Use of EVOH for Food Packaging Applications Rafael Gavara1, Ramón Catalá1, Gracia López Carballo1, Josep P. Cerisuelo1, Irene Dominguez2, Virginia Muriel-Galet3, Pilar Hernandez-Muñoz1 1 Packaging Group, Instituto de Agroquimica y Tecnologia de Alimentos, CSIC, Av. Agustin Escardino 7, 46980 Paterna (SPAIN), Phone: +34-963900022, email: rgavara@iata.csic.es 2 Analytical Chemistry of Contaminants Research Group, Dep. Chemistry & Physics, University of Almería, Agrifood Campus of International Excellence, ceiA3, E-04120 Almería (SPAIN) 3 ITENE, Valencia Parc Technologic, Albert Einstein 1, 46980 Paterna (SPAIN) Keywords: Active packaging, antimicrobial, antioxidant, ethylene-vinyl alcohol copolymer, EVOH, hydrophilic, oxidation, packaging, permeation, oxygen barrier, retorting, scavenger Abstract: Ethylene-vinyl alcohol copolymers (EVOH) are a family of thermoplastic polymers with application in many industrial sectors including packaging and, especially, food packaging. The main characteristic of EVOH copolymers for packaging applications is their outstanding barrier to gases (oxygen, carbon dioxide, …) and organic vapors (food aroma). EVOH is applied in multilayer structures for bags, trays, cups, bottles, squeezable tubes or jars to protect oxygen-sensitive products. However, the hydrophilic nature of EVOH must be taken in consideration in package design to minimize water uptake and polymer swelling during thermal treatment processes such as retorting. This article reviews EVOH applications in barrier packaging and in active packaging.

INTRODUCTION Ethylene vinyl alcohol copolymer, EVOH, was introduced in Japan by Kuraray Co. in 1972; they use the abbreviation EVAL. Since then, the use of EVOH has been increasing and presently, is one of the most widely applied packaging materials in the high barrier packaging industry, thanks to its excellent properties, in particular its extraordinary barrier to gases and organic vapors. EVOH is also heavily used due to its ease of processing in the most common manufacturing processes used by the polymer industry, and particularly, the packaging industry. EVOH is used in rigid packaging made by extrusion blow molding, sheet thermoforming and co-injection processes. EVOH can also be coextruded in blown film, cast film and extrusion coating processes to make flexible packaging. EVOH copolymers are the most widely used polymers to provide barrier to packages for food and beverage requiring a very low permeability to gases (like oxygen or carbon dioxide) and organic vapors for its conservation. On the other hand, although it is a synthetic material coming from petroleum, it can be easily recycled in the polyolefin regrind stream using existing infrastructure. ETHYLENE VINYL ALCOHOL COPOLYMERS (EVOH). COMPOSITION AND GENERAL CHARACTERISTICS The ethylene-vinyl alcohol copolymer, EVOH, whose general chemical structure is given in Figure 1, is obtained from the hydrolysis of ethylene vinyl acetate copolymer (EVA) which transforms the acetate groups into alcohol ones. The EVOH copolymer chains are formed by the constitutional units of both monomers statistically distributed (Iwanami and Hirai, 1983).

Figure 1. Structure of ethylene-vinyl alcohol copolymer Polyethylene -PE- is a semi-crystalline polymer of hydrophobic nature, which provides an excellent barrier to water, but is extremely permeable to oxygen and CO2. Polyvinyl alcohol -PVOH- is a hydrophilic semicrystalline polymer, with extremely low permeability in dry conditions, but soluble in water and difficult to process. EVOH copolymer combines the hydrophobic character of ethylene-based polymers and the hydrophilic behavior of PVOH. The presence of OH groups in the structure of the polymer chain, substituting an equivalent number of hydrogen atoms in the polyethylene chain, has remarkable effects on the EVOH films. The very polar OH group increases the intermolecular forces between polymer chains and, at the same time, makes the polymer hydrophilic. On the other hand, the OH group is small enough so that the polymer can maintain the crystalline characteristics of polyethylene even though it is distributed randomly in the chain (Aucejo, 2000).

