Active and Intelligent Packaging Materials

4.52

Active and Intelligent Packaging Materials

L-T Lim, University of Guelph, Guelph, ON, Canada © 2011 Elsevier B.V. All rights reserved.

4.52.1 4.52.2 4.52.2.1 4.52.2.1.1 4.52.2.1.2 4.52.2.2 4.52.2.2.1 4.52.2.2.2 4.52.2.2.3 4.52.3 4.52.3.1 4.52.3.2 4.52.3.3 4.52.4 4.52.4.1 4.52.4.2 4.52.4.2.1 4.52.4.2.2 4.52.4.2.3 4.52.5 References

Introduction Antimicrobial AFP Controlled Release of Volatile Antimicrobial Agent from a Carrier Ethanol emitter Allyl isothiocyanate emitter Antimicrobial AFP Based on Direct Contact with Food Coating and immobilization of antimicrobials on package surface Antimicrobial polymer Incorporation of active agent within packaging structure Vapor and Gas Scavengers Oxygen Scavenger Ethylene Scavenger Controlled Release of Food Preservatives Intelligent Packaging Oxygen Indicator with Controlled Activation Quality Indicator Indicator to monitor microbial activities Indicator for detecting meat spoilage Ripeness indicator technologies for fruits Conclusion: Acceptance and Safety issues of AFP and IFP

Glossary active food packaging Packaging system that interacts dynamically with the product it contains and/or the surrounding environment it exists, which results in the activation of some mechanisms to extend the shelf-life or quality of the product. electrospinning A forming technology that uses electrostatic force to draw polymer into ultrathin fiber that has diameter ranging from tens to hundreds of nanometers. intelligent food packaging Packaging system that provides information, in real time, about the history or

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actual quality status of the product it contains through an internal or external indicator/sensor. modified atmosphere packaging Package system of which the headspace gas is modified such that its composition is different from that of air. nanomaterial A material with at least one of its dimensions smaller than 100 nm. shelf-life Shelf-life is the period of time during which a product remains acceptable under certain storage conditions. The acceptance criteria can be based on sensory, nutritional value, or safety attributes.

4.52.1 Introduction Packaging is an integral part of food-preservation system to maintain, and sometimes, to enhance the quality of food during distribution. It played an important enabling role in globalization of food economy by stabilizing food products so that they can be distributed over long distances. Besides the protective function, packaging also provides product-relevant information and adver­ tisement to communicate with consumers. Packaging also provides end-use convenience (e.g., reduces food preparation time, is easy to open, is re-sealable, has ergonomic design, etc.) that often influences consumers’ purchase decision. Shelf-life is defined as the time after packaging during which a product remains acceptable under certain storage conditions and acceptance criteria. The shelf-life for a given product is closely related to its inherent properties, environmental condition to which the product is exposed during transportation and storage, as well as the efficacy of the packaging system employed [1]. In order to be commercially viable, a product must be reasonably stable during distribution to ensure that its quality attributes remain acceptable at the time of consumption. The stability of a product will dictate how it is distributed (e.g., air vs. land freight; refrigerated vs. ambient) which will have profound implications on the distribution cost. From a food producers’ perspective, packaging also has strategic importance to differentiate their products from those of competitors’.

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Package

External factors

External factors

Package headspace

Active component

Intelligent component

FOOD

Action

Decision

Enhanced protection

Enhanced communication

Figure 1 Conceptual representation of advanced food packaging system showing the enhanced protection and communication roles.

Recently, there is also a growing trend of consumers for minimally processed and preservative-free foods, which can present considerable challenges during distribution due to reduced product stability. From a broader perspective, increasing regulatory requirements, environmental awareness, and more recently threat of bioterrorism also affect how companies package their products [2]. Driven by these social–economic demands, many packaging innovations have taken place over the past decade. One area that has undergone considerable growth is active food packaging (AFP). Instead of passively protecting the products as in a conventional food package, AFP dynamically protects the food product that it contains. Some AFP systems are capable of interacting with the product and/or surrounding environment, and in response carry out desirable functions to enhance the protective role of the package. The motives for this enhancement are to extend product shelf-life, maintain or enhance product quality, and ensure safety. Another innovation that has received considerable commercial interests is intelligent food packaging (IFP). The hallmark for this type of packaging is that it contains an external or internal indicator to provide information about the history and/or the quality of the product. In essence, this technology expands the communication functionality of a package in a more dynamic way. For instance, instead of relying on open-dating mechanism such as best before label to indicate product freshness, IFP provides a visual cue to reflect the actual state of the product, taking into account potential abuses such as package failure and temperature fluctuation encountered during distribution. For food producers, IFP can potentially enhance the flow of information during the entire product supply chain cycle, such as inventory flow, time–temperature history, water activity, pH, etc. [3]. In summary, the uniqueness of IFP lies in its ability to enhance communication, while AFP enhances the protective role of a food package (Figure 1). Depending on the intended functionality of a package, the active and intelligent concepts may or may not be mutually exclusive [3]. While the objectives of AFP and IFP are different, they share one similarity in that one or more interactive mechanisms are involved. These mechanisms may be mechanical, chemical, thermal, electromagnetic, and biological in nature. In this article, emerging technologies in AFP and IFP will be discussed.

4.52.2 Antimicrobial AFP Food-supply chains today rely on highly efficient centralized production to achieve economy of scales and better utilization of processing residuals. Nevertheless, centralized production can also pose food safety concerns due to the large volume of products handled and the requirement of transporting products over a long distance to the points of sales. These trends, along with consumer’s preference for minimally processed products, are some of the contributing factors to many recent outbreaks of foodborne illnesses. In order to increase the microbiological keeping quality and safety of food products, one of the strategies is to incorporate antimicrobial mechanisms into food-packaging materials to inhibit the growth of spoilage and pathogenic microorganisms. In general, antimicrobial AFP can be classified into two categories. In nonmigratory AFP, active agents are immobilized on the surface of the package surface or some sorts of carrier, requiring the product to be in direct contact with these surfaces. By contrast, active releasing AFP systems involve migration of antimicrobial agents to the products. Selected examples are discussed below.

4.52.2.1

Controlled Release of Volatile Antimicrobial Agent from a Carrier

Modified atmosphere packaging (MAP) refers to packages for which the headspace gas composition is modified such that it is different from the composition of air, or it is replaced with other gases. Conventional MAP is based on flushing the headspace of the product with the desirable gas mixture before the package is sealed. In comparison, AFP employs scavenger and emitter technologies to modify the headspace gas composition to achieve optimal product shelf-life. In antimicrobial AFP, one or combinations of volatile antimicrobial agents are released into the package headspace to inhibit the proliferation of spoilage microorganisms. For products which are susceptible to postprocess surface contamination, this method has an advantage that high concentration of

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antimicrobial can be achieved where it is most needed on the surface. Antimicrobial agents are usually temporarily immobilized in polymeric carriers. Their release may be triggered by one or more stimulants, such as moisture present in the headspace.

