The demand for minimally processed, easily prepared and ready-to-eat ‘fresh’ food products, globalization of food trade, and distribution from centralized processing pose major challenges for food safety and quality. Recently, extensive studies have been conducted to develop non-thermal processing techniques (PEF, HHP, IR, UV, US) as replacements for thermal processing in order to keep the freshness of food along with extending its shelf life. Although some of these technologies are capable of decontaminating of food, they are energy-intensive and require costly equipment; hence, their yet relatively limited commercial applications. However, the use of proper packaging technology to minimize food losses and provide safe and sound food products has always been the focus of food packaging. Nanotechnology has potential to influence the packaging sector greatly. Nanoscale innovations in the forms of pathogen detection, active packaging, antimicrobial packaging and barrier formation are poised to elevate food packaging to new heights. This chapter lays foundations for the role of antimicrobial polymer nanocomposites in food packaging (Emamifar et al., 2010; Brody et al., 2008). This chapter briefly describes new food packaging and focus mainly on the applications of nanotechnology in food packaging. Effort will be made to considerable extent to explore the potential of using antimicrobial polymer metal nanocomposites for food packaging industry. Moreover, the antimicrobial performance of the most common nanoparticles based on zinc oxide, titanium oxide and nanosilver applied directly to plastic films, will be described. This chapter also demonstrates the effect of nanocomposite packaging containing Ag and ZnO on the shelf life of fresh orange juice and on inactivation of
2. Innovative food packaging
Traditional food packaging is meant for mechanical supporting of otherwise non-solid food, and protecting food from external influences. This principal function of packaging involves retardation of deterioration, extension of shelf-life, and maintenance of quality and safety of packaged food. Packaging protects from environmental influences causing deterioration of foods and beverages such as heat, light, the presence or absence of moisture, oxygen, pressure, enzymes, spurious odors, microorganisms, insects, dirt and dust particles, gaseous emissions, and so on. Prolonging shelf-life involves application of various strategies such as temperature control; moisture control; addition of chemicals such as salt, sugar, carbon dioxide, or natural acids; removal of oxygen; or a combination of these with effective packaging (Restuccia et al., 2010). In recent years, new food-packaging systems have been developed as a response to trends in consumer preferences towards mildly preserved, fresh, tasty and convenient food products with a prolonged shelf-life. In addition, changes in retail practices, such as centralization of activities (e.g. preparation of retail packs of fresh meat and sliced vegetables) and the globalization of markets resulting in longer distribution distances, present major challenges to the food-packaging industry to develop packaging concepts that extend shelf-life while maintaining the safety and quality of the packaged food. Traditional systems are reaching their limits with regard to further extension of shelf-life of packaged food. To provide this shelf-life extension, and to improve the quality, safety and integrity of the packaged food, innovative active and intelligent packaging concepts are being developed. Active and intelligent packaging may be defined as follows:
Active packaging changes the condition of the packaged food to extend shelf-life or improve food safety or sensory properties, while maintaining the quality of the packaged food.
Intelligent packaging systems monitor the condition of packaged foods to give information about the quality of the packaged food during transport and storage (De Kruijf et al., 2002).
Important examples of active packaging include oxygen scavengers, carbon dioxide emitters/absorbers, moisture absorbers, ethylene absorbers, ethanol emitters, flavor releasing/absorbing systems and antimicrobial containing films and for Intelligent packaging include time-temperature indicators, leakage indicators and freshness indicators (Ozdemir et al., 2004 ).
