4.1 Introduction Antimicrobial packaging is one of many applications of active packaging (Floros et al., 1997). Active packaging is the packaging system which possesses attributes beyond basic barrier properties, which are achieved by adding active ingredients in the packaging system and/or using actively functional polymers (Han and Rooney, 2002). Antimicrobial packaging is the packaging system that is able to kill or inhibit spoilage and pathogenic microorganisms that are contaminating foods. The new antimicrobial function can be achieved by adding antimicrobial agents in the packaging system and/or using antimicrobial polymers that satisfy conventional packaging requirements. When the packaging system acquires antimicrobial activity, the packaging system (or material) limits or prevents microbial growth by extending the lag period and reducing the growth rate or decreases live counts of microorganisms (Han, 2000). Compared to the goals of conventional food packaging such as (i) shelf-life extension, (ii) quality maintenance, and (iii) safety assurance which could be achieved by various methods, antimicrobial packaging is specifically designed to control microorganisms that generally affect the above three goals adversely. Therefore some products, which are not sensitive to microbial spoilage or contamination, may not need the antimicrobial packaging system. However, most foods are perishable and most medical/sanitary devices are susceptible to contamination. Therefore, the primary goals of an antimicrobial packaging system are (i) safety assurance, (ii) quality maintenance, and (iii) shelf-life extension, which is the reversed order of the primary goals of conventional packaging systems. Nowadays food security is a big issue and antimicrobial packaging could play a role in food security assurance. 4 Antimicrobial food packaging J. H. Han, The University of Manitoba, Canada All antimicrobial agents have different activities which affect micro- organisms differently. There is no ‘Magic Bullet’ antimicrobial agent effectively working against all spoilage and pathogenic microorganisms. This is due to the characteristic antimicrobial mechanisms and due to the various physiologies of the microorganisms. Simple categorisation of microorganisms may be very helpful to select specific antimicrobial agents. Such categories may consist of oxygen requirement (aerobes and anaerobes), cell wall composition (Gram positive and Gram negative), growth-stage (spores and vegetative cells), optimal growth temperature (thermophilic, mesophilic and psychrotropic) and acid/ osmosis resistance. Besides the microbial characteristics, the characteristic antimicrobial function of the antimicrobial agent is also important to understand the efficacy as well as the limits of the activity. Some antimicrobial agents inhibit essential metabolic (or reproductive genetic) pathways of micro- organisms while some others alter cell membrane/wall structure. For example, lysozyme destroys cell walls without the inhibition of metabolic pathways and results in physical cleavages of cell wall, while lactoferrin and EDTA act as coupling agents of essential cationic ions and charged polymers. Two major functions of microbial inhibition are microbial-cidal and microbial-static effects. In the case of microbial-static effects, the packaging system has to possess the active function of maintaining the concentration above the minimal inhibitory concentration during the entire storage period or shelf-life in order to prevent re- growth of target microorganisms. Traditional preservation methods sometimes consist of antimicrobial packaging concepts, which include sausage casings of cured/salted/smoked meats, smoked pottery/oak barrels for fermentation, and bran-filled pickle jars. The basic principle of these traditional preservation methods and antimicrobial packaging is a hurdle technology (Fig. 4.1). The extra antimicrobial function of the packaging system is another hurdle to prevent the degradation of total quality of packaged foods while satisfying the conventional functions of moisture and oxygen barriers as well as physical protection. The microbial hurdle may not contribute to the protection function from physical damage. However, it provides tremendous protection against microorganisms, which has never been achieved by conventional moisture and oxygen barrier packaging materials. Antimicrobial functions which are achieved by adding antimicrobial agents in the packaging system or using antimicrobial polymeric materials show generally three types of mode; (i) release; (ii) absorption; and (iii) immobilisation. Release type allows the migration of antimicrobial agents into foods or headspace