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antimicrobial agents for food packaging applications
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Accepted Manuscript
Antimicrobial agents for food packaging applications
Suet-Yen Sung, Lee Tin Sin, Tiam-Ting Tee, Soo-Tueen Bee, A.R. Rahmat, W.A.W.A. Rahman, Ann-Chen Tan, M. Vikhraman
PII: S0924-2244(13)00160-X
DOI: 10.1016/j.tifs.2013.08.001
Reference: TIFS 1474
To appear in: Trends in Food Science & Technology
Received Date: 22 December 2012
Revised Date: 26 July 2013
Accepted Date: 3 August 2013
Please cite this article as: Sung, S.-Y., Sin, L.T., Tee, T.-T., Bee, S.-T., Rahmat, A.R., W.A. Rahman,W.A., Tan, A.-C., Vikhraman, M, Antimicrobial agents for food packaging applications, Trends in FoodScience & Technology (2013), doi: 10.1016/j.tifs.2013.08.001.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service toour customers we are providing this early version of the manuscript. The manuscript will undergocopyediting, typesetting, and review of the resulting proof before it is published in its final form. Pleasenote that during the production process errors may be discovered which could affect the content, and alllegal disclaimers that apply to the journal pertain.
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• Antimicrobial packaging is multifunctional by reducing harmful microbial activity in food
• Antimicrobial packaging helps to increase food safety. • Antimicrobial packaging reduces food wastage and improves food shelf life. • Bio-based antimicrobial agents in packaging provide extra safety for health.
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Antimicrobial agents for food packaging applications 1
2
Suet-Yen Sung1, Lee Tin Sin1, Tiam-Ting Tee1, Soo-Tueen Bee1, A. R. Rahmat2, W.A.W.A. 3
Rahman2, Ann-Chen Tan1, Vikhraman, M1. 4
1 Department of Chemical Engineering, Faculty of Engineering and Science, Universiti 5
Tunku Abdul Rahman, Jalan Genting Kelang, 53300 Setapak, Kuala Lumpur, Malaysia. 6
2 Department of Polymer Engineering, Faculty of Chemical Engineering, Universiti 7
Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia. 8
9
*Corresponding author. Tel: +60 3 4107 9802; Fax: +603 4107 9803. 10
Email: [email protected] and [email protected] 11
12
Abstract 13
14
Foods contamination leading to spoilage and growth of pathogenic microorganisms 15
can happen when exposed to environment during slaughtering, processing, packaging and 16
shipping. Although traditional food preservation methods such as drying, heating, freezing, 17
fermentation and salting can extend food shelf-life, it is not consummate especially to inhibit 18
the growth of pathogenic microorganisms that may endanger consumers’ health. 19
Antimicrobial packaging is a novel development that incorporates antimicrobial agent into 20
polymer film to suppress the activities of targeted microorganisms. However, antimicrobial 21
packaging is still an extremely challenging technology and there are only a few 22
commercialized products found in the market. This review focuses on analyzing the 23
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antimicrobial agent development for the past decades till recent technology. The information 24
about performance of antimicrobial packaging such as microbiology performance and 25
physico-mechanical properties of the packaging film were discussed. It is expected such 26
information would provide an overview as well as promote the development of antimicrobial 27
packaging in the food related field and industry. 28
29
Keywords: Antimicrobial; Packaging; Polymer 30
31
1. Introduction 32
33
It is well known that the primary functions of packaging are to isolate foods from 34
external environment and protects the foods against deterioration by the actions of 35
microorganisms, moisture, gases, dusts, odors as well as mechanical forces (Cooksey, 2010). 36
The exposures of these degradation agents tend to reduce the shelf-life of foods as well as 37
affecting consumers’ health when foods are contaminated by microorganisms related food-38
borne diseases. Contamination could occur anywhere when food is being exposed to open 39
environment such as during slaughtering, post processing, distribution, shipping, and storage 40
or retail display stages. Thus, a good packaging should act as a barrier system to reduce 41
passage of surrounding contaminants into foods. Meanwhile, the packaging must be inert, 42
non-toxic, impermeable to microorganisms and strong enough to withstand possible amount 43
of mechanical forces from easily rupture. In addition to the functions of extending shelf-life 44
and maintaining food quality, packaging also important for marketing and advertising, 45
standardizing, provides useful information to consumers while making products more usable 46
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and convenience (Cooksey, 2010). In short speaking, packaging functions for containment, 47
protection, convenience, and communication. 48
49
Commonly, microbial contamination is the main reason for food spoilage. Traditional 50
methods of preserving foods include fermentation, drying, adding antimicrobial agents 51
(organic acids, plants, and salts), thermal processing, freezing, refrigeration, modified 52
atmosphere and irradiation have been employed long times ago. All these methods possess 53
their own limitation especially when attempting to apply on fresh meats (Quintavalla & 54
Vivini, 2002). Inasmuch, varieties of packaging systems have been developed as alternatives 55
to preserve foods for different attributes and applications. For example, overwrap packaging 56
are designed for short term refrigeration storage, whereas modified atmosphere packaging 57
(MAP) or vacuum packaging are utilized for long term storage. 58
59
In addition to the public demand for foods with extended shelf-life, the safety of foods 60
has become the major consideration especially after the World Trade Center tragedy 61
happened in year 2001. Public started to concern on foods and water supplies as a form of 62
bioterrorism (Nestle, 2003). Besides, food-borne pathogenic microorganisms’ outbreaks issue 63
has been frequently occurring without effective solution. Hence, the current trend of food 64
packaging system are concerning about developing more innovative approaches to inhibit 65
pathogenic microbial activities in foods. Plenty of products such as active packaging and 66
intelligent packaging have been developed to meet crucial safety requirements. For instance, 67
antimicrobial (AM) packaging which is under the family of active packaging is made from 68
packaging system containing AM agents. The usage of AM packaging has more advantages 69
compared to direct adding of AM agents onto foods because AM agents added on food 70
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surfaces by sprays or drips are not effective enough to inhibit microorganisms. This is due to 71
rapid diffusion of AM agent into foods and denaturization of the active substances by food 72
constituents which reduce the reactivity of AM agent. Whereas, AM packaging offers slow 73
and continuous migration of AM agent from packaging material to food surfaces which 74
enable AM agent to maintain at high concentration over a long period (Quintavalla, & Vicini, 75
2002). 76
77
Basically, the purposes of AM agents in packaging are to provide safety assurance, 78
shelf-life extension and quality maintenance on food. AM packaging are able to inhibit 79
spoilage and suppress food-borne illness microbial that potentially contaminates food 80
products (Hotchkiss, 1997). Generally, AM packaging is designed to address one property or 81
requirement of the foods. For example, for the purpose of extend food shelf-life, the AM 82
agents chosen must be able to inhibit the food native spoilage microorganisms. In the past 83
decades, large numbers of AM food packaging products were developed from novel plastic 84
materials and AM agents. Most of the products are proven able to control the growth of 85
microbial and prolong food shelf-life effectively. However, there are only a few 86
commercialized products found in the market. This may be due to several reasons such as 87
strict safety and hygiene regulations, limited consumer acceptance on product effectiveness 88
and high cost. Table 1 lists numerous commercial products of AM food packaging. 89
90
The most popular commercialized products available in the market are those 91
packaging that use volatile gas-form AM-agents such as chlorine dioxide, ethanol and sulfur 92
