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Development of edible films and coatings from alginates and carrageenans
Accepted Manuscript Title: Development of edible films and coatings from alginates and carrageenans Author: Elham Tavassoli-Kafrani Hajar Shekarchizadeh Mahdieh Masoudpour-Behabadi PII: DOI: Reference:
S0144-8617(15)01054-1 http://dx.doi.org/doi:10.1016/j.carbpol.2015.10.074 CARP 10491
To appear in: Received date: Revised date: Accepted date:
20-7-2015 20-10-2015 21-10-2015
Please cite this article as: Tavassoli-Kafrani, E., Shekarchizadeh, H., and Masoudpour-Behabadi, M.,Development of edible films and coatings from alginates and carrageenans, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.10.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights (for review)
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1. Edible films and coatings can protect the product. 2. Such films can be a carrier of different additives. 3. Alginates and carrageenans films have good barrier and mechanical properties.
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*Manuscript
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Development of edible films and coatings from alginates and carrageenans
2 Elham Tavassoli-Kafrani, Hajar Shekarchizadeh*, Mahdieh Masoudpour-Behabadi
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Abstract
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Technology, Isfahan 84156–83111, Iran
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The use of renewable resources, which can reduce waste disposal problems, is being explored to produce biopolymer films and coatings. Renewability, degradability, and edibility make such films particularly suitable for food and nonfood packaging applications. Edible films
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and coatings play an important role in the quality, safety, transportation, storage, and display of a wide range of fresh and processed foods. They can diminish main alteration by avoiding
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moisture losses and decreasing adverse chemical reaction rates. Also, they can prevent spoilage and microbial contamination of foods. Additionally, nanomaterials and food additives, such as flavors, antimicrobials, antioxidants, and colors, can be incorporated into edible films and
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coatings in order to extend their applications. Water-soluble hydrocolloids like polysaccharides
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usually impart better mechanical properties to edible films and coatings than do hydrophobic substances. They also are excellent barriers to oxygen and carbon dioxide. Recently, there has been much attention on carrageenan and alginate as sources of film-forming materials. Thus, this
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Department of Food Science and Technology, College of Agriculture, Isfahan University of
review highlights production and characteristics of these films. Keywords: Edible films; Edible coatings; Alginates; Carrageenans
1. Introduction
The quantity of packaging materials has been increasing by 8% annually (MuiznieceBrasava, Dukalska, & Kantike, 2011). Less than 5% of all plastics are being recycled, leading to a high accumulation of plastics in the environment (Espitia, Du, Avena-Bustillos, Soares, & McHugh, 2014). Besides, increasing consumer concerns on food safety led to development of biodegradable, edible, and renewable films and coatings suitable for food and nonfood packaging applications (Alves, Costa, & Coelhoso, 2010; Espitia et al., 2014). However, due to the low cost *
Corresponding author. Tel.: +98 311 3913369; Fax: +98 311 3912254; E–mail: [email protected].
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of synthetic polymers, biodegradable materials had been ignored (Hambleton, Voilley, & Debeaufort, 2011). Today, with traditional agricultural commodities being a source of filmforming material, wide commercialization of biopolymer films has gained more significance (Arvanitoyannis, 2010).
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Biopolymers such as polysaccharides, proteins, and lipids can be used for the formation of edible films and coatings (Albert, & Mittal, 2002; Espitia et al., 2014; Lee, Shim, & Lee,
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2004). They can be used as complement or replacement of traditional materials in order to reduce traditional polymeric packaging (Barreto, Pires, & Soldi, 2003; Mate, & Krochta, 1998).
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Edibility and biodegradability are the most beneficial characteristics of edible films and coatings. Edibility of films and coatings could be achieved if films and coatings components including biopolymers, plasticizers, and other additives be food grade ingredients. Meanwhile,
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all the processes and equipment should be acceptable for food processing. To claim biodegradability of films and coatings, their toxicity and environmental safety must be evaluated
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by standard analytical protocols (Han, J. H., & Aristippos, G. (2005). An edible coating is a thin layer of edible material formed as a coating on a food product (Kang, Kim, You, Lacroix, & Han, 2013), while an edible film is a preformed thin layer, made of
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edible material, which can be placed on or between food components (Espitia et al., 2014;
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Guilbert, Gontard, & Gorris, 1996). The main difference between these 2 food systems is that the edible coating is applied in liquid form on the food, usually by immersing the product in the solution of edible material, and edible film is first molded as solid sheets, then applied as a
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wrapping for food products (Falguera, Quintero, Jiménez, Muñoz, & Ibarz, 2011). The concept of employing edible films and coatings for foods dates back to 1950s. Their growing application is attributable to reduction of moisture loss, adverse chemical reactions (Baldwin & Wood, 2006; Osorio, Molina, Matiacevich, Enrione, & Skurtys, 2011), spoilage, and microbial contamination (Arvanitoyannis, 2010). Additionally, they can be used for controlled release of food additives (Barreto et al., 2003). Edible coatings are also effective as a postharvest treatment to preserve fruit quality (Valeroet et al., 2013). Hydrophobic substances such as resins, waxes, or some insoluble proteins are better moisture barriers, but water-soluble hydrocolloids like polysaccharides and proteins usually impart better mechanical properties (tensile strength and elongation at break) to edible films and coatings than do lipids and hydrophobic substances (Arvanitoyannis, 2010). They also are excellent barriers to oxygen and carbon dioxide (Nussinovitch, 2009) because of their tightly
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packed and ordered hydrogen-bonded network structure (Atarés, Pérez-Masiá, & Chiralt, 2011; Bonilla, Atarés, Vargas, & Chiralt, 2012). So, they can be used to extend the shelf-life of foods by preventing dehydration, oxidative rancidity, and surface-browning (Dhanapal et al., 2012). Besides, food hydrocolloids can act as nutritious food ingredients. Some health benefits include
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lowering risk factors for cardiovascular disease, for immune function, for weight management, and for intestinal problems (Viebke, Al-Assaf, & Phillips, 2014). Table 1 shows the main
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hydrocolloids that can be used for the preparation of edible films and coatings. The uses of alginates and carrageenans in edible films and coatings are summarized in Table 2.
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In recent years, there has been much attention on carrageenan and alginate as sources in edible film formation (Cian, Salgado, Drago, Gonzalez, & Mauri, 2014). According to FDA, carrageenans and alginates are GRAS materials thereby they have been passed the mentioned
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standards and are considered as edible films and coatings. To the best of our knowledge, there is no review article about carrageenan and alginate films and coatings. Thus, this review highlights
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production and characteristics of these films. 2. Alginate
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Alginate is an appealing film-forming compound because of its non-toxicity,
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biodegradability, biocompatibility, and low price (Vu, & Won, 2013). Its functional properties, thickening, stabilizing, suspending, film-forming, gel-producing, and emulsion-stabilizing have been well studied (Dhanapal et al., 2012; Zactiti, & Kieckbusch, 2006).
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Alginic acid was first discovered in 1881 by Stanford. Around 1923, Thornley, in Orkney, UK, established a briquette business based on using alginate as a binder for anthracite coal dust. He moved to San Diego, USA, and by 1927 his company was producing alginate for use in sealing cans. After some difficulties, the company changed its name to Kelp Products Corp. and in 1929 it was reorganized as Kelco Company. Alginate production was established in the United Kingdom and Norway during 1934 to 1939 and after World War II, respectively (ITC 1981). The 2 largest producers, Kelco Company in the USA and Alginate Industries Ltd, in the UK were acquired by Merck and Co. Inc., USA. These combined companies produce about 70% of the world's alginate (McHugh, 1987). To extract the alginate, seaweed is broken into pieces and stirred with a hot solution of an alkali, usually sodium carbonate. After about 2 hours, the alginate dissolves as sodium alginate to give very thick slurry, and it contains undissolved parts of seaweed, mainly cellulose. The
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solution is diluted with very large amounts of water. Then, the solution is forced through a filter cloth in a filter press along with a filter aid such as diatomaceous earth. The last step is precipitation of the alginate from the filtered solution, either as alginic acid or calcium alginate (McHugh, 2003). Pretreatment (before alkaline extraction) of the seaweed with acid leads to a
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more efficient extraction, a less colored product, and reduced loss of viscosity during extraction because lower amounts of phenolic compounds are present (McHugh, 1987).
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Based on its linear structure, alginate can form strong films and adequate fibrous structures in the solid state, hence, it is considered a good filmogenic material (Blanco-Pascual,
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Montero, & Gómez-Guillén, 2014). Alginates have many applications. They can be used in foods for limiting dehydration of meat (Varela, & Fiszman, 2011), fish and fruits (Hambleton et al., 2011), in beverage industries as thickening, gel-forming and colloidal stabilizing agents
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(Liakos et al., 2013), in nonfood industries such as textile printing, as well as manufactories welding rods, binders for fish feed, immobilized biocatalysts (carrying enzymes ), release agents,
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and paper, and also for pharmaceutical and medical uses (Skurtys et al., 2010; Vu, & Won, 2013). Alginates can also be employed as matrix polymers for encapsulation of drugs, proteins, cells, and DNA (Ashikin, Wong, & Law, 2010).
