Antioxidative Packaging System

CHAPTER

Antioxidative Packaging System

6 Dong Sun Lee

Department of Food Science and Biotechnology, Kyungnam University, Masanhappo-gu, Changwon, South Korea

CHAPTER OUTLINE Introduction ............................................................................................................111 Exclusion of oxygen ................................................................................................112 Antioxidant packaging.............................................................................................116 Controlled release in antioxidant packaging .............................................................123 Conclusion .............................................................................................................126 References .............................................................................................................126

Introduction Oxidation is often a primary quality factor limiting the shelf life of a wide variety of foods. Compared to microbial spoilage, chemical oxidation is a slower process and is dominant in dry and/or fatty foods. The oxidation reaction starts combining with unsaturated fatty acids in the presence of a catalyst, such as iron, copper, enzymes, heat, or light. It is a chain reaction consisting of initial, auto-oxidative propagation and final termination stages. Once it has started, self-acceleration of the process dominates. Through the stages of oxidation, conjugated dienes, hydroperoxides, alkanes, alkenes, aldehydes, and ketones are produced to give rancid odors and flavors finally reacting with other functional groups to damage the physical properties of foods. Particularly, cross-linking of aldehydes with amino groups in proteins may cause structural damage and textural change. Inhibition of lipid oxidation with antioxidant agents or active packaging is of great importance in protecting foodstuffs with high amounts of unsaturated fatty acids from possible quality deterioration and providing the required shelf life.

Innovations in Food Packaging. DOI: http://dx.doi.org/10.1016/B978-0-12-394601-0.00006-0 © 2014 Elsevier Ltd. All rights reserved.

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The strategy to protect the food from oxidation in packaging has been removal and exclusion of oxygen by oxygen scavengers or a barrier layer and delivery of antioxidants to the food surface through slow release from the packaging material. Protective edible coatings may also be used as an oxygen barrier layer and medium of antioxidant delivery. Scalping of catalytic compounds from food onto the packaging layer may also be a possible method for certain foods, but it has not yet been tried notably. Light-barrier packaging is also known to have the effect of reducing oxidation in packaged foods. In terms of active packaging interacting positively with the environment and food, oxygen control inside the container and designed antioxidant delivery to food are the two major methods and thus are discussed in this chapter. The main characteristics of passive packaging, such as providing good oxygen and light barriers, are assumed to be provided as the basis for the active packaging.

Exclusion of oxygen Because oxygen is a critical reactant for oxidation reaction, removal of oxygen from the package headspace inhibits or stops its progress, helping to prevent accumulation of reaction products such as off-flavor compounds. Figure 6.1 shows the general dependence of oxidation rate on oxygen concentration in the headspace of a food package, which is commonly described as Ro 5

V m ½O2  K 1 ½O2 

where Ro is oxidation rate, [O2] is oxygen concentration, and Vm and K are constant. 1 0.8

Relative rate

112

0.6 0.4 0.2 0

0

5

10

15

20

O2 concentration (%)

FIGURE 6.1 General dependence of oxidation rate of fatty foods on O2 concentration in food package.

Exclusion of oxygen

The kinetic dependence of oxidation in Figure 6.1 illustrates the benefits of persistent oxygen exclusion in food packages by vacuum, modified atmosphere (MA), and oxygen scavenger packaging. Oxygen removal achieves the desired preservation of foods, and its effectiveness can be enhanced by use of oxygen scavengers. Vacuum or MA packaging of fatty foods usually attains an oxygen-free internal atmosphere instantly by mechanical vacuuming or gas flushing. Even though vacuum and MA conditions are ensured by hermetic gas-barrier packaging materials, residual oxygen remaining after mechanical packaging operation might have the chance to initiate oxidation. An oxygen scavenger can be added to remove the residual oxygen within a short time. Initially available oxygen levels of 0.3 to 3.0% are often encountered in vacuum and MA packages but can be removed to attain oxygen concentrations below 0.01% rather quickly (e.g., 1 hour) through the addition of oxygen scavengers to the package (Day, 2008; McMillin, 2008). A properly designed oxygen scavenger can remove even the smallest amount of oxygen potentially permeated through a package layer. Basically, oxygen scavengers are inorganic or organic oxidizing substrates. Determining the amount of oxygen scavenger to be included in a food package should take into account the oxygen gas initially present inside it (and in the food matrix) and permeated through its wall during the shelf life. The stoichiometric relationship of thermodynamically spontaneous reactions in the scavenger system must then be considered to estimate the capacity of oxygen absorption. For example, 1 gram of iron in an iron-based oxygen scavenger can be estimated to absorb about 300 mL of oxygen with a supply of 0.43 gram water, based on the stoichiometric relation of iron oxidation: 4Fe 1 3O2 1 6H2 O-4FeðOHÞ3

