Food packaging is a critical component of food technology that deals with the protection and preservation of diverse food products [1]. It has been reported that food packaging represented a global market size of £303 Billion in 2021 with a compound annual growth rate (CAGR) of 5.5% until 2030, and formed about 69% of the overall consumer packaging market [2]. Petrochemical plastics have achieved widespread success as packaging materials in the sector, owning 99% of the market share due to optimal properties such as oxygen barrier capabilities, high tensile, and tear strength. Other characteristics, such as a high Water Vapor Transmission Rate and biodegradability, is less prevalent in packaging, with biodegradability found in only 0.64% of all plastic materials. All these properties guard against external degradation agents and prevent the internal loss of nutrition in food products, assuring food quality at every level of the supply chain, from producers to end users (Figure 1).

As shown in Figure 2, synthetic plastic polymers such as polypropylene (21%), polyethylene (18%), polyvinyl chloride (17%), high-density polyethylene (15%), and polyethylene terephthalate dominate the food packaging market globally [1]. Between 1950 and 2015, an estimated 7.8 billion tons of plastics were created worldwide, with approximately 4.6 billion tons ending up in landfill or being wasted [3]. Polyethylene, the most manufactured and discarded synthetic polymer globally, is the major generator of two greenhouse gases—methane and ethylene [4]. Methane emissions contribute to climate change and can harm aquatic life by changing the oxygen levels and pH of water, whereas ethylene emissions can be hazardous to plants and animals and have an impact on crops and biodiversity, and there is some evidence that it may play a role in cancer development [4].

Plastics account for 10% of the global oil output, with single-use plastics accounting for more than one third of all plastics produced in 2017 [5]. Several environmentally hazardous disposal methods, such as incineration and landfill, are currently being employed to deal with the overflow of plastics [4]. As a result, sustainable, safe, and non-toxic food packaging options are highly desirable to ensure a transition to more environmentally friendly packaging materials in the food industry.

Bioplastics represent an innovative category of plastics derived from natural sources, such as chitosan, agar, alginate, and polylactic acid (PLA), among others. They are positioned as an environmentally friendly alternative to non-biodegradable synthetic plastics due to their reduced reliance on fossil fuels, faster biodegradability, and lower carbon footprint [6]. Bioplastics are made of biopolymers and biodegradable reinforcing agents [7]. The ability of bioplastics to return to the ecosystem, either through the natural breakdown of organic waste by microorganisms or composting, rather than accumulating in landfills, is an important differentiating factor compared to non-biodegradable synthetic plastics.

Even though bioplastics provide an alternative to synthetic packaging, they are regularly combined with petrochemical plastics. This is due to their weak mechanical qualities and moisture sensitivity, which are listed as contributing causes to their restricted utilization in food packaging [6]. To overcome the challenges around bioplastics, extensive research has been conducted to embed nanomaterials in food packaging materials, leading to the development of active packaging materials. Several metallic nanoparticles, most notably silver, aluminum, and zinc, have been shown to improve qualities such as tensile strength, Water Vapor Permeability, and biocidal activity [8]. Moreover, silver nanoparticles (AgNPs) have been found to have a strong antibacterial effect against foodborne pathogens such as bacteria, parasites, and viruses [9].

The antimicrobial activity of AgNPs in food packaging can help tackle two major global challenges:

• Food and beverage waste: Excess food production used to compensate for waste could be used to help feed the 811 million people worldwide experiencing chronic undernourishment [10];
• Foodborne infections: 550 million cases and 230,000 deaths worldwide each year could be avoided by providing sustainable and effective food packaging technology [9].

These challenges are aligned with the United Nations Sustainable Development Goal 12 to reduce food waste along production and supply chains in order to promote a more sustainable economic model [11]. Food waste is typically generated by food products which have a short shelf-life; studies have shown a wide spectrum of microorganisms being responsible for food deterioration [9], increasing the challenge to finding a one-stop solution to prevent highly nutritious foods from degrading as quickly, especially in warmer climates. As a result, $1.2 trillion is wasted globally each year from food and beverage detorioration [10]. It is believed that over 1.3 billion tons, or one third of all food produced for human use, is wasted annually [10]. This accounts for up to 10% of global greenhouse gas emissions [10].