EVOH copolymer is produced by two companies, Kuraray (http://www.kuraray.com/products/plastic/eval.html) and Nippon Gohsei (http://www.soarnol.com/eng/index.html). They are commercially available with ethylene molar fraction ranging between 24 and 48%. The properties of the copolymer are highly dependent on the percentage of the two components. Thus, EVOH copolymers having an ethylene content below 42 mol% have a monoclinic crystal structure; the crystals are small, dense and with a degree of packaging similar to that of PVOH. These copolymers have higher gas barrier, higher melting temperature and higher thermoforming temperatures than PE making them more challenging to process. EVOH copolymers with an ethylene content between 42 and 80 mol% have a hexagonal crystal structure with larger and less dense crystals than those of the monoclinic structure and allowing lower processing temperatures (especially deflection temperature and therefore lower thermoforming temperatures) (Cerrada et al., 1998). With this morphology, the barrier properties to gases present a considerable reduction. In general, EVOH copolymers have excellent gas barrier properties, about 200 times higher than those of oriented polyamides and about 15 times than those of polyvinylidene chloride (PVdC). EVOH copolymers have a molecular structure with high symmetry, allowing strong hydrogen bonds between polymer segments (Lagaron et al., 2004). This fact causes high cohesive energy and restrictions on the movement of chain segments necessary for the diffusion of oxygen. With the increase of the ethylene content in the copolymer, the inter- and intermolecular H-bond interactions decrease, facilitating the movement of the chains and resulting in an increase in permeability. Besides their excellent barrier properties, EVOH offers other characteristics which make it applicable to different applications. They exhibit very good resistance to solvents, organic fats and vapors, possess good transparency and gloss that make their optical properties comparable to those of oriented polypropylene (OPP) and oriented polyester (OPET), and their anti-static properties make them useful for electronic packaging applications. In addition, they have good thermal resistance and very good printing properties. However, as already mentioned, the presence of hydroxyl groups in the polymer chain confers a hydrophilic nature to EVOH. The greater the percentage of vinyl alcohol comonomer, the greater the influence of water on its barrier properties. Therefore, EVOH has excellent barrier properties in dry conditions but, in the presence of water, its permeability increases (Zhang et al., 1999, Aucejo et al., 1999). For the aforementioned reasons, EVOH is commonly sandwiched between hydrophobic materials such as PE or PP in most applications by lamination or more commonly coextrusion processes. EVOH is used industrially as a component of flexible, semi-rigid and rigid packages (Wachtel et al., 1985, Catala et al. 2015). APPLICATIONS OF EVOH FOR FOOD PACKAGING High barrier materials

For their proper preservation during storage and commercialization, many foods require the protection of an effective barrier that substantially restricts the passage of gases and vapors. The presence of oxygen and highly reactive oxygen species that are generated by different mechanisms in the surroundings of packaged foods is responsible for the oxidation of lipids and other food components, as well as for enzymatic activity and growth of aerobic microorganisms. Likewise, the presence of moisture fosters, among other mechanisms of deterioration, the growth of bacteria and fungi and substantial modification of food texture. To control these problems, it is essential to reduce or even better eliminate the presence of gases and vapors that are involved in the deteriorative reactions of packaged foods. Only adequately sealed glass and metal packaging materials provide a complete barrier to these agents; in plastic materials, there are always permeability and sorption phenomena that, to a greater or lesser extent, allow the exchange of gases, vapors and radiation in the food / packaging / environment system.

For food products that are not sensitive to the action of oxygen or moisture, or when the product does not require an extended shelf life, the protection offered by commodity polymeric packaging materials is generally sufficient. But for many others, it is necessary to provide a higher barrier to maintain the desired quality and shelf life. High barrier polymers are those polymers which have a high resistance to gas transmission. There is not a standardized criterion for fixing a permeability classification, although commercial practice takes oxygen permeability of packages as the basis of this classification with oxygen permeability equal to or less than 1.0 mL 20µm m-2 day-1 atm-

1 at 20°C and 65% relative humidity being considered as a high barrier package. This is approximately equivalent to 0.0001 barrer, i.e. 0.0001 x 10-10 [mL cm cm-2 s-1 (cm Hg)-1] = 1 x 10-14 [mL cm cm-2 s-1 (cm Hg)-1] (recalculated from Foster 2009).

