Applications of Plastic Films for Modified Atmosphere Packaging of Fruits and Vegetables: A Review

Transcription

1 Food Eng Rev (2009) 1: DOI /s Applications of Plastic Films for Modified Atmosphere Packaging of Fruits and Vegetables: A Review S. Mangaraj Æ T. K. Goswami Æ P. V. Mahajan Published online: 15 July 2009 Ó Springer Science+Business Media, LLC 2009 Abstract Modified atmosphere packaging (MAP) of fresh produce relies on the modification of atmosphere inside the package achieved by the natural interplay between two processes: the respiration rates of the commodity and the permeability of the packaging films. MAP has been a proven technology to meet the consumer s demand for more natural and fresh foods, which is increasing day by day. Because of its dynamic phenomenon, respiration and permeation take place simultaneously, and it is necessary to design the MAP system and select the matching films to achieve desired atmosphere early and maintain as long as possible. To meet the desired film characteristics for MAP, the different plastic films are either laminated or coextruded. In this modern world, the packaging films of required gas transmission properties are made available through advanced technology. Although the MAP industry has an increasing choice of packaging films, most packs are still constructed from four basic sustainable polymers: polyvinyl chloride (PVC), polyethylene S. Mangaraj T. K. Goswami (&) Department of Agricultural and Food Engineering, Indian Institute of Technology, Kharagpur, West Bengal , India tkg@agfe.iitkgp.ernet.in S. Mangaraj sukhdev1875@rediffmail.com P. V. Mahajan Department of Process and Chemical Engineering, Faculty of Science, Engineering and Food Science, University College, Cork, Ireland p.mahajan@ucc.ie S. Mangaraj Central Institute of Agricultural Engineering, Nabibagh, Berasia Road, Bhopal, MP, India terephthalate (PET), polyproylene (PP) and polyethylene (PE) for packaging of fresh produce. Polystyrene has also been used but polyvinylidene, polyester and nylon have such low gas permeabilities that they would be suitable only for commodities with very low respiration rates. Keywords Modified atmosphere packaging Polymeric films Gas permeation Film properties Packaging films Introduction The principal roles of food packaging are to protect the food products from outside influences and damage, to contain the food and to provide consumers with ingredients and nutritional information [39]. The goal of food packaging is to contain food in a cost-effective way that satisfies industry requirements and consumer desires, maintains food safety and minimizes environmental safety [102]. Modified atmosphere packaging (MAP) of commodity refers to the technique of sealing actively respiring produce in polymeric film packages to modify the O 2 and CO 2 levels within the package atmosphere. It is often desirable to generate an atmosphere low in O 2 and/or high in CO 2 to influence the metabolism of the product being packaged and the activity of decay-causing organisms to increase storability and/or shelf life [16, 37, 63, 69, 81, 118]. In addition to atmosphere modification, MAP vastly improves moisture retention, which can have a greater influence on preserving quality than O 2 and CO 2 levels. Furthermore, packaging isolates the product from the external environment and helps to ensure conditions that, if not sterile, at least reduce exposure to pathogens and contaminants [26, 50, 64, 79, 95, 132, 139, 143].

2 134 Food Eng Rev (2009) 1: MAP is the replacement of air in a pack with a single gas or mixture of gases, either naturally or artificially. The proportion of each component is fixed when the mixture is introduced to the package. No further control is exerted over the initial composition, and the gas composition is likely to change with time owing to the diffusion of gases into and out of the product, the permeation of gases into and out of the pack, and the effects of product and microbial metabolism [36, 37, 81, 157]. MAP technology, which utilizes only the natural components of air, has achieved public acceptance due to these two trends. MAP has the advantages that synthetic chemicals are not used, no toxic residue is left, and there is little environmental impact, particularly if the plastic films used can be recycled. Recent advances in the design and manufacture of polymeric films with a wide range of gas-diffusion characteristics have also stimulated interest in MAP of fresh produce. In addition, the increased availability of various absorbers of O 2,CO 2 [81], water vapour [144] and C 2 H 4 [18, 62, 78, 139] provides possible additional tools for manipulating the microenvironment of MAP. Nearly all products are packaged at some point in their life cycle. Plastic films are widely used in packaging, and continue to grow in use as more and more applications switch to flexible packages such as modified atmosphere packaging (MAP). In these packages, plastic films may be used alone or in combinations to serve the basic packaging functions of containment, protection, communication and utility in the delivery of quality products to the consumer [67]. MAP is a dynamic process where respiration of the product and permeation of gases thorough the packaging film occurs simultaneously. The composition of the atmosphere within a package results from the interaction of a number of factors that include the permeability characteristics of the package, the respiratory behaviour of the plant material, and the environment [22, 45, 76, 81, 95, 149]. The films making up the package are selected to have specific permeability characteristics, and changes in these characteristics over time, temperature, and humidity follow known physical laws [1, 49, 95]. The environment can be controlled to provide specific conditions. In contrast to these known and controllable factors are the often unknown and uncontrollable responses of the plant material. The plant specie, cultivar, cultural practices, stage of development, manner of harvest, tissue type, and postharvest handling all contribute and influence the response of the material to the generated atmosphere. The scope of plant responses can be further modified by initial