4.52.2.1.1

Ethanol emitter

Ethanol is a well-known antimicrobial agent. In a typical ethanol emitter, ethanol is temporarily adsorbed on silicon dioxide powder enclosed in a sachet constructed with permeable materials, such as a paper and ethyl vinyl acetate (EVA) copolymer laminate. When the sachet is deployed in a hermetically sealed package, sorption of water by the silicon dioxide causes the displacement of ethanol, triggering the release of ethanol into the headspace [4]. One of the commercial examples here is EthicapTM ethanol emitter from Freund Industrial Co. Ltd. Franke et al. investigated the effect of EthicapTM ethanol emitter on the shelf-life of prebaked buns. They observed that mold growth was delayed up to 13 days when exposing prebaked buns packaged in low-density polyethylene bags, as compared to 4–6 days for the untreated controls. Smith et al. investigated the efficacy of EthicapTM against Saccharomyces cerevisiae in apple turnover products. They reported that yeast growth was completely suppressed at 0.9aw and 1.52% (v/v) ethanol vapor, extending the shelf-life to more than 21 days at 25 °C [5]. Other commercial examples of ethanol emitters include AntimoldTM, and NegamoldTM (Freund Industrial Co. Ltd.), OitechTM (Nippon Kayaku Co. Ltd.), ET PackTM (Ueno Seiyaku Co. Ltd.), and AgelessTM type SE (Mitsubishi Gas Chemical Co. Ltd.).

4.52.2.1.2

Allyl isothiocyanate emitter

Another antimicrobial compound that has been investigated for AFP application is allyl isothiocyanate (AITC). This is a naturally occurring antimicrobial compound found in plants from the Cruciferae family, such as horseradish and mustard. To a lesser extent, AITC is also found in leafy vegetables of Brassica genus such as cabbage. Its antimicrobial properties have been well demonstrated in the literature [6–13]. One of the AITC-based commercial antimicrobial AFP systems is pressure-sensitive labels from MitsubihiKagaku Foods Corporation, Japan. In this system, AITC is temporarily impregnated in the adhesive layer of the label. To prevent premature release of the antimicrobial volatile, the adhesive layer is protected by a high barrier film, such as metalized polyester. To activate the label, the protective film is removed, and the label is affixed to either the exterior or interior of a food package, depending on the AITC permeability of the package structure. Polyolefin polymers, such as polyethylene and polypropylene, are highly permeable to AITC. By applying the antimicrobial label on the exterior of the package, AITC permeates through the package structure, desorbs from the inner package surface, and emits into the package headspace (Figure 2). For packaging structures that are relatively impermeable to AITC, such as poly(ethylene terephthalate) and polystyrene, the label will need to be attached to the interior surface. In this case, the label is constructed of AITC-permeable film to allow gradual release of AITC from the adhesive layer. Both of these modes of delivery provide a sustained release of AITC to achieve continual antimicrobial effects. In a study, Winther and Nielsen investigated the efficacy of Wasaouro interior labels for shelf-life extension of Danish Danbo cheese packaged in thermoformed poly(ethylene terephthalate) containers [14]. They reported that the use of one AITC label (20 mm × 20 mm) extended the shelf-life to 13 weeks, as compared to 4.5 weeks for the control samples packaged in the absence of antimicrobial label. When two AITC labels were used, the shelf-life was further extended to 28 weeks. Cheese samples stored for up to 12 weeks in the presence of AITC label had an unacceptable mustard flavor due to AITC absorption into the cheese. Interestingly, the off-flavor decreased to an acceptable level between 12 and 28 weeks, presumably due to reaction with nucleophiles (e.g., –SH and –OH groups) present in cheese [14]. This study concluded that AITC-based AFP may be a good alternative to MAP for cheese.

AITC barrier film AITC impregnated adhesive AITC permeable film Exterior label

Internal label

AITC vapor

AITC permeable polyolefin (polypropylene, polyethylene) film Figure 2 Controlled release of AITC vapor into food package headspace to inhibit the growth of microorganism. Adapted from http://www.mfc.co.jp/wasaouro/e/products/labels.html.

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Syringe Polymer solution

Positive electrode

Spinneret

Infusion pump

DC power supply

Polymer jet

Taylor cone

Non woven fibrous mat

Grounded (or negatively charged)

Rotating drum collector

Figure 3 Schematic diagram showing the major components of a lab-scale electrospinner.

Recently, fibrous membranes prepared from electrospun fibers have been investigated as carriers for controlled release of AITC for active packaging application. Unlike the typical melt spun fibers that are formed by mechanical extrusion, this method uses electrostostatic force to draw polymer into fiber of diameter ranging from tens to hundreds of nanometer. (Figure 3). During the electrospinning process, due to the charge repulsion on the surface of the fiber which induces jet instability, the polymer undergoes chaotic whipping motion, thereby draw the fiber into submicron diameter [15–17]. By virtue of their small diameter, electrospun fibers exhibit much larger surface-to-volume ratio as compared with conventional melt spun fibers and continuous film. This property enhances the surface activity of electrospun fiber, making them an ideal carrier for delivery of bioactive agents. Vega Lugo and Lim leveraged the moisture sensitivity of electrospun fibers prepared from soy protein isolate/poly(ethylene oxide) (SPI/PEO) blend and poly(lactic acid) (PLA) electrospun fibers for controlled release of AITC [18]. Briefly, their spinning dope is made up of 15% SPI dissolved in 1% NaOH solution. PEO (0.6%) and Triton X-100 (1%) were added as process aid and emulsifier, respectively. The solution was heated for 2 h at 60 °C to denature the protein, followed by the addition of β-cyclodextrin and stirred for 30 min. After cooling to 24 ± 2 °C, AITC was added at a 0.4:2.4 AITC:β-cyclodextrin molar ratio and treated with a sonicator to form an AITC-cyclodextrin complex. The solution was electrospun using a setup similar to that shown in Figure 3 using a single 20-gauge blunt stainless steel spinneret which was positioned 26 cm from a stationary collector plate. In their experiment, the spinneret was grounded, while the collector plate was connected to the positive electrode of a 20 kV DC power supply. At 3% AITC, the fibers obtained were 281 ± 43 nm in diameter. At 0% RH, the release of AITC from the SPI fibers was negligible. However, AITC release was triggered when the fibers were tested at 50% and 75% RH (Figure 4). Compared with the direct addition of AITC to the protein solution, the AITC-cyclodextrin approach resulted in greater loading of AITC (~70% higher) due to the reduced

AITC concentration (µg l–1)

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Time (h) Figure 4 Interactive behavior of AITC-encapsulated SPI–PEO fibers toward relative humidity. The electron scanning micrograph illustrates the morphology of typical electrospun fibers at 3% AITC loading and 12% SPI concentration, with average diameter of 281 ± 43 nm.