dioxide. These AM agents are often enclosed separately in sachets/pads that are attached to 93
the internal part of the package. The AM agents will release in vapor form to the headspace 94
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of packaging to contact with food products (Appendini, & Hotchkiss, 2002). Commercial 95
product includes Microgarde™ as marketed by BarrierSafe Solutions International Inc., USA 96
exists in stickers or sheets form containing sodium chlorite and acid precursors. When 97
humidity or moisture in the air is high, it enters the Microgarde™ sheet and initiates the 98
solid-state dry reaction subsequently producing chlorine dioxide that diffuses throughout the 99
package to inhibit microbial contamination and control odour. Microgarde™ is found 100
effective to inhibit aerobic total viable count on iceberg lettuce. Another commercial product 101
called Ethicap™ from Japan, developed by Freund Corp., is ethanol vapor emitter. Ethicap™ 102
is a paper wafer contains ethanol-silicon dioxide in acetic acid that sandwiched between 103
layers of plastic films that permeable to ethanol. The ethanol vapor released into packaging 104
headspace is able to retard mold growth. This technology is often applied for bakery and 105
dried fish products. 106
107
Besides, silver compound is also employed as the AM agent in food packaging film. 108
Silver is generally recognized by public as hygiene-improving and antitoxic material. In fact, 109
silver substituted Zeolite products have been widely accepted by public when applied in 110
plastic films for food packaging. Examples of commercialized products included Zeomic™, 111
AgIon®, Novaron® and Cleanaid™. Sodium ions of Zeolites are substituted by silver ions to 112
inhibit a wide range of microorganisms such as bacteria and mold by disrupting the microbial 113
enzymes activities (Appendini, & Hotchkiss, 2002). For example, Zeomic™, product of 114
Sinanen Zeomic Co. is able to control the growth of gram-positive bacteria (methicillin-115
resistant S. aureus), gram-negative bacteria (E. coli and Pseudomonas aeruginosa), and fungi 116
(Aspergillus niger and Penicilium nigricans). Zeomic™ has been widely applied in chopping 117
board, food packaging films, glove and lunch box. 118
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On the other hand, sanitizer and fungicide are non-food grade AM agents that allowed 119
to be added into food packaging material. These AM agents are usually immobilized through 120
covalently bonded to polymer materials for limiting the migration of AM agents into food 121
products. Example of commercialized product that incorporates sanitizer to food packaging 122
system is Microban® which is developed by Microban International Ltd in USA. Microban® 123
utilizes triclosan as AM agent and this triclosan-incorporated food packaging was approved 124
by European Union who regulates that migration of triclosan to foods should not exceed 125
5mg/kg (Quintavalla, & Vicini, 2002). There were a wide range of microorganisms claimed 126
sensitive to this packaging. Cutter (1999) investigated on the effectiveness of Microban® 127
product with 1500ppm triclosan in plastic against population of meat surface related 128
microorganisms. By using plate overlay assay method, it was found that this product 129
inhibited the growth of S. Typhimurium (ATCC 14028), S. aureus (ATCC 12598), B. 130
thermosphacta (ATCC 11509), B. subtilis (ATCC 605), E. coli (ATCC 25922), S. flexneri 131
(ATCC 12022) and several strains of E. coli O157:H7. However, when tested on refrigerated, 132
vacuum-packaged meat surfaces, that product did not effectively reduce the number of 133
bacteria strains. 134
135
Among the plant extract AM agents, only wasabi extract AM products have been 136
commercialized. The antimicrobial packaging used wasabi extract called Wasapower™ is 137
well known developed by Sekisui Plastics Co. in Japan. Wasabi is a traditional food that 138
often served by Japanese together with sushi. They believe wasabi has the ability to kill 139
microorganisms on raw meat. Volatile allyl isothiocyanate (AIT) is the main AM component 140
in wasabi extract that inhibits bacteria such E. coli and S. aureus as well as fungi A. niger and 141
P. italicum. 142
143
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2. Antimicrobial Packaging Material 144
145
AM packaging are divided into two major groups called biodegradable packaging and 146
non-biodegradable packaging. Most synthetic polymers are non-biodegradable and being 147
superior candidates for usage as food packaging material with the advantages of low cost, 148
low density, inert, excellent barrier properties, good mechanical strength, high transparency, 149
ability to be heat-sealed and easy to be printed on. The properties of common plastic 150
materials used for food packaging are summarised in Table 2. The most widely used plastics 151
in packaging included low density polyethylene (LDPE), linear low density polyethylene 152
(LLDPE), high density polyethylene (HDPE), polypropylene (PP), ethylene vinyl acetate 153
(EVA), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC). 154
However, these petrochemical products possess negative impact to the environment. It causes 155
landfill depletion, environmental pollution and high energy consumption of manufacturing 156
process. The diffusion of additives from polymers into food products could also endanger 157
human health. Therefore, there has been a shift towards increasing usage of biodegradable 158
materials in recent years. 159
160
Generally, biodegradable AM films were produced by natural polymer possesses 161
inherently AM reactivity or through addition of AM agents into natural polymer. Examples of 162
renewable biopolymers are polysaccharides, proteins, gums, lipids and their complexes, 163
derived from animal and plant origin. Most of the biodegradable-based packaging researches 164
are focused on the blending of thermoplastic starches (TPS) with biodegradable polyesters 165
such as polycaprolactone (PCL), polylactic acid (PLA), polyhydroxybutyrate-co-166
hydroxyvalerate, polybutylene succinate-adipate, poly(butylenes adipate-co-terephtalate), and 167
poly(hydroxyl ester ether). Among the biodegradable ingredients, PLA is the most widely 168
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used polymer. PLA is a biodegradable and biocompatible thermoplastic which is produced by 169
fermentation from renewable resources. It can also be synthesized either by condensation 170
polymerization of lactic acid or by ring opening polymerization of lactide in the presence of a 171
catalyst (Sin et al., 2013). PLA-based films could perform better than other AM films 172
because of several advantages, such as competitive price, regulation matter and eco-173
friendliness. On the other hand, a natural polymer called chitosan possesses inherent AM 174
activities. It is one of the ideal degradable films but costly to produce due to hassle process 175
yields limited amount. The popularity of chitosan in AM development is mainly due to its 176
inherent AM properties and biodegradability. It can be used as AM agent by its own or 177
blending with other natural polymers as the structural support as well as cost reduction. 178
Meanwhile, the poly(vinyl alcohol) (PVA) is a special synthetic polymer with biodegradable 179
characteristic. Generally, it is blended with different synthetic and natural polymers because 180
it is innocuous, non-carcinogenic, good biocompatible properties, hydrophilic, water-soluble 181
and chemically stable. Tripathi, Mehrotra and Dutta (2009) suggested that chitosan-PVA 182
blends possess advantages in the biological characteristics due to biological compatibility in 183
respective chitosan and PVA blends. 184
185
Besides the biodegradable materials that discussed previously, edible films made from 186
edible biopolymer (e.g.: proteins, lipids, polysaccharides) and food-grade additives (e.g.: 187
plasticizers, AM agents, colorant, flavors, emulsifiers) are also the good candidate for 188
fabrication of AM films. By incorporation of AM agents and antioxidants into films, food 189
products can be protected from microbial growth, moisture migration and nutrient oxidation. 190
The application of edible films is mostly on nuts, candies and fruits. The recent studies on 191
AM edible films are summarized in Table 3. 192
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3. Antimicrobial Agents-Matrixes Incorporation Methods 193
194
There are plenty of researches on AM packaging; however up to date the development 195