inside packages, and inhibits the growth of microorganisms. The antimicrobial agents can be either a solute or a gas. However, solute antimicrobial agents cannot migrate through air gaps or over the space between the package and the food product, while the gaseous antimicrobial agents can penetrate through any space. Absorption mode of antimicrobial system removes essential factors of microbial growth from the food systems and inhibits the growth of microorganisms. For example, the oxygen-absorbing system can prevent the growth of moulds inside packages. Immobilisation system does not Antimicrobial food packaging 51 release antimicrobial agents but suppresses the growth of microorganisms at the contact surface. Immobilisation systems may be less effective in the case of solid foods compared to the liquid foods because there is less possibility for contact between the antimicrobial package and the whole food products. 4.2 Antimicrobial agents There are many antimicrobial agents that exist and are widely used. To be able to use antimicrobial agents in the foods, pharmaceuticals and cosmetic products, the industry must follow the guidelines and regulations of the country that they are going to use them in, for example, FDA and/or EPA in the United States. This implies that new antimicrobial packaging materials may be developed using only agents which are approved by the authorisation agencies as examples of FDA-approved or notified-to-use within the concentration limits for food safety enhancement or preservation. Various antimicrobial agents may be incorporated in the packaging system, which are chemical antimicrobials, antioxidants, biotechnology products, antimicrobial polymers, natural antimicrobials and gas (Table 4.1). Chemical antimicrobial agents are the most common substances used in the industry. They include organic acids, fungicides, alcohols and antibiotics. Fig. 4.1 Hurdle technology in antimicrobial packaging system compared to the conventional packaging system. 52 Novel food packaging techniques Table 4.1 Antimicrobial agents and packaging systems Antimicrobials Packaging materials Foods Microorganisms References Organic acids Benzoic acids PE Tilapia fillets Total bacteria Huang et al., 1997 Ionomer Culture media Pen. spp., Asp. nige Weng et al., 1997 Parabens LDPE Simulants Migration test Dobias et al., 2000 PE coating Simulants Migration test Chung et al., 2001a Styrene-acrylates Culture media S. cerevisiae Chung et al., 2001b Benzoic & sorbic acids PE-co-met-acrylates Culture media Asp. niger, Pen. spp. Weng et al., 1999 Sorbates LDPE Culture media S. cerevisiae Han and Floros, 1997 PE, BOPP, PET Water, cheese Migration test Han and Flores, 1998a; b LDPE Cheese Yeast, mould Devileghere et al., 2000a MC/palmitic acid Water Migration test Rico-Pena and Torres, 1991 MC/HPMC/fatty acid Water Migration test Vojdana and Torres, 1990 MC/chitosan Culture media Chen et al., 1996 Starch/glycerol Chicken breast Baron and Summer, 1993 WPI Culture media S. cerevisiae,. Ozdermir, 1999 Asp niger, Pen. roqueforti CMC/paper Cheese Ghosh et al., 1973, 1977 Sorbic anhydride PE Culture media S. cerevisiae, moulds Weng and Chen 1997; Weng and Hotchkiss, 1993 Sorbates & propionates PE/foil Apples Firmness test Yakovlleva et al., 1999 Acetic, propionic acid Chitosan Water Migration test Ouattara et al. 2000a Enzymes Lysozyme, nisin, EDTA SPI, zein Culture media E. coli, Lb. plantarum Padgett et al., 1998 Lysozyme, nisin, WPI Culture media L. monocytogenes, Sal. Rodrigues and Han, 2000; EDTA, propyl paraben typhimurium, E. coli, B. Rodrigues et al., 2002 thermosph., S. aureus Table 4.1 Continued Antimicrobials Packaging materials Foods Microorganisms References Immobilised lysozyme PVOH, nylon, Culture media Lysozyme activity test Appendini and Hotchkiss, cellulose acetate 1996; 1997 Glucose oxidase Fish Fields et al., 1986 Bacteriocins Nisin PE Beef B. thermosph. Siragusa et al., 1999 HPMC Culture media L. monocytogenes, Coma et al., 2001 S. aureus Corn zein Shredded cheese Total aerobes Cooksey et al., 2000 Nisin, lacticins Polyamide/LDPE, Culture media M. favus, An et al., 2000 L. monocytogenes Nisin, lacticin, salts Polyamide/LDPE Culture media M. flavus Kim et al. 2000 Nisin, EDTA PE, PE-co-PEO Beef B. thermosphacta Cutter et al., 2001 Nisin, citrate, EDTA PVC, nylon, LLDPE Chicken Sal. typhimurium Tatrajan and Sheldon 2000 Nisin, organic Acrylics, PVA-co-PE Water Migration test Choi et al., 2001 acids mixture Nisin, lauric acid Zein Simulants Migration test Hoffman et al., 2001 Nisin, pediocin Cellulose casing Turkey breast, L. monocytogenes Ming et al. 1997 ham, beef Fungicides Benomyl Ionomer Culture media Halek and Garg, 1989 Imazalil LDPE Bell pepper