Titanium Dioxide–Based Nanocomposites

Titanium dioxide (TiO,) possesses a property of a high refractive index and is resistant to discoloration. It tends to give opacity to papers, coatings, plastics, and films, among other items. In the cell membrane of microorganisms, polyunsaturated phospholipids are present. On the TiO, surface, the photocatalytic reaction of TiO, generates the hydroxyl radicals (_OH) and reactive oxygen species (ROS), ultimately resulting in the oxidation of polyunsaturated phospholipids, conclusively results into the inactivation of that microorganism. The exhibited photocatalytic property is stronger in nano form in comparison with that of the micro-sized particle. Many researchers (Babaei-Ghazvini et ah, 2018; Oleyaei et ah, 2016; Vejdan et ah, 2016) have noted blending TiO_zNPs with suitable packaging material to increase its efficiency. Whey protein film was blended with TiO,NPs to obtain a nanocomposite food package, and the improved mechanical strength of the film was observed. The variation of TiO, concentration affected the tensile strength and water barrier properties of the film (Zhou et al., 2009). Long et al. (2014) studied the disinfection of gram-negative and gram-positive bacteria, mainly Salmonella typhimurium and Listeria monocytogenes, respectively, as these are considered pathogenic bacteria found in meat and meat products, by Ti0₂ photocatalytic effect by varying ultraviolet (UV) exposure time and concentration. Xing et al. (2012) observed antibacterial activity of the Ti0₂-polyethylene film against Escherichia coli and Staphylococcus aureus and found that the nanocomposite film exhibited more effective antibacterial activity for S. aureus.

Silver-Based Nanocomposites

Silver NPs (AgNPs) possess distinctive properties such as thermal conductivity, catalytic activity, and chemical stability; they also exhibit antimicrobial and antifungal properties which lend to their application in various fields, such as textiles, medical applications, consumer products, and others (Akter et ah, 2018; Gonzalez et ah, 2015). De Moura et ah (2012) prepared nanocomposite film by blending hydroxy- propyl methylcellulose (HPMC) with AgNPs and found that the developed nanocomposite film had excellent mechanical and barrier properties; it also exhibited antimicrobial properties against an E. coli culture. It is also revealed that AgNPs act as antimicrobial agents and increase the mechanical properties of the film. This strengthens the point that AgNPs find their potential application in the field of food packaging. Zhou et ah (2011) developed the nanostructured low-density polyethylene (LDPE)/Ag₂0 film bags, which decreased microbial spoilage in apple slices. After 6 days, the quality of the apple slices was found to be deteriorated when packaged in conventional LDPE bags while those slices stored at 5°C in an LDPE/Ag₂0 bag were acceptable after 12 days. In a few cases, two or more antimicrobial reinforcements are also combined to produce the synergistic effect of microbial inactivation. It is also found that LDPE nanocomposite packaging materials containing silver and zinc oxide (ZnO) inactivated Lactobacillus plantarum in orange juice at 4°C (Emamifar et ah, 2010). Besides, the AgNPs retain higher antimicrobial activity in L. plantarum when compared with ZnONPs, especially for longer storage times. Similarly, orange juice packed in a nanocomposite of LDPE filled with a powder containing Ti0₂and nanosilver was found to have a longer shelf life without any deterioration (Emamifar et ah, 2010). Moreover, in this context, Zhou et ah (2012) prepared nanosilver/gelatin/carboxymethyl chitosan hydrogel by a green and simple fabrication method, that is radiation-induced reduction and cross-linking. The hydrogels exhibited an antibacterial effect on E. coli.

Zinc Oxide–Based Nanocomposite

ZnO is extensively used in numerous applications such as food additives. ZnO has been regarded as generally recognized as safe (GRAS) material by the Food and

Drug Administration (FDA). Out of various known metals, ZnO has been proved to be extremely toxic towards diverse microorganisms (Espitia et al., 2012; Stoimenov et al., 2002). For the inactivation of foodborne pathogens, Li et al. (2009) performed on ZnO-polyvinyl chloride (PVC) and proved to be efficient towards E. coli and S. aureus. Akbar and Anal (2014) found highly against either of them when they used ZnONPs against two pathogens, S. typhimurium and S. aureus. Towards the food- borne pathogens present in ready-to-eat poultry meat, for an active packaging application, an ZnONP-loaded film of calcium alginate was formulated.