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evaporative loss during spinning. The interactive behavior of these antimicrobial nanofibers may be promising in active packaging applications for foods to ensure that the release of AITC takes place only under elevates humidity condition, such as within the food package. Conceivably, a similar concept can be applied for encapsulation of other antimicrobial agents for shelf-life extension of food. One of the potential shortfalls for volatile antimicrobial AFP systems is that the absorption of active compounds by food may result in off flavor. Therefore, the deployment of AFP involving volatile release will require careful product selection. For instance, ethanol emitter may be ideal for brown and serve products for which they are heated before consumption, such as pizza crusts, cakes, breads, and biscuits. The antistaling properties of ethanol vapor may also present an additional advantage of applying this type of AFP in bakery products [4]. The garlic-like odor of AITC may be suitable for certain products such as raw beef, cured pork, sliced raw tuna, cheese, egg sandwich, noodle, pasta, and bread [13, 14, 19]. In certain jurisdictions, the migrated antimicrobial may be considered as a food additive, and accordingly, the use of this type of AFP may be subjected to regulatory additive limits. To address the direct food contact and potential accidental ingestion of the active patch, one recent patent describes a method to segregate active compounds with an external patch that prevents the direct contact of such compounds with the food content, while allowing a controlled exchange between the patch contents and the package interior [20].

4.52.2.2

Antimicrobial AFP Based on Direct Contact with Food

In packages where headspace is limited or absent, the antimicrobial effect is achieved by direct diffusion of active compounds from the active surface to food product. This mode of delivery is also needed when the antimicrobial compounds are not volatile. Here, the active agent is either incorporated into the packaging material during conversion (e.g., casting, extrusion, and injection molding) or the food-contact surface of the package is treated to render it antimicrobial. In this section, we review selected active packaging examples that exhibit this characteristic.

4.52.2.2.1

Coating and immobilization of antimicrobials on package surface

Glucose oxidase (GOX) from molds such as Asperigillus and Penicillium spp. catalyzes the oxidation of glucose to form H2O2 and D-gluconic acid, both of which exhibit antimicrobial properties. Vartiainen et al. [21] covalently attached GOX onto the surface of biaxially oriented polypropylene (BOPP) films. The enzyme immobilization was based on generating amino and carboxyl groups on the surface of BOPP film using NH3 or CO2 plasma. The plasma treatment was carried out in a dielectric barrier discharge reactor at atmospheric pressure using a 20 kV/200 mA AC power supply. The covalent attachment of GOX to the amino and carboxyl groups was achieved by using glutaraldehyde or N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride, respectively, under different pH conditions. Vartiainen et al. reported that the procedure produced films that remained active over 30 days of storage. They reported 2- to 6-log reductions in Bacillus subtilis and 5-log reductions in Escherichia coli when these bacteria were exposed to the GOX-immobilized active films on growth media. The antimicrobial BOPP films may have potential to be exploited in antimicrobial packaging film applications. Using an electrospinning technique, Zhou and Lim immobilized GOX in nonwoven PLA membranes to activate a natural antimicrobial mechanism in raw milk, known as lactoperoxidase (LP) system, to improve the keeping stability of the product [22]. In the presence of trace amounts of hydrogen peroxide (H2O2), this antimicrobial system catalyzes the oxidation of thiocyanate (SCN–, another component present in milk), generating hypothiocyanate ion (OSCN–) and other intermediate products that possess antimicrobial properties. The oxidation of sulfydryl (SH) group in microbial enzymes and proteins by these products is believed to be responsible for the antimicrobial actions of the LP system [23–25]. In their studies, Zhou and Lim immobilized GOX in electrospun spun PLA fibers to form an active membrane which is capable of catalyzing the oxidation of glucose in milk, generating H2O2 which in turn activates the LP system. The procedure for preparing GOX-in-PLA emulsion spinning dope is summarized in Figure 5. The dispersion of aqueous GOX in PLA-chloroform solution was achieved with the aid of sorbitan monopalmitate (SMP) and sonication. The emulsion was electrospun using a 20-gauge stainless steel spinneret attached to the

PLA resin

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emulsion (1, 2 and 10% w/w)

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Figure 5 Preparation procedure of GOX-in-PLA emulsions for electrospinning into PLA composite fibers. GOX, glucose oxidase; SMP, sorbitan monopalmitate; DMF, dimethylformamide.

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Time (h) Figure 6 Activation of LP system at 21 ± 2 °C in milk using GOX-in-PLA microfibers in skim milk samples without fortification or fortified with glucose and/or SCN−: A, Without fortification; B, 1.46 mM glucose; C, 0.49 mM SCN−; and D, 1.42 mM glucose + 0.47 mM SCN−.

positive electrode of a 10 kV DC power supply. The spinneret was positioned 16 cm from a grounded stainless steel plate covered with a low-density polyethylene film. Scanning electron micrographs showed that 1% SMP and 0.2% aqueous GOX-in-PLA emulsion provided the most consistent fiber morphology with an average diameter of 1132 ± 415 nm. These fibers exhibited about 19 times greater enzymatic activity as compared to film cast from the same emulsion, which was attributed to the larger surface area of the electrospun fibrous membrane. The exposure of fresh milk to GOX-in-PLA active membrane caused an increase in OSCN– concentration for up to about 9 h, showing that the naturally occurring SCN− and glucose in milk were adequate to activate the GOX- and LP-catalyzed reactions (Figure 6, A). Milk fortified with glucose resulted in a higher OSCN− concentration (Figure 6, B), whereas those fortified with SCN− resulted in a similar OSCN− profile as the unfortified milk (Figure 6, C). When the milk samples were fortified with both glucose and SCN−, substantially higher OSCN− concentrations were observed (Figure 6, D). These results seem to suggest that glucose was a limiting factor in the GOX reaction. It is anticipated that the efficacy of GOX-in-PLA fibers would be enhanced in lactose-free or lactose-reduced milks, in which glucose contents are higher than in the regular milk. From quality standpoints, at the end of the 24-h test period, offodor, coagulation, and discoloration were detected in the control milk (not treated with GOX-in-PLA fibers). For milk samples treated with GOX-in-PLA fibers, the extent of deterioration was less severe. By contrast, samples fortified with glucose and SCN− showed no visible deterioration or detectable off-odor. This OSCN–-controlled release concept may be applied in active packaging milk, in which a small patch is attached to the inner milk-contact surface of the package wall to extend the shelf-life of milk. Although high concentration of H2O2 is toxic, at 100 μM or less and in the presence of LP and SCN–, mammalian cells are protected from this toxicity [23]. Accordingly, Food and Agriculture Organization of the United Nations and International Dairy Federation have developed protocols to add thiocyanate and hydrogen peroxide in milk, in the forms of sodium thiocyanate and sodium carbonate peroxyhydrate, respectively, during transportation and storage of milk in regions of the world where refrigeration infrastructures are lacking [26].

