Preparation, characterization and antibacterial applications of ZnO-nanoparticles coated polyethylene films for food packaging

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Abstract

The present work describes the preparation of ZnO nanoparticles loaded starch-coated polyethylene film. The presence of ZnO nanoparticles was confirmed by surface plasmon resonance (SPR), X-ray diffraction (XRD) studies and transmission electron microscopy (TEM). The ZnO loaded film was tested for its biocidal action against model bacteria Escherichia coli using zone inhibition and killing kinetics of bacterial growth methods. This newly developed material bears potential to be used as food packaging material to prevent food stuff from bacterial contamination.

Highlights

Polyethylene films have excellent mechanical strength and moisture barrier properties. ► Use of polyethylene based films can make an appreciable contribution towards fighting against environmental hazards. ► In addition, use of ZnO nanoparticles as antimicrobial agent, makes these films more eco-friendly, thus, these films can be used as antibacterial food packaging material.

Introduction

Recently, there has been a tremendous research work on the development of food packaging films that can be employed for controlling the microbiological decay of perishable food products [1], [2], [3]. The water is often one of the components of food stuff, which also contains carbohydrates, proteins, fats and mineral salts. Under suitable conditions of temperature and moisture, the interactions between these constituents can cause browning and lipid oxidation, among other reactions, and can provide the appropriate conditions for microbiological growth [4]. The antimicrobial packaging films have shown great potential to control growth of food borne pathogens, including Listeria monocytogenes, Escherichia coli 0157; H7 and Salmonella typhimurium [5]. These films are usually loaded with antimicrobial agents, which on coming in contact with food stuff, act upon food-born microorganisms and inhibit their growth. These agents belong to a wide spectrum of organic/inorganic compounds [6], essential oils [7], bacteria-originated antibacterial proteins (bacteriocins) [8], enzymes [9], fruit extracts [10], etc. These antibacterial agents have shown great potential in inhibiting microbial growth in food stuff. However, the development of new resistant strains of bacteria to current antibiotics [11] has led to the search for new bactericides that can effectively reduce the harmful effects of microorganisms. With the emergence of nanotechnology, the search for effective biocidal agents has focused on the development of nanostructures of coinage metals like silver, copper, zinc and gold [12].

Out of these, silver and gold cannot be used on industrial scale due to their high cost. Therefore, current research work focuses metal oxides like ZnO as antibacterial agents to prevent bacterial growth. Recently, there have been several reports regarding the antimicrobial activity of ZnO nanoparticles [13]. It has been reported, on the basis of preliminary growth analysis, that ZnO nanoparticles have higher antibacterial effects on microorganisms like Salmonella aureus than other metal oxide nanoparticles [14]. Similarly, Tam et al. [15] have reported antibacterial activity of ZnO nanorods prepared by hydrothermal method. ZnO exhibited fair activity against E. coli and Bacillus atrophaeus, but it was considerably more effective in the latter case (at 15 mM versus 5 mM concentration, respectively, showing zero viable cell count). For both organisms, damage of cell wall was observed.

Although biodegradable polymers like starch, gelatin, and cellulose are the best raw-materials for preparation of packaging films, their hydrophilic nature has confined their applications to laboratory experiments only. On the other hand, polyethylene-based films show excellent mechanical strength, hydrophobicity and moisture barrier properties which are some essential features for a good packaging film. However, they suffer from lack of functionalities, to attach antibacterial agents.

In the present work, we have reported in situ preparation of ZnO nanoparticles onto starch-coated polyethylene film and investigated antimicrobial action of film against E. coli. Zinc oxide has frequently been used in food stuff. It decomposes into Zn2+ ions after going into human body. Wheat proteins, fortified with ZnO have been proved to possess good zinc absorption [16].

Section snippets

Materials

Starch and polyethylene films were obtained from HiMedia Laboratories, Mumbai, India. Zinc sulphate, sodium hydroxide, nutrient agar, agar–agar Type-1, and nutrient broth were received from CDH Chemicals, Mumbai, India. Standard cultures of the organisms were provided by the Department of Biotechnology, Govt. Model Science College, Jabalpur, India. Millipore water was used throughout the investigations.

Coating of starch onto polyethylene film

The coating of starch onto polyethylene films was carried out at room temperature. Known

Preparation of ZnO nanoparticle loaded SCP film

The method involves in situ formation of ZnO nanoparticles within starch network on polyethylene film is a novel approach. When SCP film is placed in Zn(II) solution, Zn(II) ions enter into starch network and are bound to electronegative or electron rich oxygen atoms present in the cellulosic rings. In fact oxygen atoms serve as binding sites for incoming Zn(II) ions. Later on, when Zn(II) ion-attached SCP film is placed in sodium hydroxide solution, Zn(II) ions are precipitated as Zn(OH)2

Conclusion

This work infers that polyethylene film can be loaded with ZnO nanoparticles using starch coating as supporting matrix. These fibers show fair biocidal action against E. coli. Finally, use of ZnO nanoparticles, as antimicrobial agent, makes these films more eco-friendly, and economically cheaper as compared to other metal nanoparticles like silver and gold.

Acknowledgements

The authors are thankful to Dr. O.P. Sharma, Head of the Department of Chemistry for providing facilities. One of the authors is also thankful to the Council of Scientific & Industrial Research (HRDG), New Delhi, for providing financial support via Research Associate Project [08/031(0009)/2010-EMR-I].

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