Miller et al., 1984 PE Cheese Moulds Weng and Hotchkiss, 1992 Polymers Chitosan Chitosan/paper Strawberry E. coli Yi et al., 1998 Chitosan, herb LDPE Culture media Lb. plantarum, E. coli, Hong et al., 2000 extract S. cerevisiae, Fusarium oxysporum UV/excimer laser Nylon Culture media S. aureus, Pseudo. Paik et al., 1998; Paik and irradiated nylon fluorescens, Kelly, T995 Enterococcus faecalis Natural extract Grapefruit seed LDPE, nylon Ground beef Aerobes, coli-forms Ha et al., 2001 extract LDPE Lettuce, soy-sprouts E. coli, S. aureus Lee et al., 1998 Clove extract LDPE Culture media L. plantarum, E coli, Hong et al., 2000 F. oxysporum, S.cerevisiae Herb extract, Ag- LDPE Lettuce, E. coli, ,S. aureus, L An et al., 1998 Zirconium cucumber mesenteroides, S. cerevisiae, Asp. spp, Pen. spp. LDPE Strawberry Firmness Chung et al., 1998 Eugenol, cinnam Chitosan Bologna, ham Enterobac., lactic acid Outtara et al., 2000b aldehyde, bacteria,Lb. sakei Serratia spp. Horseradish Paper Ground beef E. coli 0157: H7 Nadarajah et al., 2002 extract Allyl PE film/pad Chicken, E. coli, S. enteritidis, L. Takeuchi and Yuan, 2002 isothiocyanate meats, smoked monocytogenes salmon Table 4.1 Continued Antimicrobials Packaging materials Foods Microorganisms References Oxygen absorber Ageless Sachet Bread Moulds Smith et al., 1989 BHT HDPE Breakfast cereal Hoojjatt et al., 1987 Gas Ethanol Silicagel sachet Culture media Shapero et al., 1978 Silicon oxide Bakery Smith et al., 1987 (Ethicap) sachet Hinokithiol Cyclodextrin/plactic Bakery Gontard, 1997 (Seiwa) sachet C10 2 Plastic films Migration test Ozen and Floros, 2001 Others Hexamethyleneteh LDPE Orange juice Yeast, lactic acid Devlieghere et al. 2000b tramine bacteria Silver zeolite, LDPE Culture media S. cerevisiae, E. coli, S. Ishitani, 1995 silver nitrate aureus, Sal. typhimurium, Vibrio parahaemolyticus Antibiotics PE Culture media E. coli, S. aureus, Sal. Han and Moon, 2002 typhimurium, Klebsiella neumoniae MC: methyl cellulose; HPMC: hydroxypropyl methyl cellulose; WPI: whey protein isolate; CMC: carboxyl methyl cellulose; SPI: soy protein isolate Organic acids such as benzoic acids, parabens, sorbates, sorbic acid, propionic acid, acetic acid, lactic acid, medium-size fatty acids and their mixture possess strong antimicrobial activity and have been used as food preservatives, food contact substances and food contact material sanitisers. Benomyl and imazalil had been incorporated in plastic films and demonstrated antifungal activity. Ethanol has strong antibacterial and antifungal activity, however, it is not sufficient to prevent the growth of yeast. Ethanol may enhance some volatile flavour compounds but also causes a strong undesirable chemical odour in most food products. Some antibiotics can be incorporated into animal feedstuffs for the purpose of disease treatment, disease prevention or growth enhancement as well as human disease curing. The use of antibiotics as package additives is not approved for the purpose of antimicrobial functions and is also controversial due to the development of resistant microorganisms. However, antibiotics may be incorporated for short-term use in medical devices and other non-food products. Antioxidants are effective antifungal agents due to the restrictive oxygen requirement of moulds. Food grade chemical antioxidants could be incorporated into packaging materials to create an anaerobic atmosphere inside packages, and eventually protect the food against aerobic spoilage (Smith et al., 1990). Since the package did not contain oxygen, the partial pressure difference of oxygen is formed between the outside and inside of packaging materials. Therefore, in order to maintain the low concentration of oxygen inside the package, the packaging system requires high oxygen barrier materials such as EVOH, PVDC or aluminum foil that prevent the permeation of oxygen. Besides the antioxidants, a multi-ingredient oxygen scavenging system, such as commercial oxygen-absorbing sachets, can be used to reduce oxygen concentration inside the package. Various bacteriocins that are produced by microorganisms also inhibit the growth of spoilage and pathogenic microorganisms. These fermentation products include nisin, lacticins, pediocin, diolococcin, and propionicins (Daeschul, 1989; Han, 2002). These biologically active peptides possess strong antimicrobial properties against various bacteria. Other non-peptide fermentation products such as reuterin also demonstrate antimicrobial activity. Besides the above food grade bacteriocins, other bacteriocins would be utilised for the development of antimicrobial packaging systems. Some synthetic or natural polymers also possess antimicrobial activity. Ultraviolet