4.52.2.2.2

Antimicrobial polymer

One of the most studied biopolymers that possess antimicrobial properties is chitosan. It is a linear polysaccharide (β-(1→4)-N­ acetyl-D-glucosamine) derived from chitin found in the exoskeleton of arthropods and cell walls of fungi and yeast [27]. The antimicrobial properties of chitosan are believed to be related to its polycationic structure that interacts with anionic components of bacteria surface (e.g., lipopolysaccharides and proteins), causing a disruption in integrity of the outer cell membrane and its barrier functions. Other antimicrobial mechanisms proposed include selective binding of trace metals required for toxins production and growth, inhibition of mRNA synthesis, and acting as enzyme inhibitor [28–30]. Because of its generally regarded as safe (GRAS) status, chitosan has been used in edible antimicrobial film coating and as a carrier of bioactive agents. For instance, using solvent casting approach, Sebti et al. [31] showed that chitosan film strongly inhibited the growth of Aspergillus niger on growth medium. Ouattara et al. [32] utilized chitosan as a carrier for acetic and propionic acids to develop antimicrobial packaging films. They incorporated lauric acid, cinnamaldehyde, or eugenol into chitosan films, which slowed down the diffusion of acetic or propionic acids in water. Pranoto et al. [33]enhanced the antimicrobial efficacy of chitosan film by incorporating garlic oil, potassium sorbate, and nisin, all of which exhibit various degrees of antimicrobial properties. Han et al. [34] investigated the use of chitosan coating to extend the shelf-life and enhance the nutritional value of strawberries and raspberries. The coating was prepared by dissolving 2% chitosan in 1% acetic acid solvent. Vitamin E and calcium were added as micronutrient supplements. The decay incidence of these berries stored at 2 °C and 88% relative humidity was reduced significantly compared to the uncoated samples. At 14 days storage, 65–83% of the uncoated strawberries were infected by molds, while the coated samples were less than 25–65%. For raspberries, after 21 days of storage, the decay incidence was 95% for the control, but only 2% for the coated samples. Moreover, chitosan-coated fruits reduced weight loss, delayed change in color, pH, and titratable acidity during storage. The different efficacy observed between strawberries and raspberries may be related to the different

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wettability and adhesion of coating between the fruit surfaces. Choi et al. [35] investigated the wettability of chitosan solutions on apple skin. Their chitosan-coating solution was made up of 1.5% chitosan dissolved in 2% acetic acid. Other components added were sodium alginate, bees wax, and coconut oil. They concluded that the addition of a surfactant (Tween 80; polyoxyethylene sorbitan monooleate) was needed to reduce the surface tension of the coating solution, which was important to enhance its wettability and improve the adhesion between the coating and apple skin. Chitosan is a moisture-sensitive polymer; by itself, the polymer will not be optimal for packaging applications where moisture barrier properties are critical. One of the possible strategies to overcome this shortfall is to integrate chitosan with other moisture barrier polymers to form a composite structure. To achieve this objective, Vartiainen et al. [36] immobilized chitosan onto BOPP film to produce an antimicrobial moisture barrier film. They employed a similar N2-plasma process as described above for grafting GOX to implant amino groups onto BOPP film surfaces. They then applied a chitosan solution (1% chitosan in 0.1 M acetic acid and 0.1% glutaraldehyde) to the activated surface, resulting in 1.8 g m–2 chitosan surface loading that resisted leaching in food simulants (<2 mg dm–2). An antimicrobial drop test on petri dish showed that the chitosan-grafted BOPP films exhibited antimicrobial activities against B. subtilis and E. coli. These films may be useful in direct food contact applications such as vacuum-packed products (e.g., sliced cheese, deli meats, and sausages).

4.52.2.2.3

Incorporation of active agent within packaging structure

Nanocomposite materials which possess antimicrobial properties have been exploited in packaging and coating applications to prevent microbial proliferation in food. TiO2 is a semiconductor which is capable of harnessing ultraviolet (UV) irradiation to initiate redox reactions which are antimicrobial. Cerrada et al. [37] incorporated 2–5% w/w TiO2 (~10 nm particle size) into ethylene-vinyl alcohol copolymers (EVOH) resulting in antimicrobial nanocomposites that are biocidal toward microorganisms (e.g., E. coli, Erwinia caratovora, Pseudomonas aureus, Zygosaccharomices rouxii, and Pichia jadini). Maneerat and Hayata coated a polypropylene film with TiO2 (average particle 7 nm) by spraying TiO2 coating solution (a mixture of peroxotitanium acid solution and peroxo-modified anatase solution) on the film and allowed to air dry for 72 h. They showed that the TiO2-coated film exhibited antifungal activity to control Penicillium rot in inoculated apple, tomato, and lemon [38]. In this study, the active films were irradiated with UVA (315–400 nm). The use of UV light may not be favorable for certain food products as UV energy can trigger the degradation of UV-sensitive components in food. To expand the photosensitivity range of TiO2 to the wavelengths of visible radiation which is less detrimental to food, titania can be doped with different metal ions of alkaline earth, such as Fe3+, Cr6+, Co3+, Mo5+, and rare earth ions [39]. Antimicrobial activity of silver is related to the release of active ion from its salts, complexes, and halides. The efficacy is thought to be due to its binding with electron donor groups containing sulfur, oxygen, or nitrogen presence in biological molecules such as proteins and DNA [40]. The antimicrobial properties of silver nanoparticles have been studied extensively for antimicrobial packaging materials. Del Nobile et al. [41] studied the effect of Ag-containing nanocomposite active packaging system on the survival of a Gram-positive spore former, Alicyclobacillus acidoterrestris, which is prevalent in acidic beverages such as fruit juices. The 200 ± 20-nm-thick coating was produced by depositing an Ag-containing PEO-like coating on polyethylene film using a plasma process at a radio frequency of 13.56 MHz using silver as a cathode. They reported that the active film inhibited the growth of A. acidoterrestris in acidified malt-extract broth and apple juice, and that the effectiveness of the active film was directly related to the amount of silver ion released into the test medium [41]. Damm et al. investigated the release of silver particles from nylon 6-nanocomposite prepared by thermally reducing silver acetate through melt extrusion at 230 oC. The nanosilver particles formed are about 10–20 nm in diameter. The release of silver ions from the nanocomposite was a zero-order process in aqueous broth media. At 0.06% w/w of silver nanoparticles loading which provided 9.5 × 10−4 mg l–1 cm–2 d–1 release rate, the silver-filled materials eliminated E. coli completely within 24 h [42]. Perkas et al. adopted a novel technique to produce silver-nylon 6,6-nanocomposite by subjecting the nylon pellets to ultrasound irradiation in solution containing silver nitrate, ammonia, and ethylene glycol. Nanocrystals of silver, 50–100 nm in size, were deposited on the surface of the pellets. Perkas et al. showed that the silver-coated pellets can be melt spun into yarn that inhibited the growth of Staphylococcus aureus and Pseudomonas aeruginosa [43]. This silver–nylon nanocomposite may be used as a master batch for the production of antimicrobial packaging materials. Several studies have shown that organo-modified-layered silicate exhibited antimicrobial properties. Rhim et al. [44] attributed the antimicrobial effects to the inherent antimicrobial activity of the quaternary ammonium group in the silicate layer that disrupts bacterial cell membranes, causing cell lysis. To enhance the antimicrobial properties, Weickmann et al. prepared hybrid dispersions containing layered silicate nanoplatelets together with silver, palladium, and copper nanoparticles (14–40 nm) immobilized on nanoplatelet surfaces. The layered silicate-supported silver and copper nanoparticles were shown to be very effective additive for inhibiting bacterial growth [45]. While self-sterilizing surfaces derived from organoclay are attractive for antimicrobial applications, the use of these additives in direct food contact application will be limited due to the migration of the surfactant. Development of nonleaching biocidal organoclays will widen the application of antimicrobial nanocomposites.

