Article Title [فارسی]
با حرکت به سمت جهانی شدن ، بسته بندی مواد غذایی نیز نیاز به زمان نگهداری طولانی تر ، کنترل کیفیت و بهداشت بر اساس استانداردهای بین المللی دارد. با پیشرفت مطالعات انجام شده در زمینه استفاده از فناوری نانو در علوم غذایی ، کاربردهای مواد نانو در زمینه های مختلف صنایع غذایی افزایش یافته است. فناوری نانو می تواند تمام این نیازها را برآورده کند و عملکردهای اساسی بسته بندی - ذخیره سازی ، محافظت ، بازاریابی و توزیع و ارتباطات را گسترش و اجرا نماید. کاربردهای فناوری نانو پلیمر در واقع می تواند مواد جدید بسته بندی مواد غذایی را با خواص مکانیکی ، مهاری و ضد میکروبی همراه با حسگرهای نانو فراهم کند تا شرایط غذایی را در حین حمل و نگهداری ردیابی و کنترل کند. در این مقاله مروری ، آخرین نوآوری ها و کاربردهای نانومواد در بسته بندی مواد غذایی با استفاده از فناوری نانو پیشرفته ، فعال و هوشمند مورد بررسی قرار می گیرد. وضعیت فعلی تجارت و درک مفهوم سلامت این فناوری ها و همچنین محدودیت های نانومواد پلیمری جدید که امکان تغییر کامل صنعت بسته بندی مواد غذایی را دارند نیز مورد بحث قرار گرفته است.
Nanotechnology is the recognition, production, and application of materials smaller than 100 nanometers on atomic, molecular, and macromolecular scales. Research in nanotechnology has grown dramatically over the past decade and many companies have specialized in making new forms of nanoscale materials with anticipated applications, including medical treatment and diagnosis, energy produc-tion, molecular calculations, and structural materials (Hulla et al., 2015 Barani et al., 2018). In 1808, nanotechnology allocated more than $16 billion in global research and development budgets employing more than 4,000,000 researchers all over the world (Hulla et al., 2015). Nanotechnology will cover more than $3 trillion of the global markets by 2020. In addition, nanotechnology industries around the world will need about six million workers by the end of the decade to support those industries (Hulla et al., 2015; Bhushan, 2016). Although nanotechnology is pervasive and attracts abundant financial resources, the food industry is one of the industries that are slowly developing nanotechnology usage. This is not surprising as the general preference for "natural" food products has historically prevented the use of emerging food technologies and nanotechnology is not an exception. However, the public opinion about the broad applications of nanotechnology vary from neutral to slightly positive (Ju et al., 2017; Shahbazzadeh et al., 2009; Ydollahi et al., 2016; Moadab et al., 2011; Aminzadeh et al., 2011). Some studies showed that consumers are still concerned about nano foods (Nasiri et al., 2019; Ahari et al., 2018). The most active areas in nanotechnology research are packaging and nanotechnology-prepared foods. The beverage packaging market in 2008 was equivalent to $4.13 billion representing an annual growth of 11.65% (Ahari et al., 2017). This is entirely consistent with some investigations that indicated people are more receptive to nanotechnology in fields other than food. The use of nanotechnology can expand and take over all the main functions of packaging, including maintenance, protection, marketing, distribution, and communication. This is why many of the largest food packaging companies in the world are actively seeking the potential of polymer nanotechnology to achieve new packaging materials with improved mechanical, inhibitory, and antimicrobial properties that can track and control food conditions during transportation and storage (Ahari et al., 2017). The vital role of nanotechnology in the food packaging process is considered as the most extensive commercial application in the food sector. In recent years, research and innovation in food packaging have increased and various film-based materials, carbon nanotubes (CNTs), and wax nanocoatings have been developed for different foods. Maodab et al. (2011) reported that nanoparticles may be beneficial in producing new materials for food packaging with improved mechanical, impermeability, and antimicrobial properties, as well as higher durability. In addition to their antimicrobial properties, nanoparticles can be used as carriers to deliver antioxidants, enzymes, seas-nings, anti-browning agents, and other materials for augmenting durability even after open-ing the package.
Being basically utilized in food packaging, the polymer nanomaterials for food packaging (PNFP) are discussed as follow:
Despite diverse advances in nutrition, there is still a risk of infection with microorganisms, such as mold, bacteria, and viruses that threaten human health. Direct consumption of antimicrobial substances in food can be harmful to the health of consumers. As a result, antimicrobial packaging has become very important. One of these packaging methods is improved packaging that entails polymer chains with a 5% weight of nanoparticles and nanocomposites used in products, such as carbonated beverage bottles, films, and edible oils. One of the most critical features of this type of packaging is its higher capacity to prevent the entry of gases, regulate the temperature, and improve resistance to food moisture. The Food and Drug Administration (FDA) has approved using nanocomposite materials in the food industry science. Polymers containing clay nanoparticles are among the first polymer nanomaterials that emerged on the market as enhanced materials for food packaging (Soltani et al., 2009; Mosadab et al., 2011).