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Alginate can be isolated from brown algae (Laminaria digitata and Ascophyllum
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nodosum) cell walls where it is present as the calcium, magnesium, and sodium salts of alginic acid (McHugh, 2003). It can also be synthesized by microorganisms (Alboofetileh et al., 2014; Blanco-Pascual et al., 2014). Alginate is a linear anionic (Vu, & Won, 2013) water-soluble
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polysaccharide (Hambleton et al., 2011) consisting of monomeric units of 1-4-linked α-Dmannuronate (M) and ß-L-guluronate (G) (Fig. 1a,b) (Alboofetileh et al., 2014; Blanco-Pascual et al., 2014). The polymer chain of alginate is made up of 3 kinds of regions or blocks in different proportions and different distributions in the chain (Fig. 1c). In these the physical properties of alginates depend on the relative proportion of these 3 blocks (Liakos et al., 2013; Xiao, Gu, & Tan, 2014). The G blocks contain only units derived from L-guluronic acid which cause greater gel strength, the M blocks are entirely based on D-mannuronic acid, and the MG blocks consist of alternating units of D-mannuronic acid and L-guluronic acid which determine the solubility of alginates in acid (McHugh, 1987). M and G blocks are known as homopolymeric blocks and MG blocks are heteropolymeric blocks (Gómez-Ordóñez & Rupérez, 2011). The relative fraction of each unit depends on species, part, and age of seaweeds from
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which the alginate is isolated (Ashikin et al., 2010). The biological source, growth, and seasonal conditions are also determinative (Zactiti, & Kieckbusch, 2006). Alginate monomer composition and sequence affect seriously the final properties of alginate gels since selective binding of ions is a pre-requisite for gel formation (Draget, &
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Taylor, 2011). The M/G ratio and distribution of M and G blocks in the chain of alginate affect the physical properties of the alginate. M/G ratios < 1 indicate a large amount of guluronic acid,
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which has the ability to form strong junctions. M/G ratios > 1 are indicative of a lower guluronic proportion, which might result in softer, more elastic structures (Blanco-Pascual et al., 2014).
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Alginate solutions can form gels either by lowering the pH below the pKa value of the guluronic residue or in the presence of divalent ions (Hambleton, Perpiñan-Saiz, Fabra, Voilley, & Debeaufort, 2012; Hambleton et al., 2011). These ions include calcium, magnesium,
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manganese, aluminum, and iron (Dhanapal et al., 2012). The affinity of alginates towards divalent ions decreases in the following order: Pb > Cu > Cd > Ba > Sr > Ca > Co, Ni, Zn > Mn
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(Pawar, & Edgar, 2010).
The most useful and unique property of alginates, which causes the strong gel or lowsoluble polymer, refers to their ability to react with polyvalent metal cations, specifically calcium
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ions (Zactiti, & Kieckbusch, 2006). These ions help the formation of association between M and
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G blocks. The length of G blocks determines the alginate ability and selectivity to form these interactions (Hambleton et al., 2012). M blocks and MG blocks are almost without selectivity. The diffusion of ions into the alginate solution causes an ion exchange process where the water
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soluble alginate (for example sodium or potassium form) has to exchange its counter-ions with Ca2+ to obtain a sol/gel transition. This ionic cross-linking will be form a rapid cold setting and heat stable gel. When forming alginate gels, two contiguous, diaxially linked guloronic residues form a cavity that act as a binding site for calcium ions (Cuadros, Skurtys, & Aguilera, 2012). This arrangement is pictured as the “eggbox” model (Fig. 2). The mechanical properties of films are directly related to the number of “eggbox” sites. As a result, film characteristics like water and mechanical resistant, barrier properties, cohesiveness, and rigidity can be improved (Zactiti, & Kieckbusch, 2006). However, the definite mechanism of alginate gelation is still under controversy. The polyvalent metal cations tend to chelate the carboxylate and hydroxyl groups of the alginate. This chelation is not just simple, but a sort of bridge between the metal ion and two carboxylate and one or more pairs of the hydroxyl groups occurs through partially ionic and partially
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coordinate bonds, respectively. Therefore, a two- stage mechanism has been suggested for such chelation by numerous studies (Fig. 3): first, the formation of strongly intramolecular linked dimer associations with important contributions from van der Waals and hydrogen bonding interactions in which the functional groups involved in chelation belong to the same chains,
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followed by the formation of weaker intermolecular dimer associations in which the carboxylate and hydroxyl functional groups are related to different chains that display no particular
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specificity, and being mainly governed by electrostatic interactions (Cuadros, Skurtys, & Aguilera, 2012; Hassan et al., 2013; Li, Fang, Vreeker, & Appelqvist, 2007).
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3 kinds of alginate blocks have different interactions with cations. M blocks bind cations externally near their carboxylate groups, while G blocks integrate cations into pocket-like structures formed by adjacent G residues. In MG blocks, cations preferentially locate in a
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concave structure formed by M-G pairs (Emmerichs, Wingender, Flemming, & Mayer, 2004). A direct mixing of alginates and polyvalent metal cations does not produce homogeneous
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gels due to the very rapid and irreversible formation of junctions between these two components. The formation of gel lumps (fish-eyes) is the result of such mixing. The only exception is for very small volumes of alginate under high shear. To overcome this problem, the ability to control
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the introduction of the crosslinking ions is essential. This control is possible by two different gel
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preparing methods: diffusion and the internal setting. In the diffusion method (Fig. 4a), crosslinking ions, for example Ca2+, diffuse from an outer reservoir into an alginate solution. In the internal setting method (sometimes also referred to as in situ gelation) (Fig. 4b), an inert ion is
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converted into an active cation by a change of pH of the alginate solution or by a limited solubility of the calcium salt source.
In the diffusion method, the Ca2+ will first cross-link the film surfaces drawing the polymer chanis closer to form a less permeable surface to the diffusion of Ca2+. Thus, the diffusion method yields gels having a Ca2+ ion concentration gradient across the thickness, while internal setting gives gels with uniform ion concentrations throughout (Pawar, & Edgar, 2010). By increasing the cation concentration during gelation of alginate, a more densely crosslinked structure will be formed which causes a less porous structure as well as reduction in water content and permeability of the gel (Aslani, & Kennedy, 1996). However, there is an optimal amount of cross-linker that can be used and further increase in amount of cation employed does not exert significant changes in the gel properties (Chan, Lee, & Heng, 2006).
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Sodium alginate among the other kinds of alginates, can form films with the properties of being water-soluble, strong, glossy, tasteless, odorless, flexible, low permeable to oxygen and oils (Xiao et al., 2014). During the formation of alginate film by diffusion setting method, the Ca2+ in the cross-
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linking solution will first cross-link the film surface drawing the polymer chains closer to form a less permeable surface to the diffusion of calcium ions into the interior. So, using an optimal
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amount of cross-linker to produce matrix with the desired characteristics (instead of a matrix with a highly cross-linked surface and a less well cross-linked interior) is essential in diffusion
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setting method. In internal setting, by using CaCO3, as the source of calcium ions, the reaction between the acid and carbonate leads to the formation of CO2 which cause formation of cavities within the film. Therefore, prepared films and coating by diffusion setting which use external
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cross-linking are thinner films with smoother surface, greater matrix strength, stiffness and permeability than films and coatings prepared by internal setting which are internally cross-
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linked (Chan et al., 2006). 3. Carrageenan
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Carrageenan and agar are 2 major groups of galactans presented in red seaweeds (de
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Araújo et al., 2011). Carrageenans are natural hydrophilic polymers (Osorio, Molina, Matiacevich, Enrione, & Skurtys, 2011) with a linear chain of partially sulphated galactans, which presents high potential of film-forming. These sulphated polysaccharides are extracted
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from the cell walls of various red seaweeds (Skurtys et al., 2010). The most usual seaweeds for extraction of carrageenans are Kappaphycus alvarezii and Eucheuma denticulatum seaweeds (McHugh, 2003). However, some scientists extract carrageenans from Hypnea musciformis (Wulfen) Lamoroux seaweeds (Cosenza, Navarro, Fissore, Rojas, & Stortz, 2014) and Solieria filiformis (de Araújo et al., 2011). The main source of commercial carrageenan is Chondrus crispus species of seaweed known as Carrageen Moss or Irish Moss in England and Carraigin in Ireland. The name Carraigin was first used by Stanford in 1862 for the extract of Chondrus crispus. It has been used in Ireland since 400 AD as a gelation agent and as a home remedy to cure coughs and colds. The term “carrageenan” is more recent and has been used by several authors after 1950 (Necas & Bartosikova, 2013; Rosa, 1972). There are 2 different methods for carrageenan production. First, is the original method, based on extraction of carrageenan to an aqueous solution and after removing the filtrate
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containing seaweed residue, carrageenan should be recovered from the solution. This method was expensive and was only used until the late 1970s–early 1980s. In the second method, seaweeds are washed to remove solid impurities before treating with alkali to extract the carrageenan. After extraction, the dilute extracts (1–2 % carrageenan) are filtered, concentrated,
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and then precipitated with isopropanol to give a fibrous coagulum. The coagulum is pressed to remove solvent and washed. It is then dried and milled to an appropriate particle size.(McHugh,
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2003).
Carrageenan has many applications in food and even nonfood industries and is a high
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value functional ingredient in foods (Hambleton et al., 2009; Necas & Bartosikova, 2013). Carrageenan can be used as stabilizer (Hsu & Chung, 1999) in dairy products such as flavored milks (Necas & Bartosikova, 2013). They also can be used in water-based foods, meat products
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(as oxygen barrier to retard lipid oxidation (Varela & Fiszman, 2011)), pet food (McHugh, 2003), infant food, and nutritional supplement beverages. The ability of suspending cocoa in
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chocolate milk at very low concentrations (ca. 300 ppm) is unique in carrageenan (Necas & Bartosikova, 2013). They mostly have been used to delay microbial growth in gels containing antimicrobial agents (Varela & Fiszman, 2011). They are also used in pharmaceuticals,
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cosmetics, printing, and textile industries (Cosenza et al., 2014). Preparation of edible films (Flores, 2011).