gas flush of the package before sealing and inclusion of chemical treatments to slow unwanted processes or reduce decay. Each of these components of the packaging process can be examined separately to better understand how each contributes to packaging strategies [70, 78, 119, 168]. The application of polymeric films for MAP are most often found in flexible package structures; they may also be used as component in rigid or semi-rigid package structures, for example, as a liner inside a carton or as lidding on a cup or tray. The plastic film used in MAP is low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC), polyester, i.e. polyethylene terephthalate (PET), polyvinylidene chloride (PVDC), polyamide (Nylon) and other suitable films [1, 4, 23, 28, 41, 43, 46, 49, 67, 81, 82, 86, 92, 98, 102, 105, 120, 129, 135, 137]. Although an increasing choice of packaging materials is available to the MAP industry, most packs are still constructed from four basic polymers: polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyproylene (PP) and polyethylene (PE), for packaging of fruits and vegetables [4, 29, 49, 80, 81, 102, 129, 160]. Polystyrene has also been used but polyvinylidene, polyester and nylon have such low gas permeabilities that they would be suitable only for commodities with very low respiration rates. However, perforating the films can expand their use to many commodities. Recent advances in the technology of manufacturing the polymeric films have permitted tailoring films for gas permeabilities needed for some fruits, vegetables and their products. MAP is most commonly used for highly perishable, high-value commodities such as apple, cherry, strawberry, litchi, raspberry, broccoli, asparagus, mushroom, capsicum, fig and for freshly cut (minimally processed) fruits and vegetables [37, 61, 80]. Required Characteristics of Plastic Films for MAP The desirable characteristics of a polymeric film for MAP depend on the respiration rate of the produce at the transit and storage temperature to be used and on the known optimum O 2 and CO 2 concentrations for the produce that will result in optimum MA conditions with a definite time period. For most produce, a suitable film must be much more permeable to CO 2 than to O 2 [20, 26, 49, 81, 92, 94]. The major factors to be taken into account while selecting the packaging materials are: (1) The type of package (i.e. flexible pouch or rigid or semi-rigid lidded tray) (2) The barrier properties needed (i.e. permeabilities of individual gases and gas ratios when more than one gas is used) (3) The physical properties of machinability, strength, clarity and durability (4) Integrity of closure (heat sealing), fogging of the film as a result of product respiration

3 Food Eng Rev (2009) 1: (5) Sealing reliability (6) Water vapour transmission rate (7) Resistance to chemical degradation (8) Nontoxic and chemically inert (9) Printability (10) Commercial suitability with economic feasibility Polymeric Film in Application for MAP Flexible plastic packaging materials comprise nearly 90% of the materials used in MAP with paper, paperboard, aluminium foil, metal and glass containers accounting for the remainder. This is largely due to the changing consumer demand where convenience, quality, safety and impact on the environment are of prime considerations. These materials provide a range of permeability to gases and water vapour together with the necessary package integrity needed for MAP (Tables 1, 2). Sometimes the films are used alone, and often they are used in combinations that provide the benefits of multiple materials. The most commonly used polymeric films for MAP are as follows: Polyolefin s Polyolefin is a collective term for polyethylene and polypropylene, the two most widely used plastics in food packaging industries. Polyethylene and poly propylene both possess a successful combination of properties, including flexibility, strength, lightness, stability, moisture and chemical resistance, and easy processibility, and are well suited for recycling and reuse [1, 23, 82, 101, 102]. Low-Density Polyethylene (LDPE) The simplest and the most inexpensive plastics made by addition polymerization of ethylene is polyethylene. Lowdensity polyethylene is the most commonly used packaging film. LDPE seals at lower and over a wider temperature range, and has better hot tack, all of which result, to a great extent, from its long-chain branching [1, 23, 114, 129]. LDPE is a good barrier to water vapour, but a poor barrier to oxygen, carbon dioxide and many odour and flavour compounds. Because LDPE is relatively transparent, it is predominantly used in film applications and in applications where heat sealing is necessary. Some properties and characteristic of LDPE are presented in Tables 1 and 2. LDPE is generally the cheapest plastic film, on a per-unitmass basis. Linear Low-Density Polyethylene (LLDPE) Linear low-density polyethylene is also most commonly used packaging films in packaging industry. The reduction of density comes about through the use of comonomers that put side groups on the main chain that act like branching in decreasing crystallinity. LLDPE is also a soft, flexible material, with a hazy appearance. At equal density and thickness, LLDPE has higher impact strength, tensile strength, puncture resistance and elongation than LDPE. Like LDPE; LLDPE has good water vapour barrier properties, but is a poor barrier to oxygen, carbon dioxide and many odour and flavour compounds [1, 23, 105]. Since LLDPE often permits considerable down gauging, it can be the lowest cost alternative on a per-use basis. High-Density Polyethylene (HDPE) High-density polyethylene is a linear addition polymer of ethylene, produced at temperatures and pressures similar to those used for LLDPE, and with only very slight branching. HDPE films are stiffer than LDPE films, though still flexible, and have poorer transparency. Their water vapour barrier is better, as is their gas barrier. However, permeability to oxygen and carbon dioxide is still much too high for HDPE to be suitable as a barrier for these permeants [28, 101, 102]. Because of the distinctly cloudy appearance of HDPE film, a small amount of white pigment is commonly