4.52.3 Vapor and Gas Scavengers When a product is packaged in air, oxygen and H2O vapor trapped in the package headspace may be undesirable to storage stability. Other food products may generate vapors during storage that can accelerate the deteriorative process (e.g., ethylene in certain fruits). Scavengers are reactive components in AFP that actively remove these undesirable gaseous compounds from the package headspace

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to confer product stability and thereby increase its shelf-life. The scavenger may be an integral part of the packaging structure or standalone system that can be deliberately attached to the interior of a package.

4.52.3.1

Oxygen Scavenger

Oxygen is deleterious for most food products as it can trigger deteriorative reactions, such as oxidative rancidity, discoloration, and nutrient degradation. Moreover, oxygen also supports the growth of aerobic bacteria, yeast, and molds, causing food spoilage. In many MAPs, the primary goal is to eliminate or reduce oxygen concentration to slow down these deteriorative reactions and inhibit microbial proliferation, thereby increasing the product shelf-life. Many oxygen scavengers are based on enclosing reactive com­ pounds within porous sachet, patch, or label. The most commonly used reagent here is iron powder. When the sachet is placed within a food package and exposed to the moisture present in the headspace, the iron powder undergoes rusting and consumes oxygen. One important aspect of oxygen scavenger is that it continuously removes oxygen dissolved in food, besides intercepting the incoming oxygen that permeated through the package, as long as the iron powder is not saturated. Iron powder has a relatively high oxygen scavenging capacity; one gram of iron is capable of removing approximately 300 ml of oxygen. Other oxygen scavenging reagents that may be used include ascorbic acid, catechol, and polyunsaturated fatty acids [1, 46]. Although sachet scavengers can be incorporated easily into food packages, they are susceptible to accidental ingestion and potential misuse by consumer (e.g., heating in microwave oven). Contamination of sachet content with food can be another drawback, especially for liquid and high-moisture products. To overcome these shortcomings, in some packaging systems, the scavenging components are incorporated into the packaging materials. One example here is UV-activated oxygen-scavenging packaging film. The scavenger is derived from unsaturated ethylene-based polymers (e.g., 1,2-polybutadienes, polyisoprene, and styrene–butadiene copolymer) that may be substituted with unsaturated fatty acids. Another co-component present is transition metal (e.g., cobalt(II) 2-ethylhexanoate and cobalt(II) neodecanoate) which catalyzes the oxidation of the unsaturated hydro­ carbons. A photosensitizer (e.g., benzophenone, acetophenone, and methyl ethyl ketone) is usually added to achieve a faster initiation reaction [47]. When the scavenger system is irradiated with UV radiation, typically with UVC of 200–280 nm, the catalyst triggers the auto-oxidation of the unsaturated hydrocarbon, consuming oxygen from the package headspace as the reaction progresses. Such system is reportedly capable of removing oxygen at a rate of 1–100 ml m–2 d–1 at 4 ˚C when measured 4 days after triggering, and 1–10 ml m–2 d–1 at room temperature 30 days after triggering [48]. Because UV-activated oxygen scavenger film does not require moisture as an activator, it can be used for dry food products, such as nuts, coffee, and other snack foods. Commercial oxygen scavenging films based on this mechanism is Cryovac® OS films. Another transition metal-catalyzed oxygen scavenger system is based on melt blending oxidizable polyamide, such as poly(m-xylyleneadipamide), with another polymer, such as poly(ethylene terephthalate), in the presence of cobalt (~200 ppm level) as a catalyst to trigger the oxidation of the nylon [49, 50]. Unlike the UV-activated system, this scavenging system is activated when the two polymers are heated and blended during extrusion. Thus, the oxygen scavenger polymer will need to be protected from oxygen during storage. Oxbar® of Constar belongs to this category.

4.52.3.2

Ethylene Scavenger

Titanium dioxide (TiO2) nanoparticles exhibit a number of unique properties that promise active and intelligent packaging applications. TiO2 is a ubiquitous white pigment used in paper, plastics, lipstick, toothpaste, pharmaceutical tablets, etc. It exists in three major different crystal structures, namely rutile, anatase, and brookite. In nanometer range, TiO2 takes on unusual properties and can be used in various applications, such as self-cleaning window glass, air and water purifications, antimicrobial coating, etc. Only rutile and anatase are important in photocatalysis. Photo-induced reactions by TiO2 are caused by the absorption of a photon with enough energy (>3.2 eV) which leads to charge separation due to the promotion of an electron from the valence band to the conduction band. This results in the formation of a hole (h+) in the valence band (Figure 7). In this photocatalytic reaction, usually electron transfer to oxygen is rate determining. The valence band hole (h+) has strong oxidation potential; it can

O2

hν Conduction band

e–

Reduction O2• – TiOH•+

Valence band

H+

Oxidation TiOH

Figure 7 Schematic representation of the overall photocatalytical process of TiO2. Adapted from Carp O, Huisman CL, and Reller A (2004) Photoinduced reactivity of titanium dioxide. Progress in Solid State Chemistry 32: 33–177.

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react with adsorbed water or surface-bound hydroxyl group to form hydroxyl radicals. The holes, •OH radicals, O2−, all play important roles on initiating many redox reactions [39, 51]. Decreasing TiO2 particle size increases the number of active surface sites and so does the surface charge-carrier transfer rate. Zhang et al. reported that there is an optimal particle size of about 10 nm for pure nanocrystalline TiO2 photocatalyst, which was attributed to the increased surface e− and h+ recombination rate as the particle size is reduced, which offsets the benefit of increased surface area. Dopants such as Fe3+ and Nb5+ have been shown to enhance catalytic reactivity [52]. TiO2 nanoparticles have been explored as a photocatalyst for the removal of ethylene in packaging of horticultural products. Ethylene is a natural plant hormone which plays an important role in the ripening and senescence of fruits and vegetables. Accumulation of this vapor can accelerate the deterioration and greatly reduce the storage life of perishable produce sensitive to it. Since horticultural products are often exposed to fluorescence light in the retail outlets and supermarkets, the light energy may be harnessed to actively remove ethylene vapor accumulated in the package headspace by TiO2-induced photocatalytic reaction. Unlike the conventional ethylene scavenger where the scavenging capacity is limited by the scavenger loading, the TiO2 catalytic approach has unlimited scavenger capacity. Based on this concept, Maneerat and Hayata [53] compared the efficacy of using TiO2 nano­ particle and microparticle-coated oriented polypropylene film, irradiated with UV black light or fluorescence light, to remove ethylene vapor in packaged horticultural products. The coating process involved dispersing the TiO2 particles in an organic solvent mixture (methyl ethyl ketone, toluene, isopropyl alcohol, and ethyl acetate) to form a TiO2 colloid suspension which was spread on the film directly and allowed to dry. They showed that the TiO2-coated film was capable of decomposing ethylene effectively and that TiO2 nanoparticles (anatase; 7 nm) provided a larger surface area for adsorption and decomposition of ethylene and higher UV radiation absorption than TiO2 microparticles (anatase; 5 mm) (Figure 8). The TiO2-coated film reduced ethylene in the package headspace by 88% when the package was exposed to UV irradiation at 25 ˚C, while 76% reduction was achieved using the fluorescent lamp at 5 oC (Figure 9). Reportedly, the exposure of tomato fruit to UV resulted in no symptoms of disorder when the fruits ripened [54]. These studies demonstrated the feasibility of using TiO2-coated packaging film as an ethylene scavenger for horticultural products. While utilization of light energy in extending produce shelf-life appears to be promising, expanded studies are needed to address the potential end-use challenges, such as the effect of available intensity and frequency of UV radiation, the influence of shadow effect, and the potential impact of food components that are UV sensitive. The photocatalytic reaction of TiO2 may be useful during postharvest preservation of fruits and vegetables to reduce the accumulation of ethylene, acetaldehyde, and ethanol in the storage atmosphere [55].