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using carrageenan is not abundant in literature because carrageenan is mostly used as a coating There are 3 main types of carrageenans differing in chemical structures and properties
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(Fig. 5): kappa carrageenan (κ-carrageenan) with a 3-linked, 4-sulfated galactose and a 4-linked 3,6-anhydrogalactose, iota carrageenan (ι-carrageenan) with a structure like the former, but with an additional sulfate ester group on C-2 of the 3,6-anhydrogalactose residue, and lambda carrageenan (λ-carrageenan) with a 2-sulfated, 3-linked galactose unit, and a 2,6-disulfated 4linked galactose unit (Cosenza et al., 2014). Carrageenans are water-soluble polymers which their solubility depend on the content of ester sulfate and the presence of potential associated cations. Higher levels of ester sulfate mean lower solubility temperature. Presence of cations such as sodium, potassium, calcium, and magnesium promote cation-dependent aggregation between carrageenan helices. The presence of anhydro-bridges in κ- and ι-carrageeenan reduces the hydrophilicity of the sugar residue and inverts the chair conformation and allows the carrageenan to undergo
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conformational transitions which are conducive to the gelation properties of carrageenans (Rhein-Knudsen, Tutor Ale, & Meyer, 2015). Iota carrageenan forms elastic and clear gels with no syneresis in the presence of calcium salts (McHugh, 2003). Its gels are thermoreversible (50–55 ˚C). The blending of iota carrageenan
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with calcium salts makes it a suitable hydrocolloid as a functional ingredient for stabilization, thickening, and gelation in dairy products such as milk gels and ice cream (Hambleton et al.,
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2009). It can form a three-dimensional network with double-helix chains. Each pair of helixes being 13.9 Å laterally spaced, therefore it has a compact, dense, and organized film structure
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(Hambleton et al., 2012). Iota carrageenan-based edible films have good mechanical characteristics. They are emulsion stabilizers and can decrease oxygen transfer, and limit surface dehydration and taste deterioration of fruits and cheeses (Hambleton et al., 2012). They can also
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be used as encapsulation agents for active substances (Hambleton et al., 2011). Lambda carrageenan dose not form gel and just forms high viscosity solutions (de Araújo et al., 2011).
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So, these polysaccharides are used as cold soluble thickeners in syrups, fruit drinks, pizza sauces, and salad dressings (Milani & Maleki, 2012). Kappa carrageenan is one of the most common forms of carrageenans, which can be used in foods. It has a double-helix conformation. The
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linear helical portions can associate and form a three-dimensional gel in the presence of
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appropriate cations. This gel is freeze-thaw stable. Kappa carrageenan is also able to interact with various food proteins through electrostatic interactions and increase their aggregation stability (Lopez-Pena & McClements, 2014). Kappa carrageenan forms strong and rigid gels
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with potassium salts as well as brittle gels with calcium salts. Kappa carrageenan gels are opaque, but by sugar addition they become clear (McHugh, 2003). The mechanism of gel formation of carrageenans which is a thermo-reversible gel depends on temperature and gel-inducing agents. Structure of carrageenans at high temperature (above 80 °C) is as random coil which is due to the electrostatic repulsions between neighboring chains. Upon temperature reducing, the conformation of chains changes to helical structure. Further cooling and presence of cations lead to intermolecular interactions between the carrageenan chains which cause aggregation of the helical dimers and formation of a stable three dimensional network. For κ-carrageenan and ι-carrageenan, typically potassium and calcium respectively stabilize the junction zones between the two helixes by binding to the negatively charged sulfate groups without hindering cross-linking of the two helices (Fig. 6). However, the charged sulfate
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esters on the other side of the λ-carrageenan cause an extensive conformation via a repulsion effect of the negative SO3− groups and inhibit gelation while promoting viscosity in the solution (Rhein-Knudsen, Tutor Ale, & Meyer, 2015). Carrageenans, especially poligeenan (degraded carrageenan) have been known to induce
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colonic inflammation, (Chen, Yan, Wang, Xu, & Zhang, 2010) and to be toxic to macrophages (Thomson & Horne, 1976). However, toxicological properties of carrageenans have been shown
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at high doses that do not occur with the food additive. Average molecular weight greater than 100,000 Da with a low percentage of smaller fragments is necessary for food grade carrageenan
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(Cohen & Ito, 2002; Prajapati, Maheriya, Jani, & Solanki, 2014). 4. Edible film and coatings applications
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An ideal edible coating forms a thin layer on the surface of the coated food product and supplies an effective barrier to water, vapour, moisture, or temperature. In addition, it does not
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absorb oxygen and forms a selective barrier to gases, significantly carbon dioxide (ChlebowskaŚmigiel et al., 2008). Alginate and carrageenan are highly hydrophilic. So, they present only a limited barrier to moisture. However, they are a good barrier to fats and oils. They are also good
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oxygen barriers and can provide protection against lipid oxidation (Varela, & Fiszman, 2011).
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The diffusion of compounds within edible films and coatings depend strongly on the physicochemical properties of the compounds such as molecular weight, structure, hydrophobicity, and polarity. Permeability of compounds is affected by the interactions between
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the compounds and the film matrices. Also the mobility of the diffusing molecule has an important role on the permeability. On the other hand the diffusion of compounds within alginates and carrageenans which are water-soluble polymers can be affected by water. Water is a solvent, so it can induce a modification of the matrix structure, such as swelling of polymers, and can modify the diffusion of molecules showing an affinity for water (Hambleton et al., 2011: Hambleton et al., 2012).
The diffusion of water soluble material is expected to occur through aqueous pores or channels in the alginate film matrix. A more cross-linked film has lower or smaller pores or channels. The distribution of cross-linkages in the matrix has a significant effect on substances permeation. Inhomogeneous alginate films impede substances permeation more than homogeneous films.
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Internal setting method produces a more uniformly cross-linked matrix. However, the liberation of CO2 forms microscopic cavities in internally cross-linked films which impede diffusion of substances. The cavities are air vesicles which disrupt liquid continuity in the matrix. The internal gelation method is useful in producing cross-linked alginate films and coatings for
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retarding or controlling release of material in comparison to diffusion setting (Chan et al., 2006). The functionality and performance of edible coatings mostly depend on the production
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method of coating and the coating ability to adhere to the product surface (Dhanapal et al., 2012). Edible films can be defined as a thin protective layer for food packaging, which can be
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eaten with food (Hambleton et al., 2009). Edible films can reduce the environmental pollution; improve the sensorial properties of packaged products, decrease moisture loss, and increase the nutritional value of the foods (Zactiti & Kieckbusch, 2006).
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Alginate and carrageenan can be used to form film coating for meat and meat products. The coatings can prevent shrinkage, microbial contamination, and surface discoloration by
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delaying moisture transport (Nussinovitch, 2009). The effect of alginate gel coating as an edible susceptor in combination with salt in microwaveable chicken nuggets was investigated by Albert et al. (2012). Results showed that salty alginate coating could act as an effective susceptor during
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heating by microwave and therefore decrease cooking times. Alginate and carrageenan based
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edible coatings are effective as post-harvest treatments to maintain quality of fruits such as tomato, peach, sweet cherry, and so on. Edible coatings are able to delay ripening and can extend shelf-life of products (Hamzah et al., 2013; Valero et al., 2013). Minimally processed products
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such as fresh cut fruits are susceptible to microbial spoilage. Edible coatings are able to control the microbial spoilage by preventing microbial proliferation and delaying respiration (Mastromatteo, Conte, & Del Nobile, 2012). Alginate and carrageenan film coatings are applied to carry different functional agents in order to improve their application. For example, alginate solution diminished dye leaching out of colorimetric oxygen indicator due to its ability to ion bind with the cation dye (Vu & Won, 2013). Fei Lue (2010) also indicated that the use of alginate coating with nisin and cinnamon could maintain quality of northern snakehead fish fillets. Application of nisin containing alginate film for sliced beef could reduce the amount of Staphylococcus aureus. It could be suggested that nisin containing edible films could control the growth of pathogens at the surface of ground beef or other meat products (Millette, Tien, Smoragiewicz, & Lacroix, 2007) .
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Hydrogels are highly hydrophilic polymer gels with the ability of swelling by absorbing and retaining large amounts of water without dissolving or losing their integrity in water. They can be used for food packaging. Rhim (2013) prepared hydrogel films composed of agar, κcarrageenan, konjac glucomannan, and nanoclay. This film showed a high water-holding and
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water vapour adsorptive capacity. These properties makes hydrogel film a suitable packaging
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film for highly moisture foods or foods contacting with high humidity condition. 5. Film/coating production methods
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Principal technologies for producing edible films are similar to those for thermoplastic structures: solvent casting and extrusion. Obviously, the conditions are different, but the principles are allied. For the coating production the main techniques are spraying and dipping
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(Skurtys et al., 2010).
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5.1. Edible film formation methods
One of the most used techniques to form edible films is solvent casting or wet process (Nussinovitch, 2009). Dispersions of edible materials are spread on a suitable base material and
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then let to be dried. During drying of the solution, solubility of the polymer decreases as a result
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of solvent evaporation, until polymer chains align themselves to form films (Skurtys et al., 2010). It is important to carefully control drying rate and environmental conditions due to their high influence on final thickness and structural characteristics of the resultant film (Flores,
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2011). Infrared drying can hasten the drying process and so is advantageous. Easy removing of film without any tearing and wrinkling is very important, which is dependent on the type of base material. For easy peeling off dried film from one edge of the base material, optimum moisture content (5–8%) is desirable (Tharanathan, 2003). The other technique is a dry process in which compression molding and extrusion are done, rely on the thermoplastic status of some polysaccharides and proteins at low moisture levels (Nussinovitch, 2009). Among three techniques of film production, only casting method exemplifies. Because films produced via this method are stand-alone. Hence, it is possible to evaluate the physical and chemical properties of film forming material alone (Lee, & Wan, 2006). Casting method has been used to form alginate films. Alginate reaction with calcium ions is fast, but casting to make film is difficult. So, 2 steps procedures are used to preparing casted
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films. The first step is to cast a partially dry alginate film solution and then immersing into a calcium solution. Alternatively, it is possible to spray calcium solution to the pre-formed film (Flores, 2011). This method was used for preparation of alginate/chitosan (Arzate-Vázquez et al., 2012), alginate/pectin (Galus, & lenart, 2013), and alginate/cellulose films (Sirvio et al., 2014).