2.1. Antimicrobial food packaging
Antibacterial agents are of relevance to a number of industrial sectors including environmental, food, synthetic textiles, packaging, healthcare, medical care, as well as construction and decoration. They can be broadly classified into two types, organic and inorganic. Organic antibacterial materials are often less stable particularly at high temperatures and/or pressures compared to inorganic antibacterial agents. This presents a potential obstacle for the product formulation. As a consequence, inorganic materials such as metal and metal oxides have attracted lots of attention over the past decade due to their ability to withstand harsh process conditions (Zhang et al., 2007). Antimicrobial packaging is a form of active packaging (Appendini & Hotchkiss, 2002). Antimicrobial food packaging materials have to extend the lag phase and reduce the growth rate of microorganisms in order to extend shelf life and to maintain product quality and safety. The need to package foods in a versatile manner for transportation and storage, along with the increasing consumer demand for fresh, convenient, and safe food products presages a bright future for AM Packaging. However, more information is required on the chemical, microbiological and physiological effects of these systems on the packaged food especially on the issues of nutritional quality and human safety. So far, research on AM packaging has focused primarily on the development of various methods and model systems, whereas little attention has been paid to its preservation efficacy in actual foods. Research is essential to identify the types of food that can benefit most from AM packaging materials (Suppakul et al., 2003). Antimicrobial packaging can take several forms including:
Addition of sachets pads containing volatile antimicrobial agents into packages.
Incorporation of volatile and non-volatile antimicrobial agents directly into polymers.
Coating or adsorbing antimicrobials onto polymer surfaces.
Immobilization of antimicrobials to polymers by ion or covalent linkages.
Use of polymers that are inherently antimicrobial (Appendini & Hotchkiss, 2002).
Among the active packaging applications, the incorporation of antimicrobials is receiving considerable attention as a means of extending the bacterial lag phase, slowing the growth rate of micro-organisms and maintaining food quality and safety.
Direct addition of antimicrobials (organic acids or their respective acid anhydrides, spice extracts, chelating agents, metals, enzymes, bacteriocins, etc.) could result in some loss of activity because of leaching into the food matrix, and cross-reaction with other food components such as lipids or proteins. Therefore, the use of packaging films containing antimicrobial agents could be more efficient, by a controlled migration of the compound into the food, not only allowingfor initial inhibition of undesirable micro-organisms, but also residual activity over time, during the transport and storage of food during distribution (Mauriello et al., 2005) Combinations of more than one antimicrobial incorporated into packaging have also been investigated. For example, it is hypothesized that compounds active against Gram-positive bacteria (i.e. lysozyme. Combined with chelating agents (i.e. EDTA. can target Gram-negative bacteria. Addition of EDTA to edible films containing nisin or lysozyme, however, had little inhibition effect on
2.2. Nanotechnology applications for food packaging
Nanotechnology focuses on the characterization, fabrication, and manipulation of biological and nonbiological structures smaller than 100 nm. Structures on this scale have been shown to have unique and novel functional properties. Consequently, interest and activities in this research area have greatly increased over the past years. According to the National Nanotechnology Initiative (2006), Nanotechnology is the understanding and control of matter at dimensions of roughly 1 to 100 nanometers, where unique phenomena enable novel applications. Encompassing nanoscale science, engineering and technology, nanotechnology involves imaging, measuring, modeling, and manipulating matter at this length scale (Weiss et al., 2006). Nanotechnology recently introduced in the food packaging industry can potentially provide solutions to food packaging challenges such as short shelf life (Emamifar et al., 2010). Nanotechnology derived food packaging materials are the largest category of current nanotechnology applications for the food sector. The main applications for food contact materials (FCMs) including:
FCMs incorporating nanomaterials to improve packaging properties (flexibility, gas barrier properties, temperature/moisture stability).
‘‘Active’’ FCMs that incorporate nanoparticles with antimicrobial or oxygen scavenging properties.
‘‘Intelligent’’ food packaging incorporating nanosensors to monitor and report the condition of the food.
Biodegradable polymer–nanomaterial composites (Chaudhry et al., 2008).