Active packaging is particularly designed for releasing or absorbing substances in the packaged food or its surroundings. Currently, active PNFP has essentially been developed for packaging applications with antimicrobial properties. Some other promising and functional applications include oxygen scavengers, ethylene removers, and carbon dioxide absorbers or emitters. The most common nano-particles used to promote antimicrobial PNFP are CNTs, metal nanoparticles, and metal oxide nanomaterials. These particles act in direct contact, but can also move steadily and react favorably with food organics. Nanoparticles of silver, gold, and zinc are metal nanoparticles with an antimicrobial function that have been studied extensively. Several commercial applications have been noted for silver nano-particles (AgNPs). Silver, which has high-temperature stability and low volatility, is a significant antifungal and antimicrobial agent at the nanoscale. Moreover, silver is claimed to be effective against 150 different bacteria (Shahbazzadeh et al., 2011).
Titanium dioxide (TiO2), zinc oxide (ZnO), silicon oxide (SiO2), and magnesium oxide (MgO) are the most studied metal oxide nanoparticles due to their ability for blocking UV and are ineffective photocatalyst agents. The use of TiO2 as an ineffective photocatalyst for surface coatings in packaging is being investigated (Kakoolaki et al., 2019). The anti-bacterial properties of ZnO and MgO nanoparticles have been recently found. The ZnO and MgO particles are expected to be more feasible and healthier for food packaging than nanosilver. Nanomaterials, namely ZnO nano-based optical catalysts for sterilization in indoor exposure, have been recently introduced. The ZnO has been reported to have an antimicrobial activity, which has an indirect rela-tionship with particle size. Although UV light is not required for antimicrobial activity (as opposed to TiO2), visible light stimulates it and its exact mechanism has not yet been determined. The ZnO nanoparticles merge in several polymers, such as polypropylene (PP), in which UV light is absorbed without heat re-radiation and improves the stability of polymer compounds. The CNTs have appropriate antibacterial properties and promote polymer network properties. Direct contact with CNT masses was shown to be lethal for Escherichia coli. The latter impact may be due to the breakdown of microbial cells and irreversible damage by long narrow CNTs (Gokularaman et al., 2017; Roy et al., 2018). The usage of CNT is currently discontinued because of the claims of some studies suggesting that CNTs are dangerous due to their toxicity to human cells, especially when being in direct contact with the skin (Shahbazzadeh et al., 2011; Ahari et al., 2017).
Nanotechnology can help active packaging to reduce the spoilage of different kinds of food by direct and indirect oxidation through combining O2 nanoabductors. For example, direct oxidation reactions cause fruits to turn brown and vegetable oils to turn sour. Food spoilage with indirect O2 action involves the spoilage of food by aerobic microorganisms. The presence of O2 in a package can initiate or accelerate oxidation reactions, which can cause food spoilage and facilitate the growth of microbes and aerobic molds. Both direct and indirect oxidative reactions reduce quality in diverse aspects such as odor and taste and cause unpleasant color changes. Oxygen abductors remove O2 (waste or input) and thereby slow down oxidative reactions. Several nanoparticles, including TiO2 nanoparticles, are used to produce O2 abductor films. Some silver-based nanoparticles that have antimicrobial activity are also able to absorb and decompose ethylene. Ethylene is a natural plant hormone produced by ripening. Removing ethylene from the packaging environment helps to elevate conservation time for fresh products, such as fruits and vegetables (Ahari et al., 2017).
Materials in contact with food are mainly used to indicate the condition of food and its packaging. This technology can alert the producer or consumer with a visible indicator of food freshness, perforated packaging, storage at the right temperature in the supply chain, and being spoiled (Cushen and Cummins, 2017). Key factors in their application are cost, strength, and compatibility with different packaging materials. The first improvements were based on devices that were used with the product in a typical standard package to perfectly control the package, time-temperature history of the product, and effective expiration date. Industries assess food expiration dates by considering the conditions of distribution and storage, especially temperature. However, we know that such conditions are not always expressed correctly and that food is constantly exposed to inappropriate temperatures. This is especially worrying about products that require a cold chain. Time-temperature indicators, which began to appear in some food products in the late twentieth century, allow producers to verify food storage at the right temperatures. These indicators fall into two categories, one of which is based on the transfer of a dye between a porous material that depends on temperature and time. The other group uses a chemical reaction, which starts when the label is applied to the package and causes color change (Omanovic-Miklicanin et al., 2016). This indicator reassures the consumers of what they are buying and enables producers to track their food on the production line. In addition, controlling food while moving into the production chain helps companies to identify weaknesses. Furthermore, micro-penetration and sealing defects in packaging systems may lead to unwanted exposure to a lot of oxygen in food products, which results in unpleasant changes. Nanoparticles can be applied as reactive particles in packaging materials to represent the package state. Nanosensors can respond to environmental changes, such as room temperature and humidity, and oxygen exposure, as well as spoiled products, or microbial contamination (Ahari et al., 2008).