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Cha, Cooksey, Chinnan, and Park (2003) prepared nisin-incorporated films and assayed the effect of casting and heat-press methods as film preparation methods and the type of edible
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films (methylcellulose (MC), hydroxypropyl methyl cellulose (HPMC), κ- carrageenan and chitosan) on antimicrobial activity of the films. Results showed that heat-pressed films exhibited
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less inhibitory zones in comparison with casting method. The antimicrobial activity of nisinincorporated MC films was most effective in heat-pressed films as well as chitosan films in the cast films category. Casting method was also used for preparation of polycaprolactone/alginate
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based antimicrobial films (Takala et al., 2013) and for production of pectin/ κ- carrageenan
5.2. Edible coating formation methods 5.2.1. Spraying method
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composite films containing organically modified nanoclays (Falguera et al., 2011).
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This method can be used for low viscosity coating solutions, which can be easily sprayed
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at high pressure (60–80 psi) (Dhanapal et al., 2012; Tharanathan, 2003).The drop-size distribution of sprayed coating-forming solution in classic spraying system can be up to 20 μm, whereas electrospraying can produce uniform particles of less than 100 nm from polymer and
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biopolymer solutions. Furthermore, the formation of polymeric coatings by spraying systems can be affected by other factors such as drying time, drying temperature, drying method, and so on. (Skurtys et al., 2010). 5.2.2. Dipping method
Among different coating formation methods, only dipping techniques can form high thick coating (Dhanapal et al., 2012). Dipping method is used to form coatings on fruits, vegetables, and meat products (Lu, Ding, Ye, & Liu, 2010; Tharanathan, 2003). Properties such as density, viscosity, and surface tension of coating solution are important to estimate the film thickness (Skurtys et al., 2010). In this method, a thin membranous film is formed over the product surface by directly dipping the product into the aqueous medium of coating formulations, removing, and allowing to air dry. Foam application method is another coating formation method. This method
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is usually used by applying emulsions. In here the foam will break by extensive tumbling action, and therefore uniform distribution of the coating solution will be over the product surface (Tharanathan, 2003). Dipping method has been used to coat papaya fruit by using κ-carrageenan (Hamzah et
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al., 2013) and to coat carrot by using sodium alginate (Mastromatteo et al., 2012).
The formation of edible coatings on minimally processed fruits is problematic due to the
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difficulty of obtaining a good coating adhesion to the hydrophilic surface of the cut fruit. Multilayer technique is used to overcome it by the layer-by-layer electrodeposition. In here 2 or
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more layers of material are physically or chemically bonded to each other (Skurtys et al., 2010). The layer-by-layer method was used to produce multilayer antimicrobial alginate-based edible coating for fresh-cut watermelon (Citrullus lanatus) (Sipahi et al., 2013).A multilayer
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coating was also prepared for controlled release of polyethylene terephthalate by using κ-
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carrageenan and chitosan (Pinheiro, Bourbon, Quintas, Coimbra, & Vicente, 2012). 5.2.3. Spreading method
In spreading method or brushing method, the coating solution is spread on the product.
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González-Forte et al. (2014) used this method to coat dog biscuits with 2 different coatings. On
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one hand, they spread sodium alginate solution with a brush on the surface of biscuits and then sprayed a solution of CaCl2 to form a gel. On the other hand, they spread a gelatinized suspension of corn starch on the biscuit.
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6. Composite films and coatings
Edible film and coating materials must have favorable physicochemical and sensory properties to satisfy the consumers' product acceptability (Wang, Marcone, Barbut, & Lim, 2012). As each individual component has some defects, so applying one component with another one can cover their defects. Composites are hybrid materials, which their properties are different from their individual components of which they are made. For more than one hundred years, many new composite materials had been produced annually (Simkovic, 2013). It is possible to improve the technological properties of edible films and coatings made of single component by using chemical reactions (such as cross linking) or physical treatments (such as ultrasound, heat or radiation) (Wang et al., 2010). However, film functionality can be improved by combining different proteins, polysaccharides, lipids, as well as synthetic polymers.
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A synergistic effect of combined features of pure components may be achieved by designing composite films and coatings. Mechanical and barrier properties of composite films and coatings depend on properties of each forming polymers and their compatibility (Bourtoom, 2008; Galus
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& Lenart, 2013). 6.1. Carrageenan/ alginate – lipid composite films and coatings
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Composite films containing both lipid and polysaccharide can form packaging with good mechanical and water barrier properties (Hambleton et al., 2012). In here, lipids can be either
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used to obtaining an emulsified film by dispersing it in polysaccharide aqueous solution and then drying or can be cast as a layer on the polysaccharide film to obtain a bilayer film (Hambleton et al., 2012; Marcuzzo et al., 2010). The prior is used in most of food industry applications because
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it can improve water barrier properties and only need one step in manufacture in comparison to 3 step for latter (Hambleton et al., 2009; Hambleton et al., 2012). In addition, by using lipids to
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form emulsified films, active molecules can be encapsulated.
Carrageenan emulsion-based film was prepared to encapsulate different aroma compounds. The fat was added to the plasticized film-forming solution composed of carrageenan
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and glycerol. A mix of 10 aroma compounds was pre-solubilized in melted fat before being
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dispersed into the film-forming solution. Glycerol monostearate (GMS) was used as emulsifier. It was found that carrageenan films as encapsulating matrixes have better performances for retention of more polar aroma compounds. They can retain volatile compounds during film
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formation, and release gradually with time (Marcuzzo et al., 2010). Hambleton et al. (2012) measured permeability, sorption, and diffusion coefficient of the n-hexanal and D-limonene aroma compounds through emulsified and non-emulsified ι- carrageenan and sodium alginate based films. In another study, Fabra et al. (2012) analyzed the release of n-hexanal and Dlimonene from ι- -carrageenan films with and without lipid. They showed that D-limonene was encapsulated in the lipid phase of edible films and its release was decreased in the salt medium. 6.2. Carrageenan/alginate- polysaccharide composite films and coatings
Alves et al. (2010) studied the barrier properties of a polymeric matrix composed of κ- carrageenan and pectin with the inclusion of mica flakes. The effect of adding water as plasticizer on barrier properties of these composite films was also studied. Results demonstrated that there is a significant decrease of CO2 and O2 permeability in the films without mica flakes.
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Furthermore, above 10% of mica flakes in the composite, the barrier properties of the films were decreased. It was interested that a significant increase of both CO2 and O2 permeability was observed in hydrated films with 25% water. Biocomposite films based on 4 different wood cellulose fibers and alginate were prepared
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by Sirvio et al.(2014) and the film properties were studied. There was a good interaction between celluloses and alginate matrix. By increasing the amount of the fiber in the biocomposite films,
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thickness, tensile strength, and specific modulus of films were increased but strain and flexibility of films decreased and the mechanical properties of films were increased by increasing the
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amount of the fiber in the biocomposite films. Osorio, Molina, Matiacevich, Enrione and Skurtys (2011) produced edible film coating for fresh blueberries by incorporation of κ-carrageenan, plasticizer, and carnauba wax emulsion into the hydroxy propyl methyl cellulose and studied the
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film structure and functional properties. An increase in film structure and functionality was seen as a result. Martins et al. (2012) developed κ-carrageenan/locust bean gum blend films by casting
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method and studied films physical properties. The addition of κ- carrageenan to locust bean gum improved the barrier properties of the films. For example water vapour permeability of the films decreased. Moreover, the tensile strength of blend films enhanced. It is suggested that hydrogen
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films properties.
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bonds interactions between κ- carrageenan and locust bean gum have a significant influence in High cost of some polymers such as pullulan has limited their use. Blending them with other abundant polymers can overcome this problem. Alginate was successfully incorporated
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into the pullulan films by Xiao, Lim, & Tong (2012). It reduced not only the cost of pullulan films, but also potentially enhanced the material properties of the blend films. 6.3. Carrageenan/alginate-inorganic particles composite films and coatings
The addition of inorganic impermeable particles (for example mica flakes) into the polymer matrix enhances the barrier properties of the films. However, it makes the films less transparent and therefore not suitable for packaging applications. Reducing the size and the amount of the particles can overcome this drawback. Nanocomposites have obtained significant attention due to their improvement in the mechanical and physical properties (Alves et al., 2011). In the research done by Alves et al. (2011), composite film composed of pectin, κcarrageenan and modified nanoclays was prepared by casting method. Results indicated that the addition of 10% organically modified nanoclay particles to the polymer matrix has a positive
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impact on the barrier properties of the films. Rhim (2013) prepared a multilayer film with agar/ κ- carrageenan/clay nanocomposite and polylactide. The result indicated that the film properties of prepared film such as optical, mechanical, barrier, and thermal stability properties were
6.4. Carrageenan/alginate-fruit purees composite films and coatings
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improved.