2.2.1. Nanocomposites food packaging
Polymer composites are mixtures of polymers with inorganic or organic fillers with certain geometries (fibers, flakes, spheres, particulates). The use of fillers which have at least one dimension in the nanometric range (nanoparticles) produces polymer nanocomposites. Three types of fillers can be distinguished, depending on how many dimensions are in the nanometric range. Isodimensional nanoparticles, such as spherical silica nanoparticles or semiconductor nanoclusters, have three nanometric dimensions. Nanotubes or whiskers are elongated structures in which two dimensions are in the nanometer scale and the third is larger. When only one dimension is in the nanometer range, the composites are known as polymer-layered crystal nanocomposites, almost exclusively obtained by the intercalation of the polymer (or a monomer subsequently polymerized) inside the galleries of layered host crystals (Azeredo, 2009). There are three common methods used to process nanocomposites: solution method,
2.2.2. Antimicrobial mechanisms of nanomaterials
The antimicrobial activity of nanoparticles may be related to several mechanisms. The nanoparticles can either directly interact with the microbial cells, e.g. interrupting transmembrane electron transfer, disrupting/penetrating the cell envelope, or oxidizing cell components, or produce secondary products (e.g. reactive oxygen species (ROS) or dissolved heavy metal ions) that cause damage (Fig 1.) (Li et al., 2008). Antimicrobial mechanisms of the major nanomaterials are discussed as followed:
Titanium dioxide (TiO2) is non-toxic and has been approved by the American Food and Drug Administration (FDA) for use in human food, drugs, cosmetics and food contact materials. Currently there is considerable interest in the self-disinfecting property of TiO2 for meeting hygienic design requirements in food processing and packaging surfaces. Bactericidal and fungicidal effects of TiO2 on
The antibacterial activity of ceramic powders has attracted attention as a new technique that can substitute for conventional methods using organic agents. Ceramic powders of zinc oxide (ZnO), calcium oxide (CaO) and magnesium oxide (MgO) were found to show marked antibacterial activity (Yamamoto, 2001). The advantage of using these inorganic oxides as antimicrobial agents is that they contain mineral elements essential to humans and exhibit strong activity even when administered in small amounts (Sawai & Yoshikawa, 2004 ). Jin et al., 2009 studied several approaches (powder, film, PVP capped and coating) for the application of nano-ZnO in food systems and concluded that nano-ZnO exhibits antimicrobial effects against
Silver has been in use since time immemorial in the form of metallic silver, silver nitrate, silver sulfadiazine for the treatment of burns, wounds and several bacterial infections. But due to the emergence of several antibiotics the use of these silver compounds has been declined remarkably. Nanotechnology is gaining tremendous impetus in the present century due to its capability of modulating metals into their nanosize, which drastically changes the chemical, physical and optical properties of metals. Metallic silver in the form of silver nanoparticles has made a remarkable comeback as a potential antimicrobial agent. The use of silver nanoparticles is also important, as several pathogenic bacteria have developed resistance against various antibiotics. Hence, silver nanoparticles have emerged up with diverse medical applications ranging from silver based dressings, silver coated medicinal devices, such as nanogels, nanolotions, etc (Rai et al., 2009). Silver has also been long known to have microbial inhibition. The antimicrobial activity of these nanoparticles may be related to several mechanisms including, induction of oxidative stress due to generation of reactive oxygen species (ROS) which may cause the degradation of the membrane structure of the cell, release of ions from the surface of nanoparticles that has been reported to cause bacterial death due to binding to cell membrane. However, the mechanism of toxicity is still only partially understood (Emamifar et al., 2011). Silver nanoparticels can damage cell membranes of microorganisms by forming “pits” on their surfaces. Moreover, they may penetrate into the cells to cause DNA damage (Morones et al., 2005). Silver ions released from the surface of these nanoparticles can interact with thiol groups in