Nanosensors in food packaging can detect certain chemical compounds, pathogens, and toxins in foods. Consequently, they are useful for eliminating the need for inaccurate expiration dates. These sensors provide real-time status of food freshness. Recent advances in smart PNFPs include oxygen markers, novelty, and pathogen sensors. There is a growing interest in the development of irreversible and non-toxic oxygen sensors to prevent the presence of nitrogen or oxygen in non-oxygen food packaging systems, namely spotted packaging (Ahari et al., 2008). Lee et al. developed a UV-activated colorimetric oxygen marker using TiO2 nanoparticles to sensitize light-reducing methylene blue with triethanolamine in an encapsulated polymer medium using UVA light (Mills and Hazafy, 2009). As soon as exposure to UV radiation the sensor turns white and remains colorless until it is exposed to oxygen and the original blue color is restored. The rate of color recovery is proportional to the amount of oxygen exposure. Mills and Hazafy used nano crystallized SnO2 as an optical sensitizer in a colorimetric indicator with variable film color dependent on the presence of oxygen (Mills and Hazafy, 2009). Furthermore, pH markers based on organically modified organic silicate nano-particles have recently been introduced (Ahari et al., 2008). Novel indicators show the quality of packaged food in response to changes in fresh food products as a result of microbiological growth. Smolander reported that a definite prerequisite for the successful development of novel indicators is an understanding of quality metabolites. The freshness sensor must be able to react sensitively to these metabolites. Freshness is judged based on a color change due to microbial metabolites generated during spoilage. It should be noted that the formation of different metabolites depends on the nature of the spoiled flora of the packaged product and the type of packaging. Sensors embedded in a packaging film should be able to detect food spoilage organisms and initiate color change to alert the consumer about the expiration date (Smolander, 2008; Smolander and Chaudhry, 2010). A list of freshness indicators that react to the presence of quality-reflecting metabolites has also been reported (Ahari et al., 2019). Several types of gas sensors have been developed, which can be used to determine the amount and type of microorganisms by evaluating their gas emissions. One of the most popular types of sensors is a metal oxide gas sensor with high sensitivity and stability (Lotfi and Ahari, 2019). Sensors based on conductive nanoparticles embedded in a non-conductive polymer network are being studied for detecting and determining food pathogens by generating a specific response pattern for each microorganism. Currently, three types of bacteria, namely Bacillus spp., Salmonella, and Vibrio parahaemolyticus must be detected using the response pattern created by these sensors. Another advance in this area is "electronic language" technology, which is composed of an array of sensors that signals food conditions. The device contains a variety of highly sensitive nanosensors detecting the gases released by spoilage microorganisms, which produce a color change that shows the freshness or spoilage of food. DNA-based biochips are also being developed and can detect the presence of dangerous detrimental bacteria or molds in food, such as meat, fish, and fruits. Other advances in this field are in the early stages of research, including devices that provide the basis for preserving packaging technology, which will begin to be protected if food begins to spoil (Lotfi and Ahari, 2019).
When food is not consumed immediately after production, it should be placed in a package with several characteristics. A package type should have the potential to protect food from contamination, dust, oxygen, light, contaminating microorganisms, moisture, and oth-er harmful destroying agents. In addition, the packaging must meet desired conditions, such as being safe, suitable for static use, cheap, light, reusable, resistant to final conditions during processing or accumulation, unsusceptible to physical factors, and impermeable to an environmental reservoir or transfer conditions. These are the prominent features of any substance which is supposed to be included in this technology (Barani et al., 2018).
A critical issue in food packaging is mobility and impermeability. No material is entirely impervious to atmospheric gases, water vapor, or natural substances that enter during food packaging, or even the packaging material itself. In some applications, strong retainers are not desirable for moving or releasing gas. For example, in packages designed for fresh fruits and vegetables, shelf life depends on access to a continuous source of oxygen for cellular respiration. Plastics are used for carbonated beverage containers and must have strong oxygen and carbon dioxide barriers to prevent the oxidation and decarbonization of beverages. In other products, the transfer of carbon dioxide is a smaller issue than both oxygen and water vapor. Regarding these complexities, a necessity is felt for quite different packaging characteristics for food products. The packaging industry only allows longer transportations between production units and consumers (Barani et al., 2018).