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Production of edible films from fruit purees has some elaborations due to the presence of film-forming polysaccharides in their combination. The mechanical and barrier properties of
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those polysaccharides combine with the sensory and nutritional properties of the fruit in these films. Azeredo et al., 2012 evaluated the properties of the edible films from alginate-acerola puree reinforced with cellulose whiskers. Results showed that the whiskers could improve the
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water vapour barrier and overall tensile properties (except by elongation) of the films.
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7. Combination of different components with edible films and coatings Edible films and coatings can incorporate other components such as antibrowning, antimicrobial, antioxidant, and texture modifier agents (Bonilla et al., 2012; Osorio, Molina,
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Matiacevich, Enrione, & Skurtys, 2011; Zactiti, & Kieckbusch, 2006), colorants, flavors,
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nutrient, spices (Kang et al., 2013; Nussinovitch, 2009; Mastromatteo et al., 2012), surfactants, emulsifiers plasticizers, and so on. (Osorio, Molina, Matiacevich, Enrione, & Skurtys, 2011). In the other hand, blending with other food additives and modifier agents can extend applications of
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edible films and coatings. This is because of the weak mechanical properties (tensile strength and tensile elongation) and poor barrier properties of biodegradable films and coatings (Lee et al., 2004; Sorrentino, Gorrasi, & Vittoria, 2007) .For example, hydrocolloids are hydrophilic materials, and so are poor moisture barriers (Alves et al., 2011; Atarés et al., 2011) This property can be compensated by adding lipids, which are very good moisture barriers (Tharanathan, 2003).
7.1. Plasticizers Plasticizers are non-volatile and low-molecular weight compounds, which are added to polymers in order to reduce brittleness, impart flow and flexibility, and enhance toughness and strength for films. As a specific definition for coatings, plasticizers impact resistance of the coating and reduce flaking and cracking by improving coating flexibility and toughness. As a
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disadvantage, plasticizers generally increase film permeability to oxygen, moisture, aroma, and oils due to reducing intermolecular attractions along the polymer chains (Barreto et al., 2003; Rojas-Graü et al., 2007; Sothomvit & Krochta, 2005). Small size, high polarity, more polar groups per molecule, and more distance between
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polar groups within a molecule are plasticizer characteristics, which enhance plasticizing effects on a polymeric system. Plasticizers are generally required for polysaccharides or proteins based
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edible films (Skurtys et al., 2010).
Monosaccharides, disaccharides, or oligosaccharides (for example. fructose-glucose
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syrups, sucrose, honey, corn syrup) are commonly used as plasticizers in film systems (Azeredo et al., 2012). Polyols (for example glycerol, sorbitol, glyceryl derivatives, and polyethylene glycols) (Galus & Lenart, 2013; Hambleton et al., 2012; Hamzah et al., 2013; Myllärinen,
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Partanen, Sppälä, & Forssell, 2002; Song et al., 2011), lipids (Flores, 2011)), and derivatives (for example phospholipids, fatty acids, and surfactants) are also used as plasticizers ( Sothomvit &
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Krochta, 2005). Water can act as a plasticizer in hydrophilic biopolymers due to disruption of
7.2. Antimicrobial agents
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hydrogen bonds between polymer chains (Xiao et al., 2012).
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The incorporation of antimicrobial agents into the edible films and coatings has demonstrated to act as a stress factor to decrease pathogen growth and to protect foodstuff against spoilage flora. The use of chemical antimicrobial agents such as benzoic acid, sodium
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benzoate, propionic acid, sorbic acid, and potassium sorbate is limited in food systems due to health concerns of consumers. (Mastromatteo et al., 2012; Skurtys et al., 2010). Therefore, consumers demand for natural and healthy preservatives is caused to use of generally recognized as safe (GRAS) compounds (Flores, 2011). The most frequently used biopreservatives for antimicrobial packaging are lysozyme and nisin (Skurtys et al., 2010). Other antimicrobial compounds include organic acids (lactic, acetic, malic, and citric acids), chitosan, the lactoperoxidase system, and some plant-derived secondary metabolites such as essential oils and phytoalexins. Cassia, clove, garlic, sage, oregano, pimento, thyme, rosemary, lemongrass, scutellaria, and forsythia suspense are examples of such plants (Flores, 2011; Mastromatteo et al., 2012; Shojaee-Aliabadi et al., 2014). Edible films and coatings with antimicrobial properties can be named as active packaging (Juck et al., 2010).
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Metal nanomaterials as a noble kind of antimicrobial agents for food packaging system have received increasing attention in recent years. Among metal nanomaterials, nanosilver has been shown to be a promising antimicrobial material. Nanosilver particles can interrupt bacterial processes such as respiration and cell division and finally lead to cell death by attaching to the
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cell membranes and penetrating into bacteria. Bactericidal activity of nanosilver enhances by release of silver ions within bacterial cells. Jiang et al. (2013) investigated the effect of
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alginate/nanosilver coating on the microbial and physicochemical quality of shiitake mushroom during storage. Results indicated that the alginate/nanosilver coating has beneficial effect on the
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quality of shiitake mushroom and therefore, could be used for its preservation and shelf-life expansion. The antimicrobial property of alginate/clay nanocomposite films containing 3 essential oils (marjoram, clove, and cinnamon) against food pathogens was studied. Results
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showed that in all films marjoram had the highest antimicrobial activity (Alboofetileh et al., 2014).
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Azarakhsh et al. (2014) studied the influence of adding lemongrass essential oil to alginate based edible coating in microbial and physicochemical of fresh-cut pineapple. Results indicated that the incorporation of 0.3% (w/v) lemongrass in coating had potential to access both
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extension in the shelf-life and maintenance the quality of fresh-cut pineapple. In another study
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Takala et al. (2013) incorporated two antimicrobial formulations into alginate films, namely, A and B. The former contained organic acids mixture, rosemary extract, and Asian spice essential oil and the latter contained organic acid mixture, rosemary extract, and Italian spice essential oil.
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The inhibitory effect of each formulation on the growth of Listeria monocytogenes, Escherichia coli, and Salmonella typhimurium was investigated on fresh broccoli stored at 4°C. Results showed a good inhibitory capacity of alginate films containing formulation A against growth of mentioned bacteria.
Olaimat et al. (2014) evaluated the bactericidal property of allyl isothiocyanate incorporated into κ-carrageenan/chitosan coating against Campylobacter jejuni on fresh chicken. They found that κ-carrageenan/chitosan coatings containing allyl isothiocyanate had excellent potential to reduce C. jejuni viability on raw chicken. Chitosan and κ-carrageenan are oppositely charged polysaccharides, so a mixture of them has good barrier properties and can cause delayed release of incorporated bioactive compounds. The effect of incorporation of antimicrobial agents including nisin, Novagard CB 1, Guardian NR100, sodium lactate, sodium diacetate, and potassium sorbate into edible coatings
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including alginate, κ-carrageenan, pectin, xanthan gum, and starch on growth control of Listeria monocytogenes in poached and deli turkey products was studied by Juck, Neetoo and Chen (2010). Results showed that alginate based antimicrobial coatings can enhance the microbiological safety and quality of turkey poultry products by inhibiting the growth of Listeria
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monocytogenes. Meanwhile, the incorporation of nicin/sodium lactate and sodium lactate/sodium diacetate into the alginate coating effectively inhibited the growth of this pathogen. In another
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study, Mastromatteo et al. (2012) investigated the effectiveness of combined use of ethanol as antimicrobial compound and alginate based coating on the shelf-life of fresh carrots packed
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under passive and active modified atmosphere packaging. Results showed that the combination of dipping in ethanol and application of an alginate coating controlled both dehydration and respiration of sliced carrots. So, it caused a good preservation of sensory properties and
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prolonged the shelf-life of carrots.
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7.3. Antioxidant agents
Adding lipids to edible films and coatings in order to reduce water vapour transfer is popular. So, incorporation of antioxidants in edible films and coatings materials leads to increase
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product shelf-life by protecting foods against oxidative rancidity, degradation, and discoloration.
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It is interested that most antimicrobial agents have antioxidant properties. Natural and synthetic antioxidants are widely used in food packaging (Skurtys et al., 2010) Phenolic compounds, vitamins E and C, essential oils (Oregano and rosemary), sodium ascorbate, citric acid, and
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ferulic acid are the most common used antimicrobial compounds (Bonilla et al., 2012; Song et al., 2011).
Blanco-Pascual et al. (2014) extracted edible film-forming materials from brown seaweeds Laminaria digitata and Ascophyllum nodosum and studied the antioxidant activity of extracts. Results indicated that Ascophyllum nodosum films had higher antioxidant activity than Laminaria digitata. In another experiment Song et al. (2011) investigated the effect of incorporation of different antioxidants (vitamin C and tea polyphenols) into the alginate based edible coating on shelf-life and quality of refrigerated bream (Megalobrama amblycephala). They found that quality of treated product with vitamin C and tea polyphenols was higher than untreated ones. Vitamin C had the best effect in the case of reducing the degree of chemical spoilage, retarding water loss, and enhancing the overall sensory values of bream.
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7.4. Antibrowning agents
Great efforts have been done to increase the shelf-life of fresh cut fruits and vegetables by preventing cut surface browning. Incorporation of antibrowning agents into the films and coatings can improve colour preservation of fruits and vegetables. Ascorbic acid, citric acid, and
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some sulfur-containing amino acids (cysteine and glutathione) have been widely incorporated into edible coatings to prevent enzymatic browning (Conte, Scrocco, Brescia, & Del Nobile,
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2009; Robles-Sánchez et al., 2013)
The ability of alginate and gellan based edible coatings to carry antibrowning agents (N-
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acetylcysteine and glutathione) and the effect of antibrowning agents on water vapour resistance of coatings for fresh-cut apples were investigated by Rojas-Graü, Tapia, Rodríguez, Carmona and Martin-Belloso (2007). Results indicated that both antibrowning agents could be carried in
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the coatings and exerted their effects on fresh-cut apples. N-acetylcysteine had positive effect on water vapour resistance in the alginate coating, while in the gellan coating, its effect was
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negatine. Citric acid as an antibrowning agent was incorporated into the sodium alginate coating. The coating was used to prolong the shelf-life of minimally processed lampascioni. Results showed that the respiratory activity and the browning process of coated lampascioni were
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delayed. Result also indicated that the coated lampascioni packaged in a polyester based produce (Conte et al., 2009).
