Antimicrobial packaging
INTRODUCTION
In the past, the functions of packaging had been limited to the roles of containment, protection of its content from environmental effects, consumer convenience, and communication of the product information. While the conventional functions of packaging was considered as passive, the new paradigm of packaging—called active packaging— has been emerged. The active packaging may be defined as a packaging system in which the product, the packaging, and the environment interact in a positive way to extend shelf-life or to achieve some characteristics that cannot be obtained otherwise (1). Antimicrobial packaging is a form of active packaging that can control the growth of microorganisms on the surfaces of foods and packaging materials and eventually reduce cell counts in the products.
Antimicrobial function of packaging materials can be achieved by providing unfavorable environmental conditions to microorganisms by eliminating growth requirements, rendering direct contact of microorganisms to the immobilized antimicrobials on the packaging material surface, or transferring antimicrobial agents originally incorporated into the packaging materials.With the above actions, antimicrobial packaging extends the shelf life of food and secure consumer safety by controlling spoilage and pathogenic microorganisms in foods.
According to the Flexible Packaging Association (2), the market size of flexible packaging in the United States in 2007 is about $23,500 million, garnering 18% $130,000 million packaging market. The largest market for flexible packaging is food industry (retail and institutional) accounting for over 57% of shipment. Active role of food packaging has been empathized for sustainable development of food packaging industry. Antimicrobial packaging would make the food packaging industry possible to achieve an innovative development with the aid of regulatory requirements such as food product liability and HACCP. Therefore, research and development trends of antimicrobial packaging systems may progress to achieving more effective antimicrobial activity with broader spectrum, utilizing natural extracts and creating new applications.
A BRIEF HISTORY OF ANTIMICROBIAL PACKAGING
Early Works on Antimicrobial Packaging
As an early stage application of antimicrobial packaging, shelf-life extension of perishable product had been achieved by applying modified atmosphere packaging (MAP) technologies that provided adverse environments of microbial growth. MAP eliminates oxygen level and elevates the concentration of carbon dioxide to inhibit the growth of aerobic microorganisms.
The antimicrobial polymeric materials were first introduced to protect biomedical devices from microbial contamination in Japan (1). A considerable progress in antimicrobial packaging technology has been made in 1990s, and various antimicrobial products was developed and commercialized in household goods, textiles, surgical implants, biomedical devices, and food packaging materials. The progress could be attributed to the development of inherently antimicrobial polymers and also to the development of antimicrobial agent that was incorporated into polymer matrix. For example, chitosan-based polymers and copolymerized acrylic with protonated amine co-monomer have been developed as antimicrobial packaging materials (3). Preservatives with antimicrobial activity play an important role in preventing microbial contamination. Many of these agents such as silver substituted zeolite, potassium sorbates, sodium benzoate, propionic acid, and acetic acid have been successfully incorporated directly into packaging materials to control the microbial contamination (4, 5).
Current Works on Antimicrobial Packaging
The use of chemical preservatives as antimicrobial agents caused public concerns on the potential risks of chemical preservatives that might migrate into food products. These agents are categorized as food additives and controlled by legislation.
Although the chemical preservatives in packaging materials had been carefully regulated by domestic authorities, the increased consumer demand for preservative free foods has rendered the development of antimicrobial packaging with natural antimicrobial agents. The natural antimicrobials and GRAS antimicrobials may include bacteriocins, enzymes, plant extracts, and natural essential oils (5, 6). They hold a great potential and represent excellent activities for controlling microbial contamination. With the effort of finding safe antimicrobial agents, considerable studies have been exerted on developing antimicrobial biopolymer matrix as a carrier for natural antimicrobial agents (6).
ANTIMICROBIAL PACKAGING SYSTEM
Antimicrobial packaging is a system that is designed to control the growth of microorganisms by extending the microbial lag phase and by reducing the growth rate, thereby extending the shelf life of perishable products and enhance the safety of packaged products (4). Antimicrobial packaging can be constructed using antimicrobial packaging materials and/or antimicrobial agents.