Old materials for food packaging encompass metal, ceramics (glass), and paper (cardboard). Although these materials continue to be used, plastic is becoming a significant alternative for food packaging due to its light weight, low cost, ease of processing and ductility, as well as considerable diversity in the physical properties of organic polymeric materials. Polymers commonly used for food packaging include polyphenols, such as PP, low- and high-density polyethylene (LDPE and HDPE), polystyrene (PS), polyethylene terephthalate (PET), and polyvinyl chloride (PVC). Accordingly, polymers have made a revolution in the food industry and provided more advantages than traditional materials which were inherently permeable to gases and other small molecules (Bradley et al., 2011; de Azeredo et al., 2011). Some polymers are better in this case than others. For example, PET is a good barrier against oxygen (6-8 nmol m-1 s-1Gpa=O2permeability), while HDPE does not protect efficiently (200-400 nmol m-1 s-1 Gpa=O2 permeability). On the other hand, HDPE shows better resistance against water vapor than PET. No common pure polymer has all the protective and mechanical properties needed for any possible use in food packaging. Therefore, mixed polymer films or polymeric compositions are usually used. For instance, ethylene vinyl alcohol (EVOH), as a water-sensitive material, can be placed between the layers of hydrophobic polymers, such as polyethylene (PE), when powerful oxygen inhibitors over a wide range of moisture and strong oxygen protectors are required (Barani et al., 2018). Moreover, directly combining polymers is effective for obtaining optimum gas barrier and mechanical properties that are not available by other polymer monomers. Cleverly integrated com-binations can result in more controllable properties of the achieved film (Shankar and Rhim, 2016). Polymer compounds and multilayer films have led to the production of packaging materials with good gas barrier properties and without the inherent limitations of many single-layer films made of solid protective polymers. However, they cost higher, are more productive, require additional additives and adhesives, which complicate their adjustment by random factors, and have recovery problems (Ahari et al., 2017). As a result, the polymer industry still tends to produce single-layer films with improved gas and mechanical properties, especially those made of biocompatible materials.
Polymeric nanocomposites (PNCs) are the newest materials for solving the problems mentioned above. The PNCs are obtained by dispersing a nanoscale neutral filler in a polymer network. Filling materials can include nano-clay or silicate plates (fra-in vide), silica nanoparticles (SiO2), CNTs, starch nanobots, graphene, cellulose-based nanofibers, nano-whiskers, chitin or chitosan particles, and other non-organic materials. Therefore, improve-ing the polymer barrier properties is the most precise application of PNCs in the food industry. The PNCs are stronger, more flame resistant, and have better thermal properties, including dots melting, glass transition temperatures, and decomposition, compared to control polymers that do not contain any nanoscale fillers. Conversions in surface wetting and hydrophobicity have also been reported (Clifton et al., 2020). Some of these improvements in physical properties can be extremely effective. For example, a layer-by-layer method is used to fabricate a PNC material composed of clay nanosheets dispersed between polyvinyl acetate (PVA) with a modulus of hardness (GPa 11±106), which is almost twice as large as pure PVA and is comparable in hardness to some Kevlar types (Ahari et al., 2017). A similar construction method has been utilized for engineering clay/poly (ethyleneimine) PNCs that preserve the textural structure of linen products when used as a coating during long burning times. Finally, PNCs should provide better opportunities for the food industry in addition to reducing costs and wastage due to the smaller volume of polymers required in packaging materials with the same mechanical properties or even better. Furthermore, nanocomposites may have environmental advantages over traditional plastics: When a nanofiller is released into a biocompatible polymer (PLA), the PLA biocomposite biodegrades faster than PLA without such additives (Segura González et al., 2018).
polymer nanotechnology for food packaging is still in a development stage. The envisaged direction is to look at the complete life cycle of the packaging (raw material selection, production, analysis of interaction with food, use and disposal) integrating and balancing cost, performance, health and environmental considerations (Figure 1). Researches on the production methods and characterization of PCNs used for food preservation began in the late 1990s. Most of the published researches from "Rus Mont Morillonite" (MMT) has been used as a nanoscale component. A wide range of synthetic polymers, such as PE, nylon, and PVC, as well as biopolymers, such as starch have been investigated. In most published studies on PCN, varying amounts of nanoclay (usually 1%-5% by weight) have been used (Kuswandi, 2016 & 2017). The silicates used in PCN synthesis have a layered structure with a layer thickness of about 1 nm. The side dimensions of these layers can be up to several micrometers. As a result, the ratio of dimensions (length to thickness ratio) of these fillers is high and includes values higher than 1000. The defined distance between these layers form clusters is known as “gallery”. Inorganic cations between layers can replace other cations, such as lithium and sodium (Zare, 2017; Bee et al., 2018).
There are usually three possible arrangements of these silicate clays that are obtained in a polymer matrix during dispersion:
Figure 1.Full packing cycle
Figure 2.Morphology of clay / polymer nanocomposites (a) Tactoid (b) inserted (c) and exfoliated.
The structure of nanocomposites is usually determined using two complementary diagnostic methods, namely X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD is used to identify an intercalated structure by determining the interlayer space (Mohammadi et al., 2014). Placing polymer chains between the layers augments the interlayer spaces and according to Bragg's law causes the peak diffraction to move to a lower angle. However, if the interlayer space becomes too large, these diffraction peaks will disappear indicating a complete exfoliation of the layer silicates in the polymer matrix. In this case, TEM is utilized to identify the separated silicate layers (Kakoolaki et al., 2019; Pooyamanesh et al., 2019).
