The Use of Zein and Its Edible Films for the Development of Food Packaging Materials

The Use of Zein and Its Edible Films for the Development of Food Packaging Materials _ Iskender Arcan, DÖHLER Natural Food & Beverage Ingredients, _Istanbul, Turkey lu, Izmir Institute of Technology, Izmir, Turkey Derya Boyacı and Ahmet Yemeniciog Ó 2017 Elsevier Inc. All rights reserved.

Introduction Some Major Characteristics of Zein Films Potential Use of Zein as an Edible Packaging and Coating Material Zein Packaging and Coating Materials Without Active Agents Zein Packaging and Coating Materials With Active Agents Natural Antimicrobials Suitable for Zein Films Natural Antioxidants Suitable for Zein Films Basic Challenges in Formulation of Zein Films Basic Strategies to Develop Active Zein Films Effective in Food Systems Conclusions Appendix A Supplementary data References

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Introduction Zein is a hydrophobic storage protein that forms 45%–50% of corn proteins (Shukla and Cheryan, 2001). It is a prolamin group protein that also exists in maize. Hydrophobic prolamins are also found in wheat (gliadin), barley (hordein), rye (secalin), and sorghum (kafirin) (Shewry and Tatham, 1990; Shull et al., 1991). However, zein attracts particular industrial interest since it is the major coproduct of the rapidly growing oil and bioethanol industries (Manley and Evans, 1943; Selling et al., 2008; Shukla and Cheryan, 2001; Wang et al., 2007). There are four different molecular fractions of zein (a-, b-, g-, and d-zein), with a-zein being the most abundant one forming almost 80% of total fractions (Shewry and Tatham, 1990; Shewry and Halford, 2003). Zein is not valuable for human nutrition due to its poor essential amino acid content, but it has unique technological properties that attract increasing interest from food scientists and technologists. The current commercial applications of zein in the food industry are limited to its use as an ingredient in chewing gum production and as a coating material for candies, as well as fresh and dried fruits and nuts (Bai et al., 2003; Lai and Padua, 1997; Shukla and Cheryan, 2001). The specific data about the amount of zein used by the global food industry are scarce, but it is known that zein production capacity between 2001 and 2011 in the United States increased from around 500 to 18,000 metric tons (a 36-fold increase) (Shukla and Cheryan, 2001; Bevill, 2011). Zein is a great candidate to be a major edible film-forming biopolymer in the food industry, and this could turn it into one of the most extensively produced and used agroindustrial plant coproducts of the world in the future. The most promising properties of zein as a biopolymer are (1) its outstanding film-forming ability, (2) its solubility in organic solvents such as ethanol, (3) its compatibility with most of the natural antimicrobials and antioxidants, and (4) suitable gas permeability characteristics of its precast films and coatings, thus enabling its application in fresh fruits and vegetables for modified atmosphere packaging (MAP) and coating purposes, respectively. Moreover, zein could be applied very easily as a food coating, simply by immersing food into film-forming solutions, or spraying or brushing film-forming solutions on the food surface (Fig. 1). Dry-cast zein films could also be used for wrapping foods or by placing on food

Figure 1

Application of zein film as a coating by brushing a zein film solution onto the surface of a cheese block.

Reference Module in Food Sciences

http://dx.doi.org/10.1016/B978-0-08-100596-5.21126-8

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The Use of Zein and Its Edible Films for the Development of Food Packaging Materials

A

Figure 2

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Application of dry-cast self-standing zein films between slices of fresh cheese (A) and at the surfaces of a beef burger (B).

surfaces or between food layers (Herald et al., 1996; Ünalan et al., 2013; Fig. 2). This article focuses on the basic principles of using zein in the development of active edible films and coatings, and discusses promising active agents that could be employed in zein film systems.