protein to induce bacterial inactivation, condensation of DNA molecules, and loss of their replication ability (Emamifar et al., 2010). Based on electron spin resonance (ESR) measurements, Kim et al., 2007 observed that the antimicrobial mechanism of Ag nanoparticles is related to the formation of free radicals and the subsequent free radical–induced membrane damage. Treatment with nano-Ag destabilized the outer membrane of bacteria. This indicates that nano-Ag can disrupt the outer membrane barrier components such as lipopolysaccharide or porins, culminating in the perturbation of the cytoplasmic membrane. Although the detailed mechanism by which nanoparticles with a diameter of 10 nm can penetrate and disrupt the membranes remains to be determined, electron microscopy and optical imaging results suggest that nano-Ag penetrate the outer and inner membranes of the treated Gram negative bacteria, with some nanoparticles being found intracellularly. Nano-Ag elicited a rapid collapse of proton motive force. This was also indicated by the observation that nano-Ag induced a massive loss of intracellular potassium. As expected, nano-Ag also decreased the cellular ATP levels, apparently resulting from the collapse of membrane potential. The rapid and complete depletion of ATP may be also indicative of a stimulation of hydrolysis of residual ATP. It is conceivable that this dissipation of bacterial membrane potential and reduction of ATP levels by nano-Ag may culminate in loss of the cell viability. In addition, nano-Ag and Ag+ ions in the form of AgNO3 appear to share a similar membrane-targeting mechanism of action. The effective concentrations of nano-Ag and Ag+ ions are at nanomolar and micromolar levels, respectively. Nano-Ag appear to be significantly more efficient than Ag+ ions in mediating their antimicrobial activities (Lok et al., 2006). Yoon et al., 2007 observed that
2.2.3. Polymer nanocomposites with enhanced photocatalytic antimicrobial properties
Modification of polymeric matrices to prevent growth or reduce adhesion of detrimental microorganisms is a highly desired objective. Hence, there is a significant interest in the development of antimicrobial biomaterials for application in the health and biomedical devices, food, and personal hygiene industries. Among several possibilities currently explored, titania (TiO2) can be spot out as a potential candidate for polymer modification with a significant number of advantages. TiO2 works under UV light excitation with energy above the corresponding band gap (ca. 3.2 ev) forming energy-rich electron-hole pairs. Once at the surface of the material, such charge carriers are able to interact with microorganisms rendering biocidal properties to the corresponding polymer-based nanocomposite films. A point of relevance is the control of the TiO2 polymorphism ensuring the presence of the anatase form, the one with biocidal capability, as well as to control primary particle size in the nanometer range, a fact that would limit scattering events among other things. Novel hybrid or nanocomposite organo-inorganic materials that combine attractive qualities of dissimilar oxide and polymer components are not simply physical blends (vide supra) but can be broadly defined as complex materials having both organic and inorganic constituents intimately mixed. The scale of mixing or, in other words, the degree of homogeneity would influence or even command the properties of the nanocomposite solid materials when the component mixture is adequately reached, typically at the nanometer range. In particular, the optimization of the component contact has been shown to be crucial in order to render TiO2-containing polymer nanocomposites with outstanding biocidal properties. Another point of importance to improve the performance of TiO2-containing nanocomposite systems concerns the optimization of light absorption and the adequate handling of subsequent charge (electron-hole) pair creation and annihilation processes. This task has been typically attempted by controlling the morphological-structural-defect characteristics of the oxide and/or by extending its absorption power into the visible region through a doping process (Kubacka et al., 2009). Metal doping has long been known to be one of the most effective ways to change the intrinsic band structure of TiO2, and consequently, to improve its visible light sensitivity as well as increase its photocatalytic activity under UV irradiation.