4.52.3.3

Controlled Release of Food Preservatives

To increase the shelf-stability, various polymers have been investigated for controlled release of active agents. One strategy here is to utilize the package itself as a carrier. In the converting industry, the addition of antioxidant to polyolefins is quite common in protecting the polymer from degradation during melt processing. The antioxidants added also confer protection against oxidation of food product contained by the packaging material. For instance, butylated hydroxytoluene (BHT) has been added to high-density polyethyelene film for the inner liner bag of breakfast cereals packaged in paper carton. The migration of BHT to the product can delay the oxidation reactions since the compound is an effective free radical scavenger. Although BHT and other synthetic phenolic compounds (e.g., butylated hydroxyanisole (BHA), tertiary butylhydroquinone (TBDQ), and propyl gallate) are effective in various

Ethylene concentration (ppm)

12 (a) 10 (b) 8 (c)

6 4 (f)

2 (e)

(d)

0 0

4

8

12

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20

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Time (h) Figure 8 Photodecomposition of ethylene using various oriented polypropylene films: (a) uncoated film, (b) 0.1% TiO2 nanoparticle-coated film, (c) 1% TiO2 nanoparticle-coated film, (d) 5% TiO2 nanoparticle-coated film, (e) 10% TiO2 nanoparticle-coated film, and (f) 10% TiO2 microparticle-coated film. Plots are recreated based on the original data reported by Maneerat C and Hayata Y (2008) Gas-phase photocatalytic oxidation of ethylene with TiO2-coated packaging film for horticultural products. Transactions of the ASABE 51: 163–168.

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Intergration of Biotechnologies

Ethylene concentration (ppm)

25 Uncoated film, black light, 25 °C

20 15

Uncoated film, fluorescent light, 5 °C

10

Coated film, black light, 25 °C

5

Coated film, fluorescent light, 5 °C

0 0

2

4

6

8

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12

14

Time (days) Figure 9 Ethylene level in the headspace of the packages uncoated and TiO2–coated film containing tomato fruits under black light illumination at 25 and 5 °C. Plots are recreated based on the original data reported by Maneerat C and Hayata Y (2008) Gas-phase photocatalytic oxidation of ethylene with TiO2­ coated packaging film for horticultural products. Transactions of the ASABE 51: 163–168.

food systems, their use is declining due to consumer preference for natural food ingredients [56]. Recently, Granda-Restrepo et al. [57] compared the efficacy of α-tocopherol, a natural antioxidant, with BHT and BHA for protecting whole milk powder, when these antioxidants were added to various multilayer co-extruded packaging films. The study showed that BHA and BHT tended to migrate to the whole milk powder quickly (71% released in 2 days and 91% in 4 days, respectively), while α-tocopherol migrating more gradually (64% in 30 days). The α-tocopherol contributed to the protection against vitamin A degradation. Gemili et al. [58] incorporated natural antioxidants, L-ascorbic acid or L-tyrosine, into cellulose acetate films. They dissolved cellulose acetate in acetone followed by the additional of aqueous L-ascorbic acid or L-tyrosine to form casting solutions. The solutions were casted on a polypropylene plate using a film applicator. They reported that increasing cellulose acetate content in the casting solution decreased the average pore size and porosity of the films, which reduced the diffusion rates of both antioxidants through the films. Partition coefficients of L-tyrosine were to be higher than those of L-ascorbic acid due to higher affinity of hydrophobic aromatic ring of this antioxidant to cellulose acetate which is also hydrophobic [58]. In another study, the same authors investigated antimicrobial cellulose acetate films incorporated with lysozyme [59]. Similarly, increasing the polymer concentration content in the casting solution decreased the porosity of the films and reduced the lysozyme release rate. By manipulating the morphology of cellulose acetate films, lysozyme activities can be adjusted for direct contact antimicrobial food packaging applications.

4.52.4 Intelligent Packaging Recent consumer demand for minimally processed and preservative-free food products, together with more centralized food production are some of the factors that contribute to increased safety challenges in food distribution chains. Thus, there is a sustained interest from food producers, retailers, and consumers to reliably and accurately evaluate the quality of food products. Intelligent packaging can meet these requirements by interacting with the packaged products and provide close to real-time feedbacks on chemical, microbiological, and physiological states of the packaged food. The interaction of IFP device with food products may be based on direct contact or via the headspace gas composition.

4.52.4.1

Oxygen Indicator with Controlled Activation

Many food products are susceptible to oxidative degradation (e.g., rancidity, discoloration, nutrient loss, etc.), causing quality deterioration and shelf-life reduction. Due to the deleterious effect of oxygen, food producers are often interested to monitor oxygen level in the package during product distribution and storage. One of the oxygen detection technologies is based on exploiting photocatalytic properties of TiO2 nanoparticles. Lee et al. [60] developed a novel oxygen-sensing ink comprised of TiO2 nanopar­ ticles (UV photosensitizer), together with a mild reducing agent, to control the activation of methylene blue ink using UV radiation. Their method employed titania nanoparticles (80:20 anatase:rutile; about 30 nm in diameter) to photosensitize the reduction of methylene blue by triethanolamine in hydroxyethyl cellulose encapsulant. By irradiating the oxygen ink sensor with UV light, the methylene blue is activated to a colorless oxygen-sensitive form. Upon exposure to oxygen, the active indicator is oxidized to give blue coloration, the intensity of which is proportional to oxygen concentration. The mechanism of this reaction can be summarized in Figure 10. In this reaction UV illumination of the TiO2 semiconductor creates an electron–hole pair which rapidly oxidizes the sacrificial electron donor irreversibly. The photogenerated electrons then reduce methylene blue to a colorless state which is oxygen sensitive. Upon exposure to oxygen, the dye is oxidized and returned to its original color [60].

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hν TiO2*

TiO2

Sacrificial electron donor

Reduced methylene blue (colorless)

O2

Oxidized methylene blue (colored)

H2O

TiO (e–)

Oxidized sacrificial electron donor

Figure 10 Schematic representation of the UV-activated oxygen indicator involving TiO2 nanoparticles. Adapted from Lee S-K, Sheridan M, and Mills A (2005) Novel UV-activated colorimetric oxygen indicator. Chemistry of Materials 17: 2744–2751.