is still not effective enough to inhibit the growth of spoilage microorganisms and extend food 196
shelf-life. Many of the AM agents used are highly sensitive to film production process 197
condition. Under high processing pressure and temperature, deformation or evaporation of 198
volatile AM agents happened (Jari, Eija, Jouni, & Raija, 2003). Nevertheless, out of the 199
plastic production technology, compression molding and blown film extrusion are commonly 200
employed for heat-stable AM agents that required mild production conditions. In order to 201
reduce the loss of AM agents when process under elevated conditions, it is crucial to employ 202
alternative approaches for rapid processing. 203
204
By using conventional processing method which involves high temperature, 205
researchers found that the loss of AM agents occurred tremendously. Ha, Kim, & Lee (2001) 206
used high temperature profile 160 – 190oC to extrude AM LLDPE-based film resulted in high 207
loss of grape fruit seed extract (GFSE) which showed no AM activity. Marino, Alfonso, 208
Mercedes, & Maria (2012) reported 25 – 44 % weight of thymol and carvacrol remained in 209
PP film when subjected to temperature 190oC for 18 minutes of hot press process. As a result, 210
antibacterial activities towards S. aureus only effective when high initial concentration (8% 211
w/w) of AM agent were used. They also found that the AM agent only retained 3.5% w/w 212
after hot press process. AM LLDPE-based film developed by Suppakul, Miltz, Sonneveld, & 213
Bigger (2002) experienced even greater loss of AM agent at about 96.7% weight after blown 214
film extrusion process. This was due to higher melting temperature of LLDPE compared with 215
LDPE. 216
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Alternative method to incorporate volatile AM agents had been studied in order to 217
prevent unnecessary loss during film processing. Some researchers produced masterbatch of 218
EVA/AM compound at relatively lower temperature prior to undergo film production process 219
(Herath, 2010; Mistry, 2006; Suppakul, Sonneveld, Bigger, & Miltz, 2007; Suppakul, 2004; 220
Cran, Rupika, Sonneveld, Miltz, & Bigger, 2010). They first mixed EVA with AM agents to 221
obtain homogeneous compound, and then mixed the compound with LDPE powder at room 222
temperature before undergo blown film process. They successfully proved that EVA has 223
effectively retained higher amount of volatile AM agent attributed to the polarity affinity of 224
hydroxyl group in EVA towards AM agents which consisted some degree of polarity (Herath, 225
2010; Mistry, 2006; Suppakul, 2004). By incorporating EVA into LDPE, EVA/LDPE film 226
produced by blown film extrusion at 150 oC retained carvacrol up to 66 % weight, thymol 227
80% weight (Herath, 2010), linalool 60% weight (Suppakul, 2004), and methylchavicol 35% 228
weight (Suppakul, 2004). Cran, Rupika, Sonneveld, Miltz, & Bigger (2010) studied on the 229
release kinetics of AM agents from EVA/LDPE film suggested that 10% weight of EVA in 230
LDPE is the most optimum concentration to achieve minimum release rate of AM agents. At 231
higher concentration, the bulky side chain of EVA could reduce crystallinity of LDPE matrix, 232
subsequently causing adverse accelerates the release of AM agent during application. 233
Besides, Herath (2010) and Mistry (2006) have studied on the addition of polyethylene glycol 234
(PEG) and PEG/EVA respectively into LDPE on the retention of AM agents. The function of 235
PEG is to assist binding of AM agents to polymer and enhance retention of AM agents. In the 236
absence of EVA, PEG was insufficient to retain AM agents due to poor interaction of PEG 237
with LDPE matrix. EVA with bulky side chain created spaces for PEG to occupy between 238
LDPE chains and hence enhancing PEG-LDPE interaction. Subsequently, a more uniform 239
dispersion of PEG in LDPE matrix could be formed and retained up to 8% weight more of 240
thymol and cavacrol respectively when compared to EVA merely. 241
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Heat-sensitive AM agents such as volatile compounds are preferable to be produced 242
by non-heating methods such as solvent compounding, electrospinning and surface coating. 243
Among these methods, coating was the most popular approach to apply AM agents onto 244
polymer surface due to simplicity of the process. In order to enhance the attachment of AM 245
onto plastic matrix, plastic films usually undergo surface modification process by corona 246
treatment or UV radiation prior application of AM agents. Normally, the whole process was 247
done in room temperature. Examples of AM-coated films studied included chitosan/essential 248
oil-coated PP film (Torlak & Nizamlioğlu, 2011), chitosan/bacteriocin-coated plastic film 249
(Ye, Neetoo, & Chen, 2008), bacteriocin-coated LDPE film (Iseppi et al., 2008), 250
cinnamaldehyde, garlic oil and rosemary oil-coated PP/LDPE film (Gamage, Park, & Kim, 251
2009), Oregano essential oil and citral-coated PP/EVOH film (Muriel-Galet, Cerisuelo, 252
Lopez-Carballo, Lara, Gavara, & Hernandez-Munoz, 2012), chitosan-coated plastic film (Ye, 253
Neetoo, & Chen, 2008), thyme and oregano-coated LDPE (Solano, & Gante, 2010) and etc. 254
Interestingly, AM film produced by elevated temperature process shows better inhibition 255
against microbial compared to coating method as reported by Solano and Gante (2010). They 256
found that AM film produced by extrusion method is more effective towards E. coli, S. 257
typhimurium and L. monocytogenes compared to ionising-coated AM film with the identical 258
amount of AM agent incorporated. The results suggest that extrusion method allowed better 259
incorporation of active compounds on the polymer. However, there were a limited number of 260
studies done in comparing the both methods; thus, the comparison of effectiveness is still 261
required further exploration. 262
263
In addition, an alternative method to prevent loss of AM activities is production via 264
cast film. It is generally produced at relatively much lower temperature without subject to 265
mechanical shear forces. However, this method is usually limited for natural polymer which 266
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having lower melting temperature. Thus, the film is usually biodegradable and edible depends 267
on the ingredients. Examples of natural polymer can be prepared using for this method is 268
starch, chitosan, alginate, WPI, PLA, PVA and etc. Researchers usually employed several 269
types of treatments in final stage of the processes to improve the mechanical properties of the 270
cast film. For instance, chitosan and potato starch film preparation involve the microwave 271
treatment (Tripathi, Mehrotra, Tripathi, Banerjee, Joshi, & Dutta, 2008) and electron beam 272
(EB) irradiation. Another example of film produced by casting technique is chitosan/PVA 273
film. The resulting film was kept in desiccators at room temperature for 24 h, and then cross-274
linked using 0.01% weight glutaraldehyde (GA) and 0.5% weight sulphuric acid (H2SO4) as a 275
catalyst for 1 h. The final film was obtained after washing with distilled water. 276
277
4. Antimicrobial Effectiveness of AM Packaging 278
279
Numerous studies found that AM packaging can effectively inhibit targeted bacteria 280
when appropriate amount of AM agents incorporated into polymer film. The effectiveness of 281
AM packaging are greater compared to direct addition of preservative agents into food due to 282
two important reasons. Firstly, the attachment of AM agents with polymer film enables slow 283
release of AM agents function over longer period. Secondly, AM activities of preservative 284
agents may experience inactivation (such as neutralization, hydrolysis, dilution and etc.) by 285
food matrixes and components when added directly into the food (Appendini & Hotchkiss, 286
2002; Muriel-Galet, Cerisuelo, Lopez-Carballo, Lara, Gavara, & Hernandez-Munoz, 2012). 287
Besides, direct addition of preservative agents into food can reduce the food quality as 288
resulted by changing the organoleptic and textural qualities of the food. Thus, AM packaging 289
are playing important roles to inhibit the growth of targeted bacteria on foods while 290
improving foods safety and prolong shelf-life without scarifying the foods quality. 291
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292
The functionality of AM agents in foods can be examined from the number of 293
microorganism growth as described by spoilage index in Figure 2 (Fung, Kastner, Lee, Hunt, 294
