Combining carvacrol and nisin in biodegradable films for antibacterial packaging applications

Highlights

• Antibacterial biodegradable films incorporating carvacrol and/or nisin were prepared by melt mixing and compression molding.

• Both carvacrol and nisin significantly reduced the biodegradable films wettability.

• Carvacrol and nisin affected in an opposite way the mechanical and thermal property of biodegradable films.

• Carvacrol and nisin release showed an anomalous and diffusion-controlled mechanism, respectively.

• The best antimicrobial performances were obtained by films loaded with 20 phr carvacrol, alone and in combination with nisin.

Abstract

In this work, the feasibility of antibacterial biopolymeric films containing carvacrol (CRV) and a nisin commercial formulation (Nis) for potential food packaging applications was investigated. As polymer matrix, a commercial biodegradable polymer formulation of Mater-Bi (MB) was chosen due to its significant food packaging applications. CRV and Nis were chosen due to their well-established antibacterial properties and their potential synergistic effect. MB/CRV, MB/Nis, and MB/CRV/Nis systems were produced by melt mixing and compression molding. The mechanical properties of the films were evaluated by tensile tests. Differential scanning calorimetry was assessed aiming at investigating the effect of the two compounds and their mixture on the thermal properties of MB. The release profile of CRV and Nis from the MB-based films was evaluated in water at 4 °C by UV–Vis measurements and it was fitted with a power-law model. The antibacterial activity of MB-based films was tested in vitro against Listeria monocytogenes, Salmonella enteritidis, Escherichia coli, and Staphylococcus aureus. The combination of CRV and Nis strongly affected the properties of the MB-based films and ensured higher antibacterial activity if compared to MB/CRV and MB/Nis systems.

Introduction

In recent years, there is growing interest in the development of biopolymeric films due to the increasing health concerns about food safety and the widespread environmental pollution caused by non-biodegradable food packaging [1], [2], [3], [4].

Films for food packaging applications should provide food protection from external contaminants and should prevent chemical, physical, and biological changes during storage [5], [6]. Conventional packaging can offer several features for food protections acting as barriers to oxygen, moisture, and light [7], [8]. Developments in materials science and bioengineering recently focused the research attention on a novel packaging technique commonly known as active packaging (AP) that can improve the maintenance of quality and enhance food safety [9], [10], [11]. Active packaging permits continuous and potentially prolonged diffusion of active compounds into the food environment reducing the needs for preservatives [12], [13], [14], [15].

Over recent years, several additives, including natural compounds, peptides, enzymes, metals, chelating agents, and antibiotics, were incorporated into bio-polymeric matrices to provide antimicrobial activity [10], [16], [17]. Among them, plant essential oils (EOs) are interesting natural antimicrobial agents to be incorporated into the biopolymeric films or membranes due to their high inhibitory potential against a wide spectrum of microorganisms [18], [19]. Among the great variety of EOs, carvacrol (CRV), commonly present as the main compound in thyme and oregano EOs, has gained greater acceptance among food technologists due to its ability to inhibit undesired pathogenic and spoilage microorganisms [20], [21], [22]. Furthermore, CRV is a “generally recognized as safe” food additive and is approved by the U.S. Food and Drug Administration (FDA) for use in foods and drinks [13], [23].

Several research papers report the synergistic effect of CRV with other antimicrobial systems. This feature potentially enables the reduction of required amounts to be applied to treated food thus reducing unwanted modifications in their physical properties [24], [25], [26]. Recently, the synergic effect of carvacrol and nisin on the survival of L. monocytogenes 10403S on sliced bologna sausages was demonstrated [26].

Nisin is a lantibiotic (i.e., a peptide-based antibiotic) commonly present in milk, produced by Lactococcus lactis. Nisin is usually used as food preservative with specific inhibitory activity against Gram positive bacteria [27], [28]. It is an authorized food additive in the European Union (EU) under Annex II of Regulation (EC) 1333/2008 for use in several food categories, and it is currently used in over 50 countries to improve food safety and extend product shelf life [26].

However, to the best of our knowledge, the effect of the simultaneous loading of CRV and nisin in the same MB-based matrix was never reported so far.

This study aims to investigate the feasibility, the physical and antibacterial performances of Mater-Bi (MB) films containing CRV and a commercial formulation of nisin (Nis) for food packaging applications. MB finds important applications (mainly for packaging), owing to its interesting mechanical properties, especially in terms of stretchability and toughness [29], as well as good processability as reported in several studies [30], [31].

MB/CRV, MB/Nis, and MB/CRV/Nis systems were prepared by melt mixing and then filmed in hot press. MB-based films wettability was investigated by water contact angle measurements. The mechanical properties of the films were evaluated by tensile tests. Differential scanning calorimetry was carried out in order to investigate the effect of the two compounds and their mixture on the thermal properties of MB. The release profile of MB-based films in water at 4 °C was evaluated by UV–Vis measurements and it was fitted with a power-law model. The antibacterial activity of MB-based films was tested in vitro against the main food-borne pathogenic bacteria.