Silver nanoparticles in polymeric matrices for fresh food packaging

2.1 AgNPs/non-degradable polymeric matrix based nanocomposite packaging

Among the non-degradable polymers polyethylene (PE), polyvinyl chloride (PVC) and ethylene vinyl alcohol (EVOH) are the most largely used to host AgNPs for food packaging.

PE is the most common plastic made by addition of ethylene in a polymerization process. Between the two basic forms of polyethylene, high density polyethylene (HDPE) and low density polyethylene (LDPE), the latter is more flexible, transparent and resistant to moisture and therefore only this form is used to make covering films suitable for fresh food storage. PVC is a transparent, stiff and ductile thermoplastic material obtained by polymerization of vinyl chloride polymer. It is an excellent barrier to acids, bases and oil and it’s largely used for food containers and packaging films (Mahdi et al., 2012). EVOH is a copolymer of ethylene and vinyl alcohol. It is a thin film with excellent resistance to oil, fat and oxygen. However, EVOH is moisture sensitive and therefore cannot be used in direct contact with liquid food (Martinez-Abad et al., 2012).

Some examples of these polymers employed after incorporation with AgNPs for packaging of different foodstuffs are reported herein.

Low-density polyethylene (LDPE) polymer matrix containing Ag and ZnO nanoparticles was studied to preserve and extend the shelf life of orange juice (Emamifar et al., 2010). This active-nanocomposite showed to be very effective as antimicrobial nanomaterial in combination with heat treatment at the pasteurization temperature (55 °C–65 °C). The antimicrobial activity of LDPE with nanosilver was significantly more effective against fungi (yeast and molds) with respect to the other active polymer matrices containing ZnO nanoparticles. The antimicrobial activity of AgNPs allowed markedly decreasing of the pasteurization temperature of orange juice by 10 °C.

Similar effects were obtained testing Ag-LDPE packages for the preservation of appearance and sensory quality of stored barberries (Motlagh et al., 2012) and for the keeping of quality of strawberries during extended storage (Yang et al., 2010). Both works showed that nano-packaging obtained with LDPE and AgNPs was able to maintain the sensory, physicochemical and physiological qualities of barberry and strawberry fruits at a higher level compared with normal packaging realized with polyethylene bags.

Food “ready-to-use” requires a refrigerator step to ensure the quality and keep the nutritional compounds. The shelf life of meat and its derivatives depends largely on the quality of the raw material. Mahdi et al. (2012) evaluated the antimicrobial effect of AgNPs-PVC nanopackaging in minced beef, during storage at refrigerator temperature (+4 °C). After 7 days of study the indications are of an inhibitory effect of AgNPs packaging on microbial growth. In particular, these inhibitory effects are better against Escherichia coli growth with respect to Staphylococcus aureus growth. Really, bacteria growth was slowed down significantly, allowing an increased shelf-life of minced meat that usually spoiled after 2 days of storage in common food packaging.

The effects of Ag and TiO₂ nanoparticles in PE polymer packaging were investigated by Metak et al. (Metak and Ajaal, 2013; Metak, 2015) on solid, liquid, high fat containing and high acidic food samples, comparing them to conventional containers. Energy-dispersive-X-ray spectroscopy (EDX) studies indicated that the Ag and TiO₂ NPs are distinctly layered and embedded within the bulk polymer instead of existing as a coating on the polymer surface. AgNPs containers showed significant antifungal activity by inhibition of the microorganism growth after several days of storage. A significantly lower rate of deterioration was observed up to day 10 in all samples examined, due to the antibacterial activity of Ag and TiO₂ NPs, compared to normal packaging.