Antimicrobial packaging systems can be classified into three types according to the mode of antimicrobial agent’s action: absorption, release, and immobilization (5). The first type is the packaging materials containing antimicrobial agents that eliminate oxygen and moisture in the packaging system by absorption. This type of packaging controls the growth of microorganisms by providing unfavorable conditions to cell growth. The second is the packaging materials containing antimicrobial agents that migrate to the surface of food materials. The antimicrobial action is achieved by the release of the antimicrobial agents from the packaging material. The third type is those containing antimicrobial agents without migration. This type of packaging needs direct contacts between packaging materials and food product since the antimicrobial agents in the packaging material does not release into foods.
The antimicrobial agents may be coated, incorporated, immobilized, or surface modified onto the packaging materials according to the characteristics of the antimicrobials and to the antimicrobial mechanisms how the agents are working (6). Thermal polymer processing methods such as extrusion and injection molding are used for heat-stable antimicrobial agents like silver substituted zeolites. Meanwhile, solvent compounding methods may be a more suitable method to combine the antimicrobial agents and the packaging materials for heat-sensitive antimicrobials such as enzymes and volatile compounds (7).
Antimicrobial Packaging Containing Oxygen Absorbing Agent
Modified atmosphere packaging (MAP) with the absence of oxygen along with elevated concentration of carbon dioxide has been used for controlling the aerobic microorganisms that are responsible for food spoilage. Generally, oxygen concentrations of 0.1% v/v or less in package headspace are required to inhibit the growth of aerobes (1). However, MAP does not guarantee this anaerobic condition in the packaging system throughout the shelf life since the flexible packaging materials show considerable gas permeabilities. Introduction of oxygen absorbing agent in the packaging system rendered active control of oxygen level in the package headspace suitable for antimicrobial purpose by removing oxygen molecules.
In general, oxygen scavenging technologies are constructed by the oxidation of oxygen absorbing agents: iron powder, ascorbic acid, photosensitive dyes, enzymes (e.g., glucose oxidase and alcohol oxidase), unsaturated fatty acids (e.g., oleic acid or linoleic acid), and immobilized yeast on a solid substrate (8). Iron-based material is known to the most effective oxygen scavenging system among the above substances. It has been reported that 1 g of iron will react with 300 ml of oxygen (8). The oxygenabsorbing antimicrobial packaging has been successfully commercialized in bakery, pasta, and meat products by applying the agents enclosed in sachets or incorporated in polymer matrix. Recently, diversified matrices for oxygen absorbents are available in film, tray, label, and closure liner as well as sachet (8). In spite of the strong antimicrobial activity of the oxygen scavenging system against aerobic microorganisms, this system does not possess the effective activity against anaerobic microorganisms.
Antimicrobial Packaging Containing Antimicrobial Agent that Migrate into Food
The packaging materials may act as a carrier for antimicrobial agents to perform their active role to control microorganisms. Some of the antimicrobial agents may be coated or directly incorporated into the packaging materials and subsequently migrate to the food system. The antimicrobial action is achieved by release of the antimicrobial agents from the packaging material. The released antimicrobial agents control the growth of microorganisms by altering cell membrane properties or by inhibiting essential metabolic pathways of the microorganisms (4–6).
Most spoilage incidents occur primarily at food surface by the contamination of aerobic microorganisms. The concentration of the antimicrobial agents above their minimal inhibitory concentration (MIC) is required to impart antimicrobial function. Without the antimicrobial packaging concept, the excess amount of preservatives such as benzoates and sorbates should be included in foods to control the spoilage microorganisms. Thus, releasing antimicrobial additives to the food surface conveniently increases the additives concentration in the food surface above the MIC while maintains the preservative concentration inside the food at sufficiently low level (4). Considering that the use of preservatives for shelf-life extension has been strictly controlled by food safety authorities, antimicrobial packaging is advantageous in reducing potential risks of consuming excess amount of food preservatives.