Dikeman and Kropf, 1980). Most of the time, food is considered spoil when exceed a total 295
microbial count of 107 CFU/g. When total count reaches 108 CFU/g and above, unpleasant 296
odor started to build up. Most researchers studied on AM packaging referred bacteria count 297
of 107 CFU/ml or g or cm2 as a standard for shelf-life indication. 298
299
4.1 Antimicrobial Packaging with Essential Oils and Plant Extracts 300
301
The AM activities of essential oils and plant extracts are well known for a long time 302
and numerous of research outcomes have been published on the AM activities of plant 303
essential oils against foodborne pathogens. Since essential oils are rich in volatile terpenoids 304
and phenolic particles, they are highly potential to inhibit a wide spectrum of microorganisms. 305
Generally, the active components of plant essential oils inhibit microorganisms through 306
disturbance of the cytoplasmic membrane, disrupting the proton motive force, electron flow, 307
active transport and inhibition of protein synthesis. Examples of plant extracts and essential 308
oils that most widely incorporated into food packaging are linalool, thymol, carvacrol, clove 309
oil, cinnamaldehyde and basil essential oils. 310
311
There are plenty of reports found related to the AM effectiveness of plant extract 312
components in food packaging. Ha et al. (2001) reported when comparing film without AM 313
agent, linear low density polyethylene (LLDPE) co-extruded film with 0.5% and 1% w/w 314
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grape fruit seed extract (GFSE) has extended the time of beef stored under 3oC resulted total 315
aerobic bacteria count of 107 CFU/g from 13 to 14 days. When the identical amount of GFSE 316
coated on LLDPE film was tested, the shelf-life of beef has extended from 9 to 14 days. 317
Meanwhile, there was no significant different between result of 0.5% and 1% GFSE in 318
LLDPE film can be observed. GFSE can effectively inhibit the growth of pathogenic bacteria 319
such as L. monocytogenes. Besides, AM activities of GFSE incorporated with soy protein 320
edible film showed a positive result towards L. monocytogenes population by reduction of 1 321
log CFU/ml after 1 hour incubation at 25 ˚C. However, E. coli and S. Typhimurium showed 322
only 0.1 and 0.2 log CFU/ml reduction respectively (Sivarooban, Hettiarachchy, & Johnson, 323
2008). 324
325
Muriel-Galet, Cerisuelo, Lopez-Carballo, Lara, Gavara and Hernandez-Munoz (2012) 326
studied on polypropylene (PP) film coated with oregano essential oil and citral. By 327
comparing with the control specimen (PP film without AM agent), oregano essential oil-328
coated PP film with 6.7% carvacrol active ingredient has successfully reduced the number of 329
E. coli, S. enteric and L. monocytogenes in salad for 1.4 log, 0.5 log and 0.36 log CFU/g 330
respectively after storage under temperature 4oC, packaging environment of 12% CO2 and 331
4% O2 for two days. Meanwhile, 5% of citral coated PP film was able to reduce the number 332
of E. coli and S. enterica by 0.36 log and 0.41 log CFU/g respectively. However, the AM 333
effectiveness of citral coated PP film is lesser compared to oregano. In addition, the citral 334
coated-PP film was not effective towards L. monocytogenes. Carvacrol and citral were 335
reported to be more effective towards gram-negative bacteria due to the high affinity. 336
Especially carvacrol can affect the cell wall lipids of gram-negative bacteria. However, this 337
was inconsistent with some researchers who claimed there were no difference between the 338
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antimicrobial effectiveness on the gram-positive and gram-negative bacteria (Quattara, 339
Simard, Holley, Piette, & Begin, 1997). 340
341
Other essential oils-coated film that have been studied extensively included allyl 342
isothiocyanate (AIT) (Nadarajah, Han, & Holley, 2005), rosemary (Han, Castell-Perez, & 343
Moreira, 2007), garlic oil (Pranoto, Rakshit, & Salokhe, 2005; Gamage, Park, & Kim, 2009) 344
and cinnamaldehyde (Han, Castell-Perez, & Moreira, 2007). By incorporating 1.2 % w/w of 345
AIT into oriented polypropylene/polyethylene (OPP/PE) film, the microbial count on sprouts 346
stored at 10oC (Gamage, Park, & Kim, 2009) has reduced significantly. Nadarajah, Han and 347
Holley (2005) also reported AIT was effective against E. coli on ground meat patties with a 348
reduction of 3 log CFU/g within 10 days of storage. Garlic oil-incorporated film exhibits 349
effectiveness only when high concentration of garlic oil was used. Gamage, Park and Kim 350
(2009) found that plastic film with 1.2% w/w garlic oil inhibited the growth of microbial on 351
sprout, whereas no significant differences when 0.6, 0.8 and 1.0 % w/w of garlic oil were 352
incorporated. However, garlic oil was reported to effectively reduce the number of gram-353
negative and gram-positive bacteria when used in small amount in edible film, where 0.2% 354
w/w garlic oil-incorporated alginate film inhibited the growth of S. aureus and B. cereus 355
(Pranoto, Rakshit, & Salokhe, 2005). 356
357
Essential oils were also incorporated into protein-based film to evaluate its AM 358
performance. Seydim and Sarikus (2006) have prepared whey protein isolate (WPI) films 359
containing 1.0 – 4.0 % (w/v) ratios of rosemary, oregano and garlic essential oils to test the 360
AM effective against S. aureus (ATCC 43300), E. coli O157:H7 (ATCC 35218), L. 361
monocytogenes (NCTC 2167), L. plantarum (DSM 20174) and Salmonella enteritidis (ATCC 362
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13076). The researchers found that when the concentration of garlic essential oil in WPI films 363
was increased to 3% w/v, a significant inhibition was observed against all tested strains. 364
Oregano and garlic essential oil-added films respectively exhibited larger inhibitory zones on 365
S. aureus, S. enteritidis, L. monocytogenes, E. coli and L. plantarum as compared to rosemary 366
essential oil-incorporated films. In addition, WPI films added with 1.5 % w/w of oregano oil 367
prolonged the shelf-life of beef by a factor of 2 while minimizing changes in beef color. 368
Emiroğlu, Yemiş, Coşkun, & Candoğan (2010) studied the AM activities of soy edible films 369
incorporated with thyme and oregano essential oils on fresh ground beef patties showed that 370
there were lack of significant inhibition on S. aureus when tested with meat even though it 371
showed a significant inhibitory effect when tested in-vitro. Pseudomonas aeruginosa 372
exhibited better result where reduction of colonies observed when the film incorporated with 373
thyme or oregano plus thyme essential oils. On the other hand, Oussalah, Caillet, Salmieri, 374
Saucier and Loicroix (2004) studied the AM effects of milk protein-based film on 375
Pseudomonas spp. and E. coli. They reported that the usage of films containing essential oils 376
significantly reduced the microorganism level in meat during 7 days of storage at 4oC. 377
Among the essential oils, oregano oil was the most effective combination that works against 378
the growth of the respective bacteria. Oregano oil:pimento oil in 1:1 (w/w) ratio was more 379
effective against the growth of E. coli than a pimento oil-coated film, whereas the AM 380
effectiveness against the growth of Pseudomonas spp. was identical for both films. 381
382
4.2 Antimicrobial Packaging with Enzyme 383
384
Lysozyme, obtained from hen egg white is an enzyme classified as natural AM agent 385
and commonly incorporated into packaging materials. Similar with other AM agents, 386
lysozyme is less effective towards gram-negative bacteria due to the existing of protective 387
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lipopolysaccharide (LPS) layer on the bacteria cell wall. Gram-positive bacteria are very 388
susceptible to lysozyme, because their membrane is made up of 90% peptidoglycan where 389
hydrolyzation of β-1-4 glycosidic linkage between the N-acetyl muramic acid and N-acetyl 390
glucosamine take place. In order to further improve the AM effectiveness of enzyme, other 391
AM agents such as detergents and chelators were usually added. 392
393
In early year, Ellison and Gehl (1991) proposed to use lactoferrin to improve the 394
activity of lysozyme towards gram-negative bacteria. Lactoferrin is a natural component of 395
milk with large cationic patches that disrupting the outer membrane permeability resulting the 396
release of LPS. It has been used in AM spray for beef carcasses decade ago. Recently, 397