(6.1)

As with the total capacity of oxygen absorption, the oxygen absorption rate is important when the speed of oxygen removal is important. The supply of oxygen or other required reactant such as moisture in an iron-based system may control the rate of oxygen absorption. Particle size or surface area of the reactant powder may be used to control the system. The film of the scavenger sachet can also be tailored with regard to gas and moisture permeability to control the supply of oxygen or other vapors as required. In the case of iron-based systems, the moisture supply from the food or self-supply from the scavenger component may be utilized to refine control of the oxygen absorption rate. Sometimes catalysts may be incorporated in the system for the purpose of controlling the reaction. Other subsidiary reactions, direct or indirect, with reactants or additives should be examined when quantifying the actual oxygen absorption rate; for example, carbon dioxide in the package atmosphere hinders the oxygen absorption of iron-based scavengers. Table 6.1 summarizes the classes of available oxygen scavenger systems. The most versatile oxygen scavengers are based on oxidation of iron powder and are available mostly in the form of sachets and adhesive labels. However, in some oxygen scavengers, iron compounds are incorporated into the polymer matrix of

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Table 6.1 Forms of Oxygen Scavenger with Oxidation Mechanisms Form

Mechanism

Sachet, label, or patch of iron powder alone or in moist paste Sachet or label of organic substrates (ascorbates, sulfites, catechol, gallic acid, polyunsaturated fatty acids, glycols) Polymer matrix (film, walls of tray and bottle, bottle closure and liner) incorporated with iron compound Polymer laminate or blend incorporated with organic substrates or reducible compounds Reactive polymer

Oxidation of iron with supply of moisture and action of optional catalysts Oxidation of organic substrates with metallic catalysis

Polymer matrix with photosensitive dye/ organic compound Immobilized enzymes Immobilized microorganisms Hydrogen gas flushed into package Polymer surface incorporating nanocrystalline TiO2

Oxidation of iron compound activated by moisture Oxidation of reducible substrates or medium (such as rubber) Oxidation of polymer components (mostly unsaturated groups) with metallic catalysis Oxidation of anthraquinone dye with UV light Oxidases in contact with specifically oxidizable substrates, glucose, or ethanol Yeast or aerobic bacterial spores in polymer structure consume oxygen Oxidation of hydrogen to water with catalyst (palladium/alumina) Oxidation of organic volatiles by photocatalytic activity of titania

Source: Compiled from Cruz et al. (2012), Lee (2011), and Rooney (2005).

the bottle wall or liner, either in the form of a simple polymer blend or as a multilayer structure. Oxidation of other metals such as magnesium has been considered as another mechanism of inorganic oxygen scavenging (Waite, 2003). Because metal-based oxygen scavengers present several problems and limitations (e.g., being detected in metal detectors, causing arcing in microwave heating), organic-type scavengers have been studied. Organic substrates that are easy to oxidize, such as ascorbic acid, catechol, and polyunsaturated fatty acids, have been used for sachets and labels and in polymer blends. The system may still contain a small amount of catalyst, such as a transition metal to control the oxidation of the substrates. Two moles of ascorbic acid are required for its oxidation to dehydroascorbic acid with consumption of 1 mol oxygen in a copper catalyst system (Cruz et al., 2012). Some organic oxygen scavengers produce carbon dioxide from their oxidation reaction, preventing package collapse due to oxygen removal by the scavenger. Enzymes of glucose and ethanol oxidases have also been proposed as oxygen scavenging systems in the presence of their respective substrates. Recently, there have been attempts to trap aerobic microorganisms with nutrients