While the main focus of current research has been to investigate the incorporation of AgNPs in bioplastics to improve their physical properties, only a limited amount of studies have looked into how these improvements affect food packaging in a holistic manner.

Due to the excellent performance of silver bionanocomposites in antibacterial activity and their ability to improve the physico-chemical properties of bioplastics, the purpose of this tliterature review is to critically examine key technological advances that are relevant to food packaging. These properties include the antimicrobial activity, barrier properties (Water Vapor Transmission Rate (WVTR, WVP), and Oxygen Transmission Rate (OTR)), mechanical properties (Ultimate Tensile Strength (UTS) and Elongation at Break (EaB)), thermal properties, and water resistance (WS, CA) of bionanocomposites containing AgNPs.

2. Silver Nanoparticles as an Active Additive

A common approach to enhancing food safety is to embed an active ingredient within a packaging material which not only inhibits microbial growth, but also enhances Water Vapor Permeability to extend the shelf-life [9]. This is achievable through nanomaterials such as AgNPs, which have a higher surface area-to-volume ratio with respect to their bulk counterpart, allowing them to easily interact with and bond to other materials. Hence, when embedding silver nanoparticles in biopolymers, they interact with:

• Gases, such as oxygen and carbon dioxide, which increase barrier capabilities [12];
• The polymer matrix of the film, forming a network of strong bonds that improve mechanical and barrier properties through weak and covalent interaction, assuring their adhesion within the biopolymers [13];
• Bacteria and other microorganisms, which inhibit their growth [9]. The mechanism of action of AgNPs against bacteria is illustrated in Figure 3;
• UV radiation, reducing the UV penetration through the biopolymer by means of their strong scattering behaviour [14].

It is important to note that the intake of AgNPs into mammalian cells is size dependent, with the aggregation of smaller-sized nanoparticles (<10 nm) causing cytotoxicity to cells [15]. Therefore, packaging manufacturers can counteract the effects of AgNPs in human cells by tuning the size of AgNPs as well as embedding them into bioplastic polymer matrices to form bionanocomposite materials. The antimicrobial activity of AgNPs, as presented in Figure 3, is influenced by key parameters, including the shape, size, and surface charge of the nanoparticle. Enhanced antimicrobial activity has been demonstrated in spherical and triangular-shaped AgNPs in comparison to cubic, platelet, decahedron, and other shapes, as increasing the surface area increases NPs’ reactivity with the microorganism’s cell membrane ^[16][17][16,17]. Moreover, previous research has demonstrated high antimicrobial properties for AgNPs sized between 1 and 30 nm. Additionally, the AgNPs’ surface charge, conferred by their coating agent, influences their interaction with biological molecules. This includes their uptake by microorganism cells, a crucial aspect governing their antimicrobial mechanism [16]. The synthesis method of AgNPs will influence their final physical properties, therefore impacting their antimicrobial efficacy. AgNP production methods can be divided into three synthesis routes: physical, chemical, and biological, as summarized in Figure 4 [18]. To achieve control over AgNPs’ size and morphology, a number of chemical and biological approaches constitute viable options. However, compared to chemical methodologies, biological methods are rapidly becoming the preferred synthesis route due to the absence of hazardous chemicals in their production [18]. Furthermore, biological methods do not require the addition of stabilizing agents during AgNPs’ synthesis, since many of the commonly investigated natural compounds can act both as reducing and capping agents. This enables the development of simpler and more cost-effective reaction strategies compared to chemical methods. Hence, the selection of the AgNPs’ synthesis method, their characteristics, and their concentration within the bionanocomposite are critical for understanding how to extend food shelf-life, as AgNPs exert different antimicrobial activities and physical properties depending on the bionanocomposite formulation.