Barbiroli et al. (2012) developed lysozyme/lactoferrin-containing paper and assessed on E. 398
coli (DSMZ 50902) and L. innocua (DSMZ 20649). When tested on gram-positive bacteria, L. 399
innocua, the presence of both the AM agents induce a significant increment of the lag phase 400
duration which prolonged from 1.86 to 6.5 h, whereas lysozyme alone only prolongs the lag 401
phase from 1.86 to 5.81 h. For gram-negative bacteria E. coli, lysozyme alone only slightly 402
increases the lag phase from 1.08 to 1.79 h whereas, lactoferrin-containing paper showed lag 403
phase increment from 1.8 to 3.25 h. 404
405
Mecitoğlu, Yemenicioğlu, Arslanoğlu, Elmaci, Korel, & Cetin (2009) proposed the 406
combination usage of lysozyme and disodium ethylenediaminetetraacetic acid (Na2EDTA) as 407
the AM agent. EDTA is a chelating agent that capable to destabilize the LPS layer of gram-408
negative bacteria. Zein films incorporated with lysozyme showed AM effect on B. subilis 409
(ATCC 6633) and L. plantarum (DSM 1954). By the addition of Na2EDTA, the films also 410
became effective on gram-negative bacteria, E. coli (ATCC 53868). Similarly, in the studied 411
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of Gemili, Yemenicioglu & Altinkaya (2009), agar disk diffusion test conducted for E. coli 412
showed significant formation of inhibition zone when using cellulose acetate film containing 413
both AM agents. By using lysozyme and Na2EDTA alone, no zone formation can be 414
observed. Unalan, Korel and Yemenicioglu (2011) studied on zein films containing lysozyme 415
(700µg cm-1) and Na2EDTA (300µg cm-1) found that through agar disk diffusion test, the 416
zone area obtained by using zein films containing both lysozyme and Na2EDTA exhibited 1.5 417
and 2.4 fold greater zones on gram-positive bacteria, L. monocytogenes, than using films 418
containing Na2EDTA or lysozyme alone on this bacterium, respectively. The zein films 419
containing combination of lysozyme and Na2EDTA and Na2EDTA alone were both effective 420
on E. coli O157:H7 and S. typhimurium. Through the challenge test, the films containing both 421
AM agents decreased total viable counts (TVC) and total coliform counts on beef patties after 422
5 days of storage compared to those of control patties. In another study, Gucbilmez, 423
Yemenicioglu and Arslanoglu (2007) utilized functional protein extracts- chickpea albumin 424
extract (CPAE) to control lysozyme distribution and release rate while improving antioxidant 425
activity in zein film. In the result, zein film incorporated with 277µg/cm2 Na2EDTA·2H2O 426
and 1400U/cm2 lysozyme tested on E. coli (ATCC 53868) showed uneven distribution of 427
these AM agents in zein films. For the addition of 530µg/cm2 CPAE into the mentioned films, 428
the AM effectiveness did not change significant but the distribution of lysozyme were 429
improved. 430
431
4.3 Antimicrobial Packaging with Chitosan 432
433
Chitosan are highly attractive to researchers due to some interesting properties such as 434
it is natural polymer, nontoxic, film-forming ability and biodegradable (Torlak, & 435
Nizamlioğlu, 2011; Cooksey, 2010). Besides, chitosan has inherent AM properties and able 436
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to inhibit the growth of wide range of food pathogens. However, owing to the poor solubility 437
of chitosan at high pH value, its application is only effective in acid medium. In the study of 438
Liu, Du, Wang and Sun (2004), chitosan solutions reduced the numbers of E. coli and S. 439
aureus by ~ 1 log in 5 minutes as shown in Figure 1. Almost all E. coli were killed in 120 440
min, but the quantities of S. aureus were not affected. 441
442
The only weakness of chitosan film is the poor mechanical properties compared to 443
synthetic polymer. Thus, chitosan can be coated on plastic film to compromise mechanical 444
properties and enhance AM activities. Torlak and Nizamlioğlu (2011) studied AM 445
effectiveness of 2% w/v chitosan coated on PP films towards Kashar cheese slice inoculated 446
with bacteria. The cheese slices were stored in vacuum packaging at 4oC for 14 days. PP 447
films without AM agent were used as control. After 14 days, control cheese exhibited 5.11, 448
4.92 and 4.88 log CFU/g of L. monocytogenes, S. aureus and E. coli O157:H7 respectively. 449
Chitosan-coated PP films found reduction count of L. monocytogenes, S. aureus and E. coli 450
O157:H7 at 0.7, 0.61 and 0.49 log CFU/g, respectively compared to PP films without AM 451
agent. Similarly, Duan, Park, Daeschel and Zhao (2007) also reported significant reduction of 452
L. monocytogenes in mozzarella cheese and camembert cheese stored at 10oC and 4oC 453
respectively. Packaging coated with 2% and 2.5% v/v chitosan under vacuum condition has 454
reduced the population of total aerobic count in pork from 6 log (control) to 3.75 and 3.61 log 455
CFU/g respectively within 14 days of storage in refrigerator (Yingyuad, Ruamsin, 456
Reekprkhon, Douglas, Pongamphai, & Siripatrawan, 2006). On the other hand, chitosan-457
coated film prepared using high pressure treatment on turkey breast was studied by Joerger, 458
Sabesan, Visioli, Urian and Joerger (2009). They reported a lower count of L. monocytogenes 459
by 3 log after one week of storage compared to plastic film without chitosan and high 460
pressure treatment. Most researchers found that chitosan are highly effective towards gram-461
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negative bacteria. This is because the positively charged chitosan molecules can interact 462
better with negatively charged teichoic acid backbone in cell wall of gram-negative bacteria. 463
464
In order to further enhance the AM properties of chitosan, other AM agents were also 465
added into chitosan-coated films. These synergistic combinations can inhibit wider range of 466
microorganisms due to the ability of chitosan to function as carrier for other additives. 467
Zivanovic, Chi and Draughton (2005) studied on bologna wrapped with chitosan that stored 468
at 10oC for 5 days. They reported chitosan films reduced the number of L. monocytogenes by 469
2 log, whereas addition of 1% oregano oil into chitosan film reduced L. monocytogenes by 470
3.6 log. By incorporating 1% oregano and clove essential oil respectively into chitosan 471
incorporated-PP film, the inhibition effects against bacteria were significant, especially when 472
added of oregano essential oil. The reduction of L. monocytogenes, S. aureus and E. coli 473
O157:H7 count compared with control film (without AM agents) was 1.31, 1.4 and 1.18 log 474
CFU/g respectively in vacuum sealed cheese stored at 4oC after two weeks of storage (Torlak, 475
& Nizamlioğlu, 2011). Ye, Neetoo and Chen (2008) enhanced the efficacy of chitosan-coated 476
film against L. monocytogenes by adding nisin, sodium diacetate (SD), sodium lactate (SL), 477
potassium sorbate (PS) and sodium benzoate (SB) into chitosan. Such combination of AM 478
agents were generally used as food preservatives and recognized as GRAS. They found that 479
chitosan-coated plastic films alone with 0.0025 and 0.0055g/cm2 of chitosan were insufficient 480
to control the growth of L. monocytogenes on ham steaks stored at 20oC although the counts 481
were consistently lower about 0.7 log CFU/cm2 than those films without chitosan. 482
Nevertheless, such films did not extend the shelf-life of ham steaks. Furthermore, the coated 483
films contained 0.01 g/cm2 of SL and 0.003 g/cm2 of PS respectively exhibited the most 484
satisfied results. The reduction of L. monocytogenes was 1.9 log CFU/cm2 more pronounced 485
when compared to chitosan-coated film after 10 days of storage under similar condition. At 486
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storage temperature of 4oC, SL/chitosan-coated film showed even greater inhibition effect 487
with a reduction of more than 5 log CFU/cm2 compared to chitosan-coated film. 488
489
Chitosan has been widely studied as AM and edible packaging application due to its 490
biodegradability characteristic. The recent development of edible chitosan films included 491
adding of AM agent such as essential oils and nisin, or blending with other materials such as 492
chitosan-glucomannan-nisin blends and chitosan-starch blends to produce highly effective 493
AM films (Du, Olsen, Avena-bustillos, Mchugh, Levin, & Friedman, 2008). In-vitro AM 494
activity of edible chitosan-starch film with Thymus kotschyanus essential oil as studied by 495
Mehdizadeh, Tajik, Rohani, & Oromiehie (2012) found that Thymus kotschyanus essential oil 496