An additional advantage of antimicrobial packaging is the sustainable antimicrobial activity. The antimicrobial agents initially included in food ingredients might be inactivated by interacting with other food components. For example, bacteriocins and enzymatic antimicrobial agent applied in the foods or onto the food surface may interact with proteolytic enzymes in food and may cause the loss of antimicrobial activities. On the contrary, incorporation of the above substances in packaging films did not cause the loss of antimicrobial activity, controlled the release of the antimicrobial agents, and maintained antimicrobial activity for fairly long periods.
Nonvolatile Migration.
The incorporated antimicrobials are migrated to the food surface in either solute or gas states. The migrating solutes are nonvolatile materials such as organic acids and their salts, enzymes, bacteriocins, fungicides and some of natural extracts, while the gases are volatiles such as alcohols, small phenolics, aldehydes, and others (4–6, 8). They moved from the film matrix to the food surface and diffused or dissolved into the food. Diffusion is a primary mechanism of nonvolatile solute transfer in the film matrix, in which relates to the release rate. The migration kinetics of nonvolatile solute followed the Fick’s second law of diffusion, where diffusion coefficient depends on the type of film materials, microstructural voids in film matrix, and environmental temperature (9, 10). The migration of the antimicrobial agents in the film to the food surface requires directly contact. The contact between the film matrix and the food surface throughout the shelf life should be assured for their migration and, consequently, their antimicrobial action. For this, the food should be a continuous matrix without any factors that interfere with the diffusional migration. This food matrix could be a liquid solution, a semisolid paste, or a smooth solid matrix without significant pores, holes, or heterogeneous particles. The antimicrobial agents in the food surface will move into the center of the food by diffusion or dissolution. The solubility and diffusion coefficients of the agent in the food are very important factors that govern the rate of agent removal in the food surface. The antimicrobial concentration in the food surface could be maintained above the MIC for their effectiveness in controlling the microbial growth when the agent transfer in the food and films are balanced (5).
Volatile Migration.
The migrating antimicrobials could be gaseous compounds released from a gas emitting material. The gaseous antimicrobials can be migrated into the food surface and its packaging headspace or into the air gap between the package and the solid food (4).
The migrating volatile gases include SO2, ethanol, essential oils, and a component of natural herb and spice extracts. The advantage of using volatile antimicrobial agents is that the packaging film does not need to contact directly to the food surface. Therefore, food matrix in this system could be highly porous foods, powdered foods, particulate foods, or shredded and irregularly shaped foods. Sometimes, however, the use of volatile antimicrobial is limited by the incompatibility of the agent with the packaging materials, or by the loss of volatile antimicrobials during incorporation into packaging by extrusion or coating (5).
The volatile antimicrobial agent initially incorporated in the packaging film or sachet material is transferred to the film surface and subsequently evaporated to the packaging headspace. The vaporized agent in the headspace is then absorbed to the food surface. Eventually, the agent concentration in the food surface is equilibrated with the volatilized antimicrobials in the headspace (4). For the antimicrobial efficacy of the agent, the agent concentration in the food surface should be maintained above the MIC of target microorganisms. The concentration of migrated agent in the food surface is determined by the release rate of the volatile agent from the packaging material and by the absorption rate of headspace volatile agents into the food. The release rate of the agent from the packaging material to the headspace depends on the volatility of the agent which represents the partitioning behavior between packaging material and headspace. The absorption rate of the agent into food depends on the solubility of the agent into the food ingredient. Therefore, the composition of the food is a very important factor of the agent’s solubility to the food materials. In general, the volatile agents are more soluble in lipid foods than in hydrophilic food materials.