Exclusion of oxygen

in polymer structures to scavenge oxygen residing in the headspace or permeating from the outside through their respiration activity (Altieri et al., 2004; Anthierens et al., 2011). Heat-resistant spores could survive the high temperature of polymer fabrication to retain their activity of respiration through rehydration and germination. Like iron-based scavengers, they also require moisture for the oxidation reaction except catechol, polyunsaturated fatty acids, and alcohol oxidase (Ahvenainen, 2003; Vermeiren et al., 1999). Scavengers such as polyunsaturated fatty acids that do not require moisture are suitable for dry foods. Moisture is often supplied to the scavenger system from the humid headspace atmosphere. Organic oxygen absorbers can be used for packaging with a high CO2 concentration where iron-based scavengers are not effective (Rooney, 2005). Modification of polymer composition by incorporating unsaturated functional groups in the structure also provides oxygen absorption (Ferrari et al., 2009; Rooney, 2005). Oxidation of unsaturated hydrocarbons in the polymer structure can act to scavenge oxygen from the package headspace. The autoxidation reaction in the polymer is triggered by ultraviolet (UV) light with the aid of transition metal catalysts. Unsaturated hydrocarbons can be combined with common thermoplastics and fabricated into the desired form (Lo´pez-Rubio et al., 2004). Particularly, polyethylene terephthalate (PET) is compounded or layered with the scavenger to provide an improved oxygen barrier in beer bottles (Galdi et al., 2008; Vermeiren et al., 2003). Some polyamides have been proposed as candidates for oxygen-consuming components in the recent patent applications. The oxygen-scavenging layer should be protected from external atmospheric air and should not release any offensive byproducts after reaction with oxygen that may be released into the food contained in the package (Lo´pez-Rubio et al., 2004). A small percentage of hydrogen gas flushed into the package headspace can scavenge oxygen to form water with oxidation and catalysis by palladium-coated surface (Rooney, 2005). The unstable nature of hydrogen makes such a hydrogen gas system difficult to rely upon in practical packaging situations. Recently, there has been some interest in the oxygen-scavenging function of polymers incorporating nanocrystalline titania (de Azeredo, 2009; Mills et al., 2006; Xiao-e et al., 2004). Those polymer films can scavenge oxygen under UV light. The photocatalytic activity of nanocrystalline TiO2 on polymer surfaces can oxidize organic substances to consume oxygen and produce carbon dioxide. A proper supply of organic substance and UV illumination are necessary for proper functioning of the oxygen-scavenging mechanism. Film incorporating photocatalytic titanium is known to inactivate microorganisms and thus has potential as antimicrobial packaging material. Oxygen scavenging is simply the blocking of oxidation reactions by eliminating a substrate in the reaction. The major portion of oxygen absorption in lipid autoxidation occurs in the propagation step (Nawar, 1996). Metal catalysis, light exposure, and photosensitization of natural pigments can boost the initiation step before the propagation step starts. Even though the dependence of oxidation on oxygen concentration (see Figure 6.1) generally holds, internal and external

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factors such as temperature, oxygen concentration, light, and metal catalysts, which may be affected by packaging conditions, can in turn affect the state of oxygen and its effect on oxidation rate (Choe and Min, 2006).

Antioxidant packaging As a way to inhibit oxidation through formulation of the food itself, antioxidants have been added to food products. Another innovative attractive approach for preventing oxidation in packaged foods is incorporation of antioxidant in packaging material and its release to the contained food in a controlled manner. Many different antioxidants synthetic or natural have been tried or tested for inclusion into food packaging materials. Table 6.2 summarizes antioxidant packaging research published in scientific journals. A recent trend is a preference for antioxidants of natural origin for inclusion in packaging structures due to consumers’ perceptions regarding the safety and acceptance of such antioxidants compared to synthetic ones. For a long time, the packaging industry has been using synthetic antioxidants such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and Irganoxs for the purpose of protecting the polymer. Antioxidants added during processing stabilized the plastic packaging material throughout its life time. Antioxidants incorporated into plastic packaging materials may also work to reduce oxygen passage through the plastic layer by reacting with permeating oxygen, thus helping to suppress the oxidative degradation of food (Lo´pez-Rubio et al., 2004). Antioxidants located in the plastic polymer can migrate directly into liquids or indirectly into solid foods and prevent oxidation in addition to their originally intended protective function (Lee et al., 2004b; Lundback et al., 2006a; Miltz et al., 1988; Wessling et al., 2001). Because synthetic antioxidants that migrate into foods may induce off-flavors in the packaged food, particularly drinking water, natural antioxidants such as α-tocopherol have been sought as replacements for the synthetic ones. Natural tocopherol antioxidants were originally added with the intention of protecting the polymer while producing less off-flavor and thus improving the sensory quality of the food, and they currently are considered to be an important ingredient for antioxidant packaging (Ho et al., 1994; Zambetti et al., 1995). With increased attention directed toward the protection of packaged food products by migrating antioxidants, a variety of methods to incorporate antioxidants, such as encapsulation, absorption, and blending, have been developed to enhance the effectiveness of antioxidant packaging systems (Gargiulo et al., 2012; Jin et al., 2009; Koontz et al., 2010b; Sonkaew et al., 2012). Basically, any antioxidant packaging is based on the release of antioxidants from the packaging material to the contained food even though some antioxidant

