**1. Introduction**

In recent years, antimicrobial packaging has attracted a great deal of attention from the food industry because of the increase in the consumer demand for chemical preservativefree products [1]. Moreover, there is growing interest in biodegradable packaging as an eco-friendly replacement for petroleum-based polymers. As abundant, renewable, environmentally friendly, and sustainable materials, biopolymers have been used to fabricate biodegradable plastics and edible films [2,3]. Among biopolymers, polysaccharides are attractive materials because of their good film-forming properties, sustainability, and abundance [4,5].

Pectin is the major structural component of the plant cell wall and is one of the largest constituents in citrus by-products. This polysaccharide is generally recognized as safe (GRAS) by the United States Food and Drug Administration (FDA), and is used in food processing, mostly as a gelling, stabilizing, or thickening agent in products such as jams, yogurt, fruit drinks, and ice cream [6]. Pectin is mainly composed of galacturonic acid units. Depending on its degree of esterification (DE), pectin can be classified as high methoxyl pectin (DE > 50%) and low methoxyl pectin (DE < 50%), with both being able to be used for film formation [7]. Some of the advantages of pectin edible films are biocompatibility, biodegradability, and non-toxicity [8]. Nevertheless, neat pectin films do not show satisfactory functionality because of their high hydrophilicity, poor mechanical and barrier properties, and low water resistance [9,10]. Hence, various strategies have been tested for improving the properties of pectin-based films, including crosslinking, blending with other polymers, and incorporating nanoparticles and essential oil nanoemulsions [7,10]. Such films can also be engineered to exhibit functionalities. For instance, pectin films incorporating silica nanoparticles extended the shelf life of strawberries, and pectin/corn flour/beetroot powder films extended the shelf life of tomatoes [11,12]. Pectin

**Citation:** Hari, K.D.; Garcia, C.V.; Shin, G.-H.; Kim, J.-T. Improvement of the UV Barrier and Antibacterial Properties of Crosslinked Pectin/Zinc Oxide Bionanocomposite Films. *Polymers* **2021**, *13*, 2403. https:// doi.org/10.3390/polym13152403

Academic Editor: Evgenia G. Korzhikova-Vlakh

Received: 2 July 2021 Accepted: 19 July 2021 Published: 22 July 2021

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**Copyright:** © 2021 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/).

films incorporating lime antioxidants hindered soybean oil oxidation [13]. Films made of a combination of pectin and hydrocolloids from chia seeds showed antioxidant properties, and electrospun pectin films were used in multilayer structures with aroma barrier properties [14,15]. Moreover, pectin films incorporating ZnO nanoparticles (ZnO-NPs) were reported to show enhanced strength and water barrier properties as well as antimicrobial effects [9].

Compared with neat pectin films, crosslinked pectin films exhibit increased tensile strength and water resistance [16]. The crosslinking mechanism of pectin with divalent metal ions such as Ca2+ and Zn2+ is explained by the "egg-box" model, in which the crosslinks are formed by divalent ions occupying electronegative cavities in the ribbon structure of carboxylic groups [17]. Hence, the functional and mechanical properties of pectin films could be improved by incorporating both calcium ions and ZnO-NPs.

ZnO-NPs exhibit advantages such as non-toxicity, availability, low cost, high ultraviolet absorption capacity, and strong antimicrobial activity [18,19]. Compared to other natural antibacterial materials such as nisin, essential oils, grapefruit seed extract (GFSE), etc., ZnO-NPs have high thermal stability and do not lose their antibacterial activity during the processing of most food packaging films such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), etc. It is thought that the antimicrobial activity of ZnO-NPs is dependent on the release of Zn+2 ions and the formation of reactive oxygen species (ROS), which damage the integrity of the microbial cells [20]. ZnO is recognized as a GRAS substance by the FDA, although no differentiation is made between the bulk and nano form. Moreover, ZnO-NPs are approved for certain food contact applications in the European Union [21].

Therefore, here, we aimed to optimize the development of crosslinked pectin-based films incorporating ZnO-NPs. The effects of calcium chloride and ZnO-NP concentration on the mechanical, physical, optical, UV barrier, and antibacterial ability of the bionanocomposite films were evaluated. In particular, the goal of this study was to produce bionanocomposite films with enhanced UV-blocking and antibacterial properties.

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

#### *2.1. Materials*

Pectin powder from citrus was purchased from Daejung Chemicals & Metals Co., Ltd. (Siheung, Korea). Glycerol was obtained from Duksan Pure Chemicals Co., Ltd. (Ansan, Korea). Calcium chloride (CaCl2) and ZnO nanopowder were purchased from Sigma Aldrich Chemicals (St. Louis, MO, USA). Nutrient broth (NB) was purchased from DB DifcoTM (Sparks, MD, USA). *Escherichia coli* ATCC 8739 and *Staphylococcus aureus* ATCC 6538 P were procured from the American Type Culture Collection (ATCC).

#### *2.2. Film Preparation*

#### 2.2.1. Crosslinked Pectin Films

To prepare crosslinked pectin films (CPF), 4 g of pectin was dissolved in 170 mL of distilled water. As a plasticizer, glycerol was added at 20% (*w/w*, based on pectin) to prevent brittleness. Then, a CaCl<sup>2</sup> solution (30 mL; 0.5%, 1%, 2%, and 5% *w*/*v*) was added to the pectin solution and mixed using a magnetic stirrer at 70 ◦C for 20 min. The film-forming solution was cooled down at room temperature for 30 min before casting it onto a Teflon-coated glass plate (13.5 cm × 25 cm) and drying at 30 ◦C in a convection oven for 48 h. The fully dried pectin films were conditioned at 25 ◦C and 50% relative humidity (RH) for 48 h before testing their mechanical properties.

#### 2.2.2. Crosslinked Pectin/ZnO Bionanocomposite Films

For the preparation of crosslinked pectin/ZnO bionanocomposite films (CPZBF), a ZnO-NP dispersion was prepared by adding ZnO nanopowder (0.5%, 1%, and 1.5%, *w/w*, based on pectin) to 170 mL of distilled water containing 20% (*w/w*) glycerol, followed by ultrasonication at 40% amplitude for 10 min. 4 g of pectin was dissolved in the homogeneous dispersion of ZnO-NPs and stirred at room temperature for 3 h. Then, 30 mL of a CaCl<sup>2</sup> solution (1% *w/w*, based on pectin) was added and mixed using a magnetic stirrer at 70 ◦C for 20 min. The film-forming solution was cooled down at room temperature for 30 min before casting it onto the Teflon-coated glass plate (13.5 cm × 25 cm) and dried as described in Section 2.2.1.