The desired antimicrobial concentration of a food surface can be attained by controlling the release rate of antimicrobial agent from the packaging material surface. When we use an appropriately selected film layer that has a specific permeability of the volatile substance, the antimicrobial’s release rate could be controlled efficiently. The microencapsulation of the volatile substance with appropriate wall materials may control the release of antimicrobials. This encapsulation method can also provide a solution against the loss of volatile antimicrobials during packaging material fabrication processes.
Antimicrobial Packaging without Antimicrobial Agent Migration
Antimicrobial packaging can also use agents that are not migrated into the food. The nonmigrating antimicrobial packaging system can be achieved using inherently bioactive polymers or developing novel packaging materials where the antimicrobials are attached to the nonactive films by either covalent bond or ionic immobilizations (4). In this antimicrobial packaging system, many antimicrobials are enzymes or chemicals that are directly and indirectly participated in the microbial inactivation function. They participate in the biological reactions that lead to the microbial inactivation or produce biologically active radicals, ions, and reactive singlet oxygen that may induce antimicrobial effects.
Since the biologically active compounds are not mobile, the incorporation of the antimicrobials in the matrix during film fabrication processes is not considered necessary. Although the incorporated antimicrobials uniformly distributed in the film matrix, their activity is limited to the film surface only. Therefore, imparting a biologically active ingredient to the film surface would be sufficient to exhibit the antimicrobial efficacy of the nonmigrating antimicrobial packaging film. Therefore, assigning an antimicrobial activity without migration of antimicrobials can be performed by immobilizing nonmigrating antimicrobials on the polymer film surface, by coating a very thin layer of active matrix on to the packaging film, or by attaching antimicrobial compounds covalently on the film surface with the aid of multifunctional ligands (11).
The active materials in the film surface may catalyze antimicrobial action or provide sufficient energies to form ions and radicals on the food surface which are capable to control microbial growth. Therefore, this type of antimicrobial film is particularly effective for the packaging with liquid food and is advantageous for regulatory compliances. Since antimicrobial agents do not migrate into the food system, the antimicrobial agents that are not permitted as food ingredient and food additives may be used for this purpose as food contact substances.
Inherently Antimicrobial Packaging Materials.
Some polymers are inherently antimicrobial, and they have been utilized in many fields currently such as biomedical instruments, filters, membranes, and packaging materials. Cationic polymers such as chitosan, poly L-lysine, lysozyme, and numerous synthetic polymers that have quaternary ammonium, phosphonium, and biguanide compounds have exhibited to be effective antimicrobial polymers.
It has been well known that chitosan (a natural polysaccharide), and its derivatives have an antimicrobial effect due to the presence of a positively charged ammonium group. Besides the natural antimicrobial polymers, antimicrobial synthetic polymers have been produced by polymerizing biologically active monomers. The widely used biologically active materials for synthetic polymers are the cationic compounds including quaternary ammonium compounds (QACs), biguanide groups, quaternary pyridinium compounds, phosphonium compounds, sulfonium compounds, and 2-(4u-thiazolyl) benzimidazol (TBZ) (Table 1). The functional groups are linked to reactive groups of monomers and, in turn, have been polymerized to form synthetic antimicrobial polymers (Figure 1). Therefore, most synthetic antimicrobial polymers can be understood as polymerized biocides, and the polymer backbone has the multiplied activity of the antimicrobial functions of attached biocides. Many of these polymers are amphiphilic and positively charged.
The antimicrobial action of polycationic compound has been considered as the disruption of cytoplasmic membrane of bacterial cell (13). Polycationic antimicrobial polymers are absorbed onto the negatively charged bacterial cell surface at physiological pH by electrostatic interaction. The absorbed polycations are bound to cytoplasmic membrane, and disrupt it. The consequent leakage of potassium ions and other cytoplasmic constituents lead to cell death. In particular, the mode of antimicrobial action of QACs is the damage of cytoplasmic membrane of bacteria through surfactant-like interaction resulting in the loss of permeability properties of the membrane (14). This means that the functional groups of the antimicrobial polymer must diffuse through the microbial cell wall, be water soluble, and have no interaction with the microbial cell wall. For this, spacer molecules that link the biologically active agents to the polymer backbone are required to allow the agent to have sufficient freedom of motion when the polymer backbone does not act as the spacer (11).
