Citation: Silva, R.R.A.; Marques, C.S.; Arruda, T.R.; Teixeira, S.C.; de Oliveira,T.V.; Stringheta, P.C.; dos Santos Pires, A.C.; de Fátima Ferreira Soares, N. Ionic Strength of Methylcellulose-Based Films: An Alternative for Modulating Mechanical Performance and Hydrophobicity for Potential Food Packaging Application. Polysaccharides 2022, 3, 426–440. https://doi.org/10.3390/ polysaccharides3020026 Academic Editor: Rajkumar Patel Received: 17 March 2022 Accepted: 9 May 2022 Published: 20 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 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/). Article Ionic Strength of Methylcellulose-Based Films: An Alternative for Modulating Mechanical Performance and Hydrophobicity for Potential Food Packaging Application Rafael Resende Assis Silva 1, * , Clara Suprani Marques 2 , Tarsila Rodrigues Arruda 2 , Samiris Cocco Teixeira 2 , Taíla Veloso de Oliveira 2 , Paulo Cesar Stringheta 2 , Ana Clarissa dos Santos Pires 2 and Nilda de Fátima Ferreira Soares 2 1 Department of Materials Science and Engineering, Federal University of São Carlos, São Carlos 13565-905, Brazil 2 Food Technology Department, Federal University of Viçosa, Viçosa 36570-900, Brazil; supraniclara@gmail.com (C.S.M.); tarsila.arruda@ufv.br (T.R.A.); samiristeixeira@gmail.com (S.C.T.); taila.oliveira@ufv.br (T.V.d.O.); stringap@ufv.br (P.C.S.); ana.pires@ufv.br (A.C.d.S.P.); nfsoares10@gmail.com (N.d.F.F.S.) * Correspondence: rafaelras@estudante.ufscar.br; Tel.: +55-(37)-99131-3471 Abstract: The growing environmental concern with the inappropriate disposal of conventional plas- tics has driven the development of eco-friendly food packaging. However, the intrinsic characteristics of polymers of a renewable origin, e.g., poor mechanical properties, continue to render their practical application difficult. For this, the present work studied the influence of ionic strength (IS) from 0 to 500 mM to modulate the physicochemical properties of methylcellulose (MC). Moreover, for protec- tion against biological risks, Nisin-Z was incorporated into MC’s polymeric matrices, providing an active function. The incorporation of salts (LiCl and MgCl 2 ) promoted an increase in the equilibrium moisture content in the polymer matrix, which in turn acted as a plasticizing agent. In this way, films with a hydrophobic surface (98 ◦ ), high true strain (85%), and low stiffness (1.6 mPa) can be manufactured by addition of salts, modulating the IS to 500 mM. Furthermore, films with an IS of 500 mM, established with LiCl, catalyzed antibacterial activity against E. coli, conferring synergism and extending protection against biological hazards. Therefore, we demonstrated that the IS control of MC dispersion presents a new alternative to achieve films with the synergism of antibacterial activity against Gram-negative bacteria in addition to flexibility, elasticity, and hydrophobicity required in various applications in food packaging. Keywords: ionic strength; food packaging; active films; antibacterial activity; plasticizing effect; methylcellulose; strain true; performance mechanical 1. Introduction Bio-based polymers have been widely studied as potential food packaging materials to replace, totally or in part, petroleum-based plastics [1–7]. However, certain features, such as higher gas/water vapor permeability, poorer UV–Vis barrier, higher costs of production, poorer mechanical properties, and higher hydrophilicity, still hinder or prevent their practical application [4,6]. In order to improve these properties and bring them closer to the those exhibited by conventional plastics, many strategies are proposed: chemical or physical modification, incorporation of additives with particular properties, such as plasticizers and nanoparticles fillers, blends of two or more polymers, and optimization of the sample preparation method [1,6,8,9]. The modulation of ionic strength (IS) through the incorporation of salts is a scarcely studied approach that appears promising regarding material enhancement. For example, by adding divalent salts into polymeric matrices, Silva et al. [3] manufactured highly flexible Polysaccharides 2022, 3, 426–440. https://doi.org/10.3390/polysaccharides3020026 https://www.mdpi.com/journal/polysaccharides