added to provide an attractive opaque white film. Typical HDPE properties are shown in Table 1. Polypropylene (PP) Polypropylene is a linear addition polymer of propylene; resins used in packaging are predominantly isotactic. PP has the lowest density of the commodity plastics, g/cm 3. Harder and more transparent than polyethylene, PP has good resistance to chemicals and is effective at baring water vapour. Its high melting point (Table 1) makes it suitable for application where thermal resistance is required. Barrier properties of PP are comparable to those of HDPE [1, 43, 49, 98, 109]. In many applications, biaxially oriented film (BOPP) is preferred. BOPP film is explicitly used in MAP of food commodity. Polyvinyl Chloride (PVC) Polyvinyl chloride films are formed by combining PVC resin, produced by addition polymerization of vinyl chloride, with plasticizers and other additives to produce a flexible film. In general, the films are quite soft and flexible, easy to heat seal, and have excellent self-cling, toughness, medium strength, excellent resistance to

4 136 Food Eng Rev (2009) 1: Table 1 Major packaging films and their typical properties Property Polyethylene films Polypropylene Polyvinyl chloride (PVC) Polyethylene terephthalate (PET) LDPE LLDPE HDPE PP BOPP Unoriented Oriented General purpose Polyvinylidene chloride (PVDC) High barrier Ethylene-vinyl alcohol (EVOH) 32 mol % ethylene 44 mol % ethylene Polyamide Nylon-6 Nylon-11 T g ( C) to?2-15 to? T m ( C) Th ( C) Density (g/cm 3 ) Tensile modulus (Gpa) To F T (Mpa) Elongation (%) , WVTR ,700 PO PCO ,900 4, ,000 2, PCO2 /P O PO PCO E P O E P CO Q PO Q PCO T g : Glass transition temperature ( C) T m : Melting temperature ( C) T h : Heat distortion temperature, at 455 kpa ( C) F T : Tensile strength (Mpa) WVTR: Water vapour transmission rate at 37.8 C and 90% RH (g lm/m 2 day) PO2 and P : permeability at 25 C for O CO2 2 and CO 2, respectively (cm 3 lm/m 2 -h-atm) P P and O2 PP : Pre-exponential factor of film for O CO2 2 and CO 2, respectively (cm 3 lm/m 2 -h-atm) E P and O2 EP : Permeability activation energy of films for O CO2 2 and CO2, respectively (kj/mole) Q PO 2 10 and Q PCO 2 10 : Permeability quotients for 10 C rise in temperature of films for O 2 and CO 2, respectively

5 Food Eng Rev (2009) 1: Table 2 Properties, environmental issue and other properties of plastic films for MAP Type of films Product characteristics/food compatibility Consumer and marketing issues Environmental issues Cost Advantages Disadvantages Advantages Disadvantages Advantages Disadvantages LDPE i. Soft, flexible and strong material ii. Good moisture barrier iii. Resistance to chemicals iv. Heat sealable and easy to seal v. Relatively transparent and predominantly used in film application vi. High ratio of CO2 to O2 permeability vii. Can be laminated and coextruded LLDPE i. Soft, flexible and strong material ii. Better impact strength, tear resistance and higher tensile strength and elongation, better resistance to environmental stress cracking, and better puncture resistance iii. Good moisture barrier iv. Good grease resistance and inert. v. Good low-temperature performance HDPE i. Flexible, strong and tough ii. Higher softening point than LDPE and superior barrier properties iii. Resistance to chemicals and moisture iv. Permeable to gases v. Easy to process and easy to form Polypropylene (BOPP) Polyesters (PET/PEN) i. Stronger, denser and more transparent than polyethylene ii. Moderate gas barrier and good water vapour barrier (high gas barrier and moisture vapour barrier than polyethylene) iii. Good resistance to chemicals iv. Excellent grease resistance v. Favourable response to heat sealing i. Excellent transparency and mechanical properties ii. Good/adequate barrier to gases and moisture and specially odours and flavours iii. Good resistance to chemical degradation, heat, mineral oil, solvents and acids. i. Light weight i. Slight haze or translucency i. Not suitable for applications involving significant exposure to heat. i. Light weight i. Slight hazy appearance i. Poor clarity i. Light weight i. Slight haze or translucency i. Light weight ii. High clarity, strength and durability as compared to LDPE i. Light weight ii. High clarity/ glass-like transparency iii. Shatter resistance i. Recyclable i. Easily recycled in semirigid form but identification and separation more difficult for films i. Recyclable i. Easily recycled in semirigid form but identification and separation more difficult for films i. Recyclable i. Easily recycled in semirigid form but identification and separation more difficult for films i. Recyclable i. Easily recycled in semirigid form but identification and separation more difficult for films Recyclable i. Easily recycled in semirigid form but identification and separation more difficult for films Low cost Low cost Low cost Low cost Inexpensive but higher cost among plastics

6 138 Food Eng Rev (2009) 1: Table 2 continued Type of films Product characteristics/food compatibility Consumer and marketing issues Environmental issues Cost Advantages Disadvantages Advantages Disadvantages Advantages Disadvantages Polyvinyl chloride (PVC) Polyvinylidene chloride (PVdC) i. Strong and transparent ii. Good gas barrier and moderated barrier to moisture vapour iii. Excellent resistance to chemicals, oils/fats and grease etc. iv. Largely used as packaging films i. High barrier to gases and water vapour ii. Heat sealable iii. Also used in hot filling, retorting, low-temperature storage, etc. Polystyrene i. High tensile strength and excellent transparency ii. Used for produce where a breathable film is required Polyamide (nylon-6) Ethylene-vinyl alcohol (EVOH) Ethylene-Vinyl Acetate (EVA) i. Strong ii. Reasonably good oxygen barrier iii Excellent odour and flavour barrier iv. Good chemical resistance iv. Mechanical and thermal properties similar to PET v. Excellent high-temp. performance i. Excellent barrier to gases, especially to oxygen, odour and flavour ii. Often used as O 2 barrier material i. Excellent transparency ii. Very good heat seal iii. Very good adhesive properties i. High clarity Recyclable i. Contains chlorine ii. Requires separating from other waste i. High gas barrier i. Maintains product quality Recyclable i. Contains chlorine ii. Requires separating from other waste i. Poor barrier to moisture vapour and gases Poor water vapour barrier i. Low moisture barrier/moisturesensitive i. Poor gas barrier ii. Poor moisture barrier i. Good clarity Recyclable i. Requires separating from other waste Recyclable i. Requires separating from other waste i. Maintains product quality for oxygen-sensitive products i. Excellent clarity ii. Mainly used as component of the sealant layer and adhesive in multilayer films Recyclable i. Requires separating from other waste Recyclable i. Requires separating from other waste Inexpensive Inexpensive but higher cost among plastics Inexpensive Relatively costly but inexpensive when used as thin films Inexpensive when used as thin films Inexpensive