Physical modification of polymers often produces biologically active packaging films. Ultraviolet or electron beam irradiation on polyamide films increases positively charged amine concentration on the film’s surface, resulting in enhanced cell adhesion potential. However, this type of film has limited microcidal effect, and microbial adsorption on the film surface diminishes antimicrobial activity. Incorporation of antimicrobial agents into this film matrix may overcome these drawbacks successfully (7).
Immobilized Antimicrobial Agents in the Packaging Film Matrix.
The immobilized antimicrobial agents are not removed from polymer surfaces. Various immobilization techniques have been developed to overcome the contact problems of antimicrobial residues to food surfaces. The antimicrobial materials used for immobilization include the hydrolysis product of a quaternary aminecontaining organosilicon salt, hexachlorophene, acriflavine, antibiotics such as streptomycins and gentamycins, and antimicrobial enzymes including lactoferrin, sulfhydril oxidase, and bile-salt- stimulated lipase. The changes in conformation and denaturation of proteins and peptides may result in reduced antimicrobial activities of antimicrobial enzymes. The active site should be protected during the film formation.
INGREDIENT OF ANTIMICROBIAL PACKAGING
Film Matrix
The antimicrobial packaging system is comprised of film matrix and antimicrobial agents. In considering the biologically active role of the antimicrobial agent in the film, the relationships between the film matrix and the antimicrobial agent would be critical, since thermal degradation of the antimicrobial agent during film fabrication and its chemical compatibility with the film material may limit the antimicrobial activity. Synthetic polymeric packaging materials are produced by thermal processing method such as extrusion. Thermally stable antimicrobial agents should be incorporated. In general, synthetic flexible films consist of many layers of different films. Lowdensity polyethylene (LDPE) has been used for the film layer that is in direct contact with the food. Therefore, LDPE has been most widely used packaging materials among the synthetic polymers for antimicrobial agent carriers. For further modification of the contact film layer, poly(ethylene-co-methacrylic acid) (PEMA) has also been explored as a film matrix (15).
Thermally unstable antimicrobial agents cannot be used with extruded synthetic polymer films, but they can be applied to the biopolymeric film matrix instead. In general, biopolymer films are formed by removing the solvent from the film solution in the cast plate. Therefore, they do not require high temperature for polymer melting. Examples of biopolymers are hydrocolloids including polysaccharide and protein, lipid, and the composite of hydrocolloid and lipid. For antimicrobial purposes, the use of biopolymeric films such as chitosan, soy protein, whey protein, corn zein, methyl cellulose (MC), and hydroxypropyl methyl cellulose (HPMC) films with various antimicrobial agents have been reported (16).
Antimicrobial Agent
The widely used method to inhibit the growth of undesirable microorganisms is the use of chemical agents exhibiting antimicrobial activity. These chemicals may be either synthetic compounds intentionally added to foods or naturally occurring, biologically derived substances. The selection of an antimicrobial agent depends primarily on its activity against the target microorganisms, compatibility with the packaging material, and the heat stability during the thermal process (4–6).
Currently, synthesized chemical preservatives have been widely employed to control the number of microorganisms in the packaged food, but increasing consumer awareness of potential health risk associated with the synthetic agent requires a substituent that is available in the nature. The natural antimicrobial agents such as bacteriocins and spice extract have long been used as constituents of foods and showed negligible toxicity. Therefore, natural antimicrobial agents are on the way of replacing the synthetic antimicrobial agents as food preservatives for direct deposition and also as ingredients of packaging materials for migration into food.
Organic Acid.
