Opacity

The opacity test was developed using a colorimeter (Konica Minolta, Japan). Data were processed by Spectra Magic NX software. Film samples with 2 cm × 10 cm were placed under aperture for opacity measurement, determined by using black (*Pb*) and white patron (*Pw*) as references. Opacity (*Op*) was calculated as the ratio between the opacity (*Pb*) and opacity (*Pw*). Both *Pb* and *Pw* were determined in five films with black and white standards (%) using Equation (3).

$$Op(\%) = \frac{P\_b}{P\_w} \times 100\tag{3}$$

3.2.4. Experimental Design and Optimization

A simplex-lattice design was used with two components and four lattices (*m*). The design points are given by *m* + 1. The five design points correspond to:

*xi* = 0, 1/*<sup>m</sup>*, 2/*m* ... *<sup>m</sup>*/*<sup>m</sup>*, where *i* is the number of components, leaving the following mixtures:

(*<sup>x</sup>*1, *x*2): (0, 1); (0.25, 0.75); (0.5, 0.5); (0.75, 0.25); (1, 0).

The mixtures are shown in Table 4.

The optimization was performed by setting the mechanical properties of a commercial polyethylene film as a target and minimizing the values of the *WVP*, and *WS* found in the blends. The desirability function (D: global; d: individual) that converts the functions to a scale between 0 and 1 was used, combining them using the geometric mean and optimizing the general metric means.

The experimental design and the optimization were performed using Minitab (2019) software.

## Statistical Analysis

We used an analysis of variance (ANOVA) to establish statistical differences between treatments. Fisher's test was used as a post-ANOVA analysis to compare the means between treatments. ANOVA and post-ANOVA were obtained using Minitab (2019) software.

## **4. Conclusions**

Cassava starch/gelatin composite films were obtained, with good mechanical properties comparable with reported values of commercial polyethylene films. The analysis of the FT-IR spectra for the composites showed the presence of hydroxyl and amide groups characteristic of starch and gelatin and with shifts of their bands at various ratios due to hydrogen bonds. Thermogravimetric analysis of the films showed that with a high amount of gelatin (T4 and T5), degradation profiles were poorly defined, characteristic of films with high flexibility. In the DSC analysis, it was evidenced that the endothermic peak Tc increased due to the presence of gelatin. Still, for T4 and T5, there was a notable decrease in its value, probably due to molecular disorder between the chains and greater flexibility.

Additionally, the morphological analysis of T2 and T3 showed minor roughness, discontinuity, and heterogeneity, and a decrease in the flexibility of gelatin compared to the heterogeneous appearance of the gelatin film (T5). The results of material stiffness through Young's modulus sugges<sup>t</sup> that the flexibility of the films increased with the increase in the amount of gelatin, decreasing, in turn, the stiffness of the material (from 2.9 ± 0.5 to 285.1 ± 10.0 MPa for formulations T1 and T5, respectively). On the other hand, the water solubility results of the cassava starch/gelatin films were between 3.67 and 21.26% for treatments T1 and T5, respectively, similar to those found for cassava starch- or cornstarchbased films with gelatin.

With all these characterization results for the composites, an optimal formulation was obtained to develop cassava starch/gelatin-based films in a 53/47 ratio, plasticized with glycerol using the casting method, that would meet the expectations of the model polyethylene film for food-packaging applications. Young's modulus, tensile strength, deformation, thermal, *WVP*, and water solubility variables were affected by the cassava starch and gelatin mixtures evaluated in the treatments. Based on the predicted values for each response in the optimization, it can be inferred that these films could be used as food-packaging material, as their mechanical properties are close to those of low-density polyethylene. Future studies could incorporate other additives to improve moisture stability properties such as *WVP*.

**Author Contributions:** Conceptualization, D.P.N.-P. and C.D.G.-T.; Funding acquisition, C.D.G.-T.; Investigation, J.I.C., D.P.N.-P., J.A.A.C. and C.D.G.-T.; Methodology, J.I.C., D.P.N.-P., J.A.A.C., J.H.M.H. and C.D.G.-T.; Resources, J.H.M.H.; Supervision, C.D.G.-T.; Writing—original draft, J.I.C., D.P.N.-P. and C.D.G.-T.; Writing—review and editing, D.P.N.-P., J.H.M.H. and C.D.G.-T. All authors have read and agreed to the published version of the manuscript.

**Funding:** The APC was funded by Universidad del Atlántico.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are available under request to the corresponding author.

**Acknowledgments:** The authors thank Universidad del Atlántico for the APC payment.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are available from the authors.