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biodegradable film showed the best performance in prolonging the shelf-life of the fresh-cut In another research, ascorbic and citric acid were used as antibrowning agents in alginate
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edible coating and their effects on color of fresh-cut mangoes was evaluated. The application of alginate coating with antibrowning agents maintained the color of fresh-cut mangoes (RoblesSánchez et al., 2013). 7.5. Other agents
Functional ingredients such as probiotics (Tapia et al., 2007), prebiotics (Rößle, Brunton, Gormley, Wouters, and Butler, 2011), minerals (Rhim, and Wang, 2014), and vitamins (RoblesSánchez et al., 2013) are other agents incorporated into the edible films and coatings in order to increase their functionalities. Edible films and coatings can also be used as a carrier to convey nutrients and nutraceuticals that are lacking or are present in only low quantity in food products (Rößle et al., 2011; Tapia et al., 2007). Flavour and pigments agents may also be incorporated
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into the edible films and coatings to improve the sensory quality of products (Skurtys et al., 2010) 8. Advantages and adverse biological effects
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As mentioned before, edible films and coatings should have no toxic effect on biological systems. Some known positive biological effects of carrageenans such as antitumor,
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immunomodulatory, anticoagulant, antithrombotic, and antiviral activities have been known. Furthermore, the antioxidant activity of all carrageenans especially lambda carrageenan is
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reported (Campo, Kawano, da Silva, & Carvalho, 2009; Prajapati et al., 2014).
Despite the positive biological effects, some researchers have focused on the adverse biological effects of carrageenans. These effects are mostly related to degraded carrageenan
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(poligeenan) which have molecular weights below 50 kDa (Chen et al., 2010; Liu, Zhan, Wan, Wang, & Wang, 2015; Prajapati et al, 2014). However, Tobacman (2001) reported the
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carcinogenic effects of undegraded carrageenans.
Degraded carrageenans are responsible for intestinal inflammation (Tobacman, 2001) and also, they are toxic to macrophages (Thomson & Horne, 1976) . Feeding 2 g/Kg body weight of
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degraded carrageenan to guinea pigs for 20-45 dayes caused ulcerative lesions (Prajapati et al,
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2014). Silva et al. (2010) evaluated the inflammatory properties of kappa, iota and lambda carrageenans. Lambda carrageenan was seen to have the most inflammatory activity and the least activity was reported for kappa carrageenan. Their results demonstrated that an increase in the
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sulfate groups led to inhanced inflammatory activity of carrageenan. However, toxicological properties of carrageenans have been shown at high doses that do not occur with the food additive. It is necessary for food grade carrageenan to have average molecular weight greater than 100,000 Da with a low percentage of smaller (Cohen & Ito, 2002; Prajapati et al, 2014).
Unlike carrageenans, the majority of articles have been just mentioned the advantages of alginate. However, it should be noted that only highly purified alginates have been reported to have no immunogenic response in mice. This means some impurities including heavy metals, endotoxins, proteins, and polyphenolic compounds may cause immunogenic responses (Lee, & Mooney, 2012). Alginates are known as dietary fiber (Houghton et al., 2015). The positive effects of alginates on intestinal absorption and colonic health have been reported by Dettmar, Stugala, &
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Richardson (2011). Some research has been reported the pharmaceutical activity of alginate molecule. This ability refers to alginates with high mannuronate residues which could induce cytokine production 10 times more than alginates containing high guluronate blocks (Lee, &
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Mooney, 2012). Conclusion
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Consumer demands for more natural foods, and also for environmental protection have excited the development of new packaging materials. Edible films and coatings have been
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received considerable attention over the last years due to their possibility to use as edible packaging materials over synthetic ones and to reduce the environmental pollution. Besides, development of edible films and coatings can reduce the post-harvest losses and also can provide
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less expensive packaging materials for industry and lower the prices of the food products. Edible films and coatings are defined as thin layers of materials used on food products that have
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important effects on their conservation, distribution, and marketing. Edible films and coatings can protect the product from mechanical damage, physical, chemical, and microbiological activities. Such films can be a carrier of antioxidants, antimicrobial, nutraceuticals, and
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flavorings agents or other additives to improve the mechanical integrity, handling, and quality of
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food products. Different biopolymers such as polysaccharides, proteins, and their blends are applied to form edible films and coatings. Among these biopolymers, alginates and carrageenans have been frequently used in recent years due to their good barrier properties to oxygen, carbon
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dioxide, and lipids as well as their superb mechanical properties (tensile strength and elongation at break). Commercialization of biopolymer films is limited due to their high sensitivity to moisture and their compatibility with other emergent stress factors such as high pressures, electric fields, ultrasound, microwave radiation, and gamma radiation. Technical information on edible films and coatings is far from adequate, so the food scientist has the formidable task of developing a film for each food application. Acknowledgments The authors would like to acknowledge the Isfahan University of Technology for providing an excellent library facilities and support for the preparation of review article.
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Figure caption: Figure 1. Structural characteristics of alginates: (a) alginate monomers, (b) chain conformation, (c) block distribution (Draget, & Taylor, 2011). Figure 2. Formation of “eggbox” during gelation of alginate (Schnepp, Wimbush, Mann, & Hall, 2010).
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Figure 3. Intermolecular and intramolecular interactions of alginate and metal cations (Hassan, Gobouri, & Zaafarany, 2013).
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Figure 4. Two gel preparing methods: diffusion setting (a) (Juarez, Spasojevic, Faas, & de Vos, 2014), internal setting (b) (Tumarkin, & Kumacheva, 2009).
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Figure 5. Chemical structures of carrageenans.
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Figure 6. The gelation mechanism of κ-carrageenan in the presence of potassium ions (Rhein-Knudsen et al.,2015).
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Table 1
Table 1. Main hydrocolloids used for the formation of edible films and coatings (Milani & Maleki, 2012; Skurtys et al.,
Principal function
Agar (E406)
Gelling agent
Alginate (E400-404)
Gelling agent
Carrageenan (E 407)
Gelling agent
Carboxymethyl cellulose (E466)
Thickener
Hydroxypropyl cellulose (E463)
Thickener and emulsifier
Hydroxypropyl cellulose (E464)
Thickener
us
cr
ip t
Type of hydrocolloid
Methyl cellulose (E461)
Thickener,
emulsifier
and
gelling agent
an
Polysaccharide
2010).
chitosan
Gelling
and
antimicrobial
agent
M
Arabic gum (E414) Guar gum (E412)
starches
Fish gelatin
Ac ce p
protein
Bovine gelatin
te
Pectin (E440)
d
Xanthan gum (E415)
Emulsifier Thickener Thickener Gelling agent Thickener and gelling agent Gelling agent Gelling agent
Pig gelatin
Gelling agent
Whey protein
-
Page 33 of 42
Table 2
Table 2. Various edible films and coatings from alginates and carrageenans Type of hydrocolloid
Application
Product
Main results
Alginate
Film
Microwaveable
Increasing
food
efficiency
Coating
Fuji apple
Less browning
Packaging(film)
Broccoli
Inhibitory effect on growth
Takala,
of pathogens
Khan, and Lacroix (2013)
warming
Alginate/clay
nanocomposite/
Film
cr
-
Coating
Alginate/Lemongrass essential oil
Hosseini, and Abdollahi
turkey products
monocytogenes
Coating
(2014)
of
listeria growth,
Ac ce p Coating
Κ-carrageenan/pectin/mica flakes
Κ-carrageenan/chitosan/
model
Film
Coating
Juck, Neetoo, and Chen (2010)
good adherent and stable film Less chemical and bacterial
Song, Liu, Shen, You, and
bream
spoilage
Luo (2011)
Extention shelf life
Azarakhsh,
fresh-cut pineapple
Carrageenan
Rezaei,
refrigerated
d
coating
growth
Inhibition
te
Alginate/antioxidant
Salmieri,
Alboofetileh,
Poached and deli
M
Alginate
Vu,
Inhibitory effect on bacterial
an
essential oil
and
Rojas-Graü et al. (2007)
us
antimicrobial compounds
Salvador,
Fiszman (2012)
agent polycaprolactone/alginate/
Albert,
ip t
Alginate/gellan/antibrowning
Source
Osman,
Ghazali, Tan, and Mohd Adzahan (2014)
Papaya
-
Reduction
moisture
loss,
Hamzah, Osman, Tan,
delayed ripening, retention
and Mohamad Ghazali
of firmness
(2013)
Improvement
of
barrier
Alves et al. (2010)
properties -
Dependent
release
of
bioactive compound (methylene
methylene
blue
on
blue)
concentration gradient and polymer
relaxation
Pinheiro et al. (2012)
of
nanolayers Cellulose/alginate
Film
-
Good mechanical and barrier
Sirvio,
Kolehmainen,
Page 34 of 42
properties
Liimatainen,
Niinimaki,
and Hormi (2014) Alginate/pectin
Film
-
Homogenous
and
Galus and Lenart (2013)
transparent film Coating
Dog biscuits
-
González-Forte,
Bruno,
ip t
Alginate
and Martino (2014)
alginate/
i-carrageenan
Film
-
Dependence of permeability,
Hambleton et al. (2011)
cr
Sodium
/aroma compound
diffusion, and structure of
us
film on aroma compound characteristics Film
-
-
isolate (WPI) /gelatin(G) Κ-carrageenan/agar/clay
Film
-
Coating
(2010).