Some Major Characteristics of Zein Films The characteristic film structure of zein consists of a meshwork composed of doughnut structures formed by asymmetric rods joined to each other (Guo et al., 2005). The film integrity is maintained mainly by hydrophobic interactions, but strong intermolecular disulfide bonds also contribute to keep the zein rods together (Argos et al., 1982; Guo et al., 2005). Thus, zein films are quite brittle and they do not show flexibility. The surface topography of zein films could be highly variable depending on the type of plasticizer and solvent used in film preparation, and the applied film making and drying methods (Ghanbarzadeh et al., 2007; Pankaj et al., 2014; Shi et al., 2009). However, it was reported that the classical zein films that are prepared in ethanol and plasticized with glycerol have a smooth surface when they are obtained with dry casting or heat pressing methods (Ghanbarzadeh et al., 2007; Pankaj et al., 2014). The scanning electron microscopy of zein films obtained by different workers showed that dry-cast zein films have a porous surface (Padgett et al., 1998; Güçbilmez et al., 2007; Wang et al., 2005), while heat-pressed zein films have a nonporous surface with no visible pores (Padgett et al., 1998). Thus, it is clear that the gas, water vapor, and aroma barrier properties of zein films could vary depending on the film making method. It is difficult to characterize the gas barrier properties of edible films since these properties are highly dependent on film formulation, film making method, film thickness as well as temperature and relative humidity (RH) of test conditions. However, Gennadios et al. (1993) provided useful data over a broad temperature range (between 7 and 35
C) about the O2 permeability of dry-cast zein films plasticized with glycerol. According to these authors the O2 permeability of corn zein films at 0% RH and 81 mm thickness could be calculated by the following equation obtained from the Arrhenius plot: Ln P ¼ 20.794–0.566 (1/T)104 (R2: 0.994; P is permeability as amol m1 s1 Pa1 and T is temperature in K) Gennadios et al. (1993) found that zein films (thickness: 81 mm) show higher O2 permeability than wheat gluten (thickness: 146 mm) and wheat gluten/soy protein composite films (thickness: 135 mm) tested at the same conditions at 0% RH and between 7 and 35
C. These authors also reported that O2 permeability values determined for their zein films were lower than those reported in the literature for edible cellulosic films and their composites with lipids (thicknesses: 50–150 mm), and for different plastic films such as low- and high-density polyethylene, polypropylene, polystyrene (thickness: 50 mm), and polyvinyl chloride (thickness: 100 mm), but higher than those of ethylene vinyl alcohol and polyvinylidene chloride (thicknesses of last two films were not reported) (results for cellulosic and plastic films were obtained at 0% RH and between 23 and 30
C) (Gennadios et al., 1993). Thus, coating of zein on the surface of some plastic films has been suggested as a new method to obtain plastic packaging materials with improved O2 and water vapor barrier properties (Tihminlioglu et al., 2010). Aydt et al. (1991) reported the average O2 and CO2 permeabilities of glycerol-plasticized dry-cast zein films (thickness: 89 mm) at 0% RH and 37.8
C, and 0% RH and 22.8
C as 34.8 (coefficient of variation: 137.4%) and 216 (coefficient of variation: 88.4%) amol m1 s1 Pa1 [0.104 and 0.645 barrer or 1010 mL (STP) cm cm2 s1 (cmHg1)], respectively. These authors also reported that zein films showed almost 10- to 180fold higher O2 and CO2 permeabilities than wheat gluten films (thickness: 140 mm) at the same testing conditions. On the other hand, Bai et al. (2003) reported that the average O2 and CO2 permeability of propylene glycol plasticized dry-cast zein films (thicknesses changed between 8 and 40 mm) at 60% RH and at 30.0
C as 1.2  1016 and 5.4  1016 mol m1 s1 Pa1 (0.358 and 1.613 barrer), respectively. Thus, all these findings showed that packaging and coating of food with zein films could provide a benefit to limit or control levels of O2 and CO2 contacting food.

The Use of Zein and Its Edible Films for the Development of Food Packaging Materials

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Water vapor permeability (WVP) of zein films has also been studied extensively by different workers. Similar to most proteinbased edible films the water vapor barrier properties of zein films are highly affected from the temperature and RH of a medium. The amount and type of plasticizer used in film making might also affect the WVP of zein films. For example, Xu et al. (2012) deter
mined a linear increase in WVP of dry-cast zein films (thickness: 100 mm) at 25 C and 80% RH from almost 1.25 to 10 1 1 1 g m s Pa (20,739 and 26,546 barrer) (data read from the curve) by gradual increase of plasticizer glycerol 1.6  10 from 0% to 30% of zein (w/w). In contrast, the same authors did not determine any significant changes in WVP of dry-cast zein films [data read from curve varied between 1.1 and 1.35  1010 g m1 s1 Pa1 (18,250 and 22,398 barrer)] plasticized with different concentrations of oleic acid between 0% and 30% of zein (w/w). In other studies, Park and Chinnan (1995) determined a WVP of 0.116  0.019 ng m m2 s1 Pa1 (19,246  3152 barrer) for glycerol-plasticized dry-cast zein films (thicknesses: 0.12–0.33 mm) tested at 85% RH and 21
C, while Aydt et al. (1991) reported the WVP of glycerol-plasticized zein films (thickness: 89 mm) as 22.6 pmol m m2 s1 Pa1 (coefficient of variation: 13%) (67,493 barrer) at 50% RH and 26.0
C. Aydt et al. (1991) also found that zein (thickness: 89 mm) and regenerated cellulose (thickness: 36 mm) films that showed the same WVP at the test conditions had almost threefold lower WVP than wheat gluten films (thickness: 140 mm). However, it is important to report that similar to most protein-based edible film, zein films typically show 80- to 2600-fold higher WVP than plastic films (Mauri et al., 2016) and they do not provide a good barrier against water vapor. Although the gas and water vapor barrier characteristics of zein films have been characterized extensively, studies on the aroma barrier properties of these films are scarce. However, different reports related to use of zein as plastic film coating and encapsulation agent suggested good aroma barrier properties of zein films. For example, Torres-Giner et al. (2010) reported that zein capsules considerably increased the retention of volatile off-flavor compounds such as propanal, 2,4-heptadienal, 2,4,7-decatrienal when they were employed for encapsulation of docosahexaenoic acid (DHA). Fabra et al. (2014) also showed that forming a zein interlayer by electrospinning between multilayer plastic films of polyhydroxyalkanoate-based materials (PHB and PHBV5), polyethylene terephthalate (PET), and polylactic acid (PLA) caused a great improvement in barrier properties of these films against the aroma compound D-limonene. The good barrier properties of whey protein films against D-limonene were also reported (Miller and Krochta, 1997), but many more studies are needed to investigate the aroma barrier properties of protein-based edible films. For example, data about permeability characteristics of edible films against major aroma compounds in fresh fruits and vegetables, herbs and spices, plant essential oils, and fermented, maturated, or smoked food could provide invaluable information about the benefits of packaging and coating with edible films including zein.