To assess the antimicrobial activity of the nanocomposites, food samples should be differentiated according to their protein contents as inactivation of silver is favored by the presence of proteins (Martinez-Abad et al., 2012). In this work silver ions were incorporated into an EVOH polymer matrix and the antibacterial efficacy was tested in contact with different kinds of food. In particular, two sets of experiments were carried out, the first with food samples with low protein content such as fresh cut fruit, fresh fruit juice and vegetables, and the second involving food with high protein content such as meat and cheese. The results of the experiments showed that in food samples with low protein content inactivation of the silver ions might occur to a lesser extent and high bactericidal effect is achieved only for 10% Ag–EVOH films. However, even with low protein samples, a 0.1% silver content in the EVOH film is not enough to ensure not to surpass the restriction limits recommended by the EFSA.

Other polymers have also been used to prepare new nanocomposites for food packaging. Toker et al. (2013) described the synthesis and characterization of a new AgNPs containing polyurethane hybrid coating which showed antibacterial activity against E. Coli and S. aureus. Youssef et al. (Youssef and Abdel-Aziz, 2013) soaked AgNPs into a polystyrene (PS) matrix to form a Ag/PS nanocomposite. The nanocomposite was characterized using SEM, TEM and FTIR and demonstrated to significantly inhibit the growth of important pathogenic gram-positive bacteria such as Bacillus subtilis and Enterococcus faecalis, gram-negative bacteria such as E. Coli and Salmonella typhimurium and yeast (Candida albicans).

2.2 AgNPs/biodegradable edible matrix based nanocomposite packaging

Biodegradable polymeric films represent an alternative option in food packaging because they may be obtained at low costs from renewable sources with no contribution to the environmental pollution. For these reasons, biodegradable polymers, which already encounter extensive use in several fields (Mazzuca et al., 2014), have been attracting much interest as alternatives to non-degradable polymers currently used in food packaging. Among them the most commonly used are polysaccharides such as cellulose, pullulan, agarose, starch and chitosan (Valencia-Chamorro et al., 2011; Dhall, 2013; Elsabee and Abdou, 2013). Cellulose binds electropositive transition metal atoms by electrostatic interactions. Consequently silver ions are adsorbed during immersion in silver nitrate and the porous structure of cellulose favors the synthesis and stabilizations of AgNPs. Absorbent pads are realized in this polymer material which is widely used in fresh food modern packaging strategies (Fernandez et al., 2010a,b). Abundant in nature and at low cost of production, cellulose is biocompatible and may be sinthesized in edible format too.

Pullulan is an edible polysaccharide polymer consisting of maltotriose units. Pullulan films are colorless, tasteless, resistant to oil and low permeable to oxygen, but high moisture sensible. In food storage, the water sensibility is a disadvantage for the microbial growth but is advantageous during cooking process (Khalaf et al., 2013; Morsy et al., 2014).

Agarose is a polysaccharide obtained from red algae. It is a linear polymer made up of the repeating unit of agarobiose, a disaccharide formed by d-galactose and 3,6-anhydro-l-galactopyranose. Agar–agar is a jelly-like substance derived from agarose. This gel is water-insoluble, no-toxic, biodegradable and non-immunogenic. For its characteristics agar–agar is used to make coatings for “ready to eat” food and has been proposed for liquid food gel packaging (Incoronato et al., 2011).

Cellulose-based absorbent pads are used as a vehicle for AgNPs formed in situ by physical and chemical reduction methods (Fernandez et al., 2010a,b). Microbiological analyses have demonstrated a microbial growth reduction during storage of beef meat (Fernandez et al., 2010a). They were carried out following different conditions: total aerobic microflora, yeast and molds were enumerated with plate count agar test performed at 30 °C–25 °C respectively; pathogens as Salmonella, E. coli (E. coli) O157:H7, S. aureus (S. aureus), and Listeria monocytogenes (L. monocytogenes) were tested by viable count cell test after incubation at 37 °C in specific culture media. Total aerobic bacteria were significantly reduced during the storage period analyzed whereas lactic acid bacteria were less sensitive. Pseudomonas spp. and Enterobacteriaceae counts resulted to be below those of the control samples (<1 log CFU/g) (Fernandez et al., 2010a). Similar results were obtained with fresh-cut melon samples. The silver loaded absorbent pads reduced the microbial loads also in cut melon (Fernandez et al., 2010b).