Antioxidant packaging

Table 6.2 Antioxidants Incorporated into Packaging Materials to Improve Food Preservation Package or Contact Condition

Antioxidant

Matrix

BHA, BHT

HDPE, LDPE, Surlyn/ EVAs, PET, PVC, PP, PS

Tocopherol(s)

LDPE, PE, LLDPE, LDPE/PP, Surlyn/ EVAs, PP, EVOH, PLA, chitosan

Irganox 1010, Irganox 1076, Irganox 1081, Santonox R, Lowninox 22M46 Quercetin

Surlyn/EVAs, LDPE, branched PE, linear PE

Tray base for turkey meat; film pieces in 95% and 10% ethanol; cut plaques in water

LDPE, LLDPE, EVOH, EVA, PP

Film pieces in 95% ethanol; bag for fried peanuts; films in sunflower oil; films in coconut oil; wrap of brined sardine

Pouch for oatmeal; pouch for freezedried food; pouch filled with soybean oil; bottle of refined vegetable oils; vacuumed film package of cheese Pouch for oatmeal; pouch for freezedried food; pouch for whole-milk powder; tray base for turkey meat; film bag filled with 95% ethanol; film bag filled with corn oil; film pieces in linoleic acid emulsion; films in coconut oil, soybean oil, or ethanol; flask of distilled water

Refs. Lee et al. (2004b), Miltz et al. (1988), Phoopuritham et al. (2012), Soto-Cantu et al. (2008), Tawfik & Huyghebaert (1999), Wessling et al. (2000, 2001) Blanco-Fernandez et al. (2012), Byun et al. (2010), Chen et al. (2012), Graciano-Verdugo et al. (2010), Granda-Restrepo et al. (2009), Heirlings et al. (2004), Koontz et al. (2010b), Lee et al. (2004b), Del Mar Castro López et al. (2012), Manzanarez-López et al. (2011), Pettersen et al. (2004), Siro et al. (2006), Wessling et al. (2000, 2001), Zhu et al. (2012a) Galotto et al. (2011), Pettersen et al. (2004), Lundback et al. (2006a,b) Chen et al. (2012), Koontz et al. (2010b), López-deDicastillo et al. (2012a,b)

(Continued)

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Table 6.2 (Continued)

Antioxidant

Matrix

Catechin

EVOH

Ascorbic acid

Cellulose acetate, EVOH

Ascorbyl dipalmitate Curcumin

Methylcellulose Methylcellulose

Ferulic acid

EVOH

Eugenol

LLDPE/zein/LLDPE

Sesamol

HDPE

Tyrosine

Cellulose acetate

Plant extracts

LDPE, PP, EVOH, methylcellulose, carboxymethylcellulose nanocomposite, gelatin

Package or Contact Condition Bag for fried peanuts; films in sunflower oil Film contacting water on single side; wrap of brined sardine Film in surfactant solution Film in surfactant solution Wrap of brined sardine Vacuum packaging of beef patties Film strip in headspace of linoleic acid vial; pouch for oat cereal Film contacting water on single side Pouch filled with soybean oil; film strip in headspace of vial with oxidation-sensitive foods; test trap cell of films; films in sodium phosphate buffer; package of lamb; wrap of blue shark; wrap of brined sardine; cover film

Refs. López-de-Dicastillo et al. (2012b) Gemili et al. (2010), López-de-Dicastillo et al. (2012a) Sonkaew et al. (2012) Sonkaew et al. (2012) López-de-Dicastillo et al. (2012a) Park et al. (2012) Zhu et al. (2013)

Gemili et al. (2010) Camo et al., 2008, Colín-Chávez et al. (2012), de Abreu et al., (2011), Gimenez et al. (2011), Gutiérrez et al. (2012), López-de-Dicastillo et al. (2012a), Nerin et al. (2008), Pezo et al. (2008), Phoopuritham et al. (2012)

Abbreviations: BHA, butylated hydroxyanisole; BHT, butylated hydroxytoluene; EVA, ethylene vinyl acetate; EVOH, ethylene vinyl alcohol; HDPE, high-density polyethylene; LDPE low-density polyethylene; LLDPE, linear low-density polyethylene; PLA, polylactic acid; PP, polypropylene.

