## 3 Results and Discussion
### Rheology of the Edible Coating Solution
#### 3.3.3 Rheological Properties of Edible Film  
The physicochemical and functional properties of edible films, such as mechanical, rheological and optical properties, are directly related to their microstructure and the interaction between film components and the drying conditions [65]. The rheological properties of edible films were evaluated in order to analyze the influence of combining chitosan with licorice extracts in the coating solutions, and to assess the particular characteristics and modifications in comparison with using only chitosan. All the films were flexible and easily detachable from the sample holder, without evident defects in the form of cracks or pores. Figure 4a shows the frequency dependence of storage modulus (G\({}^{\prime}\)) and loss modulus (G\({}^{\prime\prime}\)) of films of Ch, Ch + LE-1 and Ch + LE-5. For Ch films, G\({}^{\prime}>>\) G\({}^{\prime\prime}\) over the entire frequency range studied, indicating that chitosan film exhibits dominant elastic behavior. G\({}^{\prime}\)>G\({}^{\prime\prime}\) in the case of Ch + LE-1, and the film presents a more robust network and exhibits a gel-like behavior. Finally, for Ch + LE-5 film, G\({}^{\prime\prime}\)> G\({}^{\prime}\), exhibiting a viscous behavior. Then, the addition of licorice phytochemicals to chitosan increases the viscous properties of film, displaying a variation in G\({}^{\prime}\). Tan \(\delta\) (= G\({}^{\prime\prime}\)/G\({}^{\prime}\)) increase with the addition of licorice extract to chitosan and values become Tan \(\delta\approx\) 1, corroborating that the viscous quality of the film is greatly affected by the addition of LE and suggesting that the films containing LE have higher flexibility compared to the chitosan film and thus, they are more suitable than pure chitosan for application as edible food coatings. The creep compliance data are shown in Figure 4b. The extensional creep curves of all films showed a typical behavior of viscoelastic material, with varying degrees of viscoelasticity. The compliance of films decreases with the addition of licorice extracts and with the percentage amount of extract added. It can be inferred that films became more flexible with the addition of LE to chitosan, indicating an interaction between LE phytochemicals with possible hydrogen bonds formation. The creep results were simulated by applying the Burger model (Equation (2)) and a satisfactory description of the essential features of the film viscoelasticity was attained, with high correlation coefficients (R\({}^{2}\) > 0.999) (Table 5). The parameters of the Burger model reflect the structure of any system, and the decreasing/increasing of these parameters shows the weakening/strengthening of the structure. The instantaneous compliance (J\({}_{o}\)) of chitosan was 3.793, and decreased to 0.482 and 0.801 for Ch + LE-1 and Ch + LE-5 respectively, indicating a reduction of rigidity or strength [66; 67]. The retarded compliance (J\({}_{1}\)) of chitosan was 1.408 mPa\({}^{-1}\), decreasing to 0.481 and 0.626 respectively, with the addition of 1% and 5% of LE, indicating a reduction of the gel cohesive force and also a reduction of the resistance to deformation caused by the three-dimensional network structure [66; 67]. Newtonian viscosity (\(\mu_{o}\)) increased significantly with the addition of LE to chitosan, revealing that  
Figure 4: Rheological properties of chitosan and chitosan-licorice edible films: (**a**) frequency sweep and (**b**) creep and recovery. mechanical strength and mobility of polymer chains increased with the addition of LE to chitosan in the coating formulation. Recovery percentages are an indirect measure of gel elasticity and the calculated values for the different coating films are given in Table 5. The recovery percentages of the films with LE extracts (83.78% and 51.09% for Ch + LE-1 and Ch + LE-5, respectively) were considerably higher than that of chitosan (35.82%).