solution to control the water uptake rate. In this respect, the film-forming solutions were prepared, the films were cast in Petri dishes (5–20 ml), and the film thickness and moisture content values were recorded. Their swelling properties were determined in two different media (water immersion and salt solution immersion tests). The viscoelastic properties of selected film-forming solutions were analyzed. The thickness of the films increased with the increased poured volume of the film-forming solution (P < 0.05). The biopolymer type and concentration had a significant effect on the physical properties of the films. All samples exhibited shear-thinning behavior and frequency dependency with elastic or viscous dominant properties.