7 Food Eng Rev (2009) 1: Table 2 continued Type of films Product characteristics/food compatibility Consumer and marketing issues Environmental issues Cost Advantages Disadvantages Advantages Disadvantages Advantages Disadvantages Relatively expensive Recyclable i. Requires separating from other waste Biodegradable hydrolysable Suitable for MAP of fresh produce Relatively expensive but costeffective for purpose. Layer separation is required Often allows for source reduction Flexible in design and characteristics i. Properties can be tailored for product needs Polylactide (PLA) Laminates/ Coextrusions chemical, resilience and clarity. Permeability is relatively high [4, 25, 49, 81, 105]. Both oriented and unoriented films are available. Properties of PVC films are listed in Tables 1 and 2. Polyesters Polyethylene terephthalate (PET), polycarbonate and polyethylene naphthalate (PEN) are polyesters, which are condensation polymers formed from ester monomers that result from the reaction between carboxylic acid and alcohol. The most commonly used polyester in food packaging is PET [1, 81, 101]. Polyethylene Terephthalate (PET) PET is commonly used in biaxially oriented form and has excellent transparency and mechanical properties. PET provides a good barrier to gases (O 2 and CO 2 ), moisture and especially to odours and flavours. The barrier properties can be enhanced by coating with PVDC. Coating or coextrusion is often used to provide good heat seal properties. It can tolerate considerably higher temperatures for short periods, such as in dual ovenable packaging for frozen foods. The main reasons for its popularity in food packaging industry are its glass-like transparency, adequate gas barrier properties, light weight and shatter resistance [1, 56, 84, 160]. Typical PET properties are listed in Tables 1 and 2. Polyvinylidene Chloride (PVDC) Polyvinylidene chloride is an addition polymer of vinylidene chloride. It is heat sealable and serves as an excellent barrier to oxygen, water vapour, odours and flavours [66, 81, 101]. The PVDC copolymer can be heat sealed and has excellent barrier to gases. However, the best barrier films generally do not provide the best heat seal capability, and vice versa, so when both heat sealability and barrier are desired, sometimes two differently formulated PVDC copolymer coatings are applied. The major applications of PVDC include packaging of poultry, cured meats, cheese, snacks food, tea, coffee, confectionary and MAP of food products. Ethylene-Vinyl Alcohol (EVOH) Ethylene-vinyl alcohol is a copolymer of ethylene and vinyl alcohol. The presence of OH groups in the structure results in strong intermolecular hydrogen bonding. EVOH is an excellent barrier to gases (especially O 2 ), odours and flavours. However, the hydrogen bonds also make it a moisture-sensitive material, and high humidity decreases

8 140 Food Eng Rev (2009) 1: its barrier capability [101, 102]. EVOH is most often used as an oxygen barrier. Typical EVOH properties are listed in Table 1. Polyamide (Nylon) Nylon films are used for specialty applications in packaging, where performance requirements justify their relatively high cost. Nylons have mechanical (excellent strength) and thermal properties (high-temperature performance) similar to PET and have similar usefulness. Nylons also provide excellent odour and flavour barrier, and reasonably good oxygen barrier [43, 81]. They are very poor water vapour barriers, and generally have a tendency to lose some barrier performance when exposed to large amounts of moisture. However, their performance is not as water-sensitive as EVOH. Owing to their relatively high cost, they are often coextruded with other plastics. Typical properties of some nylon films are given in Table 1. Nylon- 6 tends to be the most used nylon packaging film in industry. Polychlorotrifluoroethylene (PCTFE) These films are considered the best available transparent moisture barriers for flexible packaging; however, they are rather expensive. Aclar films can be laminated to paper, polyethylene, aluminium foil or other substrates. The film is heat sealable, and can be thermoformed. Aclar blister packages are often used for unit packages for highly moisture-sensitive pharmaceuticals [67]. Polyvinyl Alcohol (PVOH) Polyvinyl alcohol polymers are produced by hydrolysis of polyvinyl acetate. Because PVOH degrades at temperatures well below melt, it cannot be processed by extrusion. Therefore, casting from a water solution is used to make the film. As produced, the film is amorphous, but orientation induces some crystallinity [113]. Ethylene-Vinyl Acetate (EVA) Ethylene-vinyl acetate is produced by addition copolymerization of ethylene and vinyl acetate. EVA has higher permeability to water vapour and gases (in comparison to LDPE). These films have excellent transparency, and provide very good heat seal and adhesive properties, with excellent toughness at low temperatures. In both lidding and base films, they are mainly used as a component of the sealant layer [141]. Ionomers The heat seal performance of ionomers is outstanding. Ionomer films have excellent clarity, flexibility, strength and toughness which make them suitable for MAP of commodities. They can be used to package sharp objects, which break through many alternative materials when subject to vibration during distribution. Ionomers have relatively poor gas barrier and tend to