In general, none of the commodity polymeric material provide the high barrier to different gases and vapors required by foods stored for long periods of time. Although aluminum foil laminates provide an almost complete barrier in their pristine state, foil laminations are very susceptible to pinholes that occur during the lamination, forming, filling, processing, transportation, storage and handling of the packaging. Multilayer structures including polyvinylidene chloride (PVdC) as a barrier layer were the first to be widely used as flexible packaging. However, the use of PVdC has been questioned by potential environmental problems (particularly in Europe), as well as difficult manufacturing procedures. The alternative, currently widely implemented, has been multilayer structures containing ethylene-vinyl alcohol copolymer -EVOH- (Catala and Gavara, 1996). The EVOH copolymer with 27% of ethylene offers the lowest permeability (ten times lower in dry conditions than PVdC), and its use has been extended in both flexible and rigid packaging. Perhaps the great limitation of EVOH copolymers is their sensitivity to moisture, which increases significantly with the molar percentage of alcohol groups in the polymer chain. The strong deterioration of the EVOH gas barrier properties caused by moisture implies that EVOH is exclusively used in the form of multilayer structures, where the EVOH layer is sandwiched between films that protect it from water, particularly polyolefins (Aucejo et al., 1999). The success of this material can be attributed to its behavior as a high barrier, together with the ability for

coextrusion in flexible and rigid structures, its reasonable cost and the lower environmental impact compared to PVdC or aluminum structures.

EVOH copolymers are commercially available for flexible and semi-rigid packages. In flexible packages, EVOH-based structures are in competition with films metallized with aluminum, aluminum oxides or silicon oxide. However, in the semi-rigid and rigid packaging market, it is the preferred material, with the only competition being novel inorganic barrier coatings (SiOx or amorphous carbon). Typically, these structures are obtained by coextrusion, but when it is desired to obtain a printed material they are made by lamination of a multilayer structure containing EVOH obtained by coextrusion to a second previously printed film material (Koros et al., 1990, Catala and Gavara, 1996, Hernandez, 1997). The number of high barrier structures containing EVOH is nearly unlimited and to make an exhaustive list a very difficult task. Instead, it is preferable to provide a list of typical structures with their main characteristics and applications such as the one presented in Table 1. Table 1. High-barrier packaging structures containing EVOH, with the most relevant characteristic and application.

Packaging item Structure Main property Application

Film for bags and lids

PE//EVOH//PE Flexibility Bag in box, MAP

PA/EVOH//PE

PE//PA/EVOH/PA//PE

O-(PA/EVOH/PA)//PE

Puncture resistance, mechanical stability

Meat products, fish products,

cheese, vacuum

OPET//PE//EVOH//PE Optical, printing Cheese, nuts, dried fruits, beef

jerky, MAP

Thermoformed

trays, cups

PS//EVOH//PE PP//EVOH//PE

PET//EVOH//PE

Rigidity Desserts, MAP

PC or PP/EVOH/PP Retortable Precooked food, preserves

Bottles and jars (coextruded or

coinjected)

PE/EVOH/PE Collapsible

Sauces

PP/EVOH/PP Retortable Preserves

PET/EVOH/PET Mechanical resistance

Alcoholic beverages

High barrier flexible films can be manufactured by coextrusion with PE as the inner and outer layers. Due to the low adhesion between PE and EVOH, a tie layer is required (represented by a double slash, //, in Table 1). Examples of PE//EVOH//PE use are bags for dry food and nuts. Polyamide (PA) provides increased mechanical resistance and diverse combinations with EVOH are commercial including coextruded, bioriented and laminated structures. Examples of applications are vacuum packaging of sausages, smoked meat or fish products. High barrier trays manufactured by thermoforming are also popular, including polystyrene, PS (expanded or not), polypropylene, PP, or