exhibited different inhibition levels against L. monocytogenes, E. coli, S. aureus and S. 497
typhimurium. The films without essential oils were not effective against S. typhimurium 498
where clear zone of inhibition was not observed. L. monocytogenes was the most sensitive 499
bacteria inhibited against the films added with kotschyanus essential oil, followed by E. coli, 500
S. aureus and S. typhimurium. As the concentration of kotschyanus essential oil increased, the 501
inhibition zone of E. coli expanded significantly. This indicates that the kotschyanus essential 502
oil has selective reactivity over the range of gram-positive and gram-negative bacteria. 503
504
4.4 Antimicrobial Packaging with Bacteriocin 505
506
Other than natural AM agents extracted from plants, animals and essential oils, AM 507
agents generated by bacteria called bacteriocin were gradually gaining popularity attribute to 508
their ability to withstand elevated temperatures and acidic environment. Bacteriocin is 509
metabolic by-product (antimicrobial peptide) produced by bacteria defense system from 510
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almost all types of bacteria. This naturally occurred activity allowed bacteria of one strain to 511
inhibit the growth of other strains in adjacent. Bacteriocin produced by lactic acid bacteria 512
(LAB) has highly acceptance from public, because LAB has been the important bacteria for 513
food fermentation for centuries. Example of traditional fermented foods that consume for 514
centuries included cheese, wine, kimchi, miso, soysauce, bean paste, and etc. Nisin, produced 515
by Lactococcus lactis bacteria, commonly present in milk is designated by Food and Drug 516
Administration (FDA) as “Generally Recognized as Safe” (GRAS) AM agent and employed 517
commercially for food preservation. It is the first isolated bacteriocin and specially used to 518
prevent the outgrowth of Clostridium botulinum spores in cheese. 519
520
Bacteriocins such as enterocins A and B, Sakacin, Enterocin 416K1, pediocin AcH 521
have been proven able to control the proliferation of L. monocytogenes in artificially 522
contaminated foods (Iseppi et al., 2008). Siragusa, Cutter and Willett (1999) reported that 523
bacteriocin-coated PE film fabricated by blown film extrusion process tested for vacuum 524
packaging of meat inoculated with B. thermosphacta has significantly inhibited the growth of 525
this bacterial. An, Kim, Lee, Paik and Lee (2000) also reported that bacteriocin-coated PE 526
film inhibits against Micococcus flavus and L. monocytogenes. Kim, Paik, & Lee (2002) 527
studied on bacteriocin-coated LDPE film found that the usage of nisin and lacticin NK24-528
coated films extended the shelf-life of oysters and beef stored at 3oC from 10 to 12 days and 529
from 9 to 13 days, respectively. Meanwhile, the shelf-life of oyster and beef extended from 5 530
to 12 days and 5 to 9 days respectively when stored at 10oC. 531
532
Besides, pediocin-plastic film was also reported to exhibit inhibition effects over 533
several pathogenic and gram-positive bacteria on foods. Particularly, their effectiveness is 534
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most prominent towards L. monocytogenes. In the research of Goff, Bhunia and Johnson 535
(1996), a preparation containing pediocin was added to sterilize raw chicken inoculated. The 536
results showed a reduction from approximately 3.0 log CFU/g of L. monocytogenes to below 537
detectable levels after 28 days of storage at 5oC, whereas the control treatment of the 538
counting reached 1.9 log CFU/g. Santiago-Silva et al. (2009) developed cellulose film 539
containing pediocin found that the film with 50% pediocin promoted an inhibition towards L. 540
innocua counting on sliced ham, whereas films with 25% pediocin and control unable to 541
suppress the microorganism growth during storage at 12oC. At the end of storage period (15 542
days), the inhibitory was 2 log cycles reduced for the sliced ham in contact with film of 50% 543
pediocin, while less than 1 log cycle for the film with 25% pediocin in relation to the control. 544
Ming, Webwe, Ayres and Sandine (1997) reported total inhibitory effects towards L. 545
monocytogenes when used cellulosic casings sprayed with pediocin on the meats (chicken, 546
beef and ham) for 12 storage-weeks at 4oC. Iseppi et al. (2008) studied on electrocin-coated 547
LDPE reported reduction of L. monocytogenes count on contaminated frankfurters that stored 548
at temperature of 4oC. During the first day, there was significant drop at more than 1 log 549
observed. Whereby, such level of decrement was maintained until the 14th day. Massani, 550
Morando, Vignolod and Eisenberg (2012) have produced multilayer-plastic films 551
incorporated with lactocin and tested on L. planturum. The films were composed of an 552
external PP layer, an internal polyamide–polyethylene structure, a barrier layer of ethylene 553
vinyl alcohol co-polymer, and a LLDPE food contact layer. The researchers reported that the 554
films’ AM activity was influenced by time and temperature. Through agar disk diffusion test 555
on L. planturum, it was found that after 7 days of storage at 30, 10 and 5oC, the inhibition 556
zones achieved 32, 66 and 100% respectively. After 14 days, no AM activity on the 557
multilayer film stored at 30 and 10oC was detected. 558
559
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4.5 Antimicrobial Packaging with Inorganic Nanoparticles 560
561
The utilization of inorganic nanoparticles as AM agents has recently gaining 562
popularity which attributed to the good stability of these materials to withstand harsh process 563
conditions such as high pressures or temperatures in plastic fabrication process. The most 564
extensively studied inorganic nanoparticles for AM purposes were titanium dioxide (TiO2) 565
and zinc oxide (ZnO). Titanium dioxide (TiO2) is non-toxic and approved by FDA for used in 566
foods, drugs and food contact materials. It is a photocatalyst widely used to inactivate wide 567
range of microorganisms. TiO2 generated hydroxyl radicals and reactive oxygen species via 568
light reaction which can inactivate microorganisms by oxidizing the polyunsaturated 569
phospholipids component of the cell membrane. Chawengkijwanich and Hayata (2008) 570
developed TiO2-coated OPP films to investigate the AM effects on E. coli in-vitro and in 571
actual case. The results showed that when two 20W black-light illumination was applied, the 572
film coated with TiO2 powder having greater AM effects with 3 log CFU/ml reduction of E. 573
coli compared to uncoated film with only 1 log CFU/ml reduction after 180 min of 574
illumination. In the actual test, a more significant result obtained where under irradiation of 575
ultraviolet A light, E. coli counts on cut lettuce stored in a TiO2-coated film bag decreased 576
from 6.4 on day 0 to 4.9 log CFU/g on day 1, while the uncoated film only exhibited slightly 577
decreased from 6.4 to 6.1 log CFU/g after 1 day of storage. TiO2-incorporated PE film 578
developed by Xing et al. (2012) using blown film extruder exhibited that, by antibacterial 579
drop-test method, the blank film did not possess any antibacterial activities where the 580
untreated TiO2 exhibited inhibition ratios of 50.4% for E. coli and 58.0% for S. aureus. 581
Whereas, the antibacterial activities of TiO2-incorporated PE film treated with ultraviolet 582
light for 1 hour improved significantly, which were 89.3% for E. coli and 95.2% for S. aureus. 583
Recently, Bodagh et al. (2013) developed TiO2-LDPE film by using blown film extruder and 584
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tested on Pseudomonas spp. and R. mucilaginosa. Through the modified in-vitro test method 585
developed by Chawengkijwanich and Hayata (2008), the number of Pseudomonas spp. 586
decreased significantly by 4 log CFU/ml and 1.35 log CFU/ml after 3 hours of ultraviolet A 587
illumination on TiO2 film and blank film respectively. Meanwhile, the number of R. 588
mucilaginosa decreased by 2 log CFU/ml and 0.64 log CFU/ml on TiO2 film and blank film 589
respectively. In an in-vivo test done on fresh pears packaged in TiO2 nanocomposite film and 590
stored under illumination by a fluorescent light lamp at 5oC for 17 days, the number of 591
mesophilic bacteria decreased from 3.14 to less than 2 log CFU/g whereby the bacteria count 592
increased from 3.19 to 4.02 log CFU/g for pears packaged in blank films. On the other hand, 593