Antioxidant packaging

on the package surface may act to scavenge the free radicals in the package headspace. Antioxidants that are not volatile or are of low volatility, such as tocopherol and ascorbic acid, are suited for food structures such as liquids and semi-solids that come into direct contact with the package surface and absorb antioxidants released from the migrated agent (Gemili et al., 2010; Wessling et al., 1999) (Figure 6.2A). Packaging that comes into close contact with the food is required for the antioxidant function to be effective. Structures such as porous solid food can be protected by the migration of volatile active agents in antioxidant packaging systems (Figure 6.2B). Volatile antioxidants, such as natural essential oils, BHT, and sesamol, are thought to work through inhibition of gas-phase oxidation reactions with headspace free radicals and subsequent autooxidation in the food matrix with indirect migration (Camo et al., 2008; Nerin et al., 2008; Pezo et al., 2008; Zhu et al., 2013). Scavenging of vapor-phase radicals has been suggested as a possible mechanism. Multilayered film structures containing volatile ingredients in the inner middle layer have also been designed for tailored release to the package headspace through the outer ply (Park et al., 2012). Antioxidants of natural origin that have been studied in the development of antioxidant packaging include tocopherol, ascorbic acid, curcumin, tyrosine, essential oils and plant extracts of barley husks, borage, cinnamon, citronella, clove, ginger, green tea, marigold, murta leaves, rosemary, oregano, and thyme (Camo et al., 2008; Colı´n-Cha´vez et al., 2012; Gemili et al., 2010; Gimenez et al., 2011; Gutie´rrez et al., 2012; Nerin et al., 2008; Phoopuritham et al., 2012; Sonkaew et al., 2012; Wessling et al., 1999). While tocopherol, ascorbic acid, curcumin, tyrosine, and their derivatives are mostly nonvolatile, essential oils and extracts usually contain both volatile and nonvolatile components. Food packaging systems that include natural antioxidants have been studied for use with liquid linoleic acid, vegetable oils, cheese, chilled and frozen flesh foods, and cereal products. The most intensively studied natural antioxidant packaging system uses tocopherol, which has been incorporated into a wide variety of polymers in the concentration range of 0.004 to 4.0%. Various forms of packaging with tocopherol have been tested for their antioxidant effectiveness with many foods. In general, mostly positive effects of tocopherol-added plastic packaging on inhibition of lipid foods have been reported in the literature. However, the antioxidant effectiveness of the active packaging film varies depending on the packaged food, packaging material, and storage conditions. According to Wessling et al. (2000), LDPE film containing 0.34 % α-tocopherol inhibited the oxidation of a linoleic acid emulsion at 6 C under the conditions of dark and open-air exposure, whereas no significant retardation of oxidation was observed at 20 and 40 C. It was also reported by Lee et al.(2004b) that a laminated pouch consisting of a HDPE layer and a heat seal layer (Surlyn/EVAs) incorporated with α-tocopherol at the concentration of 0.007% failed to retard the oxidation of a dry solid food with water activity of 0.3 and containing 0.36% (w/w) linoleic acid at 45 C. When used as a pouch for oatmeal, LDPE film with 0.07% or 0.4% α-tocopherol added did not

119

Direction of antioxidant

´

 

Antioxidant compound



Direction of antioxidant

Package headspace



Package headspace

Antioxidant compound





Liquid food Package wall Diffusion in polymer Dissolution at package/liquid interface ´ Desorption at package/headspace interface Dispersion or diffusion in liquid Desorption at liquid/headspace Dispersion in the headspace (A) Direct contact system

 Package wall

Solid food

Diffusion in polymer Desorption at package/headspace interface Adsorption on food

(B) Indirect contact system

FIGURE 6.2 Release of antioxidant from packaging material to antioxidative food packaging system. Adapted from Arabi et al., 2012.