absorb water readily. They have also relatively high cost compared to films such as ethylene-vinyl acetate [105]. Polycarbonate Films Polycarbonate films have excellent transparency, toughness and heat resistance, but high cost. They have some use in skin packaging, food packaging where exposure to high temperatures for in-bag preparation is required, and medical packaging. Polystyrene Polystyrene is another thermoplastic film with excellent transparency, with a high tensile strength but a poor barrier to moisture vapour and gases [1, 19, 81, 116]. It is often used in window envelopes and window cartons. Because of its low gas barrier, it can be used for produce where a breathable film is required. Polystyrene alone is brittle, but it can be blended or generally biaxially oriented to get required properties. In heavier gauges, polystyrene is widely used for transparent thermoformed trays. Cellulose-Based Plastics Cellulose-based plastics such as cellulose acetate, cellulose butyrate, cellulose propionate and copolymers are also used to a relatively small extent, most often as sheet rather than film. Their high price and water sensitivity limit their usefulness [116]. Biodegradable Polymers Biodegradable polymers are derived from replenishable agricultural feedstocks, animal sources, marine food processing industry wastes, or microbial sources. Biodegradable polymers are made from cellulose and starches [107]. Cellophane is the most common cellulose-based biopolymer. Starch-based polymers include amylose, hydroxylpropylated starch and dextrin. Other starch-based polymers are polylactides (PLA), polyhydroxyalkanoate (PHA), polyhydroxybuterate (PHB), and a copolymer PHB and valeric acids (PHB/V). Made from lactic acid formed from microbial fermentation of starch derivatives, polylactide

9 Food Eng Rev (2009) 1: does not degrade when exposed to moisture [10, 102, 146] (Table 2). In addition, biodegradable films can also be formed from chitosan, which is derived from the chitin of crustacean and insect exoskeletons. Chitin is a biopolymer with a chemical structure similar to cellulose [155]. Edible films, thin layer of edible materials applied to food as a coating or placed on or between food components, are another form of biodegradable polymer. They serve several purposes, including inhibiting the migration of moisture, gases and aromas and improving the food s mechanical integrity or handling characteristics, aimed to achieve MAP conditions [102]. At present, bioplastics are more expensive than petroleum-based polymers, so substitution would likely result in increased packaging cost. Commercialization of bioplastics is underway. Polylactide are commercially produced from natural products (corn sugar). After the original use, the polymer can be hydrolysed to recover lactic acid, thereby approaching the cradle-to-cradle objective (that is, imposing zero impact on future generation). In addition, Wal-Mart Inc uses biopolymers by employing polylactide to package fresh and cut produce [15, 102]. Multilayer Plastic Films In many cases, the best combination of packaging attributes at the lowest cost is achieved by using a combination of materials. Therefore, plastic packaging films are often combined with one another or with other materials such as paper or aluminium through processes such as coating, lamination, coextrusion and metallization. Coating Coating is commonly used to add a thin layer of a plastic on the surface of another plastic film or, more commonly, on a non-plastic substrate such as paper, cellophane or foil. The coating may be applied as a solution, a suspension or a melt. Common reasons for using coating in flexible packaging are: to impart heat sealability for plastics that are not heat sealed easily; to provide moisture protection for paper or cellophane; to improve barrier properties; and to provide protection from direct contact of the base material with the product [12, 13, 22, 149]. PVDC copolymer coatings are often used to improve barrier and heat sealability. Lamination Lamination is the process of combining two webs of film together [102, 129]. In flexible packaging applications, lamination is often used to combine a plastic film with another film, paper or foil. A variety of lamination methods are used. When plastic films are involved, either as a substrate or as an element in the finished structure, the laminating adhesive is often a low-density polyethylene, applied by extrusion, and the process is known as extrusion laminating. When paper is contained in a flexible package, it is most often being used for its excellent printability, along with its ability to impart substance and strength. Another significant use of lamination is to produce a web with buried printing. Coextrusion Coextrusion results in the production of a multiplayer web without requiring initial production of individual webs and a separate combining step. The melted polymers are fed together carefully to produce a layered melt, which is then processed in conventional ways to produce a plastic film or sheet. When only plastics are being used in a flexible packaging structure, coextrusion is generally preferred to lamination, unless buried printing is involved. A major advantage of coextrusion over lamination is its ability to incorporate very thin layers of a material, much thinner than those that can be produced as a single web. This is particularly important for expensive substrates, such as those often used to impart barrier properties. The amount of the expensive barrier resin used need only be enough to provide the desired performance [1]. The thinness of the layer is not limited by the need to produce an unsupported film and handle it in a subsequent lamination step. Metallization Metallization is a way of applying a thin metal layer on a plastic film. In commercial packaging practice, the metal being deposited is almost always aluminium. Metallized films have significantly enhanced barrier characteristics, and are usually chosen for this reason. In addition to gas barrier, metallized film provides an essentially total light barrier [67]. Barriers and Permeation The mechanism by which substances travel through an intact plastic film is known as permeation. It involves dissolution of the penetrating substance, the permeant, in the plastic, followed by diffusion of the permeant through the film, and finally by evaporation of the permeant on the other side of the film, all driven by a partial pressure differential for the permeant between the two sides of the film [82, 87, 105, 115, 127, 129]. The barrier performance of the film is generally expressed in terms of its permeability coefficient or