Sorbic acid, benzoic acid, propionic acid, and their salts have been widely used for preventing microbial deterioration of food. They are on the food additive list in many countries and have been used under the control of regulation for their potential health risk. High polarity of organic acids causes them to be incompatible with the apolar LDPE. Acid anhydrides were thought to be more compatible than free acids and their salts because of their lower polarity. Organic acid antimicrobials could be incorporated into the biopolymeric film for their compatibility.
Inorganic Metallic Ion.
Silver Ion.
Silver ions in the microbial cell inhibit metabolic enzymes and have strong antimicrobial activity. Silver ions are generally incorporated into polymer films in the form of silver substituted zeolite, an ionic bonded metallic compound in which the sodium ions present has been substituted with the silver ions. Silver-substituted zeolite is thermo-stable and can be applied to the thermal process of synthetic polymers like polyethylene, polypropylene, and nylon as well as biopolymeric films (11, 17, 18). Silver ions are released from the silver-substituted zeolite incorporated in the film matrix to the food materials for antimicrobial action (18).
Titanium Ion.
Titanium dioxide (TiO2) is nontoxic and has been approved by the U.S. Food and Drug Administration for the use in foods and food contact materials (19). Ultraviolet energy shifts the energy level of TiO2 to the excited state. On its way back to ground state, TiO2 releases absorbed energy to the food material (20). The released energy may form very reactive radicals and singlet oxygen which have antimicrobial function. In this system titanium ion is not a migrating agent but trapped in the film structure (20). Currently there is considerable interest in the self-disinfecting property of TiO2 to satisfy hygienic design requirements in food processing equipment surfaces (21, 22).
Natural Compounds.
The naturally occurring antimicrobials are enzymes, peptides, oils, and other materials that have been present self-defense functions against microbial contamination. Typical examples are bacteriocins in lactic acid bacteria, lysozymes in egg white, flavonoids in plant extracts, and essential oils.
Bacteriocins.
Bacteriocins are small bacterial peptides that show strong antimicrobial activity against closely related bacteria. Antimicrobial effectiveness of bacteriocins in food is related to specific antimicrobial activity of the bacteriocin, amount of the bacteriocin used, number of microorganisms, process conditions, interaction with food components, and pH and temperature of the product (23).
Nisin is a polypeptide produced by Lactococcus lactis spp. It has been approved as a food additive with GRAS status in over 50 countries worldwide. It has a relatively broad spectrum of antimicrobial activity against various lactic acid bacteria and other Gram-positive bacteria (24). It is particularly effective against heat-resistant bacterial spores of Clostridium botulinum (25). It is not effective against gram-negative bacteria because it cannot penetrate outer lipid membrane of cell to the cytoplasm (24). Use of nisin in conjunction with ethylenediamine tetraacetic acid (EDTA) may increase the antimicrobial effectiveness against Gram-negative bacteria (26). Nisin has been incorporated into biopolymer films such as soy protein, corn zein, chitosan, and methylcellulose films for antimicrobial edible coating applications (27, 28). The thermal processing in film production may reduce antimicrobial activity of nisin (28).
Lysozyme.
Lysozyme, also known as muramidase or Nacetylmuramichydrolase, is a relatively small enzyme from hen egg white. Lysozyme is effective against spoilage and pathogenic microorganisms by disintegrating the cell wall structure. Antimicrobial activity of lysozyme is attributed to the function that catalyzes the hydrolysis of the b-1, 4 glycosidic bond between N-acetylmuramic acid and 2-cetyl-amino-2-deoxy-D-glucose residues in the bacterial cell wall (29, 30). Lysozyme demonstrates a strong antibacterial potential against Gram-positive bacteria such as Listeria monocytogenes (30). Lysozyme has been applied to biopolymer films such as chitosan, whey protein, fish gelatin, corn zein, and sodium alginate films in either purified or unpurified forms alone and in combination with EDTA and other bacteriocins (31, 32). Antimicrobial activity of immobilized lysozyme in polyvinyl alcohol film also has been reported against various spoilage and pathogenic bacteria (33).
Plant Extracts.