A different type of cellulose was used by de Moura et al. (2012) who incorporated AgNPs into a hydroxypropyl methylcellulose (HPMC) matrix for applications in food packaging. The matrix showed good antibacterial properties against E. coli and S. aureus.

The addition of essential oils of spices (rosemary and oregano) already known as antibacterial agents as well as AgNPs may further improve the safety and the quality of foods. Khalaf and coworkers (Khalaf et al., 2013) studied the antimicrobial activity of edible pullulan films incorporated with either nanoparticles (AgNPs, ZnO NPs) and essential oils (EOs) such as oregano oil or rosemary oil, in turkey deli meat conservation. The results demonstrated that AgNPs and oregano oil edible films resulted to be more active than ZnO NPs and rosemary oil. The antimicrobial activity was studied against the Gram-positive bacteria L. monocytogenes and S. aureus during 7 weeks of storage at different temperatures (4°, 25°, 37°, 55 °C). The optimum temperature condition for storage in pullulan edible films with AgNPs and EOs was 4° and 25 °C. Food samples packaged with pullulan film with AgNPs and oregan oil demonstrated a significant decrease in the population of both bacteria.

In a similar work Morsy et al. (2014) demonstrated the activity of AgNPs incorporated into pullulan films in addition to essential oils to control pathogens such as S. aureus and L. monocytogenes on meat and poultry products.

Agar hydrogel was also used as hosting matrix for NPs (Incoronato et al., 2011). Silver montmorillonite (Ag-MMT) nanoparticles, obtained replacing Na⁺ ions in the natural sodium montmorillonite with Ag⁺ ions, were loaded into an agar hydrogel to prolong the shelf life of Fior di Latte cheese. The results confirmed the inhibitory effect exerted by the Ag-MMT matrix on the growth of Pseudomonas spp.

Among the edible materials used to incorporate AgNPs for food packaging creating a biopolymer matrix also stabilizing agents such as salts of the anionic polysaccharide alginic acid (sodium alginate) have been successfully used (Fayaz et al., 2009). Film-gels made with sodium alginate show good tensile strength, flexibility, resistance to tearing and they are oil-proof. The low temperature used in the alginate gel coating synthesis minimizes the inactivation of the antimicrobial agent and preserves the foodstuff characteristics. An odium alginate film incorporating silver nanoparticles was synthesized and investigated demonstrating a reduction in fruit and vegetable decay rate after several storage days (Fayaz et al., 2009). Antibacterial activity was demonstrated against microorganisms of both Gram-positive and Gram-negative types. In another work, sodium alginate was used with calcium ions as active coating loaded with silver montmorillonite nanoparticles to realize a significant shelf life prolongation of fresh-cut carrots (Costa et al., 2012). In other works, sodium alginate was used to form a bio-composite coating to preserve quality of Fior di latte cheese (Gammariello et al., 2011; Mastromatteo et al., 2015).

Also starch was used as edible film. A nanostructured starch based film containing clay and AgNPs has been recently proposed (Abreu et al., 2015). In their native forms, starches are organized into granules and show poor mechanical properties and high water affinity. The presence of AgNPs, which can be easily complexated by the large number of hydroxyl groups of the biopolymer, showed to improve mechanical and gas barrier properties with excellent antimicrobial results.

In another paper, starch and sodium alginate are mixed together to form an edible film with improved mechanical properties (Maizura et al., 2007). Alginate has a potential to form biopolymer films because of its unique colloidal properties but exhibits poor water resistance because of its hydrophilic nature. The mixture of the two biopolymers allowed to improve the mechanical properties of the film. In this paper lemongrass oil was added to the film which resulted to be effective in inhibiting the growth of E. coli. The antimicrobial effect resulted to be enhanced in the films containing also glycerol compared to films without glycerol.