10 142 Food Eng Rev (2009) 1: permeability. For one-dimensional steady-state mass transfer, the permeability coefficient is related to the quantity of permeant, which is transferred through the film as represented by the equation: P ¼ Q:x ð1þ A:t:Dp where P is the permeability coefficient, Q is the amount of permeant passing through the material, x is the thickness of the plastic film, A is the surface area available for mass transfer, t is the time, and Dp is the change in permeant partial pressure across the film. Hence the permeability coefficient (P) is the proportionality constant between the flow of the penetrant gas per unit film area per unit time and the driving force (partial pressure difference) per unit film thickness. The amount of gas penetrating through the film is expressed in terms of either moles per unit time (flux) or weight or volume of the gas at STP. Commonly, it is expressed in terms of volume. It can be shown that the permeability coefficient (P), as defined by Eq. 1, is equal to the product of the Fick s law s diffusion coefficient, D, and the Henry s law s solubility coefficient, S (P = DS), in situations where these laws adequately represent mass transfer (ideally dilute solutions, diffusion independent of concentration): The permeability coefficient, under these circumstances, is a function of temperature, but is not a function of film thickness or permeant concentration. Concept and Theoretical Approach Gases and vapours can permeate through materials by macroscopic or microscopic pores and pinholes or they may diffuse by a molecular mechanism, known as activated diffusion. In activated diffusion, the gas is considered to dissolve in the film at one surface, to diffuse through the film by virtue of concentration gradient, and to reevaporate at the other surface of the packaging film. The equilibrium and kinetics considerations governing mass transfer are applicable here as well. The gas transport properties through polymers can be described by three parameters, the diffusion coefficient, the permeability coefficient, and the solubility. These terms are interrelated although the precise nature of the correlation is dependant on the type of diffusion that occurs. Generally the Fickian diffusion process is considered for gas transport in polymer. The diffusion is the speed with which a gas molecule penetrates through the polymer. The diffusion coefficient D is based on Fick s first law of diffusion. It states that the flux J in the x direction is proportional to the concentration gradient ðoc=oxþ: J ¼ D oc ð2þ ox The flux, J, is the volume of substance diffusing across unit area in unit time, independent of the state of aggregation of the polymer. This first law is applicable to diffusion in the steady state, that is, where concentration is not varying with time. The change in concentration with the time at a distance x into a thin film sheet, where the flux is in the x direction alone, is given by oc ox ¼ oj X ð3þ ox Substituting the values of Eq. 2 in Eq. 3 we have oc ox ¼ o ox Doc ð4þ ox If D is independent of concentration then Eq. 4 can be written as oc ox ¼ D o2 c ox 2 ð5þ The permeability coefficient, P, concerns the steadystate flux, J, of gas passing through the polymer and the pressure difference across it, which gives the driving force: J ¼ P p 1 p 2 ð6þ x where p 1 and p 2 are the partial pressures on opposite sides of a film of thickness x. P is expressed in cm 3 of gas at STP per cm 2 of film, unit cm of film thickness per second for a pressure difference of 1 atm. The solubility, S, is defined as the amount of dissolved gas in the polymer divided by the volume of the sample for 1 atm of gas on the sample surface ðc 1 c 2 Þ ¼ Sp ð 1 p 2 Þ; When p 2 % 0, c 2 % 0 then the above equation can be written as c 1 ¼ Sp 1 and when p 1 ¼ 1 atm ð7þ S ¼ c 1 p 1 where c 1 is the concentration in the sample when the equilibrium is reached. Equation 6 obeys Henry s law when S is independent of p, and hence Eqs. 2, 6 and 7 can be combined and written as P ¼ DS ð8þ The solubility S is expressed in cm 3 of gas at STP per cm 3 of the solid at a pressure of 1 atm. (cm 3 STP/cm 3 atm). The diffusivity or diffusion coefficient D is expressed as the diffusion of penetrants in cm 2 across the film at STP per

11 Food Eng Rev (2009) 1: sec (cm 2 /s). P is expressed in cm 3 of gas at STP per cm 2 of film, unit cm of film thickness per second for a pressure difference of 1 atm (cm 3 STP-cm/cm 2 s atm). Gas Permeation or Gas Transmission Conceptually, gas permeability coefficient is the same as gas transmission rate. The GTR is defined as the volume of gas that passes through a sample of unit area under unit pressure differential, at a given temperature and film thickness, with the rate being determined after the gradient of the recorded volume time curve becomes constant. The gas transmission rate is usually expressed for total thickness of the film while gas permeability is expressed on the basis of per unit film thickness. For composite films, it is more appropriate to use gas transmission rate values since permeation in composite films does not vary linearly with film thickness, usually. For some single material (polymer) films also, the relationship is not linear either. In such cases extrapolation may be erroneous [90, 115]. Measurement of Gas Permeability There are many methods for measuring permeability of gases and it is not possible to review them in detail here. Two major types of methods used for the measurement of gas transmission rates are as follows [11, 82, 129]: Pressure-Increase Method/Differential Pressure Principle The principle involved is that the test specimen is placed between the upper and lower chambers and clamped tightly. The lower pressure chamber (lower