Unlike typical oxygen indicators which lack control of the reaction starting point, this approach is more robust because the indicator is in a stable state prior to activation. The approach also provides a more consistent reading of the color response as the sensor can be activated just before use. It is also possible to activate the indicator in situ within a sealed package, provided that the package is transparent to UV radiation. The indicator can also be reset for repeated use by irradiating the tested sensor with UV light, as long as the reducing agent is not depleted. The bandgap of anatase TiO2 is 3.2 eV, implying that photons with wavelength of smaller than 388 nm are capable of activating the oxygen indicator system. This characteristic can sometime present a problem during end use when the package is exposed to fluorescent tubes in the food display cabinets and shelves, since some of these light sources have an emission peak in UVA region (320–400 nm) of the electromagnetic spectrum. This is undesirable since the indicator may be re­ activated uncontrollably on the shelf. To address this problem, Mills and Hazafy [61] developed an oxygen indicator ink systems similar to the ones described above, except that nanoparticulate tin(IV) oxide was used as the semiconductor photosensitizer, instead of TiO2. Here, the bandgap for SnO2 is 3.65 eV, corresponding to 340 nm. With SnO2 as a photosensitizer, the indicator can only be activated by UVB light (280–320 nm), which is largely absent in light sources commonly used in food display cabinets. This type of oxygen indicator may be useful for MAP to provide a real-time indication on the integrity of the package. Besides, by covering the oxygen indicator with a series of barrier films of different thicknesses (Figure 11) or different oxygen permeability coefficients, it may be possible to develop an indicator which reflects the duration of oxygen exposure. Here, indicators covered by thicker films or films of higher oxygen barrier properties will turn blue slower than those of thinner thickness and lower oxygen permeability. This low-cost and printable oxygen indicator may be useful for the identification of expired products. As shown in Figure 7, the photocatalytic reaction resulted in electrons which reduce the adsorbed oxygen to generate superoxide ions (O2•−) which in turn reduce to hydrogen peroxide and then water. Thus, one consequence of the photocatalytic reaction is the consumption of molecular oxygen. Xiao-e et al. tested the oxygen removal efficacy of UV-illuminated nanocrystalline TiO2 coated on glass and plastic substrates. Efficient deoxygenation was achieved in a closed photoelectrochemical cell for a variety of different TiO2/polymer nanocomposites. Under aerobic conditions, first-order oxygen reduction reaction was reported with a rate constant of 70 s-1 [62]. This shows that the TiO2 system may be promising for active oxygen removal to achieve a reduced oxygen environment such as one in MAP.

Normalized absorbance

1.0 0 µm 15 35 60 130 230

0.8

0.6

0.4

0.2

0 0

10000

20000

30000

40000

Time (s) Figure 11 Normalized absorbance at 610 nm for UV-activated oxygen indicator covered with poly(ethylene terephthalate) films of different thicknesses. Plots are created based on the original data published by Lee S-K, Sheridan M, and Mills A (2005) Novel UV-activated colorimetric oxygen indicator. Chemistry of Materials 17: 2744–2751.

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Intergration of Biotechnologies

4.52.4.2

Quality Indicator

With increasing consumer demand for minimally processed and mildly preserved ready-to-eat foods, safety is becoming more critical due to reduced product stability. Thus, there is a sustained interest from producers, retailers, consumers, and food safety agencies to accurately determine food product quality. To this end, intelligent packaging systems that are capable of providing information on product quality, in real time, have been developed. One approach here is to apply indicators to food packaging that changes color upon reacting with compounds which are associated with the deteriorative reactions in food to provide visual cue on product freshness.

4.52.4.2.1

Indicator to monitor microbial activities

Many food-spoilage reactions result in changes in pH. Based on this phenomenon, a number of studies have leveraged the application of pH-sensitive dyes as quality indicators in intelligent packaging. For instance, Nopwinyuwong et al. [63] developed a colorimetric indicator to detect the spoilage of an intermediate-moisture dessert product, by using pH reagents which react with CO2 produced by spoilage microorganisms. The indicator coating solution was comprised of methylcellulose binder, poly(ethylene glycol) 400 plasticizer, bromothylmol blue, and methyl red as pH indicators. The solution was cast onto nylon/LLDPE support film, allowed to dry and then followed by applying another cellulose-based topcoat. Placed within a glass jar that contained an intermediate moisture content dessert product, the indicator strip changed from light green to orange-red color, as the product underwent spoilage with concomitant release of CO2 into the headspace (Figure 12). The reaction is based on the formation of carbonic acid that dissociated in water to give hydroxonium ions, which in turn reacted with the pH reagent [63]. Accordingly, the indicator is suitable only for intermediate to high-moisture content products. Since the solubility of CO2 in water decreases with increasing temperature, the color response of the detector is also expected to be temperature dependent. In uncontrolled environ­ ments, the reliability of such indicators may be problematic.

4.52.4.2.2

Indicator for detecting meat spoilage

A similar quality indicator concept was adopted by Pacquit et al. [64, 65] for intelligent packaging of raw fish. Using pH indicating disposable labels, they monitored the release of alkaline volatile amines (e.g., trimethylamine, dimethylamine, and ammonia) as fish samples spoil. The label was prepared by entrapping within cellulose acetate matrix a bromocresol green pH-sensitive dye that changes color when it reacts with the volatile amines. A quarternary ammonium salt was added to prevent leaching of the dye.

Light orange Green

Orange-Red

Orange

Dark yellow

Yellow

Red 3.5

8 Mesophililc bacteria

7

3

Yeast and mold Osmophilic yeast

2.5

Carbon dioxide log cfu g–1

5 2 4 1.5 3

Concentration (% v/v)

6

1 2 0.5

1 0

0 0

2

4

6

8

10

Time (day) Figure 12 Changes in microbial counts of a dessert product (500 g), golden drop, packaged in 1000 ml glass jar. The right vertical axis is the corresponding CO2 concentration in the headspace within the package during storage. Plots are recreated based on the original data from Nopwinyuwong A, Trevanich S, and Suppakul P (2010) Development of a novel colorimetric indicator label for monitoring freshness of intermediate-moisture dessert spoilage. Talanta 81: 1126–1132.

Active and Intelligent Packaging Materials

Yellow Light green

Green Blue green

Log count (cfu g–1)

Blue

Total viable count

1.6

7 6

1.4 Pseudomonas

5

1.2

4 3

Indicator response

1.0

2 0

10

20

30

40

50

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Normalized indicator response (a.u.)

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Blue

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Time (h) Figure 13 Correlation of bacterial growth and fish quality indicator in whiting filet samples at 21 °C. Adapted from Pacquit A, et al. (2007) Development of a smart packaging for the monitoring of fish spoilage. Food Chemistry 102: 466–470.