polyethylene terephthalate, PET, in the outer layer, and PE in the inner layer to provide heat sealability. PP//EVOH//PP structures for thermoformed trays are of paramount importance since they can be considered the polymeric alternative to glass or metal packaging for retortable products. Bottles and jars with twist-off closures or screw caps are manufactured by coextrusion blow molding or coinjection-stretch-blow molding for different applications from preserves to alcoholic beverages. In recent years, there has been a considerable increase in the use of retortable plastic containers for the packaging of pre-cooked food and preserves. Among their advantages over more traditional materials such as glass or metal, can be mentioned the reduction of heat treatment, the improvement of the sensory and nutritional quality of the food, as well as greater convenience. The potential market for high quality precooked products has driven the introduction of barrier plastics and the creation of new categories within food packaging. In this way, new market trends have promoted the development of new plastic materials with barrier properties capable of satisfying the requirements imposed by food processing companies. EVOH copolymers provide an excellent barrier to oxygen, gases and organic compounds permeation under dry conditions, but at high humidity, the oxygen permeation of the EVOH increases considerably. During the retorting process, the combined effect of temperature and humidity reduces the protection of the inner and outer layers of the structures and the EVOH absorbs large amounts of water, strongly reducing their barrier characteristics. After retorting, the external layers recover their water barrier, impeding the release of the water gained during processing (Lopez-Rubio et al., 2003). Therefore, EVOH copolymers are not ideal polymers from the point of view of their processability and resistance to humidity. Considerable research in this field has focused on better understanding and resolving these limitations, including the use of water permeable external layers (polycarbonate, PC) or the design of asymmetric structures in which the external PP layer is thinner to accelerate the water release. Adopting these measures, novel jars are being designed for retorting mainly manufactured by coinjection or coextrusion and blow molding, as mentioned previously; closures are similar to those of glass jars or tinplate cans (twist-off, or metallic double seam). Active materials There is an increasing interest in the development of new packaging technologies where packaging materials present a higher degree of involvement in the preservation of food than that of providing a barrier against external factors. Active packaging is understood as a food / packaging / environment system which acts in a coordinated way by either modifying the composition of the package headspace, or modifying the composition and/or characteristics of the food to improve its quality or safety (Catala and Gavara, 2001, Lopez Rubio et al., 2004). These positive actions are performed by the release of substances into the food or headspace, or absorbing / retaining undesirable components from the food or headspace.

EVOH copolymers have been used as a vehicle for the incorporation and release of substances for the development of active materials. The hydrophilic character and the presence of polar groups in the EVOH provides an adequate interaction with the active agents incorporated and provides a good triggering mechanism for initiation of the activity for its technological utilization in active food packaging. The high sensitivity to humidity, which can certainly be a problem when high barrier is required, can be used in a positive way for the development of active packaging systems. Indeed, the exposure of the active EVOH to humidity (from the packaged food) can reduce the polymer barrier and initiate the release of the agent, thus producing the desired positive effect on the food for which it has been designed. It is of the utmost importance that the active material has an initiation mechanism to give rise to its activity, thus avoiding the action of the active agent prior to the packaging of the food, which could produce the early exhaustion of the activity. Hydrophilic materials such as EVOH may therefore be suitable to form part of active containers since, due to their hydrophilic nature, when they come into contact with the food, the polymer matrix swells, allowing the release of the active compound. Some developments have been proposed for active packaging using EVOH copolymers as a basis for the control of different problems of food quality deterioration. Several designs focused on microbiological control, oxidation reduction or elimination of undesirable substances in food are reviewed below (Catala et al.,2015). Active materials with antimicrobial properties Microbial growth is the main cause of food spoilage and a factor limiting quality, safety and shelf life. Conventional physico-chemical conservation techniques (sterilization, pasteurisation, irradiation, etc.) and the incorporation of different antimicrobial substances applied directly to food, are routinely used for microbial control. A good alternative for reducing microbiological deterioration of food is the incorporation of antimicrobial substances into the packaging material from which these agents are released in a controlled and sustained manner on the packaged food, thus constituting an active antimicrobial package. Active packaging may have a microbicidal or microstatic effect. In the first case, active packaging eliminates the microbial load present in the food or prevents its growth. In the second case they can prolong the latency phase or inhibit the microbial growth. In general, the antimicrobial active materials are developed by incorporating the active agents in conventional synthetic polymers, mainly polyolefins, from which they are released by migration processes. EVOH copolymers are being used as matrices that incorporate the agent for the design of active antimicrobial packaging technologies (Gavara et al, 2015). Kubacka et al. (2009) and Martinez-Abad et al. (2012), have developed materials with silver nanoparticles in EVOH 32% as base polymer, with application in food packaging.