Nanocomposites are usually produced by soluble method, in-situ/interlamellar polymerization, and melt processing.
Many companies have made PCNC packaging materials commercially available because they are not relatively expensive to manufacture. Some companies, such as Nanocore offer a wide variety of polymer nanocomposites in bullet form, in addition to several commercial product lines, including Aegis (Honeywell, USA), Durethan (LANXESS Deutschland GmbH, Germany), Imperm (ColorMatrix Co., USA), nano stuff (Nylon Co., USA), and Nanoseal (Nanopack Inc., USA). Pooyamanesh et al. presented a complete summary of one of the active companies in this field (Pooyamanesh et al., 2019).
The most common PCNC available products are dedicated to particular applications, including the food and beverage industries. For example, Brewing Miller Company uses these products to make plastic bottles, which are resistant to the ingress of both oxygen and CO2. Another exciting usage of PCNC technology is found in the US Army Natick Soldier Research, Development, and Engineering Center (NSRDEC), which saves signi-ficant time and money on the possible use of PNC plastics for meals ready-to-eat (MREs) packaging. In addition, MREs are surprisingly hard to store, and the strength of PCNC-based packaging may only be able to meet this (Pooyamanesh et al., 2019).
Given the number of studies that consider food packaging as a possible endpoint for PCNC research, the number of researchers who examine these substances in storage or health tests using real food components is surprisingly small. An investigation observed that PET, PHBV, and PHB-based PCNCs at 5.5 wt% relative passages (Polymer/Pcomposite) of limonene d, an essential aroma compound in citrus fruits, were 3.2, 1.6, and 8.8 in size, respectively (Sasikala and Umapathy, 2018). This shows that PCNC packaging materials are not much similar to skin flavors, colors, or food odors. Another study was more concerned with the behavior of entire food systems demonstrating that the number of germs and mold on apple slices diminished dramatically over 10 days when packaged in iPP/PNCCaCo3 films. On the other hand, those stored in a pure isotactic PP experienced an elevation in overall mesophilic microflora during the same time (Avella et al., 2007). In addition, this review showed that apples stored in PNC packaging had ripened more because of ethylene gas retention and had less oxidation than apples stored in common PP packages (Mohammadi et al., 2016).
In terms of the safety of PNCs-made packaging materials, some concerns are raised regarding the permeability of packaging materials to foods. Investigations on the accessibility of PNC materials are controversial and limited. A survey in 2005 revealed that vegetables in contact with clay/starch nanocomposite films did not change in terms of iron and magnesium content. However, they had higher levels of silicon. The authors argued that the main elements of clay nanoparticles did not have a significant entry into food or the entry was within the limits of current EU regulations (Avella et al., 2005). Another study showed that OB Uvitex, a common additive used in polyolefins with European approval for food contact use, has a very slow distribution in oily and aqueous foods, such as PNCMMT/wheat-gluten film. It was found to be more than 60% in LLDPE films subjected to the same test (Mauricio-Iglesias et al., 2010). Furthermore, the PNCMMT/wheat-gluten film allows aluminum and silicon to enter foods at levels well within the limits set out in European regulations. Finally, a paper showed that the distribution of triclosan and trans-, trans-1, 4-diphenyl-3, 1-butadiene (DPBD), as two other common additives in polyamide/clay PCNCs, is slower than pure polyamide (Türe et al., 2012; Nataraj et al., 2018). Although the mentioned studies suggest that PCNCs may moderate the entry of possibly harmful additives into foods, the health literature is fragmented at this moment. More detailed testing should be performed on PCNCs because various food and beverage companies use them to package their products. Importantly, the toxicological data of clay nanoparticles are still inaccessible as strategies for identifying and classifying clay nanoparticles and other nanoparticles in complex food networks are being developed. One study found that layered silicate nanoparticles had little cell and gene toxicity even when were part of a diet given to mice (measured acute oral poisoning, mean lethal dose, LD50et al., 2016; Nataraj et al., 2018). However, the authors of this study have only tried one type of clay and morphology. Therefore, its application for a general proposition remains unclear. In addition, the PCNC films are found to increase the entry of nano clay composites into quasi-foods when films are under high pressures, which is an increasingly popular food sterilization/preservation method. Another study showed that MMT clays undergo new chemical and structural changes at pressures below 300 MPa. It was concluded that such changes should be considered when montmorillonite-polymer bond-ed composites are in contact with food. As a result of these assumptions, while PCNCs may represent the next revolution in food packaging technology, steps remain to be taken to ensure that consumers are safe from any potential risks posed by these substances (Mohammadi et al., 2016).