Indicator solutions were prepared in water and cast onto an optically clear poly(ethylene terephthalate) sheet and allowed to dry. The indicator labels were sandwiched between polytetrafluoroethylene gas permeable membranes to protect the indicator from water vapor condensates. The color response was correlated with changing microbial load (total viable count and Pseudomonas spp.). The sensor thus can be used to track the increase in volatile amines in the package headspace (Figure 13). The response time of color change was found to be relative humidity dependent as the protonation–deprotonation of dye requires a proton transport medium between the acidic dye (proton donor) and basic ammonia (proton acceptor) [65]. Nevertheless, in sealed package for raw fish, this may not present a limitation since the humidity will remain relatively constant. Raw fish quality indicators which are based on similar concept are already commercially available, such as FreshTag® from COX Technologies. Smolander et al. [66] developed an innovative approach for monitoring the freshness of poultry meat. During spoilage of meat, H2S is produced by microorganisms such as pseudomonas, psychrotrophic anaerobic clostridia, Enterobacter, and Alteromonas. By using myoglobin as a detector for H2S, these investigators correlated the color change of the indicator with degree of spoilage in raw poultry samples packaged in modified atmosphere packages flushed with 40% CO2/60% N2. In vitro testing, when exposed to H2S, indicators with 0.5 mg/indicator resulted in most prominent color change from brown to green. When 2 mg/indicator concentration was used, the indicator first turned bright red and thereafter green. However, the total change of color, as measured by ΔE = (ΔL2 + Δa2 + Δb2)1/2 (where L, a, and b are HunterLab lightness, green-red, and blue-yellow tristimulus values, respectively), was highest when 4.0 mg/indicator concentration was used, indicating that higher myoglobin concentration may be more suitable if a indicator color change were measured using an instrument. During in vivo testing of MAP-packaged meat, the color response was reportedly dependent on oxygen concentration. In packages where O2 concentration was low at around 1%, the myoglobin turned brown to bright red as H2S concentration increased. When the packages were punctured at the end of the experiment, the color of the indicator turned green due to the reaction with atmospheric oxygen to form sulphymyoglobin. The indicator may be useful to detect the onset of spoilage in raw poultry meat.

4.52.4.2.3

Ripeness indicator technologies for fruits

Although quality of food products usually deteriorates with the passage of time, others improve during storage and eventually become unacceptable. Many fruits belong to the latter category. For instance, pears are harvested before ripe and allowed to undergo postharvest ripening, during which the product can exhibit various organoleptic properties, from crisp/sour to soft/juicy. Because this ripening process reveals little visual cue, the consumer tends to rely on pressing the fruit and/or sniffing for aroma to evaluate the state of ripeness. Such actions inevitably will result in product damage at the retail level. To overcome this problem, various ripeness indicators have been developed. One patent application discloses a general process to employ a visual indicator that reflects the maturity of maturing products, such as incorporating the indicator in a label and adhering to the maturing product. The indicator chemistries may be based on diffusion technology, oxidative reactions, silver salt redox reactions, enzymatic reactions, and/or electronic exposure indicator [67]. One such commercial sensor is known as ripeSense®, developed in New Zealand by Jenkins Group (self-adhesive lebels supplier) and HortResearch (New Zealand Crop Research Institute). The indicator label is attached inside the lid of a transparent thermoformed clamshell packaging which holds four pears. A product label is printed with an indicator scale ranging from red (crisp), orange (firm), and yellow (juicy).

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Intergration of Biotechnologies

The proprietary reagent in the indicator reacts with the aroma compounds released by pear as it ripens. The sensor is initially red in color, but changes to orange and finally yellow as it reacts with the aroma compounds given off by the fruit. By visually evaluating the color of the indicator label, consumers can select fruits of various ripeness degrees that match their preference and be able to estimate the remaining product shelf-life. For instance, if the sensor is red, the fruit is at the beginning of its ripening process and has a longer shelf-life compared to those with yellow indicator. By knowing the degree the ripeness, the indicator also allows producer and consumer to decide when to slow down the ripening process by refrigeration once it has reached the desired degree of ripeness [68]. Similar indicators have been developed for kiwifruit, melon, mango, and avocado. In one patent application, a method was disclosed to apply ripeness indicator printed on small label that can be adhered to the skin of a fruit to better present the condition of the produce and avoid possible adverse effects from local air convention currents [67]. Another patent application disclosed a similar approach based on ethylene detection involving color change of KMnO4 and molybdenum chemistries. Here, ethylene is readily oxidized by KMnO4 (purple) to form manganese oxide (brown) and ethylene glycol (i.e., Baeyer test to detect unsaturated hydrocarbons). Ethylene can also reduce ammonium molybdate ((NH4)6MO7O24) (yellow) catalyzed by palladium sulfate to MO3O8 (blue) [69]. These color changes form the basis of ethylene detection, which is one of the vapors given off during the ripening of fruits.

4.52.5 Conclusion: Acceptance and Safety issues of AFP and IFP In active AFP, the presence of chemicals in close proximity to the food products, potential misuse, and accidental ingestion may cause consumer and legislative concerns. Innovative packaging design to avoid these problems, such as integrating the active components within the packaging structure, may help address these concerns. Another aspect of quality indicator is potential false negative and positive responses, which will have safety and cost implications. The use of an intelligent device, in conjunction with conventional open dating system, may be still necessary to avoid this potential issue. For simplicity, in many studies, antimicrobial AFPs are often tested using synthetic media. However, food components may affect the activity of antimicrobial in many ways. For instance, silver-substituted zeolites are less active in nutrient-rich media, since lysine, sulfate, sulfides, and other sulfur-containing amino acids can interact with silver and weaken its antimicrobial activities [29]. Consider chitosan as another example; due to its reactive polycationic properties, when chitosan film is applied to food products, its interactions with other food components such as proteins, fats, and other anionic substances in foods are highly probable, and may weaken its efficacy. Thus, antimicrobial properties demonstrated in laboratory using growth media need to be further tested in real products to elucidate its efficacy, under conditions that are realistic to the distribution chain. Because of their intentional interaction with food, AFP and IFP may pose new challenges in terms of evaluating their safety due to the absence or inadequacy of existing standards that are established for conventional packaging systems. While the use of nanomaterials may result in enhanced packaging performance, concerns have been raised in terms of their safety, especially on whether any components of the packaging material migrate to food and whether the migrant substances are safe. Because nano­ sized particles have much greater surface area than microsized particles, they tend to be more chemically active. Thus, assessment on the interaction of a nanomaterial with biological systems in terms of toxicity will not be identical as with their bulk counterparts [70]. The major challenge to the risk assessment of nanomaterials is that such data are not widely available. These uncertainties and lack of a clear communication of risks and benefits can generate concerns among the public. Adding to this complication, food and packaging safety regulations varied from jurisdiction to jurisdiction. For instance, compared to Japan, USA, and Australia, the use of AFP and IFP in the European market is less receptive due to the more stringent European regulations in food packaging. This trend may also relate to the more conservative consumer behaviors in Europe regarding innovations in food [71]. From a consumer acceptance standpoint, the perception and willingness to adapt new technologies will be important determi­ nants for success implementation of AFP and IFP. Therefore, there is a need for caution in proceeding with introducing new packaging technologies. Ongoing public education by the food industry will be essential to the process. Besides consumer perception, cultural influence in different marketplace will also affect the acceptability of nanotechnology-driven products. For instance, a study reported that the Swiss public tends to be less receptive to the use of nanotechnologies in food than populations in China [72]. While there are many new developments in AFP and IFP technologies, ultimately, they must be accepted by the end users (consumers, retailers, and producers) and approved by food and health legislative agencies before they would gain widespread acceptance.

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