Fernandez-Saiz et al. (2010), have studied the antimicrobial properties and activity of cast films prepared with blends of low molecular weight chitosan and EVOH (with 29 and 32 mol% ethylene). The films developed exhibit optimum performance with good optical properties, resistance to moisture, better water barrier and excellent antimicrobial activity in vitro assays. In an extensive study focused on the development of an antimicrobial active container for fresh vegetable salads, antimicrobial bags were designed by applying EVOH 29% coatings containing oregano and citral essential oils on bioriented PP films (Cerisuelo et al., 2012, Muriel Galet et al., 2012, Muriel-Galet et al., 2013a). The prepared films showed a relevant growth reduction of salad microflora, as well as inhibition of the E. coli, S. enterica and L. monocytogenes pathogens inoculated in salad. Likewise, the kinetics and extent of antimicrobial mass transport were characterized and modeled. The developed model was employed for the optimization of the package design in order to ensure a controlled release of the agent, allowing the maintenance of the necessary concentration to prevent deterioration during shelf life. This model could easily be extended to other packaging systems if experimental data are available on all parameters involved to fulfill the mathematical requirements demanded by the model. In fact, Cerisuelo et al. (2013) applied the same methodology to study mass transfer in a package for fresh salmon with a PP// EVOH//PP laminate and to optimize the active packaging design. Muriel-Galet et al. (2013b), Muriel-Galet et al. (2014b), and Muriel-Galet et al. (2014a) proposed the incorporation in EVOH 29 and EVOH 44% molar matrices of non-volatile agent such as LAE (lauramide arginine ethyl ether), polylysine or lysozyme. The developed materials showed great activity against several pathogenic microorganisms and improved stability of an infant formula milk. Active materials with antioxidant properties Together with microbial spoilage, oxidation processes are the most common form of food deterioration. In particular, foods with high lipid content, and especially those with highly unsaturated fatty acids, are susceptible to deterioration by this mechanism. The consequences of food oxidation include the development of undesirable odors and flavors, alteration of color and texture, and reduction of nutritional value when some vitamins and polyunsaturated fatty acids are degraded (Kanner and Rosenthal, 1992). In order to control the oxidation of packaged foods, it is essential to reduce or even better eliminate the presence of oxygen, as well as highly reactive oxidizing species (ROS) such as superoxide free radicals, hydroxyl, singlet oxygen, etc. which are spontaneously generated by different mechanisms in the package headspace and that are involved in oxidation reactions of lipids and other food components. Vacuum or modified atmosphere packaging, together with the use of packages with the appropriate gas and light barrier characteristics, or the direct addition to food of antioxidant substances are the solutions commonly used by industry.

An alternative option that can provide good results is the use of active antioxidant packaging. In this technology, agents capable of scavenging oxygen or reactive oxidizing species present in the headspace are incorporated in the packaging material. Their mechanism of action includes their releasing to the food in a controlled manner with adequate kinetics, and with the advantage that such release would results in an accumulation of the agent at the food surface, which is normally the most affected by oxidation (Gomez-Estaca et al., 2014) or by scavenging ROS generated in the headspace (Lopez-de-Dicastillo et al., 2012b) Granda-Restrepo et al. (2009b) and Granda-Restrepo et al. (2009a) have studied the development of active packaging with LDPE//EVOH//HDPE films obtained by coextrusion, and the incorporation of α-tocopherol and butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) for both the protection of polyolefin degradation during processing and the reduction of riboflavin degradation by migration of α-tocopherol to milk. The use of EVOH as a base material for the development of active antioxidant materials has been extensively studied by Lopez-de-Dicastillo et al. (2010), Lopez-de-Dicastillo (2011), Lopez-de-Dicastillo et al. (2012b) and Lopez-de-Dicastillo et al. (2012a). In these studies, natural substances such as the flavonoids quercetin and catechin and other polyphenolic compounds with high antioxidant capacity, have been used as active agents incorporated in EVOH 44 and 29 mol% films prepared with different manufacturing techniques. The release kinetics of antioxidants in model media and antioxidant activity in fatty foods such as fried peanuts or sunflower oil were monitored. According to the results, these EVOH films could be used to limit food oxidation.

Active materials for the scavenging of undesired food components Some foods contain compounds such as cholesterol, lactose or biogenic amines whose presence degrades their sensory or nutritional quality or makes them unsuitable for consumption. The presence of these compounds could in many cases be controlled by the incorporation into the container some absorbent substance such as zeolite, molecular sieve, clays, active carbon or cyclodextrins. López-de-Dicastillo et al. (2010a) and Lopez-de-Dicastillo et al. (2011) studied the development of active materials capable of retaining undesirable substances, such as milk cholesterol or peanut oxidation byproducts. In this work, cyclodextrins, oligosaccharides known for their capacity to scavenge hydrophobic substances, were incorporated into EVOH films and used for the packaging of milk or fried peanuts. The results showed that the active EVOH films were able to decrease the content of cholesterol in whole milk, and to reduce the hexanal accumulation during oxidation of packaged peanuts.

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