it was observed a reduction of yeast count from 2.45 to less than 2 log CFU/g obtained by 594
fruits packaged in TiO2 films, whereas for blank film, the yeast count increased from 2.1 to 595
3.37 log CFU/g. Such results proven that the nanoparticles-incorporated plastic films 596
prepared by extrusion could be effectively used in fruit packaging applications. 597
598
The applications of ZnO nanoparticles coating systems have recently attracted a great 599
deal of attention due to its AM activity towards both the gram-positive and gram-negative 600
bacteria. In the study of Li, Xing, Li, Jiang and Ding (2010), ZnO-coated PVC films 601
exhibited significant AM effects on E. coli and S. aureus compared with blank film. When 602
tested on films that been exposed to ultraviolet light for 3 hours with the present of identical 603
amount of ZnO nanoparticles, the inhibition improved by 29.8% for E. coli and 26.0% for S. 604
aureus. This result indicated that ZnO breakdown products were the key AM particles that 605
inhibit the growth of bacteria. Tankhiwale and Bajpai (2012) also developed PE film that 606
coated with ZnO. The ZnO-coated PE film without treatment with light could also retard the 607
growth of E. coli by showing a clear zone of inhibition around the film during agar disk 608
diffusion test, whereas a clouded bacteria was observed in the blank film. The inhibition 609
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activities were further confirmed by the kinetics of bacterial growth which showed that the 610
growth rate of bacterial was suppressed appreciable in the solution containing ZnO-coated 611
film. 612
613
5. Effects of Antimicrobial Agents on Mechanical and Barrier Properties 614
615
The incorporation of AM agents into polymer can adversely affect the physical 616
properties, mechanical integrity and thermal stability of packaging when the AM agents used 617
are not compatible with the polymer. Whereas, AM agents that are compatible with 618
packaging materials can impregnate well into spaces between polymer chains. In other words, 619
it does not influence the film properties when reasonable amount is added (Han, 1996). 620
Therefore, the study of polymer chemistry and structure are important in predicting the 621
influence of particular AM agents on the packaging. Subsequently, the selection of AM 622
agents, packaging polymers and incorporation methods can be more effective. 623
624
Some researchers have thoroughly investigated the packaging mechanical properties 625
when substantial amount of AM agents incorporated. The mechanical properties of film as 626
represented by tensile strength (TS) and elongation at break (Ɛ) are the most important 627
properties to be analyzed and benchmarking. It measures film strength and stretchability prior 628
to breakage when extension force applied. The incorporation of additives into molten 629
polymers other than crosslinking agents generally detriment the packaging tensile strength 630
due to the additives is usually small components that tend to occupy between polymer chains 631
and cause slippage when external forces being applied. This phenomenon is called 632
plasticizing effect which can be proven by comparing AM agent incorporation methods 633
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between both coating method and extrusion method. Solano and Gante (2010) compared both 634
methods by adding 4% w/w Oregano essential oil and thyme essential oil into LDPE film. 635
Through the coating method, polymer chains were radiated and applied with layers of 636
essential oils. When comparison was done on blank film, coated film has no significant 637
changes in mechanical properties. Whereas, the essential oil incorporated-LDPE film 638
produced by using single screw extruder was slightly inferior in tensile strength as compared 639
to film without essential oil that produced by the similar method. 640
641
Generally, TS did not reduce significantly when small amount of AM additives 642
applied. Mistry (2006) reported that the incorporation of 2% w/w linalool and thymol 643
respectively into LDPE did not significantly affect the mechanical properties of the film. This 644
was due to the presence of small amount of AM does not interrupt the crystalline phase and 645
intermolecular interaction of polymer chains. Similar result was also found when incorporate 646
1% w/w Oregano essential oil and thyme essential oil respectively into LDPE film (Solano, 647
& Gante, 2010). Besides, the AM particle size is also an important factor that affects the 648
mechanical characteristics of the polymer films. Most of the time, particles in nano size are 649
unlikely to interrupt the polymer chains morphology. In contrast, some specialty mineral 650
types’ AM agents may enhance the film stiffness. Xing et al. (2012) studied on PE film 651
incorporated with 2% w/w TiO2 nanoparticles produced by single-screw extruder found that 652
both the TS and Ɛ were higher when compared to film without additive. The similar result 653
obtained by Li, Hong, Li, Zheng and Ding (2009) when carried out study on polyurethane 654
(PU) films incorporated with 2% w/w ZnO nanoparticles. This can be explained with the so-655
called exfoliation effect helps to strengthen the interactions of polymer chains and 656
nanoparticles. 657
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658
Biodegradable films generally possess weak mechanical strength while the addition of 659
AM agents would further weakening the films. According to Kechichian, Ditchfield, Veiga-660
Santos and Tadini (2010), when cinnamon and clove powders were incorporated into films 661
made by cassava starch, both the TS and Ɛ decreased dramatically. Kiam wood extract-662
incorporated hydroxypropyl methylcellulose (HPMC) film was having weaker TS compared 663
to blank film due to the incompatibility characteristics of kiam wood extract and HPMC. The 664
incomplete dispersal of kiam wood extract produced heterogeneous film structure thus 665
weakened the film properties (Maria et al., 2007). 666
667
Another important property that should take into account is water vapor transmission. 668
Water vapor transmission represents the ease of moisture to penetrate and pass through a 669
material (Li, Li, Kennedy, Peng, Yie, & Xie, 2006). It is important for a packaging to have 670
good water vapor barrier properties not only to prevent excessive water loss from foods, but 671
also resists moisture from atmosphere to migrate into foods. Water can accelerate 672
microorganisms’ growth and reduce shelf-life of foods. Water vapor transmission of AM 673
packaging is very much dependent on the hydrophilic-hydrophobic ratio of AM-matrix 674
material. Hydrophilic material tends to increase packaging water vapor transmission. 675
Suppakul, Miltz, Sonneveld and Bigger (2006) studied on basil extract-EVA/LDPE film 676
manufactured by extrusion method. They found that a reduction of water vapor transmission 677
from 13.7 (blank EVA/LDPE) to 10.5 and 5.2 g/m2/day respectively for linalool-incorporated 678
LDPE film and methylchavical-incorporated LDPE film. Methylchavical reduces water vapor 679
transmission greater than Linalool due to higher hydrophobicity characteristic. Jari, Eija, 680
Jouni and Raija (2003) reported the addition of 5% EDTA with hydrophilic characteristic in 681
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LDPE film has increased water vapor transmission from 5.85 to 6.6 mg/dm2/day. The water 682
vapor transmission values are in relation to the concentration of EDTA added. 683
684
For biodegradable film, water vapor transmission was dependent on AM agents’ 685
hydrophilicity as well. Kechichian, Ditchfield, Veiga-Santos and Tadini (2010) obtained the 686
lower value of water vapor transmission for cassava starch films added with cinnamon and 687
clove powders attributed to the hydrophobic properties of these ingredients. Maria et al. 688
(2007) studied on alginate-apple puree edible film (AAPEF) reported the incorporation of 689
essential oil and oil compounds such as lemongrass, cinnamon oil, citral, oregano oil, 690
carvacol and cinnamaldehyde did not affect the water vapor permeability of the edible film. 691
Similarly, addition of Oregano essential oil into WPI film developed by Zinoviadou, 692
Koutsoumanis and Biliaderis (2009) did not change the water vapour permeability of film 693
though 1.5 % w/w of oil was added. These may be due to essential oils are mostly terpene-694
like compound which is water resistant. According to Pranoto, Rakshit and Salokhe (2005), 695