Improving film properties
Rhim (2013)
UV-activated
Reduction of dye leaching
Vu and Won (2013)
oxygen indicator
into water
M
/polylactide Alginate
Wang, Auty, & Kerry,
an
Sodium alginate (SA) /whey protein
Film
alginate-acerola puree/cellulose
Alginate-apple essential oils
Film
Ac ce p
Κ-carrageenan/locust bean gum
puree/
plant
-
te
Alginate/pullulan
d
films
Film
Film
-
Moisture sensitive
Xiao,
Lim,
and
Tong
(2012) Improving barrier properties
Martins et al. (2012)
and tensile strength -
Improvement of tensile and
Azeredo, Miranda, Rosa,
water
Nascimento,
vapor
barrier
properties of films -
Antimicrobial
and
de
Moura (2012)
activity,
no
Rojas-Graü et al. (2007)
adverse effect on mechanical properties of film
alginate/nano-Ag
Coating
Shiitake
Spoilage
mushroom
improvement
Reduction, of
sensory
Jiang, Feng, and Wang (2013)
atribiutes, lower weight loss i-carrageenans
film
Encapsulating
Suitable
aroma compound
encapsulation
for
flavor
Hambleton,
Fabra,
Debeaufort,
Dury-Brun,
Page 35 of 42
and Voilley (2009) i-carrageenans/ aroma compound
film
-
Controlled release of aroma
Marcuzzo,
componds,
Debeaufort, and Voilley
encapsulation
Coating
cinnamaldehyde
fresh-cut
Shelf-life
watermelon
extention,
Sipahi,
Castell-Perez,
Inhibitory effect on growth
Moreira,
Gomes,
of
psychrotrophics,
Castillo (2013)
Robles-Sánchez,
ip t
Alginate/beta-cyclodextrin/ trans-
(2010)
cr
capacity
Sensidoni,
and
coliforms, yeast and molds Coating
Fresh-cut
Kent
Coating
Alginate/chitosan
Film
te
glucomannan
compound in water
breasts
Ac ce p
agar/k-carrageenan/konjac
aroma compound
d
isothiocyanate/ mustard extract
Fast
chicken
Film
Fresh spinach
release
Rojas-
Graü, Odriozola-Serrano, González-Aguilar,
and
Martin-Belloso (2013)
Encapsulating
Fresh
high
antioxidant activity
M
film
κ-carrageenan/chitosan/allyl
retention,
an
mangoes
i-carrageenan
Color
us
Alginate /ascorbic acid/ citric acid
of
aroma
Fabra, Chambin, Voilley, Gay,
and
Debeaufort
(2012) Inhibition of C. jejuni, lactic
Olaimat,
Fang,
acid bacteria, and aerobic
Holley (2014)
and
bacteria growth Antimicrobial
and
Rhim and Wang (2013)
antifogging film with high water holding capacity
-
Rougher structure
Arzate-Vázquez
et
al.
(2012)
Page 36 of 42
Ac ce p
te
d
M
an
us
cr
ip t
Figure 1
Page 37 of 42
Ac ce p
te
d
M
an
us
cr
ip t
Figure 2
Page 38 of 42
Ac ce p
te
d
M
an
us
cr
ip t
Figure 3
Page 39 of 42
Ac ce p
te
d
M
an
us
cr
ip t
Figure 4
Page 40 of 42
Ac ce p
te
d
M
an
us
cr
ip t
Figure 5
Page 41 of 42
Ac ce p
te
d
M
an
us
cr
ip t
Figure 6
Page 42 of 42
S0144-8617(15)01054-1 http://dx.doi.org/doi:10.1016/j.carbpol.2015.10.074 CARP 10491
To appear in: Received date: Revised date: Accepted date:
20-7-2015 20-10-2015 21-10-2015
Please cite this article as: Tavassoli-Kafrani, E., Shekarchizadeh, H., and Masoudpour-Behabadi, M.,Development of edible films and coatings from alginates and carrageenans, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.10.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights (for review)
Ac
ce pt
ed
M
an
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1. Edible films and coatings can protect the product. 2. Such films can be a carrier of different additives. 3. Alginates and carrageenans films have good barrier and mechanical properties.
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*Manuscript
1
Development of edible films and coatings from alginates and carrageenans
2 Elham Tavassoli-Kafrani, Hajar Shekarchizadeh*, Mahdieh Masoudpour-Behabadi
3 4 5
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Abstract
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Technology, Isfahan 84156–83111, Iran
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The use of renewable resources, which can reduce waste disposal problems, is being explored to produce biopolymer films and coatings. Renewability, degradability, and edibility make such films particularly suitable for food and nonfood packaging applications. Edible films
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and coatings play an important role in the quality, safety, transportation, storage, and display of a wide range of fresh and processed foods. They can diminish main alteration by avoiding
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moisture losses and decreasing adverse chemical reaction rates. Also, they can prevent spoilage and microbial contamination of foods. Additionally, nanomaterials and food additives, such as flavors, antimicrobials, antioxidants, and colors, can be incorporated into edible films and
d
coatings in order to extend their applications. Water-soluble hydrocolloids like polysaccharides
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usually impart better mechanical properties to edible films and coatings than do hydrophobic substances. They also are excellent barriers to oxygen and carbon dioxide. Recently, there has been much attention on carrageenan and alginate as sources of film-forming materials. Thus, this
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6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Department of Food Science and Technology, College of Agriculture, Isfahan University of
review highlights production and characteristics of these films. Keywords: Edible films; Edible coatings; Alginates; Carrageenans
1. Introduction
The quantity of packaging materials has been increasing by 8% annually (MuiznieceBrasava, Dukalska, & Kantike, 2011). Less than 5% of all plastics are being recycled, leading to a high accumulation of plastics in the environment (Espitia, Du, Avena-Bustillos, Soares, & McHugh, 2014). Besides, increasing consumer concerns on food safety led to development of biodegradable, edible, and renewable films and coatings suitable for food and nonfood packaging applications (Alves, Costa, & Coelhoso, 2010; Espitia et al., 2014). However, due to the low cost *
Corresponding author. Tel.: +98 311 3913369; Fax: +98 311 3912254; E–mail: [email protected].
1 Page 2 of 42
of synthetic polymers, biodegradable materials had been ignored (Hambleton, Voilley, & Debeaufort, 2011). Today, with traditional agricultural commodities being a source of filmforming material, wide commercialization of biopolymer films has gained more significance (Arvanitoyannis, 2010).
ip t
Biopolymers such as polysaccharides, proteins, and lipids can be used for the formation of edible films and coatings (Albert, & Mittal, 2002; Espitia et al., 2014; Lee, Shim, & Lee,
cr
2004). They can be used as complement or replacement of traditional materials in order to reduce traditional polymeric packaging (Barreto, Pires, & Soldi, 2003; Mate, & Krochta, 1998).
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Edibility and biodegradability are the most beneficial characteristics of edible films and coatings. Edibility of films and coatings could be achieved if films and coatings components including biopolymers, plasticizers, and other additives be food grade ingredients. Meanwhile,
an
all the processes and equipment should be acceptable for food processing. To claim biodegradability of films and coatings, their toxicity and environmental safety must be evaluated
M
by standard analytical protocols (Han, J. H., & Aristippos, G. (2005). An edible coating is a thin layer of edible material formed as a coating on a food product (Kang, Kim, You, Lacroix, & Han, 2013), while an edible film is a preformed thin layer, made of
d
edible material, which can be placed on or between food components (Espitia et al., 2014;
te
Guilbert, Gontard, & Gorris, 1996). The main difference between these 2 food systems is that the edible coating is applied in liquid form on the food, usually by immersing the product in the solution of edible material, and edible film is first molded as solid sheets, then applied as a
Ac ce p
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
wrapping for food products (Falguera, Quintero, Jiménez, Muñoz, & Ibarz, 2011). The concept of employing edible films and coatings for foods dates back to 1950s. Their growing application is attributable to reduction of moisture loss, adverse chemical reactions (Baldwin & Wood, 2006; Osorio, Molina, Matiacevich, Enrione, & Skurtys, 2011), spoilage, and microbial contamination (Arvanitoyannis, 2010). Additionally, they can be used for controlled release of food additives (Barreto et al., 2003). Edible coatings are also effective as a postharvest treatment to preserve fruit quality (Valeroet et al., 2013). Hydrophobic substances such as resins, waxes, or some insoluble proteins are better moisture barriers, but water-soluble hydrocolloids like polysaccharides and proteins usually impart better mechanical properties (tensile strength and elongation at break) to edible films and coatings than do lipids and hydrophobic substances (Arvanitoyannis, 2010). They also are excellent barriers to oxygen and carbon dioxide (Nussinovitch, 2009) because of their tightly
2 Page 3 of 42
packed and ordered hydrogen-bonded network structure (Atarés, Pérez-Masiá, & Chiralt, 2011; Bonilla, Atarés, Vargas, & Chiralt, 2012). So, they can be used to extend the shelf-life of foods by preventing dehydration, oxidative rancidity, and surface-browning (Dhanapal et al., 2012). Besides, food hydrocolloids can act as nutritious food ingredients. Some health benefits include
ip t
lowering risk factors for cardiovascular disease, for immune function, for weight management, and for intestinal problems (Viebke, Al-Assaf, & Phillips, 2014). Table 1 shows the main
cr
hydrocolloids that can be used for the preparation of edible films and coatings. The uses of alginates and carrageenans in edible films and coatings are summarized in Table 2.