Among various dopants, noble metals (especially Ag) have received much attention for this purpose. Among various dopants, noble metals (especially Ag) have received much attention for this purpose. It is generally believed that Ag nanoparticles enhances photoactivity of TiO2 by lowering the recombination rate of its photo-excited charge carriers and/or providing more surface area for adsorption. Visible light absorption by surface plasmon resonance of Ag nanoparticles is also thought to induce electron transfer to TiO2 resulting in charge separation and thus activation by visible light (Akhavan, 2009). A route to simultaneously influence both light absorption and charge handling is based on the so-called plasmonic photosystems. As detailed above, TiO2 is excited by near-UV irradiation and a metal such as Ag shows a very intense localized surface plasmon (LSP) absorption band in the near-UV-visible region. Adequate handling of the LPS resonance can allow extending the absorption light into the visible region of the electromagnetic spectrum and, due to the enhancement of the electric near-field in the vicinity of the Ag, would allow boosting the excitation of electron-hole pairs. An overall improvement of the oxide-polymer nanocomposite performance upon excitation on a region ranged from the near-UV (above ca. 280 nm) to the visible light (below ca. 500-525 nm) can be thus envisaged through a plasmonic effect. This would yield highly efficient systems, with improved performance with respect to TiO2-alone nanocomposites, and having the potential of working under sunlight and/or diffuse artificial light typical of human environments. A last point to mention is the concomitant degradation of the polymer matrix by effect of the charge carriers; this has been proved to be limited by addition of small amounts of titania, typically below 5 wt. % (Kubacka et al., 2009). On the other hand high release level of silver, especially for silver based bulk materials, leads to shortening the effective life of antibacterial activity. If Ag nanoparticles and nanostructures with high antibacterial activities are immobilized on porous matrixes, the release time of silver can be delayed for a long time so that these kinds of silver-supported materials will be of great potentials for bactericidal application (Akhavan, 2009). Yao et al., 2007, demonstrated that Ag, as a typical antimicrobial metal, could be homogeneously deposited onto the surface of TiO2 photocatalyst thin films under UV light illumination. The amount of Ag deposited was controllable as desired by controlling only the UV irradiation time. The Ag/TiO2-coated silicon catheters possessed significant bactericidal activity against
3. Preparation antimicrobial nanocomposite films containing Ag and ZnO
As a case study a laboratory trial by Emamifar et al., 2010, was conducted to producing antimicrobial nanocomposite films containing Ag and ZnO. Film grade LDPE resin pellets were directly mixed with each of the antimicrobial agents including P105 powder (a combination of 95% TiO2 powder, with an average particle diameter of about 250 nm, which acts as a base for doping of nanosilver plus 5% metal nanosilver with particle diameters of about 10 nm) and ZnO nanoparticle powder with an average particle diameter of about 70 nm (Figs 2a, b) separately and the mixture was fed into a twin-screw extruder machine to be cut into masterbatch nano-granules. Proper amounts of masterbatch resins were then added to pure LDPE resin pellets into a single-screw blowing machine to fabricate the final nanocomposite film (50 µm thick) with the desired nanomaterial concentrations (0.25 and 1% for nano-ZnO & 1.5 and 5% for P105). Dispersion quality of nanomaterials into the polymer matrix film was monitored using the Transmission Electron Microscope (Philips CM 200 kV, The Netherlands). Figs (3a, b, c, d) show TEM images of nanocomposites LDPE containing different concentrations of P105 and ZnO nanoparticles. As shown in Figs (3a, b), P105 particles are well distributed in the polymer matrix. However, a slight agglomeration is observed by increasing the concentration of particle powders to 5%. The TEM image of nanocomposite LDPE+0.25% nano-ZnO indicates that the particles are well dispersed in the polymer matrix exhibiting nanometer-scale aggregates ranging from 10 to 200 nanometers with an average size of 70 nm (Fig 3. c). As the nano-ZnO content increases to 1%, the quantity of the agglomerates increases and their size becomes more uneven (Fig 3. d).