Potential Use of Zein as an Edible Packaging and Coating Material Zein Packaging and Coating Materials Without Active Agents Due to their suitable gas barrier properties, dry-cast zein packaging materials and zein coatings without active agents (e.g., antimicrobials, antioxidants, bioactive agents) are employed mainly to suppress respiration rates of fresh fruits and vegetables. Rakotonirainy et al. (2001) tested dry-cast self-standing zein films on lids of glass containers for the MAP of vegetables. Other workers employed zein as coatings to suppress respiration rates and extend the shelf life of fruits such as tomatoes, mangoes, pears, and apples (Table 1). Similarly, zein coatings have also been employed to suppress the sprouting of seeds. Other major applications of zein coatings include suppression of growth of aerobic microorganisms (mainly fungi), prevention of microbial contamination, and reduction of oxidative changes in foods. Less frequently, zein coatings have been applied to prevent oil uptake of food during frying or to reduce leaching of micronutrients in enriched rice.

Zein Packaging and Coating Materials With Active Agents Due to the growing demand to obtain packaged foods with better shelf life and quality, active packaging incorporating antimicrobials (antimicrobial packaging) and antioxidants (antioxidant packaging) has become one of the most rapidly developing research areas in food technology. Studies related to antimicrobial packaging have mainly focused on minimally processed fresh produce and ready-to-use food since microbial outbreaks originating from these products continue to increase (De Roever, 1998). Antioxidant packaging has focused on foods susceptible to lipid oxidation (Lee, 2013). The incorporation of bioactive compounds into packaging materials is also a novel packaging concept (bioactive packaging) that targets delivery of these compounds into foods to provide health benefits for the consumer (Lopez-Rubio et al., 2006). However, this field needs further studies to clarify the effects of film processing, and film and food components, ingredients, and additives on bioavailability of specific bioactive compounds. The major advantage of using zein as an edible film material is that it could be effectively employed both for active packaging (antimicrobial and antioxidant packaging) and bioactive packaging. This is due to the compatibility of zein with a majority of the natural antimicrobials, antioxidants, and bioactive compounds. Zein owes its compatibility to a lack of sufficiently charged groups in its structure that prevent formation of attractive charge–charge interactions with active and bioactive compounds that have a net positive (e.g., nisin and lysozyme) or negative (phenolic compounds) charge at the pH of most foods. This helps maintain the solubility and activity of the incorporated active and bioactive agents in the film and ensures delivery of these agents onto the food surface. The highly hydrophobic nature (water insolubility) of the zein film matrix is also an important factor that minimizes reactions between zein and the incorporated active or bioactive compounds. Different antimicrobial chemicals such as organic or inorganic acids, metals, alcohols, ammonium compounds and amines, and different synthetic antioxidants such as butylated hydroxyanisole,

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The Use of Zein and Its Edible Films for the Development of Food Packaging Materials

Table 1

Examples of applications with zein coatings and films without active agents

Major target(s)

Product

Application method

References

To delay ripening/to reduce moisture loss

Apple Pear Mango Mango Tomato Broccoli

Bai et al. (2003) Scramin et al. (2011) Gol and Rao (2014) Hoa et al. (2002) Zapata et al. (2008) Rakotonirainy et al. (2001)

Mashed potato balls

Coating – – – – Use of glass jar sealed with precast film Coating – Use of pouch from precast zein/soy protein laminate (bilayer) film Coating

Mallikarjunan et al. (1997)

Sugar beet and broccoli seeds Date Cheese-based dessert

Coating Coating Coating

Assis and Leoni (2009) Mehyar et al. (2014) Guldas¸ et al. (2010)

Semihard cheese

Coating

Pena-Serna et al. (2016)