Antimicrobial activity of phytochemicals in the extracts of spices and herbs has been demonstrated by many researchers (5, 6, 31). In many cases, the concentration of biologically active component in spices and herbs and their essential oils are too low to be effectively used for packaging applications due to their limited sensory acceptance at high concentration level (23). They may be used in junction with other biologically active agents and contribute to total hurdle technology system for microbial control. Many phytochemical compounds in essential oils and extracts responsible for antimicrobial activity are phenolic compounds.
- Essential Oils. Plant extracts of coffee, green tea, spices, and herbs (i.e., cinnamon, cloves, mustard seed, oregano, rosemary, thyme and vanillin) have been evidenced for their antimicrobial activity against a wide spectrum of microorganisms (5, 34). The antimicrobial activities of the spices are associated with phenolic compounds in their essential oil fraction. The antimicrobial essential oil components of cinnamon and cloves are eugenol and cinnamaldehyde, respectively. Terpenes such as carvacrol, p-cymene, and thymol are the major volatile components of oregano and thyme (23). Many essential oils are volatile and can be applied to volatile migration when they are incorporated into the packaging film (31).
- Allyl Isothiocyanate (AIT). Allyl isothiocyanate is a naturally occurring nonphenolic volatile compound responsible for the flavor of horseradish, wasabi, and mustard. Volatilized AIT has an effective antimicrobial activity against variety of pathogens when used at low concentration. AIT causes metabolite leakage by affecting cell membranes (35). Antimicrobial activity of AIT-incorporated gelatin film, nylon 6,6 film, and PVDC/PVC copolymer film has been reported against spoilage and pathogenic microorganisms (36).
COMMERCIAL APPLICATIONS
Commercialized Antimicrobial Packaging
Even though many research works have been conducted all over the world, there are only a few commercialized products of antimicrobial food packaging materials. This is because of strict hygienic regulation on food packaging, high price, and limited consumer perception and acceptance on their effectiveness. The area of food preparation and utensil products has less strict regulation in applying the antimicrobial materials into real practices and has seen a higher number of commercial products—for example, kitchen board and gloves. Table 2 lists some examples of commercial products available in market. The list is not exclusive and covers only those observed in the writers’ eye for the last 20 years. Some products available in a country or area may not be allowed for sale in other countries because of the difference in food safety regulation.
In a practical sense, ethanol emitter and plant extracts have acquired wider acceptance, depending on people’s attitude and corresponding country’s legislation. Some ethanol emitters combine the oxygen-scavenging capacity to inhibit aerobic spoilage organisms. Ag-containing ceramic materials have been applied in plastic films and containers where people generally recognize silver as hygiene-improving and antitoxic material. SO2 generator may be used in postharvest handling and storage of agricultural products, where agricultural safety practices are applied. A ClO2-generating plastic tube, Knick’n’- cleans, which is activated by bending to mix two fluids in it, is marketed for the use in refrigerators for a limited time period of about 30 days. A ClO2-emitting film (Micro- Gardet) claimed to produce antimicrobial microatmosphere for 10 days keeping freshness of foods and breaking down ethylene gas. Until 2008, ready-to-eat food applications of this system await FDA clearance in the United States. Triclosan is not approved as a food additive and is incorporated into a plastic master batch, which is converted into food-contact household equipments such as cutting board and hand gloves.
Edible coating being a component of food may be applied for antimicrobial function when it contains safe bioactive compounds, organic acids, or edible essential oils of plant or spice origins. Because the coating may be understood as a food component, there would be no barrier of application in the regulations if all active/inactive ingredients are food grade. The potential edible coating matrices include chitosan, hydroxy propyl methyl cellulose, and alginate gels (29).