4. Applications of antimicrobial polymer nanocomposites for fresh orange juice packaging
Orange juice is one of the most globally accepted fruit products. Demand for natural orange juice with high quality in terms of nutritional value, physicochemical properties and sensory characteristics with minimal or no heat treatment has increased considerably (Bull et al., 2004; Souza et al., 2004). Natural orange juice, even kept under refrigeration, has a short shelf-life due to increasing microbial spoilage (Emamifar et al., 2010). Therefore,
Emamifar et al. 2010 evaluated the capabilities of ZnO and Ag nanoparticles filled LDPE nanocomposite packaging as a new approach to preservation and prolonging shelf life of orange juice by using a antimicrobial nanocomposite (0.25 and 1% for nano-ZnO & 1.5 and 5% for P105) and pure LDPE films 15×10 cm in size, similar to Doypack packaging commonly used for packaging fruit juice. The packages were immediately wrapped in aluminum foil and sanitized at 95oC for 2 min. After cooling and under a sterile laboratory hood, 175 ml of fresh orange juice was poured into each package and sealed by the heat sealer. Packages containing orange juice were stored in dark and cool conditions (4oC). The samples were evaluated in duplicate for their microbiological and sensory characteristics immediately after packaging and after 7, 28, and 56 days of storage.
Mean initial population immediately after packaging was determined to be 4.93 log cfu/ml for yeast and moulds and 4.83 log cfu/ml for total aerobic bacteria in orange juice. The variations in the population of yeast and moulds and total aerobic bacteria are shown in Table 1. In packages made from pure LDPE, the mean population of yeast and moulds increased whereas that of total aerobic bacteria decreased after 7 days of storage. It can be observed that yeast and moulds are better adapted to orange juice under refrigeration than
bacteria. In the LDPE +5% P105 sample, significance decreases were observed over 7 days of storage in total count and yeast and moulds population compared with LDPE +0.25% nano-ZnO packages and LDPE pure packages containing the same concentration of nano-ZnO. Table 1 shows that the level of population of yeast and moulds and total count increased to 6.47 log cfu/ml and 6.37 log cfu/ml, respectively, after 28 days of storage in LDPE pure packages. The shelf life of fresh orange juice is defined as the time to reach a microbial population of 6 log cfu/ml (Raccach & Mellatdoust, 2007). The mean population of total aerobic bacteria and yeast and moulds remained below 6 log cfu/ml after identical storage times in all the packages expect for the LDPE +1% nano-ZnO one. By increasing nano-ZnO concentration to 1%, the antimicrobial activity of the film decreased (Table 1).
This is due to the agglomeration of nanoparticles during the processing of the film (Fig 3. d). In contrast, by increasing the nanosilver concentration, the antimicrobial activity of the film increased (Table 1). The reduced antimicrobial activity of ZnO powder might be related to the increasing particle size, which might decrease the generation of H2O2 from the surface of ZnO powder (Yamamoto, 2001). No significant differences were observed in total aerobic bacteria between LDPE +1.5% P105 and LDPE +0.25% nano-ZnO packages, whereas LDPE +1.5% P105 showed a higher antifungal activity compared with LDPE+0.25% nano-ZnO after 28 days of storage. It appears that antifungal activity of nanocomposites containing nanosilver is significantly (p<0.05) higher than that of nano-ZnO. Sawai et al., 2004 have concluded that ZnO, CaO, and MgO powders have satisfactory antimicrobial effects against broad spectrum microorganisms but that ZnO has a poor antimicrobial effect on
Based on our results (Table 1), the antimicrobial effect of Ag nanoparticels is much higher than that of ZnO nanoparticels. However, it seems that LDPE+5% P105 has a significantly (p<0.05) higher antimicrobial activity compared with other nanocomposites over 28 days of storage for orange juice at 4oC. This is while previous studies have shown a shelf life of up to 14 days for natural, cold orange juice (4oC) (Bull et al., 2004). Yeast, moulds and bacteria exhibit different levels of susceptibility to antimicrobial nanoparticles. The shelf life of natural orange juice has been observed to depend mainly on yeast growth in cold storage (Zanoni et al., 2005). Microbial population increased with increasing storage time to 56 days