Antimicrobial food packaging 51 All antimicrobial agents have different activities which affect micro- organisms differently. There is no "Magic Bullet antimicrobial agent effectively orking 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 psychotropic) 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 notAll 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