or excimer laser irradiation can excite the structure of nylon and create antimicrobial activity. Among natural polymers, chitosan (chitin derivative) exhibits antimicrobial activity. Short or medium size chitosan possesses quite good antimicrobial activity, while long change chitosan is not effective. Chitosan has been approved as a food ingredient from FDA recently; therefore, the use of chitosan for new product development as well as a natural antimicrobial agent would become more popular. The use of natural plant extracts is desirable for the development of new food products and nutraceuticals, as well as new active packaging systems. Some plant extracts such as grapefruit seed, cinnamon, horseradish and clove have Antimicrobial food packaging 57 been added to packaging systems to demonstrate effective antimicrobial activity against spoilage and pathogenic bacteria. More use of natural extracts is expected because of the easier regulation process and consumer preference when compared to the chemical antimicrobial agents. Gaseous antimicrobials have some benefit compared to the solid or solute types of chemical antimicrobial agents. They can be vaporised and penetrated into any air space inside packages that cannot be reached by non-gaseous antimicrobial agents. An ethanol sachet is one example of a gaseous antimicrobial system. Headspace ethanol vapour can inhibit the growth of moulds and bacteria. The use of chlorine dioxide has been permitted with no objection notification from FDA recently and can be incorporated into packaging material. Chlorine dioxide shows effective antimicrobial activity and some bleaching effect. Allyl isothiocyanate, hinokithiol and ozone have been incorporated into packages and demonstrated effective antimicrobial activity. However, the use of these reactive gaseous agents has to be considered after careful studies of their reactivity and permeability through packaging materials. Since most antimicrobial agents have different antimicrobial mechanisms, the mixture of antimicrobial agents can increase antimicrobial activity through synergic mechanisms when they do not have any interference mechanisms. Therefore, the optimisation study on the combination of various antimicrobials will extend the antimicrobial activity of the mixture and maximise the efficacy and the safety of the antimicrobial packaging system. 4.3 Constructing an antimicrobial packaging system Antimicrobial agents can be incorporated into a packaging system through simple blending with packaging materials, immobilisation or coating differently depending on the characteristics of packaging system, antimicrobial agent and food. The blended antimicrobial agents can migrate from packaging materials to foods, while the immobilised agent cannot migrate. Fig. 4.2 explains the antimicrobial systems and their releasing profiles. Systems (A) and (B) release antimicrobial agents through diffusion, while systems (C) and (D) release volatile antimicrobial agents by evaporation. Fig. 4.2 presents (A) One-layer system: the antimicrobial agent is incorporated into the packaging material or chemically bound on the packaging material by immobilisation. (B) Two-layer system: the antimicrobial agent (outer layer) is coated on the packaging material (inner layer), or the antimicrobial matrix layer (outer layer) is laminated with the control layer (inner layer) to control the release rate specifically. (C) Headspace system: the volatile antimicrobial agent initially incorporated into the matrix layer releases into the headspace. Headspace antimicrobial agent is partitioned with the food product by equilibrium sorption/isotherm. (D) Headspace system with control layer: the control layer specifically controls the permeation of the volatile antimicrobial agent and maintains specific headspace concentration. 58 Novel food packaging techniques As mentioned above, the gaseous (volatile) agents can evaporate into the headspace of the packaging system and reach the food. As examples of incorporation processes, the antimicrobial agents were impregnated into packaging materials before final extrusion (Han and Floros, 1997; Nam et al., 2002), dissolved into coating solvents (Rodrigues and Han, 2000; Rodrigues et al., 2002), or mixed into sizing/filling materials of paper and paperboards (Nadarajah et al., 2002). Chemical immobilisation utilises the covalent binding of the agents into chemical structures of packaging materials when regulation does not permit migration of the agents into foods (Appendini and Hotchkiss, 1996; 1997; Halek and Garg, 1989; Miller et al., 1984). The immobilised antimicrobial agents will inhibit the growth of microorganisms on the contact surface of packaged