3. Formulation and Bionanocomposites Manufacturing

Bionanocomposites are composite materials comprising two fundamental components: biopolymers that constitute the bioplastic matrix and embedded nanostructures capable of imparting unique properties which re-enforce the polymeric material. These nanostructures can be organic, encompassing polysaccharides, proteins, or synthetic colloids, or inorganic, including substances such as silica, noble metal oxides, or ceramics. Various bioplastic formulations have been produced in the literature. In particular, polysaccharide-based bionanocomposites are the most investigated due to their unique chemical–physical properties, and relative low cost provided by the abundance of their main sources. Indeed, polysaccharides are naturally abundant, are generally non-toxic and circular, and meet the criteria for packaging production [19]. These biopolymers, unlike lipids which are commonly subjected to peroxidation reactions leading to the loss of their main structure, exhibit a higher thermal stability. However, they are highly susceptible to moisture and have limited mechanical resilience [19]. These include agar [20], chitosan [13], and hydroxypropyl methylcellulose (HPMC) [21] in combination with multiple components such as gum tragacanth/HMPC/beeswaxes (GT/HMPC/BW) [22] and agar/banana powder [23]. Understanding their formulation is critical in order to develop optimal bionanocomposite materials, as their formulation will impact the final material properties as well as theour understanding of how AgNPs interact with the polymers themselves. Physical properties that affect food safety and shelf life must be considered when designing food packaging. To address these issues, three solutions have been suggested:

• The addition of different reinforcing chemicals into polysaccharide matrices such as cellulose, lignocellulose, or micro/nanocrystals [24];
• The combination of different polymers which produce blends or multilayer films [25];

Furthermore, plasticizers are commonly used in bioplastics to overcome brittleness and prevent cracking and chipping of the biopolymer during handling and storage. Plasticizers are substances with a low molecular weight and high volatility, such as glycerol or sorbitol. These can reduce intermolecular interactions and boost polymeric chain mobility, resulting in a drop in the material glass transition temperature via protein structural change [26]. It has been demonstrated that adding oleic acid to glycerol improves the mechanical and barrier properties of edible films and coatings [27]. This mixture creates polyglycerol-esters through the esterification of pendant hydroxyl groups with fatty acids. Polyglycerol-esters are commonly employed as additives, modifiers, and emulsifiers in products that contain immiscible food ingredients [28]. The selection of components is critical to the food packaging quality of the bionanocomposite, where the base formulation of the bioplastic will not only define the starting point of the physical, chemical, and biological properties of the film, but is also critical in understanding the interaction of the AgNPs with the bioplastic formulation itself, which also depend on how the AgNPs are integrated into the biopolymer matrix. AgNPs can be embedded into bionanocomposites using a variety of techniques, including in situ synthesis [29], solution blending (melt blending and solution casting) [30], and electrospinning [31]. The first method involves the synthesis of AgNPs within the biopolymer itself by adding a silver salt to a solution containing a reducing agent before its addition into a bioplastic material. Melt blending mixes AgNPs with a bioplastic in its molten state. On the other hand, in the solution casting method, a bioplastic is dissolved in a solvent, allowing the nanoparticles to be dispersed in the solution and mixed with the polymer before being cast into sheet materials. Finally, AgNPs can be electrospun to produce nanofibers and then integrated into a biopolymer solution. Overall, the choice of embedding technique will depend on the specific packaging material applications and the desired properties of the final product as well as the economic viability of such a process. For example, thermal extrusion methods will be the preferred choice for thermoplastic polymers such as Poly (Lactic Acid) (PLA) bionanocomposite production but not for agar-cellulose materials for which in-situ synthesis and mixing are preferred due to the high water affinity of AgNPs once stabilised by hydrophilic capping agents. A fundamental aspect to assure an increase in bionanocomposites activities is represented by the AgNPs’ uniform distribution within the biocomposites, in order to maintain an homogeneous action over the bionanocomposites’ surface and avoid AgNP aggregations, which can cause a lack of activities. To achieve homogeneity and uniform dispersion, manufacturers employ different techniques during the processing of the material:

• The use of dispersant or surfactant ^[32][33];
• Mechanical mixing during the extrusion process for thermoplastic polymers ^[33][34];

• The inclusion of inorganic additives, such as AgNPs
• ^[19][25][19,25].

• Sonication to break down agglomerates and ensure proper dispersion
• ^[34][35];
• The fine tuning of processing conditions, such as temperature, pressure, and mixing time to optimize AgNP dispersion.

Finally, the AgNPs’ high stability at different conditions, in particular with regard to pH and temperature, is a required feature when they are embedded within bionanocomposites used in food packaging materials to ensure food safety, regulatory compliance, consumer acceptance, and the long-term performance of the packaging. AgNP aggregates can lead to changes in material performance, compromising the integrity and functionality of the packaging ^[35][36].