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.
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