Antioxidant packaging

produce a clear effect on the production of volatile oxidation products in the food, and had lower antioxidant ability than film to which 0.03% BHT was added (Wessling et al., 2001). Coextruded multilayer film (HDPE/EVOH/LDPE) manufactured with an inner LDPE layer of 4% tocopherol and a light-barrier property could protect vitamins in packaged whole-milk powder better than film with 1.5% BHA or 1.5% BHT (Granda-Restrepo et al., 2009). LDPE film bags initially containing 1.9 and 3.0% α-tocopherol extended the shelf life of corn oil by 4 weeks at 30 C compared to the shelf life of 12 weeks for control bags without the antioxidant, based on the hexanal content (Graciano-Verdugo et al., 2010). Multilayer (PP/polyamide/PE) films with 0.01% α-tocopherol in the PE layer have been used as the base web of trays for frozen turkey meat and have been found to be helpful in retarding the oxidation progress of the meat for 12 months, as measured by thiobarbituric acid-reactive substances and hexanal content (Pettersen et al., 2004). Quercetin has been incorporated for protection of the tocopherol and polymer matrix during processing (Koontz et al., 2010a; Lo´pez-de-Dicastillo et al., 2012b). Because of the presence of a hydrophilic group in its molecular structure, quercetin can be readily incorporated into hydrophilic polymers such as EVOH and can be released upon aqueous or alcoholic food stimulation to some extent (Lo´pez-de-Dicastillo et al., 2010). Its nonvolatile property also aids in its retention in the polymer during high-temperature processing. Quercetin has thus emerged as a synergist or aid for effective release profiles and improved antioxidant ability in combination with tocopherol in dual-antioxidant packaging (Chen et al., 2012). Catechin, an active component of green tea, has properties similar to those of quercetin and was shown to be an effective antioxidant ingredient in active packaging to retard the oxidation of sunflower oil, fried peanuts, and brined sardines (Lo´pez-de-Dicastillo et al., 2012a,b). Other nonvolatile antioxidants of ascorbic acid, ascorbyl dipalmitate, ferulic acid, cucumin, and tyrosine incorporated into polymers such as EVOH, cellulose acetate, and methyl cellulose demonstrated antioxidant activity when they came in contact with food solutions or fatty foods (Gemili et al., 2010; Lo´pez-de-Dicastillo et al., 2012a; Sonkaew et al., 2012). Electrostatic interactions between the antioxidant and polymer have been used to explain the release rate of antioxidants into foods and thus the resulting antioxidant activity. Ascorbic acid antioxidants in film were found to be stable before their release, but upon release into aqueous food simulants they were unstable (Lo´pez-de-Dicastillo et al., 2012a). Maintaining a consistent level of antioxidant in the food matrix can be achieved by controlled release from the polymer film, which serves as an antioxidant reservoir. While the volatility of the antioxidants hinders their retention in plastic packaging materials fabricated through high-temperature polymer processing such as extrusion, the portion retained after processing may be more effective, in the gas phase, at reaching the sites of oxidation in the solid food matrix, thus contributing to inhibition of the oxidation progress. Synthetic antioxidants of BHA and BHT, which have been used to protect polymers during film manufacture, have some

121

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degree of volatility and could migrate to the contained solid food through the gas phase (Miltz et al., 1988; Wessling et al., 2001). Volatile extracts, essential oils, or active compounds of plants or spices have been incorporated into polymers and tested for their application to food packaging (Nerin et al., 2006; Park et al., 2012; Pezo et al., 2008). Antioxidant films from rosemary, oregano, clove, and cinnamon could play a role in inhibiting oxidative quality deterioration for systems with both indirect and direct contact between the packaged food and the film (Camo et al., 2008; Nerin et al., 2008; Phoopuritham et al., 2012). Because essential oils or extracts of plants or spices have the dual functions of antioxidation and microbial inhibition, plastic packaging materials with those components added may offer both antioxidant and antimicrobial activities. A multinational European research project, NAFISPACK (Natural Antimicrobials for Innovative and Safe Packaging), conducted by a consortium consisting of several research and industrial partners under the Seventh European Framework Programme for Research and Technological Development, has contributed to the development of antioxidant packaging using natural resources. It needs to be mentioned that some compounds of plants or spices may have pro-oxidant effects, thus reducing the antioxidant ability of active packaging materials incorporated with essential oils. Finding the optimal loading to release the desired concentration range to the food matrix is important in the design and application of antioxidant packaging incorporated with natural antioxidants. Volatile antioxidant compounds of natural sources have also been used as additive ingredients in food packaging materials. Sesamol, an active compound from sesame seeds, could be incorporated into the middle layer in multilayer coextruded films, such as LLDPE/HDPE/HDPE and HDPE/HDPE/EVA (Zhu et al., 2013). The retention of sesamol in the extruded film was found to be 0.06% and 0.12% from initial loadings of 0.1% and 0.2%, respectively, with 35 to 39% loss, which occurred due to vaporization to the air. Even though the highly volatile nature of sesamol resulted in fast release from the package to the headspace (completion within 25 hours at 10 C), the antioxidant action of sesamol-incorporated film continued over extended periods for a liquid food system of linoleic acid (10 days at 23 C) and solid food system of oat cereal (1 year at 23 C). Eugenol included in a multi-ply film has been shown to migrate to the atmospheric phase and retard oxidation of the packaged beef patties (Park et al., 2012). Edible films or coatings added with natural antioxidants have been applied to fresh produce and dried foods (Das et al., 2013; Gimenez et al., 2011; Han et al., 2008; Lin et al., 2008). While an edible coating on the fresh produce itself reduces respiration and water loss, it can become more helpful by inclusion of an antioxidant to preserve the antioxidant and biochemical quality of the produce. Edible coatings on dried fatty foods are expected to provide a barrier layer against moisture and oxygen but may also be designed to deliver antioxidants to the food surface in a controlled manner. An edible tocopherol-loaded chitosan film prepared for sustained release through chemical interactions between the aNH2