Roasted peanuts

Coating

Wambura and Yang (2010)

To delay respiration rate/to maintain firmness and color To prevent leaching of micronutrients To develop oxygen barrier edible pouch To reduce fat uptake and moisture loss during deep-frying To delay sprouting and germination To delay maturation To inhibit total viable bacteria, and total yeast and mold To reduce ripening rate/moisture loss/ microbial contamination To delay initiation of rancidity

Iron-fortified rice Calcium-fortified rice Olive oil

Mridula and Pooja (2014) Mridula et al. (2015) Cho et al. (2010)

butylated hydroxytoluene, propyl gallate, and tert-butylhydroquinone can be incorporated into edible or plastic packaging materials (Appendini and Hotchkiss, 2002; Suppakul et al., 2003; Jamshidian et al., 2012). However, health concerns of consumers and environmental problems present a great challenge to the food industry in using natural antimicrobial and antioxidant compounds in edible packaging materials (Appendini and Hotchkiss, 2002; Han, 2005). The examples of different active zein films developed by using natural or chemical agents are presented in Table 2. However, it is important to note that this article is mainly focused on using natural antimicrobials, antioxidants, and film ingredients in the development of active zein films.

Natural Antimicrobials Suitable for Zein Films

To obtain antimicrobial zein-based materials by using coatings and self-standing dry-cast films, natural generally recognized as safe (GRAS) status antimicrobials such as lysozyme, polylysine, and nisin have been employed successfully (Dawson et al., 2000; Güçbilmez et al., 2007; Hoffman et al., 2001; Janes et al., 2002; Mecito glu et al., 2006; Padgett et al., 1998; Teerakarn et al., 2002; Table 2

Examples of applications of zein coatings and films with active agents

Active agent(s)

Major target(s)

Product

Application method

References

Lysozyme

To inhibit Listeria monocytogenes growth To inhibit L. monocytogenes growth and lipid oxidation To reduce total viable counts and total coliform counts To inhibit L. monocytogenes growth To inhibit L. monocytogenes growth To inhibit Salmonella enterica serovar Typhimurium To inhibit total viable count To repel granary weevils (Sitophilus granarius) To limit postharvest fungal decay To inhibit L. monocytogenes growth To reduce decay rate and to prevent softening

Fresh cheese

Precast films were placed on both surface

Ünalan et al. (2013)

Beef patty (uncooked)

Precast films were placed on both surface Coating

Ünalan et al. (2011)

Marcos et al. (2007)

Cherry tomato

Precast films were placed on both surface Coating

Yun et al. (2015)

Fish balls (surimi) Cereal

Coating Film cast on paperboard

Lin et al. (2011) Germinara et al. (2010)

Apple

Yõlmaz et al. (2016)

Turkey frankfurter

Coating (with electrospun mats of zein nanofibers) Coating

Jamun fruit

Coating

Lysozyme þ catechin þ gallic acid Lysozyme þ Na2EDTA Nisin or Nisin þ calcium propionate Enterocin A and B from Enterococcus faecium Cinnamon or mustard essential oils Nisin or Nisin þ Na2EDTA Propionic acid Curcumin Nisin, sodium diacetate, sodium lactate Cysteine, ascorbic acid, jamun leaves extract

Ready-to-eat chicken Sliced ham (cooked)

Janes et al. (2002)

Lungu and Johnson (2005) Baraiya et al. (2015)

The Use of Zein and Its Edible Films for the Development of Food Packaging Materials