The use of silver as an antimicrobial agent in food and beverage storage has a long history. The wide range of antimicrobial activity and relatively low cost of silver make it a suitable case for active water disinfectant in developing countries (Hadrup and Lam 2014, Akter et al., 2018). In 2009, the FDA amended the Food Additives Regulations to allow the direct addition of silver nitrate, as a disinfectant, to commercially bottled water at a maximum concentration of 17 kg/g (Akter and Sikder, 2018).
Aside from applications in the food industry, silver has long been used as a disinfectant. Hippocrates, the ancient Greek physician, used silver powder on wounds to accelerate healing. However, perhaps the most significant antimicrobial advantage of silver is that silver can be easily incorporated into countless materials, such as fabric and plastic, making it particularly useful for applications where a broad and stable antimicrobial activity is desirable and traditional antimicrobials are useless. In the food industry, silver-containing plastics are incorporated in almost everything from refrigerated liners to cutting boards and food storage containers. In addition, silver has been a revolution in the medical device industry with an increase in silver-plated urinary catheters, cardiovascular implants, esophageal tubes, bandages, sutures, and other instruments on which the growth of bacteria endangers the patient's life. Up to now, the FDA has approved more than ten silver-containing zeo-lites or other materials for use as food contact materials for disinfection, as well as many silver-plated medical devices (Hadrup and Lam, 2014; Deshmukh et al., 2019).
Despite the long history of silver as an antimicrobial agent, the mechanism of this activity remains a subject of active research. Some suggested explanations include interference with vital cellular processes by binding to sulfhydryl or disulfide functional groups on the surfaces of membrane proteins and other enzymes, disruption of DNA replication, and causing oxidative tension (oxidation) by catalyzing the formation of reactive oxygen species (ROS) (Cvjetko et al., 2017; Deshmukh et al., 2019). However, there is no consensus on which of these mechanisms is most important. For example, Dibrov et al. provided evidence that the binding of silver, especially to membrane proteins, disrupts ion and proton transport across membranes (Dibrov et al., 2002). Another study found that silver ions penetrate the cell, where they interfere with ribosome activity and disrupt the production of several key enzymes responsible for energy production (Yamanaka et al., 2005).
Cell wall damage due to silver (Ag) binding to membrane proteins and DNA shrinkage has been observed in E. coli and Staphylococcus aureus. DNA shrinkage in response to the presence of Ag ions has been stated as a defense mechanism, which, while protecting DNA from damage, limits the ability of cells to proliferat (Batarseh, et al., 2004). In contrast, some reported that silver complexes of glutamic and tartaric acids are actively involved in DNA cleavage, and suggested that the binding of Ag ions to membrane enzymes and proteins plays a relatively minor role in silver antimicrobial activity. Gram-negative bacteria, such as E. coli are generally more sensitive to silver treatment than gram-positive bacteria, namely S. aureus because the transfer of positively-charged silver ions across the membrane of gram-positive bacteria, which is thicker and rich in outer peptidoglycans, is slower than the thinner membranes of gram-negative species (Guo et al., 2019; Rezvani et al., 2019). Finally, there is evidence suggesting that the antibacterial activity of silver zeolites could be attributed to silver ability for catalyzing the production of ROS, which causes cell death as the result of oxidative stress (Bakhsheshi-Rad et al., 2018).
Since the first published reports of the antimicrobial properties of silver colloids, AgNPs have been identified as potent agents against many species of bacteria, including E. coli, Enterococcus faecalis, S. aureus, S. epidermidis, V. cholerae, Pseudomonas aeruginosa, P. putida, P. fluorescence, Shigella flexneri, B. anthracis, B. subtilis, B. cereus, Proteus mirabilis, Salmonella enterica, S. typhimurium, Micrococcus luteus, Listeria monocytogenes, and Klebsiella pneumoniae. Moreover, AgNPs are effective against the strains of these species which are resistant to potent chemical antimicrobials, including methicillin-resistant S. aureus, methicillin-resistant S. epidermidis, vancomycin-resistant enterococcus, and extended-spectrum β-lactamase-producing Kleb-siella (Bakhsheshi-Rad et al., 2018; Tang and Zheng 2018). In addition, AgNPs are toxic for some fungi, such as Candida albicans, Aspergillus niger, Trichophyton mentagrophytes, and yeast isolated from bovine mastitis, as well Chlamydomonas reinhardtii algae and Thalassiosira weissflogii phytoplankton. Further-more, they inhibit at least two viruses, namely the human immunodeficiency virus and smallpox (de Souza et al., 2019).