Controlled release in antioxidant packaging

groups of chitosan and the aOH groups of tocopherol could exhibit a radical scavenging activity for a long time (Blanco-Fernandez et al., 2012). Currently, there is considerable interest in biodegradable packaging due to environmental concerns, and incorporation of natural antioxidants into biodegradable polymers such as polylactic acid (PLA) has been studied to add value and compete better with synthetic polymers. One example is PLA with tocopherol incorporated (Arabi, 2012; Byun et al., 2010; Manzanarez-Lo´pez et al., 2011). Many natural essential oils contain ingredients having both antioxidant and antimicrobial activities (Sacchetti et al., 2005); therefore, packaging materials or coatings with natural essential oils incorporated may play the dual role of antioxidant and antimicrobial preservation (Abdollahi et al., 2012; Camo et al., 2008; Kanatt et al., 2012; Ponce et al., 2008). There is a need for active packaging that provides both antioxidant and antimicrobial functions. Foods susceptible to both microbial spoilage and oxidative deterioration may benefit from packaging with antioxidant and antimicrobial functions. Active compounds retaining both antioxidant and antimicrobial activity may be incorporated into the packaging matrix with the desired release profile. Or two or more compounds, each with antioxidant or antimicrobial activity, may be embedded together into the packaging layer. Lee et al. (2004a), for example, incorporated antimicrobial nisin and antioxidant α-tocopherol into a polymer matrix. Han et al. (2004) applied an edible coating consisting of antimicrobial chitosan and antioxidant α-tocopheryl acetate to frozen strawberries and raspberries. Siripatrawan and Harte (2010) fabricated antimicrobial and antioxidant chitosan film with the addition of antioxidant green tea extract.

Controlled release in antioxidant packaging Antioxidant released from antioxidant packaging to the food or contacting the food on the package surface directly or indirectly is thought to act to scavenge free radicals and singlet oxygen, inactivate metal ions, and stop the chain reactions (Choe and Min, 2009; Frankel, 1996; Pezo et al., 2008). The release rate or profile is reported to be a significant variable in controlling the antioxidant effectiveness of antioxidant packaging. Antioxidant packaging systems with tocopherol have been studied intensively in terms of controlled-release packaging. As shown in Figure 6.3, a supply of antioxidants through direct addition or antioxidant packaging can extend the induction period of oxidation products such as conjugated dienes or hydroperoxides. The induction period in fatty foods (usually equivalent to shelf life) is dependent on the supply of the tocopherol, typically showing a maximum peak during optimal supply (Zhu et al., 2012b). Too slow a release or too little addition does not provide sufficient antioxidant activity to protect fatty foods, and too fast a release or too much addition delivers more tocopherols than free radicals produced in early oxidation, thus resulting in the formation of dimers or other products from the excess tocopherols themselves. This

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CHAPTER 6 Antioxidative Packaging System

Induction period

800 Conjugated dienes or hydroperoxides (mmol/kg)

124

700 600 500 400

Release rate or initial supply amount

300 200 Induction 100 period 0

0

5

Control

10

Antioxidant addition or packaging

15 Time (days)

20

25

30

FIGURE 6.3 Effect of antioxidant addition or packaging on induction period of oxidation products as compared to control without any antioxidant. Inner panel shows general trend of induction period affected by supply of tocopherol to the food system through direct addition or release from antioxidant packaging.