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Ünalan et al., 2011). The possibility of using heat pressing and thermal extrusion methods to obtain active zein films has also been investigated, but these methods are not compatible with most of the natural active agents that lose their antimicrobial activity during thermal processing (Dawson et al., 2003; Padgett et al., 1998; Selling, 2010). Hen egg white lysozyme is one of the antimicrobials most compatible with zein films. This enzyme shows its antimicrobial activity by splitting the bonds between N-acetylmuramic acid and N-acetylglucosamine of the peptidoglycan (PG) layer in the bacterial cell wall. It is also extremely stable not only in ethanolic zein film–making solutions, but also in dry-cast zein films kept under refrigeration (Mecito glu et al., 2006). Lysozyme is effective mainly on Gram-positive bacteria, but it is not directly effective on Gram-negative bacteria that have a protective lipopolysaccharide (LPS) layer around their PG layer. Some Gram-positive bacteria such as Staphylococcus aureus also show extreme resistance against lysozyme. In fact, lysozyme also boosts the protective biofilm formation capacity of S. aureus (Suda gõdan and Yemenicio glu, 2012). Thus, antimicrobial packaging conducted by using lysozyme might not provide any protection against S. aureus. In contrast, lysozyme is a very effective natural antimicrobial agent against the critical pathogen Listeria monocytogenes (Duan et al., 2007; Min et al., 2005; Ünalan et al., 2011). Therefore, current use of lysozyme in antimicrobial packaging is concentrated mainly in antilisterial film production. The combined application of lysozyme with chelating agents such as EDTA also enables inactivation of major pathogenic Gram-negative bacteria such as Salmonella Typhimurium and Escherichia coli O157:H7 by the enzyme (Ünalan et al., 2011). EDTA (used in the form of disodium EDTA in packaging) destabilizes the LPS layer of Gram-negative bacteria and exposes their PG layer to enzymatic action. Lysozyme has been successfully combined with EDTA in zein films (Mecito glu et al., 2006; Güçbilmez et al., 2007). However, EDTA, a chemical food additive, should be employed very carefully since its limited concentrations allowed in the food systems could be neutralized rapidly in the presence of metal atoms. Another natural GRAS status antimicrobial agent tested successfully in zein film systems is nisin (Padgett et al., 1998; Ünalan et al., 2011). This antimicrobial peptide, a bacteriocin obtained from lactic acid bacteria, has been used extensively by the food industry for a long time to inhibit Clostridium spp. (Carvalho et al., 2007; Delves-Broughton et al., 1996; Thomas et al., 2002). Nisin is a cationic peptide and it owes its antimicrobial action to its ability to interact with the anionic phospholipids at the bacterial gõdan and Yemenicio glu, 2012). surfaces and to form pores and dissipate proton motive forces at the bacterial membrane (Suda Nisin is effective on Gram-positive bacteria, but it shows no antimicrobial effect on Gram-negative bacteria since their LPS forms a barrier for this antimicrobial. However, nisin is a highly effective antimicrobial for critical Gram-positive bacterial pathogens such as S. aureus and L. monocytogenes (Suda gõdan and Yemenicioglu, 2012). Thus, this natural antimicrobial peptide is employed in antimicrobial packaging when effective antistaphylococcal and antilisterial films are needed. The nisin-containing antimicrobial films are quite suitable for dairy products, which are at risk of contamination by both S. aureus and L. monocytogenes (Oliver et al., 2005; Pinto et al., 2011; Cucarella et al., 2004; Jakobsen et al., 2011; Borucki et al., 2003; Hunt et al., 2012). However, reports about possible nisin adaptation of L. monocytogenes (Harris et al., 1991) suggested that combination of nisin with lysozyme in antimicrobial films could be a smart solution against bacterial resistance problems. There are different findings in the literature about the potential synergy of combined nisin and lysozyme in food systems against S. aureus and L. monocytogenes (Chung and Hancock, 2000; Gill and Holley, 2000; Mangalassary et al., 2008; Sobrino-López and Martín-Belloso, 2008; Takahashi et al., 2012). Thus, a combination of nisin and lysozyme in zein films might be a very effective solution to inhibit critical Gram-positive pathogenic bacteria in foods. ε-Polylysine obtained commercially from aerobic fermentation by Streptomyces albulus is another antimicrobial peptide tested in the zein film systems (FDA, 2004; Geornaras and Sofos, 2005; Hiraki et al., 2003; Ting et al., 1999; Ünalan et al., 2011). In Japan, application of ε-polylysine has been approved for sliced fish and fish surimi, boiled rice, noodle soup stocks, noodles, and cooked vegetables (Hiraki et al., 2003). In the United States, FDA has recognized ε-polylysine as GRAS for use in cooked rice and sushi rice (FDA, 2004). This antimicrobial peptide is formed by 25–35 L-lysine residues and is effective on major Gram-positive and Gramnegative food pathogenic bacteria such as L. monocytogenes, E. coli O157:H7, and S. Typhimurium (Geornaras and Sofos, 2005; Geornaras et al., 2007). According to Chang et al. (2010), ε-polylysine could also be used in meat products against critical pathogenic bacteria such as L. monocytogenes, E. coli O157:H7, and S. Typhimurium. ε-Polylysine owes its antimicrobial activity to its polycationic and surface active nature that enables its interaction with bacterial membranes, and its ability to interact with the bacterial DNA when it penetrates into cells (Ho et al., 2000; Liu et al., 2015). The reported synergy of ε-polylysine with nisin against E. coli and S. aureus (Liu et al., 2015) also suggests that it is possible to increase the effectiveness of antimicrobial zein packaging by using carefully selected combinations of natural antimicrobials.