garlic oil with hydrophobic characteristic did not significantly affect the water vapour 696
permeability on chitosan film. However, the addition of potassium sorbate (KS) into 697
chitosan-tapioca films increased WVP of the film. Even though chitosan possesses higher 698
hydrophobicity compared to tapioca starch, the addition of potassium sorbate (KS) with 699
hydrophilic hydrogen bond has increased the hydrophilic:hydrophobic ratio of the film 700
(Maria, Silvia, Carmen, Juan, & Lia, 2009). Hydrophilic additives like kiam wood extract 701
increased the water vapour permeability when the content of kiam wood extracts was higher 702
(Jutaporn, Suphitchaya, & Thawien, 2011). Nevertheless, addition of lactic acid (LA) into 703
whey protein isolated (WPI) film did not change the water vapor permeability (WVP) (Oscar 704
et al., 2012). 705
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706
Other than the characteristic of AM agent, the method of incorporating AM also 707
affected the changes of water vapour transmission. AM coated film possesses lower water 708
vapour transmission since coating could prevent part of moisture from passing through 709
packaging. Xing et al. (2012) reported water vapour transmission value of TiO2-incorporated 710
PE film produced by extrusion method increased from 18.1 (blank PE) to 20.1 g/m2/day. It 711
was probably due to the TiO2 nanoparticle caused irregularity of the crystallinity structure of 712
PE thin film, subsequently promoting the moisture passing through the PE film. In contrast, 713
addition of ZnO nanoparticles by coating method decreased water vapour transmission from 714
128 to 85 g/m2/day. 715
716
6. Conclusion and Future Trends 717
718
Nowadays, people are getting more alert on safety of food additives added in foods. It 719
is undeniable that some controversies have been arisen in the last few years on the safety of 720
synthetic additives. Natural AM as one of the food preservative ingredient is expected to 721
substitute synthetic additives for fulfilling the demand of the consumers on safer food 722
additives. Concurrently, researchers who are involved in the development of AM films are 723
thoroughly working on utilizing the natural compounds like plant extracts as the alternatives 724
solution for AM packaging. However, the development and application of natural AM active 725
packaging is limited due to some reasons like insufficient knowledge regarding the efficacy 726
of AM in polymer films, economic impact, consumer acceptance and unavailability of 727
specific regulations about active packaging (Kechichian, Ditchfield, Veiga-Santos, & Tadini 728
(2010). Thus, based on the review above, the ideal AM polymer films should meet several 729
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important factors, whereby the most important consideration in food industry is that process 730
of making AM film should be simple and within the reasonable cost. Secondly, the film 731
should be chemically stable for long term usage and storage. Besides, the AM film shall act 732
as water barrier and would not decompose easily to maintain the AM effect throughout 733
packaging period. Importantly, the AM packaging films should not be harmful to handle as 734
well as do not release toxic particles into foods. 735
736
In addition, researchers can also venture into developing specialty AM agents which 737
can be regenerated upon loss of activity and biocidal to recognizable food pathogens in 738
respective foods. At the mean time, it is also important to investigate the maximum allowable 739
amount of antimicrobials used in food packaging so that the additives in foods are not over-740
dosed. All the requirements are crucial in order to ensure the AM packaging can be widely 741
used and accepted by the consumers. For more comprehensive approach, the application of 742
AM agents should be coupled with biodegradable packaging materials. This is because the 743
current global trend highly recognized the usage of degradable packaging film, typically 744
when the biodegradable materials are derived from renewable resources. Innovative 745
technology in the development of food preservation and safety paired with biodegradability 746
are the trend in the near future to meet the eco-friendliness expectation. Hence, experts from 747
different disciplinary such as food technologists, microbiologists, chemists, polymer 748
technologists, chemical engineers, as well as environmental scientists are in great demand to 749
work towards a notable implementation and commercialization of eco-friendly AM 750
packaging films. Such AM degradable films are promising food packaging materials because 751
its biodegradability provides sustainable development for the modern community. 752
753
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Acknowledgement 754
755
This review was prepared as part of the project supported by Ministry of Science, 756
Technology and Innovations (MOSTI) of The Federal Government of Malaysia-Putrajaya 757
under ScienceFund 03-02-11-SF0128. 758
759
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Table 1: Commercial products of AM food packaging
Active Compound Matrix Application Trade Name
Silver substituted Zeolite LLDPE, PE, PVE, rubber Film, Wrap, milk containers,
paperboard cartons
AgIon®, Zeomic™, Cleanaid™,
Novaron®
Chlorine dioxide Polyolefin Film, sachet MicroGarde™, Microsphere™
Ethanol Silicon dioxide Sachet Ethicap™
Sulfur dioxide Laminated plastic sheet with
Na2S2O5
Sheet or pad for postharvest
storage of grape fruits
Uvasy™
Triclosan Polymer, rubber Food container Microban®
Wasabi extract Encapsulation in
cyclodextrin
Coated PET film, tablet Wasapower™
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Table 2: Properties of some common plastic materials used in food packaging (Bastarrachea, Dhawan, & Sablani, 2011; Ščetar,
Kurek, & Gali ć, 2010)
Packaging material Water vapour
permeability (x 1014),
g m m-2 s-1 Pa-1
O2 permeability (x 107),
mL m m-2 day-1 Pa-1
Tensile
strength,
MPa
Elastic
modulus,
GPa
Impact
strength,
J/m
Light
transmission,
%
Poly(vinyl chloride) PVC 18.279 0.198 – 0.790 42 – 55 2.8 180 – 290 90
Poly(vinylidene chloride)
PVDC
1.161 0.001 – 0.030 49 – 98 0.2 – 0.6 - 90
Polypropylene (PP) 2.321 – 4.642 4.936 – 9.869 27 – 98 1.18 43 80
High density polyethylene
(HDPE)
1.741 – 3.482 7.127 19 – 31 0.98 373 -
Low density polyethylene
(LDPE)
6.673 – 8.704 44.756 7 – 25 0.15 – 0.34 375 65
Ethylene vinyl acetate,
EVA
6.673 – 17.118 21.220 6 – 19 - 45 55 – 75
Polyamide, PA 5.803 – 114.314 0.010 – 0.098 49 – 69 0.7 – 0.98 50 -60 88
Poly(ethylene
terephthalate), PET
5.803 – 22.921 0.098 – 0.494 157 – 177 3.5 100 88
Polystyrene, PS 11.315 – 45.552 9.869 – 14.805 31 – 49 2.7 – 3.4 59 92
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Table 3: Antimicrobial incorporated into edible polymer film
Polymer Antimicrobials Targeted
Microorganism References
S. aureus Thyme
E. coli
E. coli O157:H7
S. aureus
E. coli
Soy edible
films
Oregano oil
E. coli O157:H7
Emiroğlu, Yemiş,
Coşkun, & Candoğan
(2010)
E. coli
S.typhimurium
S.aureus
Alginate-based
edible films
Garlic oil
B. cereus
Yudi, Vilas, & Sudip
(2005)
E. coli O157:H7
S.aureus
S. enteritidis
L. monocytogenes
Whey protein
edible films
Garlic oil
L. plantarum
Seydim & Sarikus (2006)
E. coli O157:H7
S. aureus
S. enteritidis
L. monocytogenes
Oregano oil
L. plantarum
E. coli O157:H7
S. aureus
S. enteritidis
L. monocytogenes
Whey protein
edible films
Rosemary oil
L. plantarum
Seydim & Sarikus (2006)
E. coli
S. aureus
lactic acid (LA)
Y. lipolytica
E. coli
Whey protein
isolate
Propionic acid (PRO)
S. aureus
Oscar et al. (2012)
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Y. lipolytica
E. coli
S. aureus
Chitooligosaccharides
(COS)
Y. lipolytica
E. coli
S. aureus
Natamycin (NA)
Y. lipolytica
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Figure 1: Effects of acetate buffer, pH 5.4 (-○-), or 0.5% (-•-) or 0.25% (-□-)
solutions of chitosan on the numbers of (a) E. coli or (b) S. aureus in cell suspensions
incubated at room temperature (Liu, Du, Wang, & Sun, 2004; published with
permission from Elsevier)
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Figure 2: Food deterioration process for liquid, solid, and general surfaces based on
total microbial count (Fung et al., 1980)
Slime development:
109 CFU/ml or cm2
Low count (no concern):
100-2 CFU/ml or cm2
Intermediate count (slight concern):
103-4 CFU/ml or cm2
High count (serious concern):
105-6 CFU/ml or cm2
Index of spoilage:
107 CFU/ml or cm2
Odor development:
108 CFU/ml or cm2
Unacceptable, too high:
1010 CFU/ml or cm2