chamber) is vacuumized first and then the whole system. When the specified degree of vacuum is reached, the lower test chamber is shut off and feeds test gas of certain pressure to the upper test chamber (high-pressure chamber). It is ensured that a constant differential pressure (adjusted) is maintained across the specimen. Hence under the gradient of differential pressure the test gas permeates from the high-pressure side to the low-pressure side. By monitoring and measuring the pressure in the low-pressure side, the various barrier parameters (permeability coefficient) of the tested specimen are calculated [89] Karel et al. [82] reported that in pressure-increase method a membrane is mounted between the high-pressure and the low-pressure sides of a permeability cell. In this method, both sides are evacuated and the membrane is degassed. Then, at zero time a known constant pressure P H of the test gas is introduced on the high side, and P L (lowside pressure) is measured as a function of time. If the measurement is continued only as long as P H (high-side pressure) remains much larger than P L (low-side pressure), the Dp remains essentially constant and the permeability coefficient can be calculated as follows: P ¼ Dp L V L 273 Dx ð9þ Dt 760 T A where P is the permeability coefficient (cm 3 -mm/ cm 2 s cm Hg), P H is the pressure introduced at the high side, P L is low-side pressure, Dp L /Dt is the steady gas pressure increment in the low-side pressure and is obtained from the slope of the increments of low-side pressure versus time plot, V L is the calibrated volume of low-side pressure of the cell, x is the thickness of the film and A is the effective permeation area. Banerjee et al. [11] measured the gas permeability of films using a laboratory-made high-vacuum apparatus with static permeation cell at 1 atm for different temperatures. The polymer film was degassed for 24 h within the permeation cell prior to the experiment. To start the measurement, desired gas pressure (P i = 1 bar) was applied instantaneously to the pressure side of the film. On the downstream side, a reservoir of constant volume was connected with a pressure transducer, so that the total amount of gas that passed the polymer film could be monitored. The time-lag method was employed for the gas transport measurements. This technique allows the determination of the mean permeability coefficient P from the steady-state gas pressure increment (dp/dt) s in the calibrated volume V of the product side of the cell. The permeability coefficient is reported in barrier and was calculated from Eq. 10 P ¼ dp dt S V P 0 P i T 0 T x A ð10þ where P is the permeability coefficient in barrier (cm 3 -cm/ cm 2 s cm Hg), dp/dt is the steady-state gas pressure increment in the calibrated volume V of the cell and is obtained from the slope of the increments of downstream pressure versus time plot, V is the calibrated volume of the product side of the cell in cm 3, T 0 is the standard temperature = K, P 0 is the standard pressure = bars, P i is the upstream side pressure (desired gas pressure to be applied at the high-pressure side of the film, i.e. 1 bar), T is the temperature of measurement in K, x is the thickness of the film in cm and A is the effective permeation area in cm 2. Laffin et al. [90] measured gas permeability of LDPE/ LLDPE films under controlled conditions of pressure, temperature and relative humidity. The test consisted of placing the film sample between a partition test cell and an evacuated manometer, with the pressure across the film at 1 atmosphere. As the gas passes through the film sample, the mercury in the capillary of the manometer is depressed. After a constant transmission rate was achieved, a plot of

12 144 Food Eng Rev (2009) 1: mercury height against time gave a constant gradient. The slope of the gradient was then used to calculate the gas transmission rates. The quantity of gas passing through the film is directly proportional to the difference in gas pressure on either side of the film and inversely proportional to the thickness of the film. In addition, it is directly proportional to the time during which permeation has occurred and to the exposed area of the sample [82, 127]. Hence, Q a At ð p 1 p 2 Þ ð11þ x Q ¼ PAt ð p 1 p 2 Þ ð12þ x where Q is the quantity of gas which passes through the film, A is the surface area in contact with the gas, t is the time, p 1 - p 2 is the partial pressure differential and x is the thickness of film. (2) Concentration-Increase Method/Equal Pressure Principle The test principle is that high oxygen flows on one side of the film and high pure nitrogen flows on the other side of the film. Oxygen molecule passes the film into the nitrogen on the other side, and is taken to the sensor by the flowing nitrogen. The transmission of oxygen is tested by analysing the concentration of oxygen detected by sensor. As for the packaging container, nitrogen flows in the container, and air or high pure oxygen covers the outside of the container. In most cases, GTR has been measured employing pressure gradient method (Dp = 1 atm.). Normally, in MAP packages the pressure gradient of 1 atm. between the internal atmosphere of the packages and the external atmospheres is a rare possibility. Certain degree of flexibility in package free volume and the presence of a pressure balancing gas, viz. N 2, help in maintaining low pressure gradient without causing considerable variation in concentration gradient. Hence the concentration-increase or concentration gradient method facilitates close simulation of the conditions under which gas transmission takes place in MAP [129, 163]. Water Vapour Permeability It is also important to calculate the water vapour transmission rate of the packaging system. In this case, the partial pressure difference for water vapour between the inside and the outside of the package is almost never constant. Simplifying Eq. 1 the rate of moisture gain or loss in the product is given by the resulting differential equation as follows [1]: dq dt ¼ 1 x P wvaðp 2 p 1 Þ ð13þ where Q is the mass of permeant (water vapour) passing through the material in g, p 1 and p 2 are the partial pressures of water vapour outside and inside