products. The coating process can produce an antimicrobial packaging system. Over- coating on the pre-packaged products or edible coating on the food itself can Fig. 4.2 Antimicrobial packaging system Antimicrobial food packaging 59 produce an extra physical barrier layer that also contains antimicrobial agents. The antimicrobial agent in the over-coating material has to penetrate through the inner liner to reach the food surface to be effective. An edible coating system has various benefits due to its edibility, biodegradability and simplicity (Krochta and De Mulder-Johnston, 1997). The edible coating may be either dry coating or wet battered coating. The dry coating can incorporate chemical and natural antimicrobials, and play the role of physical and chemical barriers as well as being a microbial barrier (Han, 2002; 2001). The wet coating system may need another wrap to avoid the loss of the wet coating materials. However, the wet system can carry many different types of functional agents as well as probiotics and antimicrobials (Gill, 2000). Lactic acid bacteria can be incorporated into the wet coating system and control the competing undesirable bacteria. This new wet coating system could be very beneficial to the fresh products, meats and poultry industries. 4.4 Factors affecting the effectiveness of antimicrobial packaging Many factors should be considered in designing antimicrobial packaging systems besides the factors described above such as antimicrobial agent charac- teristics, incorporation methods, permeation and evaporation. Extra factors include specific activity, resistance of microorganisms, controlled release, release mechanisms, chemical nature of foods and antimicrobials, storage and distribution conditions, film/container casting process conditions, physical and mechanical properties of antimicrobial packaging materials, organoleptic characteristics and toxicity of antimicrobials, and corresponding regulations. An antimicrobial agent has its own specific inhibition activity against each microorganism. Therefore, the selection of antimicrobial agent is dependent on its activity against a target microorganism. Due to the characteristics of food products such as pH, water activity, compositions and storage temperature, the growth of potential microorganisms that can spoil food products are predictable. The antimicrobial agent has to be selected by the inhibition activity of the agent against the targeted potential microorganisms in the environmental conditions of the packaged foods. The design of an antimicrobial packaging system requires controlled release technology and microbial growth kinetics. When the migration rate of an antimicrobial agent is faster than the growth rate of the target microorganism, the antimicrobial agent will be depleted before the expected storage period and the packaging system will lose its antimicrobial activity because the packaged food has an almost infinite volume compared to the volume of packaging material and the amount of antimicrobial agent. Consequently the microorganism will start to grow after the depletion of the antimicrobial agent. On the other hand, when the release rate is too slow to control the growth of the microorganism, the microorganism can grow instantly before the antimicrobial 60 Novel food packaging techniques agent is released. Therefore, the release rate of the antimicrobial agent from the packaging material to food is specifically controlled to match the release rate with the growth kinetics of the target microorganism. Figure 4.3 also shows the importance of the mass transfer kinetics and growth kinetics in the cases of a film-packaging system and a coating system. Antimicrobial agents which have been incorporated in the package material (A) or coating material (B) will migrate into the foods during storage and distribution. Packaging system (A) mostly has contaminating microorganisms on the surface of the food product inside the package, while coating system (B), which has been coated by the antimicrobial material, may have the contaminating microorganisms on the surface of coating layer. The migration of antimicrobial agents from the package into the food product is an essential phenomenon to inhibit the growth of microorganisms on the surface of food products. While the concentration of antimicrobial agents is maintained over the m.i.c. (minimal inhibitory concentration) on the food surface, the system actively presents effective antimicrobial activity. However, the migration of the incorporated antimicrobial agents from the coating layer into the food product dilute the concentration in the coating layer. Compared to the volume of the coating layer, the coated food has