Natural Antioxidants Suitable for Zein Films Natural phenolic antioxidants (pure phenolic compounds, crude or purified phenolic plant extracts, essential oils, and tocopherols) are antioxidant compounds with the most potential; they are compatible not only with the zein material itself, but also with the ethanol used in zein film preparation. The main advantage of phenolic active agents is that in addition to their antioxidant activity, some of them are also effective antimicrobial compounds. The antioxidant activity of phenolic compounds originates from their free radical scavenging and iron chelating capacity (Perron and Brumaghim, 2009). However, the phenolic compounds owe their antimicrobial activity to multiple mechanisms including complex formation with cell walls, membrane disruption, inhibition of bacterial adhesion, or inactivation of bacterial enzyme systems (Cowan, 1999). The use of natural phenolic compounds in edible films including zein has become quite popular since these active agents improve not only the microbial and oxidative quality of foods, but also show different benefits on human health when foods enriched with phenolic compounds are consumed regularly (Lopez-Rubio et al., 2006; Wang et al., 2012). Although, zein contains minimal amounts of ionic groups, hydrogen bonds formed

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The Use of Zein and Its Edible Films for the Development of Food Packaging Materials

between carbonyl groups of zein and hydroxyl groups of phenolic compounds cause binding of part of the added phenolic compounds by the zein film matrix (Alkan et al., 2011). However, a significant amount of phenolic compound remains soluble in the films if the amount of phenolic compound incorporated into films is higher than the phenolic binding capacity of zein (Arcan and Yemenicio glu, 2011; Ünalan et al., 2013). Moreover, it is important to report that the binding of some phenolic compounds into the zein film matrix might cause positive changes in its mechanical properties. Arcan and Yemenicio glu (2011) showed that the hydroxycinnamic acids such as gallic acid and ferulic acid; hydroxybenzoic acids such as p-hydroxy benzoic acid; and flavonoids such as (þ)-catechin act as natural plasticizers for the zein film matrix. The exact mechanisms of zein plasticization by the phenolic compounds have not been fully clarified. However, it is thought that the phenolic compounds act similarly with most plasticizers that use their hydroxyl groups to form hydrogen bonds with the biopolymer to increase the free volume of the film matrix (Arcan and Yemenicio glu, 2011; Sothornvit and Krochta, 2005). The reduction of hydrophobicity of the film matrix as a result of hydrophilic phenolic hydroxyl groups is also an effective factor that contributes to the plasticization of zein films (Alkan et al., 2011; Arcan and Yemenicio glu, 2011). One of the phenolic candidates with the most potential for antioxidant packaging is catechin; some members such as (þ)-catechin have been extensively tested not only in zein films (Arcan and Yemenicioglu, 2011, 2014; Ünalan et al., 2013), but also in other types of edible films (Ku et al., 2008a, 2008b). (þ)-Catechin is a potent antioxidant and it can be combined with lysozyme since it has no negative effect on antimicrobial activity of this enzyme in zein films (Ünalan et al., 2013). Catechins have been increasingly used as food antioxidants since they are well characterized for their health benefits and are already part of the human diet due to their abundancy in plant-origin food (Yõlmaz, 2006). It is important to note that foods enriched with green tea extract, a perfect source for catechins such as catechin, epicatechin, epicatechin gallate, and epigallocatechin gallate, is now in particular demand by consumers (Perumalla and Hettiarachchy, 2011; Yõlmaz, 2006). Almost half of the catechins in green tea extracts are formed by epigallocatechin gallate that is an effective antimicrobial against critical bacterial pathogens (Staphylococcus spp. and Salmonella spp.) (Perumalla and Hettiarachchy, 2011). Thus, films containing catechins might show both antimicrobial and antioxidant activity. Other potential phenolic compounds tested extensively in zein films are GRAS essential oils such as eugenol and citral (Alkan and Yemenicio glu, 2016; Khalil and Deraz, 2015; Park et al., 2012). However, it should be noted that the distinctive odor and taste of essential oils are great disadvantages in using these effective antimicrobials and antioxidants in many food systems. It is strongly suggested that the films incorporated with essential oils should also contain suitable flavor compounds such as strawberry or vanilla aroma to mask their undesired odor and taste (Gutierrez et al., 2009). Nevertheless, this strategy also has limitations since aroma compounds should also be compatible with the target food system. The use of grape seed extract in zein films is also problematic due to its oligomeric proanthocyanidins that are red colored and have an astringent taste (Perumalla and Hettiarachchy, 2011). On the other hand, rosemary extract that contains mixture of diterpene phenolics, carnosol, and carnosic acid could be very suitable for zein films due to its minimal odor and taste compared to other phenolic extracts (Madhavi and Salunkhe, 1996; Reische et al., 1998). Further studies are needed to find and test odorless and tasteless phenolic extracts within edible zein films.

