Metal oxide nanoparticles for safe active and intelligent food packaging

Highlights

• Easy to fabricate, safe and cost-effective nanomaterials for food smart packaging.

• Antimicrobial biomaterials for food packaging are developed from metal oxide nanoparticles.

• Oxygen and ethylene molecules from the headspace of food packaging are absorbed.

• The safety of packaging material is evaluated on human cells, intestinal barrier, and microbiota.

• Packaging for indicating food quality are developed utilizing metal oxide nanoparticles.

Abstract

Background

Food safety and food security remain the major concern of consumers and the food industry. Bacterial contamination continues to be a crucial food safety issue. Smart packaging incorporates both active and intelligent components. Intrinsic antibacterial activity, oxygen and ethylene scavenging (active) and the sensing (intelligent) properties of metal oxide nanoparticles are in research focus for application in smart food packaging, especially bio-nanocomposite films.

Scope and approach

Metal oxide nanoparticle properties are closely linked to their morphology resulting from the synthesis process. In this review, we cover current innovative synthesis methods for obtaining metal oxide nanoparticles and current incorporation techniques used to obtain smart (active and/or intelligent) packaging, focusing on bio-nanocomposites, commonly used metal oxides and future mixed metal or doped metal oxides. Taking into account safety, we focus on current legislation, and methods for risk assessment due to particle release from the packaging material and a summary of cytotoxic studies of metal oxide nanoparticles on human cells and the gut microbiota.

Key findings and conclusions

Antimicrobial effectiveness of metal oxide nanoparticles is highly dependent on morphology as a result of the synthesis method. Solution casting and electrospinning are innovative methods applied to synthesize metal oxide incorporated biopolymer films for active packaging with improved mechanical and barrier properties combined with active components (antimicrobial, ethylene scavenging). Metal oxides show sensitivity and selectivity to most gases produced during food spoilage. In selection of metal oxide for smart packaging, particle migration and cytotoxic activity are key issues requiring careful and detailed characterization.

Introduction

The food industry is under constant and crucial pressure to provide appetizing and safe food products. To satisfy these consumer demands, the food industry regularly improves both the food quality and packaging technology. Food packaging is essential in maintaining the safety and quality of products from processing and manufacturing, through handling and storage until it reaches the consumers. Petroleum-based plastic materials (like polyethylene terephthalate, polypropylene, polystyrene) are usually used to envelop food in order to protect its content from contamination and spoilage and to facilitate its transport and storage. However, plastic materials cannot fully protect food from the environment and, thus, cannot completely ensure product quality and safety. In addition, plastic undergoes continuous fragmentation, and may create micro- and nano-plastics that have potential toxic impacts on human health. Plastic pollution has increased due to the COVID-19 pandemic (Silva et al., 2020). To improve plastic inability to stop light, oxygen and other gases from penetrating and reaching the consumables and causing their degradation, as well as to prolong shelf-life of food and protect human health, novel materials are employed to envelop food products.

Starting from the beginning of the twentieth century, three main approaches have been applied to improve food packaging. The first approach consists in improving plastic polymers by mixing them with other materials. Doping or formation of nanoparticle-polymer composites improves mechanical properties of the packaging material, which can enforce the temperature and humidity resistance properties or improve oxygen barriers (Khajavi et al., 2020). Biopolymers, as ecologically sound “green” materials often suffer from degradation and mechanical issues so application of these materials in food packaging can be accomplished in the form of nanocomposites. The second approach aims to develop “active packaging” in which particles added to the packaging material interact directly with food and protect it from UV, oxygen, ethylene or microbiological contamination (Rai et al., 2019; Vilela et al., 2018). Active packaging systems can be classified as active scavenging systems (absorbers) that remove undesired elements from the product, such as moisture, carbon dioxide oxygen, ethylene and odour and active releasing systems (emitters) that release into the packaging in the form of antioxidants, carbon dioxide or antimicrobial compounds (Yildirim et al., 2018). Finally, the third approach develops “intelligent packaging”, which allows real-time monitoring of food safety (Müller & Schmid, 2019; Rai et al., 2019). For this, sensing elements are combined with the packaging material to transform the food envelope into a miniaturized device for tracking. Intelligent packaging may provide monitoring of food freshness and quality, its storage condition, and, in that way, improve safety and convenience, and help to extend food shelf-life. Thus, enhanced functionality of food packaging is obtained by smart packaging that includes both active and intelligent components, as shown in Fig. 1.

Nanomaterials and nanoparticles are used in the development of all three advanced packaging approaches. Adding nanomaterials including nano-metal oxides to different polymers to form nanocomposites can make packaging lighter, stronger and less permeable (Y. Huang, Mei, Chen, & Wang, 2018). Nanomaterials with an intrinsic antimicrobial activity incorporated in active and intelligent packaging contribute to extending the shelf-life of products by keeping food safe from harmful and spoilage bacteria, fungi and viruses, and by providing freshness during longer storage time. Metal oxide nanoparticles (NPs) have unique properties and morphology and a great potential for application in food industry NPs in nanocomposite packaging can perform oxygen and ethylene scavenging and UV- blocking as part of active packaging functions contributing to extending the product shelf life (Gaikwad, Singh, & Lee, 2018; Gaikwad, Singh, & Negi, 2020).

The objective of this review is to provide an overview of the methodologies and procedures carried out in earlier literature on the development of active and intelligent packaging utilizing metal oxide nanoparticles. As the physicochemical properties of nanoparticles and their stability in nanobiocomposites are essential for the development of packaging films we describe the state-of-the art techniques for nanoparticle synthesis, characterization and incorporation in polymers. Antibacterial properties of active packaging containing metal oxides and current available data on the antiviral aspect is presented. Antifungal and antiviral activities, also significant for food protection, are briefly mentioned. To point out that the cytotoxicity of nanoparticles is the main barrier for their applications in food packaging, we provide a condensed assessment of toxicity of metal oxide nanoparticles at the level of cells, mucus and microbiota. It is noteworthy that new regulations, consumer attitudes and acceptability, the societal involvement and impact, have been comprehensively described in some recent reviews (Garcia, Shin, & Kim, 2018; Omerović et al., 2021). Finally, an overview of the current research covering the potential for utilizing metal oxide nanoparticles in smart packaging for oxygen and ethylene scavenging, moisture control and in food safety sensors is also given.