There is a disagreement about the toxicity of AgNPs for bacterial cells. The most conservative view is that silver atoms separate from the surfaces of AgNPs and cause cell damage by precisely the same mechanisms observed for conventional silver antimicrobials. Some studies showing that AgNPs are more toxic than the equivalent of isolated silver ions refer to the "Trojan horse" model for the toxicity of engineered nanoparticles (Cho et al., 2018), according to which AgNPs play an efficient role in transferring large amounts of silver ions into cells in a short time. A study confirmed the hypothesis that AgNPs are only Ag+ carriers demonstrating that these particles are ineffective in slowing the growth of E. coli species resistant to Ag+ (Cho et al., 2018; de Souza et al., 2019). In addition, E. coli cells exposed to 9% nm AgNPs presented the same abnormality in transmembrane potentials and decreased ATP levels similar to E. coli cells exposed to AgNO3 with the lower absolute molar concentrations (μM vs. mM). It is important to note that the chemical nature of silver leads to antibacterial effect, and gold nanoparticles (AuNPs) with similar size have no efficient antimicrobial activity (Vazquez-Muñoz et al., 2017; Cho et al., 2018). While AgNPs probably act as a source of Ag+ ions, they may also have additional antimicrobial mechanisms. For example, when the released Ag+ concentration is uniformed, the AgNPs seem to be more toxic to algae than the equivalent doses of AgNO3. In contrast to the above-mentioned study, another report indicated that AgNPs are very influential against silver-resistant strains of Proteus mirabilis and E. coli, and highlights the fact that particles with different sizes, shapes, or other properties may behave differently even in the same system. There is also evidence that AgNP levels effectively catalyze the formation of free radicals in bacterial cells, which can lead to cell death through oxidative stress (Vazquez-Muñoz et al., 2017; Smith et al., 2018).
However, perhaps the most astonishing evidence that AgNPs are toxic to microorganisms through mechanisms different from Ag ions is the result of researches by Elechiguerra et al. These authors revealed that the concentration of released Ag+ ions from AgNP was under the tested conditions was not sufficient to fully justify the toxicity of AgNPs (Elechiguerr et al., 2005). Above all, these authors could show that AgNPs bind to membrane proteins, form cavities, and cause other morphological changes (Figure 3). The AgNPs were also observed to react with phosphorus DNA groups. Morphological changes (cavitation) were observed in the cell membrane as a result of exposure to bacterial AgNP independently by Sundi and Salopek-Sondi (Sondi and Salopek-Sondi, 2004), which were more likely to bind AgNP to membrane surfaces causing lipopolysaccharides to adhere and subsequently structural integrity and impenetrability to be lost. Therefore, perforated membranes become more porous resulting in disrupted molecular and ionic transport, as well as accelerated AgNPs entering into the cell that can cause more damage to DNA and other cellular components inside the cell.
In summary, AgNPs are powerful broad-spectrum antimicrobials as the minimum inhibitory concentration of 2 mL/kg-4 has been reported for AgNPs with a diameter of 45-50 nm against E. coli, V. cholera, S. flexneri, and at least one strain of S. aureus that compete with the antibacterial properties of penicillin against non-resistant strains. In addition, this power can be efficiently modified with the particular physical effects provided by nanomaterials. For example, Akhavan and Ghaderi (Akhavan and Ghaderi, 2009) showed that silver nanowires, when exposed to external electric fields, have about 18.5% to 63% better antimicrobial power due to the elevation in silver ion production at the wire terminals. Furthermore, Fuertes et al. (Fuertes et al., 2011) demonstrated that the optical excitation of AgNPs coated with a thin layer (1-2 nm) of porous silica at visible light frequencies that resonate with AgNP surface plasmon bands significantly increased the antimicrobial activity against E. coli by ROS photosynthesis or photocatalyzed by the release of silver ions. It is also a reversible effect that provides a portal in photo switchable antimicrobial behavior. Such studies suggest that an electric field or external light source can be a controllable, non-invasive sterilization method if silver nanostructures are embedded into food storage containers.
Figure 3. Mechanisms of Silver Nanoparticle Bacteriocidicity. (A and B) Silver nanoparticles (AgNPs) are lethal to bacteria in part because they damage cell membranes.
One of the greatest advantages of mineral or inorganic nanoparticles over molecular antimicrobials is their ease of use in polymers to form available antimicrobials (Pozdnyakov et al., 2016). This is especially true regarding the controlled diffusion properties of AgNPs, which can be designed for the long-lasting durability of strong antimicrobial agents. There-fore, silver/polymer nanoparticles are attractive materials for usage in medical devices, as well as food packaging materials to maintain durability. While silver zeolites have been utilized for some time to make antibacterial polymer composites, AgNP nanocomposites augment stability and slow down the release of silver ions into stored foods, which is essential for stable antimicrobial activity. For example, when the antimicrobial activity of SiO2/AgNP nanocomposite materials was compared with the antimicrobial activity of silver zeolite (Ag) and a SiO2/AgNO3 composite, the last two materials had about ten times more severe antimicrobial response. However, nanocomposi-tes made a longer period of activity possible (Pozdnyakov et al., 2016; Prozorova et al., 2019). Although a zeolite material may provide an excellent immediate effect, the stable antimicrobial activity of nanocomposite is more suitable for food packaging that requires transport distance or long storage life. Note that in the case of uncoated AgNPs, the SiO2/AgNP composite material was found to be effective against a wide range of bacteria and fungi. It was more effective against gram negative bacteria than gram positive ones and can be incorporated into a PP polymer matrix to create antibacterial films for food-contact containers (Prozorova et al., 2019).