phenomenon has led to development of the concept of a target release rate, which must be considered during antioxidant packaging design (Zhu et al., 2012b). Package variables such as the plastic material used, film thickness, and antioxidant loading can be used to control the release rate or profile of the antioxidant to the food system. The release profile of tocopherol from packaging polymer to fatty liquid foods is governed by its diffusion in the polymer, where diffusivity is an important independent variable. Mathematical models of diffusion are a valuable tool to predict the release profile into food systems, and appropriate equations for specific boundary conditions may be referred to (Crank, 1975; Piringer, 2000). An example analysis using the diffusion model has been reported by Lee and Yam (2013) and is presented in Figure 6.4. There seems to be a particular range of tocopherol loading and diffusivity that provides a longer shelf life. Loading that is too low (e.g., 50 μmol/kg) or diffusivity that is too low (e.g., 10217 m2/s) cannot provide a long enough shelf life. For a given film thickness and tocopherol loading, the diffusivity of tocopherol depends on the blending ratio of the polymer, which was found to be an important variable in controlling the induction period of linoleic acid systems (Zhu et al., 2012a). However, optimal diffusivity differs with tocopherol loading, as higher loading shifts the optimum diffusivity to the lower side (see Figure 6.4). Several attempts have been made to reduce tocopherol diffusivity in polymers (Gargiulo et al., 2012; Heirlings et al., 2004; Siro et al., 2006). According to the analysis of Figure 6.4, the advantage of low diffusivity may apply with high loading, but not when a small amount is included in the film. On the other hand, acceleration of tocopherol release with increased diffusivity could be achieved by the addition of a

Controlled release in antioxidant packaging

Shelf life (day)

30

Loading of 500 µmol/kg

Loading of 125 µmol/kg Loading of 50 µmol/kg

20

10 Zero loading 0 10–17

10–16

10–15

10–14

10–13

2

Diffusivity (m /s)

FIGURE 6.4 Estimated shelf life of oxidative food as the time for hydroperoxides to reach 41.25 mmol/ kg as a function of tocopherol diffusivity and loading in film of 76.2 μm thickness (based on the concentration in food of total releasable amount). Estimation is done by the method and conditions of Lee and Yam (2013).

chain extender, polypropylene glycol-block-polyethylene glycol-block-polypropylene glycol, into polypropylene film (Del Mar Castro Lo´pez et al., 2012). Even though trends like those shown in Figure 6.4 generally hold true for antioxidant packaging, a number of design variables can alter that picture. Tocopherol loading in the polymer, for example, may be restricted by migration levels permitted by food safety regulations and by its compatibility with the antioxidant. Tocopherol diffusivity is determined by the plastic polymer used, which should have the appropriate physical and barrier properties for practical applications. Commonly used LDPE and PP food packaging films have tocopherol diffusivity in the range of 10216 to 10214 m2/s at 30 to 40 C (Zhu et al., 2012a). Optimized antioxidant packaging can be achieved by looking into the dependence of the oxidation process on controllable design variables for given package conditions. For a film of given diffusivity, an optimum loading providing the longest shelf life can be sought. The various diffusivities for available plastic materials may be examined to find the best material that will provide the longest induction time or shelf life. Based on Figure 6.4, a low tocopherol loading would be desirable with a high-diffusivity film, and high tocopherol loading would be best with a low-diffusivity film. To a limited extent, film thickness can also be used as a variable for changing the release rate; however, in terms of controlled-release antioxidant packaging, a thicker film with higher loading would be costly. Polymer incorporated with antioxidant in antioxidant packaging serves as a good reservoir for supplying antioxidants to the food system, where the antioxidants interact with the food or degrade, thus inhibiting or retarding oxidation. Releasing the preserved antioxidant in a manner harmonized with food oxidation was found to be a way of maximizing the effectiveness of the antioxidant

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packaging system. Even though the usefulness of controlled release has been studied and confirmed for model food systems, still much work needs to be done with regard to its application in practical packaging of oxidation-sensitive foods. The kinetics of antioxidant delivery to the food, antioxidant degradation in the food, and oxidation as a function of real-time antioxidant concentration must all be considered in packaging design. Antioxidant packaging design can be optimized through collaboration of packaging material research and food preservation research.

CONCLUSION Oxidation is a major contributor to food quality deterioration, thus limiting shelf life. Its rate and extent can be reduced or stopped by antioxidative packaging systems. Natural antioxidants have been studied for their potential to be incorporated into packaging polymers and released to packaging contents in a controlled manners. Maximum effectiveness of antioxidative packaging systems can be achieved by tailoring the packaging design to release the incorporated antioxidant in balance with lipid oxidation, such that antioxidant diffusivity, loading, and package layer thickness are important variables. Oxygen exclusion by a barrier layer should be ensured or assumed for an effective system. Other functions such as antimicrobial activities offered by the same incorporated compound or other additives in the packaging may be added to make a packaging system more powerful when needed.

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