Basic Challenges in Formulation of Zein Films One of the most challenging steps in zein film development is film formulation that needs extensive preliminaries to determine optimal film-making concentrations of zein, different aiding agents (such as plasticizing, cross-linking, and emulsifying agents), active agents, and composite and blend film-making agents that are frequently used to modify zein film morphologies. It is also important to employ suitable homogenization and dispersion methods to mix film components effectively. However, the most critical step to obtain useful zein films is the selection of a plasticizing agent and determination of its optimal effective concentration. Although zein has excellent film-forming abilities, its coatings and films suffer from classical brittleness and flexibility problems. Thus, the zein film formulation process should focus mainly on mechanical properties of the obtained films. The brittleness problem of zein films appears particularly during storage of coated or packaged foods. It is highly undesirable to observe cracks and disintegrated parts on the surface of zein-coated food or food wrapped by a dry-cast zein film. Small pieces of film left within the package or at the food surface after removal of zein film before consumption are similarly undesirable. However, these problems originate from the hydrophobic nature of zein films, and they should be eliminated or reduced by use of a suitable plasticizer in film making. Many studies have been conducted to solve the classical brittleness problem of zein films using different ingredients such as organic acids, sugars, alcohols, fatty acids, synthetic polymers, and cross-linkers (Ghanbarzadeh et al., 2006; Kim et al., 2004; Lai and Padua, 1997; Lawton, 2004; Sessa et al., 2008; Woods et al., 2009). However, there are still very limited studies that provide a fully applicable practical solution to the flexibility and brittleness problems of zein films. One of the promising studies is that of Arcan and Yemenicio glu (2011) who reduced zein film brittleness and obtained flexible films using different pure phenolic compounds. Similar promising results were also obtained by incorporating different phenolic-rich essential oils into zein films (Alkan and Yemenicio glu, 2016; Fig. 3). The strong antimicrobial and antioxidant activity of zein films plasticized with phenolic compounds and essential oils is a great advantage in controlling microbial and oxidative changes in foods (Alkan and Yemenicioglu, 2016; Ünalan et al., 2013). However, it is important to note that although the use of phenolic-based plasticizers results in flexible zein films showing elongations over 100%, the concentrations of phenolic compounds to achieve the desired mechanical properties are too high. Thus, these applications need to use GRAS status essential oils and phenolic compounds compatible with

The Use of Zein and Its Edible Films for the Development of Food Packaging Materials

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Figure 3 Plasticization of zein film by incorporation of eugenol (2 mg/cm2) (Notes: Control film break down when it was pulled from both sides; Eugenol-containing film showed elongation when it was pulled from both sides).

the aroma and flavor characteristics of foods. Another alternative strategy to reduce the brittleness problem of zein films is to use polar and amphiphilic plasticizing agents such as glycerol and oleic acid, respectively (Scramin et al., 2011; Xu et al., 2012). Although glycerol and oleic acid make a limited contribution to increase zein film flexibility, these plasticizers help reduce film brittleness problems by controlling the size and development of microscopic cracks. Glycerol is highly popular as a zein film plasticizer since this GRAS substance is tasteless and odorless, does not significantly change film morphology, and could be employed in zein films up to 35% of zein by weight (Güçbilmez et al., 2007; Xu et al., 2012). On the other hand, the surface active agents such as lecithin might be used in film-making solutions to mix constituents that are more hydrophilic or hydrophobic than zein (Arcan and Yemenicioglu, 2014). The use of lecithin is particularly beneficial when composites or blends of zein are obtained with highly hydrophobic waxes and fatty acids, respectively (Arcan and Yemenicioglu, 2013, 2014).

Basic Strategies to Develop Active Zein Films Effective in Food Systems A carefully designed antimicrobial packaging material improves not only the safety of packaged foods by inhibiting pathogenic bacteria, but also prolongs their shelf life by controlling spoilage flora using minimum amounts of active compounds (Appendini and Hotchkiss, 2002). However, there are many factors that are important in the effectiveness of the developed antimicrobial zein films in foods. Some of these factors include (1) selection of a suitable antimicrobial agent, (2) presence of sufficient amounts of a free soluble antimicrobial agent within the films, (3) release rate of antimicrobial agent from film onto food surface, (4) rate of diffusion for antimicrobial agent from food surface to depths within the food, (5) microbial load of food and its susceptibility to microbial spoilage, (6) level of food surface contamination by critical bacterial pathogen(s), and (7) presence of supporting hurdles (chilling, modified atmosphere or vacuum packaging, nonthermal preservation, protective cultures). However, the selection of the suitable antimicrobial agent is one of the most critical steps in active film development studies. The antimicrobial agent selected should be a proven antimicrobial against the major key pathogen(s) at different conditions (pH and storage temperature) and should be highly stable in the film and the food at both commercial (þ4
C) and domestic refrigeration temperatures (between þ4 and 10
C). In a risky food product (e.g., beef burger, hamburger, smoked salmon, eggs, fresh cheese, fish, poultry) the target of active packaging could be one or several major pathogenic bacteria such as L. monocytogenes, E. coli O157:H7, S. Typhimurium, or Campylobacter jejuni (Alkan et al., 2011; Ünalan et al., 2011, 2013). However, in some cases, specific spoilage fungi (e.g., Botrytis cinerea for table grapes, Penicillium commune for cheese, and Penicillium species for bread) might also be targeted to prevent microbial-origin economic losses (Kure et al., 2001; Suhr and Nielsen, 2004; Zoffoli et al., 1999). In all cases, the choice of the most potent applicable antimicrobial against the target microorganism(s) is strategically important. The selection of a suitable antioxidant agent is similarly critical for the success of antioxidant packaging. It is important to employ a proper antioxidant considering the amount, form (fat or oil), fatty acid profile and distribution (a fatty layer, marbled or emulsion) of lipids within the food, and the presence of food constituents involved in lipid oxidation (lipoxygenase, naturally existing antioxidants and prooxidants, and metal atoms). However, for fresh minimally processed fruits and vegetables, enzymatic browning is the primary concern, and antioxidants (ascorbic acid and derivatives) are employed to prevent formation of quinones by polyphenoloxidase (PPO) and to inhibit PPO (Yemenicio glu, 2016). Another strategically important factor is the release (delivery) rate of antimicrobial agent(s) from the film onto the food surface, the most susceptible part of the food to microbial contamination and spoilage (Ünalan et al., 2013). The controlled release of antimicrobials from edible films is particularly important for successful antimicrobial packaging. A sufficient antimicrobial effect cannot be achieved unless the release rate of the antimicrobial compound from the packaging material to the food surface is adjusted. A very rapid release of antimicrobials onto the food surface causes diffusion of antimicrobials deep into the food, and this leaves

