**1. Introduction**

Plastic materials are widely used globally due to their low production cost and excellent barrier, mechanical and thermal properties. Such synthetic plastics are used in various areas and products, such as food packaging, supermarket bags, toys, electronic devices, kitchen appliances, automotive parts and medical devices, among many others. The extensive use of plastic materials derived from petroleum, coupled with their inadequate disposal, has negatively impacted the environment. It is estimated that the global consumption of plastic exceeds 700 million tons per year [1]. To reduce and avoid this environmental problem, the use of polymers obtained from renewable sources to produce plastic materials is being more widely considered, mainly due to their rapid biodegradation and viability of being compostable. Polymers that can produce biodegradable packages/films can be polysaccharides such as starch, chitosan, cellulose and their derivatives, alginate and pectin;

**Citation:** Díaz-Cruz, C.A.; Caicedo, C.; Jiménez-Regalado, E.J.; Díaz de León, R.; López-González, R.; Aguirre-Loredo, R.Y. Evaluation of the Antimicrobial, Thermal, Mechanical, and Barrier Properties of Corn Starch–Chitosan Biodegradable Films Reinforced with Cellulose Nanocrystals. *Polymers* **2022**, *14*, 2166. https://doi.org/10.3390/ polym14112166

Academic Editor: Evgenia G. Korzhikova-Vlakh

Received: 27 April 2022 Accepted: 18 May 2022 Published: 26 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

proteins such as gelatin, zein, and collagen; or polyesters such as polyhydroxybutyrate, polylactic acid, polycaprolactone [2–6].

The use of starch as a packaging material has been widely investigated in recent years due to its high availability in nature, multiple sources of production, and economic viability [7–9]. Meanwhile, chitosan, i.e., a biopolymer obtained from waste from the fishing industry, is a product with significant added value which can be obtained in large quantities [10–12]. It has been reported that the protonated amino groups of chitosan promote the formation of intermolecular bonds with the structural matrix of starch films, giving rise to a material with better mechanical performance, good thermal stability, and better water resistance [13,14]. This biopolymer has demonstrated antimicrobial, antifungal, anti-inflammatory, and antioxidant properties, making it an excellent alternative for the production of active materials for various industries such as food, packaging, and medicine [12,15]. Chitosan is a widely investigated polymer for the preparation of composite materials due to its excellent compatibility with compounds of both natural and synthetic origin, promoted by amino, hydroxyl, and carboxyl groups [13,15]. With the mixture of both biopolymers, new, low-cost packaging materials could be generated that could also improve the quality of packaged products.

Recent studies have evaluated the use of nanometric compounds for the reinforcement of materials made from natural polymers. It has been found that reducing the particle size of the reinforcing materials promotes more homogeneous dispersion and increases the specific surface of the reinforcements, such as nanocellulose and nanoclays [16–19]. Nanocellulose is produced from cellulose, the most abundant polymer in nature, which can be obtained from the residues of agricultural byproducts. Cellulose is subjected to processes that can be enzymatic, mechanical, or chemical, in which its amorphous regions are eliminated, giving rise to a more crystalline and ordered nanometric structure [20–22]. Nanocellulose is a renewable material which is friendly to the environment, and due to its size, it can provide exciting advantages for packaging materials. The use of nanocomposites improves the thermal, mechanical, and barrier properties of packaging materials by using lower reinforcement loads (1 to 5% by volume) compared to when micrometric or larger size reinforcements are used [23]. In addition to improving the physicochemical properties of the materials, some nanocomposites have exhibited significant antimicrobial and antioxidant capacities, which would benefit the food packaging industry [5,17,22,24,25]. The food industry is currently trying to transition to cleaner products by reducing the number of additives and preservatives of synthetic origin, promoting research using various types of preservatives and antibacterials. The use of nanoparticles has been widely investigated as a substitute for various antibacterials, mainly to reduce and combat the global problem caused by the extensive use of antibiotics, which generate resistance in microorganisms [26,27] The antibacterial activities of nanomaterials depend on the properties of the nanoparticles and the bacteria of interest. Therefore, more research should be carried out, since there are many types of nanoparticles with different effects on microorganisms, depending on their nature, morphology, size, and composition.

Various studies have been carried out on the development of packaging materials using natural polymers in pure form or as mixtures, as well as nanometric materials for the improvement of physicochemical properties. The polymer–nanomaterial relationship plays a fundamental role in the development, functional properties, and application of such materials. Previously, it was possible to develop starch–chitosan composite materials with good handling and barrier characteristics; however, when seeking to incorporate a nano-sized material such as cellulose crystals, the information reported in the literature was inconsistent (0.5 to 50% *w*/*w*) [20,23,28,29]. This is due to the wide ratio when nanomaterials are incorporated into packaging materials, since their effect depends on the type of polymer, its composition, and its interactions with the matrix. As such, each nanomaterial must be evaluated to find the appropriate concentration for the material to be used.

The objective of this study was to evaluate the antimicrobial, thermal, water vapor barrier, and mechanical performance of corn starch–chitosan-based films when a reinforcing material such as cellulose nanocrystals (CNC) was incorporated at different concentrations (0, 0.5, 2.5, 5, 7.5, and 10% *w*/*w* biopolymers). Nanocrystals could substantially improve the properties of these biodegradable films and, in the future, be a viable alternative to synthetic food packaging.

#### **2. Materials and Methods**