almost infinite volume. Therefore, the migration will deplete the antimicrobial agent inside the coating layer, reduce the concentration below m.i.c. and eliminate the antimicrobial activity of the coating system. The migration of the incorporated antimicrobial agents contributes the antimicrobial effectiveness in the case of packaging systems. On the other hand no migration is beneficial to the coating system. The chemical nature of antimicrobial agents is also an important factor. Some agents are soluble in water but some are not. If water-soluble agents are mixed into plastic resins to make antimicrobial films, special consideration of film properties should be involved to obtain high quality films. Due to the hydrophilic nature of the agents compared with the hydrophobic nature of (a) Packaging system (b) Coating system Fig. 4.3 Antimicrobial packaging and edible coating systems Antimicrobial food packaging 61 plastics, the plastic extrusion process may interfere with various problems including hole creation in the films, powder-blooming, the loss of physical integrity and/or the loss of transparency. Therefore the compatibility of antimicrobial agent and packaging material is an important factor. Most antimicrobial chemicals change their activity with respect to pH. The pH of packaging systems depend mostly on the pH of packaged foods. Therefore, consideration of food composition with the chemical nature of the antimicrobial agent is important as well as the consideration of packaging material properties with the chemical nature of the agents. The solubility of the antimicrobial agents to the foods is also a critical factor. If the antimicrobial agent is highly soluble in the food, the migration profile will follow the unconstrained free diffusion (Fig. 4.4), while the very low solubility creates the monolithic system. The left side (grey coloured) is an antimicrobial packaging material and right side (white) is a food. The dashed line indicates m.i.c. of antimicrobial agents. Unconstrained free diffusion model (A) shows the highly soluble antimicrobial agent positioned in the packaging material migrating into the food layer and the concentration of the antimicrobial agent inside the package decreases as migration continues. The concentration of the antimicrobial agent on the surface of the food (C s ) decreases as the concentration inside the package decreases and eventually reduces below the m.i.c. losing the antimicrobial activity. Monolithic system (B) consists of not- very-soluble (or lower affinity) migrants to the food layer. In this system, the concentration of antimicrobial agent on the surface of food (C s ) is much lower than that of soluble migrants. The concentration is highly dependent on the Fig. 4.4 Changes in the concentration of antimictobial agent in two different antimicrobial packaging systems. 62 Novel food packaging techniques solubility of the antimicrobial agent in the food. Until complete depletion of the antimicrobial agent in the package, the surface concentration (C s ) is maintained as a constant concentration (actually maximum solubility) maintaining constant antimicrobial activity, while the total amount of antimicrobial agent inside the package decreases. The migration profile of antimicrobial agents should be fully understood and able to be controlled to achieve predictable antimicrobial effectiveness during entire shelf-life. The importance of controlled release principles is described in Fig. 4.5. The integrated areas ( C s time) below the concentration lines of both systems have a similar value assuming an equal amount of initial antimicrobial agents. However, system (B) has the longer period of effectiveness in which concentration is above the m.i.c. compared with the period of system (A). Storage and distribution conditions are important factors. The conditions include storage temperature and time. This time-temperature integration affects the microbial growth profile. To prevent microbial growth, a storage period at the favourable temperature range for microbial growth should be avoided or minimised during the whole period of storage and distribution. In the case of controlled atmosphere storage or modified atmosphere packaging, active gas permeation through the packaging materials should be controlled to maintain optimum gas composition during the whole period of storage and distribution. When the gas composition is altered through unexpected gas permeation or seal defect, microorganisms that are not considered as target microorganisms may spoil the packaged foods. Film/container casting methods are important to maintain antimicrobial effectiveness. There are two casting methods; one is extrusion and the