Effectiveness of Antimicrobial Food Packaging
Many studies showed potential benefits of antimicrobial packaging for fresh fruits and vegetables, milk, beverages, cheese, ham, meat and fish (37, 38). Packaging materials with nisin, chitosan, or acids typically resulted in marginal microbial count reduction of 1.5–2.5 log10 compared to control (37). The reduction or suppression of microbial growth by the antimicrobial packaging is varied too much to conclude that any specific system or condition is universally optimal or better than others. It is generally accepted that the results in microbial media or buffer solutions cannot be directly applied to the real food applications. Conducting test with real food sample is needed for practical or commercial application.
The effectiveness of microbial inhibition by antimicrobial packaging is generally improved when combined with other preservation hurdles such as modified atmosphere, low pH, high-pressure treatment, and low temperature (38–40). Analysis of microbial spoilage kinetics with antimicrobial packaging showed that the microbial growth was suppressed more with lower storage temperature (41). The microbial inhibition of antimicrobial packaging at the low temperature could be represented by low cell concentration, extended lag time, and reduced growth rate.
When the temperature dependence of microbial lag time was analyzed by square root kinetic model [equation (1)], antimicrobial packaging showed higher temperature dependence (increased b value) with increase of hypothetical minimum growth temperature (increased Tmin) (41):
where T is the temperature (in 1C), b is slope parameter representing temperature effect, and Tmin is respective theoretical minimum temperatures for growth estimated by extrapolation of the regression line to the temperature axis (Figure 2).
The effectiveness of antimicrobial packaging may vary with food type, release kinetics of antimicrobial agents, and environmental factors. Many antimicrobial packaging systems that show effective activity with microbial culture media may not work with real foods. Some food components such as fatty acids, amino acids, sulfates, and/or enzymes may diminish the activity of the antimicrobial packaging films (11, 42). Food–package interaction may change the mechanism and effectiveness of microbial inhibition of the antimicrobial packaging system (29). There are possibilities that the incorporation of antimicrobials into the plastic material may change its mechanical, barrier, and optical properties. Sensory properties of food may also be affected by food–package interaction. For example, film with plant extract such wasabi essential oil may alter flavor of the contained food. The antimicrobial packaging may have to be designed, selected, and customized for each food type (43). The packaging system has to be selected after consideration on the primarily concerned target microorganism, spoilage organisms, its activity spectrum, microbial inhibition mode, food properties, release of active agent, and storage conditions.
Regulations
Antimicrobial packaging to be marketed in the United States is subject to the food additive approval process if its components are reasonably expected to migrate to foods for effective microbial inhibition (4). There are two processes of food contact substance approval in FDA: food contact notification process and food additive petition process. Packaging material that has no intended technical effect on the food may be approved by the former process, in which a manufacturer should notify the FDA to market a new product at least 120 days before its introduction and can sell it unless the FDA objects to the notification. However, the material that exerts antimicrobial effect on the food through migration or controlled release needs to be processed through the food additive petition process. Food contact substance notification is specific only to the manufacturer named in the notification, and it does not apply generically to the product category. Due to complicated and strict regulation on approval of antimicrobial food packaging materials, their applications in food packaging is very limited in the United States. Chlorine-dioxide-releasing material seems to reach the commercial stage (42). Some silver incorporated materials are approved for food-contact purpose (6).
Until 2004, plastic packaging regulations in the EU did not allow any specific provisions for antimicrobial packaging and, thus, required that any potential antimicrobial component in the antimicrobial food packaging should be covered by ‘‘positive lists,’’ which would have specific migration limit (44). The plastic packaging material should also have met overall migration limit, typically 60mg per kg food. In 2004, active and intelligent packaging including antimicrobial food packaging has been allowed in the EC Framework Regulation 1935/2004. Further detailed requirements and specification on adopting antimicrobial packaging will soon come out, to include positive lists of authorized substances and/or materials and articles.
General food packaging regulations in Japan made allowances for active packaging emitting ethanol and wasabi extract volatiles to preserve the bakery products and prepared foods. Australia and New Zealand legislation permits silicon dioxide sachets containing ethanol and flavors specifically (45), which may act to inhibit microbial growth.