8

The Use of Zein and Its Edible Films for the Development of Food Packaging Materials

the critical food surface unprotected. In contrast, too slow a release of antimicrobials from films prevents them reaching the critical inhibitory concentration for the target bacteria at the food surface. The release rate of the antimicrobial should also be compatible with its neutralization rate in the food system (due to interactions with the food components), with the growth kinetics of the target pathogenic or spoilage microorganisms, and with the expected food shelf life (Appendini and Hotchkiss, 2002; Han, 2005). Due to presence of pores at its surface and the ability to show limited swelling, rapid release of antimicrobials from zein films is observed when they are incorporated with hydrophilic antimicrobials (Mecito glu et al., 2006). An effective strategy to sustain release rates of hydrophilic active agents from zein films is to ensure that the film hydrophobicity is increased while its porosity is reduced (Arcan and Yemenicio glu, 2013, 2014). The combination of zein with a more hydrophobic film-making agent by composite formation or by blending increases the film hydrophobicity and this helps control film swelling in aqueous media. The trapping of antimicrobial agent in the particles (or microspheres) of a hydrophobic agent could also delay its release rates. Moreover, a composite structure could also increase film tortuosity, and this might sometimes cause a limited reduction in the diffusion coefficient of the antimicrobial within the film (Ozdemir and Floros, 2003). A composite zein film is simply obtained by incorporation of waxes (such as beeswax, carnauba wax, or candelilla wax) into ethanolic film-forming solutions with effective homogenization applied in the presence of emulsifiers (such as lecithin) and sufficient heating to melt waxes (Arcan and Yemenicioglu, 2013). On the other hand, hydrophobic blended films of zein could be obtained by mixing fatty acids (such as oleic acid) into film-forming solutions by effective homogenization in the presence of emulsifying agents (Arcan and Yemenicio glu, 2014). In both composite and blended films of zein, sufficient control of the antimicrobial’s release rate is achieved only when film porosity is reduced by modification of film morphology. (þ)-Catechin that is an effective plasticizer of zein films also reduces porosity of these films by modifying film networking and morphologies (Arcan and Yemenicio glu, 2013, 2014; Ünalan et al., 2013). Thus, this phenolic compound is a critical film component for zein films that target controlled release of antimicrobials and antioxidants.

Conclusions This article discusses the importance of zein as an edible film-forming agent and describes the basic principles of active zein film and coating development. A particular emphasis was put on applicable methods of using natural active agents and film ingredients having a GRAS status since edible films obtained by using synthetic chemicals and food additives will find very low applicability in the future. Zein is a great candidate as an edible film-forming agent not only because it is a valuable by-product of the rapidly growing bioethanol and oil industries, but also due to its excellent film-forming ability, good solubility in solvents such as ethanol, and compatibility with many natural antimicrobials and antioxidants. However, for zein to become a universal edible film-forming agent, more efforts are needed to overcome challenges such as highly variable grades and purity of commercial zein preparations, classical brittleness problems, and a lack of alternative economically feasible commercial methods to obtain self-standing zein films. The current technology and knowledge enable commercial applications of zein-based films as active food coatings. However, the growing interest in active self-standing zein film production suggests that this valuable by-product could play important roles in the future as a biobased alternative to oil-based polymers.

Appendix A Supplementary data Supplementary video related to this article can be found at http://dx.doi.org/10.1016/B978-0-08-100596-5.21126-8.

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