other is solvent casting. In the case of extrusion, the critical variables related to residual antimicrobial activity are extrusion temperature and specific mechanical energy input. The extrusion temperature is related to the thermal degradation of the antimicrobial agent, and the specific mechanical energy indicates the severity of the process conditions that also induce the degradation of the agents. In the case of the wet casting method using solvent to cast films and containers such as cellulose films and collagen casing, the solubility and reactivity of the antimicrobial agents and polymers to the solvents are the critical factors. The solubility relates to the homogeneous distribution of the agents in the polymeric materials, and the reactivity connects to the activity loss of the reactive antimicrobial agents. Physical and mechanical integrity of packaging materials is affected by the incorporated antimicrobial agents. If the antimicrobial agent is compatible with the packaging materials and does not interfere with the polymer-polymer interaction, a fair amount of the antimicrobial agent may be impregnated into the packaging material without any physical and mechanical integrity deterioration (Han, 1996). However, the excess amount of antimicrobial agent that is not capable of being blended with packaging materials will decrease physical strength and mechanical integrity (Cooksey et al., 2000). Polymer morpho- logical studies are very helpful in predicting the physical integrity decrease by Antimicrobial food packaging 63 adding the antimicrobial agent into the packaging material. Small size antimicrobial agents can be blended with polymeric materials and may be positioned at the amorphous region of the polymeric structure. If the high level of antimicrobial agent is mixed into the packaging materials, the space provided by the amorphous region will be saturated and the mixed agent will start to interfere with the polymer-polymer interactions at the crystalline region. Although there is no physical integrity damage observed after a low level of antimicrobial agent addition, optical properties can be changed by losing transparency or changing colour of the packaging materials (Han and Floros, 1997). Since the antimicrobial agent is contacting the food or migrating into food, the organoleptic property and toxicity of the antimicrobial agent should be satisfied to avoid quality deterioration and to maintain the safety of the packaged foods. The antimicrobial agents may possess strong taste or flavour, such as a bitter or sour taste as well as an undesirable aroma, that can affect the sensory quality adversely. In the case of antimicrobial edible protein film/coating applications, the allergenicity or chronic disease of the edible protein materials, such as peanut protein, soy protein and wheat gluten, should be considered before use (Han, 2001). From all the foregoing, the most critical factor that should be considered in designing an antimicrobial packaging system is regulation. The use of an antimicrobial agent is regulated by the various regulatory agencies, for examples, FDA, EPA and USDA in the United States. An antimicrobial agent is an additive of packaging material, not a food ingredient. However, most antimicrobial agents migrate into packaged food therefore the package additive should satisfy all the regulations for food ingredients. The use of an antimicrobial agent should be classified as one of package additive, food contact substance or food ingredient. However, all three categories are applied to a new antimicrobial packaging system in terms of regulatory aspects. The use of natural antimicrobial agents such as plant extracts is a very challenging method because it is simple to deal with the permission process compared with chemical antimicrobial agents. Fig. 4.5 Concentration profile at the surface of foods: (A) unconstrained free diffusion system and (B) monolithic system. Dashed line indicates m.i.c. 64 Novel food packaging techniques 4.5 Conclusion Antimicrobial packaging systems can inhibit the growth of spoilage and pathogenic microorganisms, and contribute to the improvement of food safety and the extension of shelf-life of the packaged food. Many factors are involved in designing the antimicrobial packaging system, however, most factors are closely related to the characteristics of antimicrobial agents, packaged foods and target microorganisms. 4.6 References AN D S, HWANG Y I, CHO S H, and LEE D S (1998), ‘Packaging of fresh curled lettuce and cucumber by using low density polyethylene films impregnated with antimicrobial agents’, J Korean Soc Food Sci Nutri, 27(4), 675–81. 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