in all the test packages, indicating the limited effect of long storage time on natural orange juice preservation. However, LDPE +5% P105 packaging has a significantly less loading level at this storage time than other packaging materials. Fig.4. shows the change of sensory attributes of natural orange juice packed in different packages. The high similarity observed in color attribute scores of the packages after 28 days of cold storage (p<0.05) indicates that the change in the color of the samples is still invisible. These results correlated well with the values of browning index presented in Table 1. Odor attribute is greatly influenced by microbial growth and may lead to fermentation in orange juice during storage. After 28 days of storage, a significant difference is observed between the odor of orange juice packed in the test packages and that in pure package except for the one containing 1% nanoZnO. Changes in the taste of packed orange juice during 28 days of storage show the positive effect of nanoantimicrobial packaging. It is obvious that there is a significant difference between LDPE +0.25% nano-ZnO packaging and other nanosilver packaging materials, the lowest score being associated with LDPE +1% nano-ZnO and pure LDPE. The sensory panelists recognized LDPE +0.25% nano-ZnO film followed by LDPE +5% P105 and LDPE +1.5% P105 as the best packaging material in terms of overall acceptability. It is noteworthy that changing orange juice flavor during storage is not only due to the growth of microorganisms but also to heating, storage time, and the common chemical interactions that occur in stored juices (Parish, 1998; Haugaard et al., 2002). Souza et al., 2004 reported that lower storage temperatures of unpasteurized orange juice gave rise to a higher sensory acceptance than the the higher temperatures for 72h. Leizerson et al., 2005 reported that the sensorial shelf life of orange juice is equal to half its microbial and 2/3 its chemical shelf life.
5. Effect of nanocomposite packaging containing Ag and ZnO on inactivation of
Lactobacillus plantarum in orange juice
Acidophilic microorganisms have been shown to be the major contaminants of citrus juices, especially, lactic acid bacteria and yeasts. Lactobacillus plantarum is capable of growing over a wide pH range and spoiling minimally processed or fresh fruit juices owing to its aciduric nature, producing a ‘‘butter’’ off-flavor and swelling of packages. Antimicrobial food packaging materials have to extend the lag phase and reduce the growth rate of microorganisms in order to extend shelf life and to maintain product quality and safety (Emamifar et al. 2011). Emamifar et al. 2011, continued their studied using nanocomposite packaging containing Ag and ZnO on inactivation of
Sterilized orange juice was inoculated with
LDPE pure packages which is higher that of in LDPE +5% P105 (8.23 log cfu/ml), LDPE +1.5% P105 (8.48 log cfu/ml), LDPE +0.25% nano-ZnO (8.56 log cfu/ml), and LDPE +1% nano-ZnO (8.73 log cfu/ml). No significant differences were observed in
6. Metal ions releasing measurement
The quantities of silver and Zinc ions in orange juice after 112 days of storage are shown in Table 2. The quantity of silver ions migrating into orange juice after 112 days, is less than its allowable concentration (10 ppm). It has been reported that silver ions at as low concentrations as 10-9 molesl-1 have an antimicrobial effect in water (Damm et al., 2006). Moreover, the quantity of Zinc ions indicated a higher rate of Zn migration than that of silver but as Zinc is proved to be a GRAS compound for food applications, its low concentration is in the acceptable range for food consumers (Jin et al., 2009)..
This study showed that application of LDPE nanocomposite packaging materials containing Ag and ZnO nanoparticles is a new approach for preserving and extending the microbial shelf life of fresh orange juice at 4oC. The quality of the packaging film including good dispersion of nanomaterials in the polymer matrix free from agglomeration was shown to be very effective on the antimicrobial effects of these packaging materials. Application of packages containing nano-ZnO prolonged the shelf life of fresh orange juice up to 28 days without any negative effects on sensorial parameters. Nanosilver had a higher antimicrobial activity on
I gratefully acknowledge considerable assistance from Parastoo Samadi (Master of Engineering in Food Science).
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