Many PNCs have been reported in experiments. For example, Sanchez Valdes et al. coated a five-layer plastic film (Polyamide=PE/tie/PA-6/tie/PE; PA-6 and tie=PE bonded with maleic anhydride) with an AgNP/PE nanocomposite layer and found an antimicrobial effect against Aspergillus niger, a common food contaminant. In addition, they found that this effect and activity depends on the coating method. The methods that resulted in a harder surface, and therefore a higher level of silver ions release, were more active than the methods that led to a softer surface. Munster et al. published several studies on PNCs AgNP/PA-6 and AgNP/PP (AgNP particle size of about 800 nm) that have antimicrobial activity against E. coli, S. aureus, Candida albicans, multi-stranded worms (Spirobis spirorbis), sea squirt (Ciona intestinalis), and algae (Ulva intestinalis) (Sánchez-Valdes et al., 2018; Grijalva-Chon et al., 2017).
To determine AgNP antimicrobial impact on food shelf life, researchers have tested AgNP/polymer nanocomposite materials using real food systems. For example, Fayaz et al. (2009) dipped and extracted sterilized carrots and pears into alginate solutions containing biosynthesized AgNPs to form "edible antibacterial films". They observed less water loss for treated carrots and pears in 10 days, which was more acceptable for the consumers judging by color, texture, and taste (Figure 8, bottom) (Huang et al., 2019; Saedi et al., 2020). In a similar study, fresh asparagus leaves and buds coated with AgNP/polyvinyl pyrrolidone nanocomposite films increased their storage life to 25 days at 2°C. In addition to lower weight loss, greener color, crisper and softer texture, covered asparagus had less microorganism growth (psychrophiles bacteria, yeast, and mold) during this period (Moradi et al., 2015). An edible film containing AgNP dispersed in glycogen has also been reported. Chinese jujube, which was placed in harder and heavier food storage bags made of AgNP/TiO2/nanoparticle PE films, presented less spoilage, browning, and slower ripening in 12 days, compared to fruits stored in control materials. Orange juice stored at 4°C in LDPE films containing P105 (a mixture of TiO2 and 10 nm nanosilver) showed a statistically significant reduction in the growth of Lactobacillus plantarum over 112 days (Ydollahi et al., 2016). In other applications, cellulosic pads containing AgNP produced from silver ions have been indicated to reduce the microbial level of exudates (secreted substances) from the beef stored in modified atmospheric conditions. Moreover, in a slice of fresh melon stored in cellulose pads containing AgNP, the number of microbes (mesophiles, psychrotrophic bacteria, and yeasts) was lower and the microbial growth retardation time was longer. Silver particles accelerate and catalyze the loss of ethylene gas. Consequently, fruits stored in the presence of AgNP have a shorter ripening time and a longer storage life (Ydollahi et al., 2016). Although many advances have been made in silver nanostructures for food packaging applications, there is still a lack of comprehensive studies in different polymer systems, and extensive investigations are required to clarify the relationships affecting the antimicrobial resistance of AgNP PNCs.
The antimicrobial properties of nanoparticles composed of other materials have been investigated. The TiO2 particles are particularly promising. In contrast to AgNP, the antimicrobial activity of TiO2 nanoparticles is photocatalyzed, and therefore, TiO2-based antimicrobials are active only in the presence of ultraviolet (UV) light. For example, it has been identified that TiO2 nanoparticles are effective against food due to pathogens under UV radiation but not in the dark (Ydollahi et al., 2016). Mohammadi et al. dispersed TiO2 nanoparticles throughout EVOH films via the ultrasonic method. They observed the lethal properties of their active light against nine microorganisms, including yeasts and bacteria mentioned in food poisoning and food spoilage (Figure 4) (2014). Another study on food packaging showed that PP films coated with TiO2 nanoparticles inhibited the growth of E. coli in fresh lettuce slices (Peiris et al., 2018). Numerous researchers have combined the antimicrobial properties of TiO2 nanoparticles with silver or AgNPs to form films or particles with enhanced antimicrobial activity (Mohammadi et al., 2014). Food packaging films containing TiO2 particles may use another advantage of protecting food against the oxide-zing effects of UV radiation while maintaining good optical clarity because TiO2 nanoparticles are effective light absorbers with short wavelengths and light stability. This technique is currently being used to protect against UV rays in sunscreens, textiles, and wood oil. However, one must be careful because some forms of titanium nanoparticles may cause photocatalysis of oxidation and decomposition of the polymer (Ahari et al., 2017).