the package, respectively, in pa and p 2 is a function of Q. The principle involved is that saturated water vapour transmits through the test specimen in a unit time under specified conditions of temperature and humidity. The transmitted mass is determined by measuring the decreasing weight of distilled water with time. In a desiccant system of measurement, silica gel is used as the desiccant and it is directly placed inside the film pouch whose P wv is measured under controlled conditions of 38 C temperature and 90% relative humidity. Water vapour permeability is computed from the measured values of the change in weights of the packages with time, employing the following equations [75] P wv x ¼ dw dt 1 A:p ð14þ where Pwv is the water vapour permeability of packaging film (g-mm/m 2 day pa), dw/dt is the weight gain by desiccant with time and is obtained from the slope of the increments of weight versus time plot, t is the time in days, w is the weight gain by desiccant in g, x is the thickness of the film in mm, A is the area of the package in m 2 and p is the water vapour pressure at 38 C in Pa. Effect of Temperature on Permeability The permeability of O 2 and CO 2 in polymeric films is temperature dependent and this dependence is commonly described by an exponential equation (Arrhenius-type equations) [49, 163]. The relationship of O 2 permeability and CO 2 permeability with temperature can be depicted by this model. The generalized form and the specific form (permeability to O 2 and CO 2 ) of the Arrhenius equations are as follows. P ¼ P P exp EP a ð15þ RT and Ea P ¼ H S þ E D ; H S ¼ H c þ H m where H S is the apparent heat of solution, E D is the activation energy for diffusion, H C is the heat of condensation, H m is the heat of mixing, P is the permeability of gas at temperature T, P P is the permeability pre-exponential factor for gas, E a P is the activation energy of permeation for gas, R is the universal gas constant and T is the absolute temperature.

13 Food Eng Rev (2009) 1: When permeability coefficients are not available at the temperature of interest, an Arrhenius relationship can be used to determine the required value, from the permeability coefficient at a nearby temperature and the activation energy. The following equation is used: E a 1 P 2 ¼ P 1 exp 1 ð16þ R T 1 T 2 where T 1 is the temperature at which P 1 is known, T 2 is the temperature at which P 2 is to be calculated, E a is the activation energy and R is the gas constant. The permeability coefficient, as indicated, is a product of the diffusion coefficient and the Henry s law solubility constant. Since these vary in different ways with temperature, Eqs. 15 and 16 are valid only over reasonably small temperature ranges. A particular concern is that permeation rates are much higher above the T g than below this temperature, and the rate of change with temperature differs. Generally, the above equations would accurately characterize a polymer s gas diffusivity/temperature behaviour, except where there are strong interactions between the polymer and the gas molecules (e.g. water vapour and hydrophilic polymers). In addition, the above equations would only predict the effect of temperature above the gas transition temperature (T g ), since most films show a discontinuity of diffusion at the transition. At or below T g, the polymer conformation is set and rotational movements responsible for diffusional properties are blocked. Therefore, Eq. 15 should never be used to calculate the permeability coefficient across a temperature range that spans T g of the plastic. Temperature Quotient for Permeability The influence of temperature on permeability of polymeric films was quantified with the Q P 10 value, which is the permeability increase for a 10 C rise in temperature and is given by the following equation: Q P 10 ¼ P 10= ð T2 T 1 Þ 2 ð17þ P 1 where Q P 10 is the temperature quotient for permeability, and P 1 and P 2 are the permeabilities at temperatures T 1 and T 2, respectively. Permeability Coefficient of Multiplayer Films Permeability coefficients for multiplayer plastic film or sheet, either laminations or coextrusions, can be calculated from the thickness and permeability coefficients of the individual layers as follows [1]: P t ¼ R i¼n i¼1 x t x ð18þ i= Pi where the subscript t indicates the value for the total structure, i indicates the value for an individual layer, and there are n layers in the structure. When two films are combined to form the film laminate, Eq. 18 can be expressed as follows [82]: 1 ¼ x 1 P la ðxp 1 Þ þ x 2 ð19þ ðxp 2 Þ where P la is the permeability of film laminate (cm 3 / m 2 h atm), P 1 and P 2, are the permeabilities of individual films, i.e. film1 and film2, respectively; x 1 and x 2 are the thicknesses of individual films, and x is the thickness of the film laminate. Effect of Sub-zero Temperature on Permeability Lambden et al. [91] investigated the effect of sub-zero temperatures on OTR of packaging films. They inferred that around 0 C, a small variation in temperature greatly alters the permeabilities and thus their prediction is not possible with Arrhenius relationship. Influence of Polymer Structure and Morphology on Permeability Salame [137] correlated polymer structure and morphology with gas permeability. Based on cohesive energy density and fractional free volume of the polymer, he derived numerical scale of permachor values (p) to predict gas permeability for non-interacting polymer-penetrant systems as given below: P ¼ ða=t O Þe Sp ð20þ where A and S are constants and T 0 is the tartuosity (oriented crystalline polymers) Packaging System in MAP The polymeric films used for MAP are of three types: (i) polymeric films without perforations or microperforated; (ii) macroperforated polymeric films and (iii) perforationmediated packaging systems. Microperforated or non-perforated polymeric films yield low O 2 and low CO 2 concentrations because the CO 2 permeability of these materials is generally 3 6 times that of O 2 permeability [49, 162]. These materials are suitable for less CO 2 tolerant commodities such as mango, banana, grapes and apples. The gas permeability in microperforated polymeric films is temperature dependent and this