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In recent years, there has been much attention on carrageenan and alginate as sources in edible film formation (Cian, Salgado, Drago, Gonzalez, & Mauri, 2014). According to FDA, carrageenans and alginates are GRAS materials thereby they have been passed the mentioned
an
standards and are considered as edible films and coatings. To the best of our knowledge, there is no review article about carrageenan and alginate films and coatings. Thus, this review highlights
M
production and characteristics of these films. 2. Alginate
d
Alginate is an appealing film-forming compound because of its non-toxicity,
te
biodegradability, biocompatibility, and low price (Vu, & Won, 2013). Its functional properties, thickening, stabilizing, suspending, film-forming, gel-producing, and emulsion-stabilizing have been well studied (Dhanapal et al., 2012; Zactiti, & Kieckbusch, 2006).
Ac ce p
63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94
Alginic acid was first discovered in 1881 by Stanford. Around 1923, Thornley, in Orkney, UK, established a briquette business based on using alginate as a binder for anthracite coal dust. He moved to San Diego, USA, and by 1927 his company was producing alginate for use in sealing cans. After some difficulties, the company changed its name to Kelp Products Corp. and in 1929 it was reorganized as Kelco Company. Alginate production was established in the United Kingdom and Norway during 1934 to 1939 and after World War II, respectively (ITC 1981). The 2 largest producers, Kelco Company in the USA and Alginate Industries Ltd, in the UK were acquired by Merck and Co. Inc., USA. These combined companies produce about 70% of the world's alginate (McHugh, 1987). To extract the alginate, seaweed is broken into pieces and stirred with a hot solution of an alkali, usually sodium carbonate. After about 2 hours, the alginate dissolves as sodium alginate to give very thick slurry, and it contains undissolved parts of seaweed, mainly cellulose. The
3 Page 4 of 42
solution is diluted with very large amounts of water. Then, the solution is forced through a filter cloth in a filter press along with a filter aid such as diatomaceous earth. The last step is precipitation of the alginate from the filtered solution, either as alginic acid or calcium alginate (McHugh, 2003). Pretreatment (before alkaline extraction) of the seaweed with acid leads to a
ip t
more efficient extraction, a less colored product, and reduced loss of viscosity during extraction because lower amounts of phenolic compounds are present (McHugh, 1987).
cr
Based on its linear structure, alginate can form strong films and adequate fibrous structures in the solid state, hence, it is considered a good filmogenic material (Blanco-Pascual,
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Montero, & Gómez-Guillén, 2014). Alginates have many applications. They can be used in foods for limiting dehydration of meat (Varela, & Fiszman, 2011), fish and fruits (Hambleton et al., 2011), in beverage industries as thickening, gel-forming and colloidal stabilizing agents
an
(Liakos et al., 2013), in nonfood industries such as textile printing, as well as manufactories welding rods, binders for fish feed, immobilized biocatalysts (carrying enzymes ), release agents,
M
and paper, and also for pharmaceutical and medical uses (Skurtys et al., 2010; Vu, & Won, 2013). Alginates can also be employed as matrix polymers for encapsulation of drugs, proteins, cells, and DNA (Ashikin, Wong, & Law, 2010).
d
Alginate can be isolated from brown algae (Laminaria digitata and Ascophyllum
te
nodosum) cell walls where it is present as the calcium, magnesium, and sodium salts of alginic acid (McHugh, 2003). It can also be synthesized by microorganisms (Alboofetileh et al., 2014; Blanco-Pascual et al., 2014). Alginate is a linear anionic (Vu, & Won, 2013) water-soluble
Ac ce p
95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125
polysaccharide (Hambleton et al., 2011) consisting of monomeric units of 1-4-linked α-Dmannuronate (M) and ß-L-guluronate (G) (Fig. 1a,b) (Alboofetileh et al., 2014; Blanco-Pascual et al., 2014). The polymer chain of alginate is made up of 3 kinds of regions or blocks in different proportions and different distributions in the chain (Fig. 1c). In these the physical properties of alginates depend on the relative proportion of these 3 blocks (Liakos et al., 2013; Xiao, Gu, & Tan, 2014). The G blocks contain only units derived from L-guluronic acid which cause greater gel strength, the M blocks are entirely based on D-mannuronic acid, and the MG blocks consist of alternating units of D-mannuronic acid and L-guluronic acid which determine the solubility of alginates in acid (McHugh, 1987). M and G blocks are known as homopolymeric blocks and MG blocks are heteropolymeric blocks (Gómez-Ordóñez & Rupérez, 2011). The relative fraction of each unit depends on species, part, and age of seaweeds from
4 Page 5 of 42
which the alginate is isolated (Ashikin et al., 2010). The biological source, growth, and seasonal conditions are also determinative (Zactiti, & Kieckbusch, 2006). Alginate monomer composition and sequence affect seriously the final properties of alginate gels since selective binding of ions is a pre-requisite for gel formation (Draget, &
ip t
Taylor, 2011). The M/G ratio and distribution of M and G blocks in the chain of alginate affect the physical properties of the alginate. M/G ratios < 1 indicate a large amount of guluronic acid,
cr
which has the ability to form strong junctions. M/G ratios > 1 are indicative of a lower guluronic proportion, which might result in softer, more elastic structures (Blanco-Pascual et al., 2014).
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Alginate solutions can form gels either by lowering the pH below the pKa value of the guluronic residue or in the presence of divalent ions (Hambleton, Perpiñan-Saiz, Fabra, Voilley, & Debeaufort, 2012; Hambleton et al., 2011). These ions include calcium, magnesium,
an
manganese, aluminum, and iron (Dhanapal et al., 2012). The affinity of alginates towards divalent ions decreases in the following order: Pb > Cu > Cd > Ba > Sr > Ca > Co, Ni, Zn > Mn
M
(Pawar, & Edgar, 2010).
The most useful and unique property of alginates, which causes the strong gel or lowsoluble polymer, refers to their ability to react with polyvalent metal cations, specifically calcium
d
ions (Zactiti, & Kieckbusch, 2006). These ions help the formation of association between M and
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G blocks. The length of G blocks determines the alginate ability and selectivity to form these interactions (Hambleton et al., 2012). M blocks and MG blocks are almost without selectivity. The diffusion of ions into the alginate solution causes an ion exchange process where the water
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soluble alginate (for example sodium or potassium form) has to exchange its counter-ions with Ca2+ to obtain a sol/gel transition. This ionic cross-linking will be form a rapid cold setting and heat stable gel. When forming alginate gels, two contiguous, diaxially linked guloronic residues form a cavity that act as a binding site for calcium ions (Cuadros, Skurtys, & Aguilera, 2012). This arrangement is pictured as the “eggbox” model (Fig. 2). The mechanical properties of films are directly related to the number of “eggbox” sites. As a result, film characteristics like water and mechanical resistant, barrier properties, cohesiveness, and rigidity can be improved (Zactiti, & Kieckbusch, 2006). However, the definite mechanism of alginate gelation is still under controversy. The polyvalent metal cations tend to chelate the carboxylate and hydroxyl groups of the alginate. This chelation is not just simple, but a sort of bridge between the metal ion and two carboxylate and one or more pairs of the hydroxyl groups occurs through partially ionic and partially
5 Page 6 of 42
coordinate bonds, respectively. Therefore, a two- stage mechanism has been suggested for such chelation by numerous studies (Fig. 3): first, the formation of strongly intramolecular linked dimer associations with important contributions from van der Waals and hydrogen bonding interactions in which the functional groups involved in chelation belong to the same chains,
ip t
followed by the formation of weaker intermolecular dimer associations in which the carboxylate and hydroxyl functional groups are related to different chains that display no particular
cr
specificity, and being mainly governed by electrostatic interactions (Cuadros, Skurtys, & Aguilera, 2012; Hassan et al., 2013; Li, Fang, Vreeker, & Appelqvist, 2007).
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3 kinds of alginate blocks have different interactions with cations. M blocks bind cations externally near their carboxylate groups, while G blocks integrate cations into pocket-like structures formed by adjacent G residues. In MG blocks, cations preferentially locate in a
an
concave structure formed by M-G pairs (Emmerichs, Wingender, Flemming, & Mayer, 2004). A direct mixing of alginates and polyvalent metal cations does not produce homogeneous
M
gels due to the very rapid and irreversible formation of junctions between these two components. The formation of gel lumps (fish-eyes) is the result of such mixing. The only exception is for very small volumes of alginate under high shear. To overcome this problem, the ability to control
d
the introduction of the crosslinking ions is essential. This control is possible by two different gel
te
preparing methods: diffusion and the internal setting. In the diffusion method (Fig. 4a), crosslinking ions, for example Ca2+, diffuse from an outer reservoir into an alginate solution. In the internal setting method (sometimes also referred to as in situ gelation) (Fig. 4b), an inert ion is
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converted into an active cation by a change of pH of the alginate solution or by a limited solubility of the calcium salt source.
In the diffusion method, the Ca2+ will first cross-link the film surfaces drawing the polymer chanis closer to form a less permeable surface to the diffusion of Ca2+. Thus, the diffusion method yields gels having a Ca2+ ion concentration gradient across the thickness, while internal setting gives gels with uniform ion concentrations throughout (Pawar, & Edgar, 2010). By increasing the cation concentration during gelation of alginate, a more densely crosslinked structure will be formed which causes a less porous structure as well as reduction in water content and permeability of the gel (Aslani, & Kennedy, 1996). However, there is an optimal amount of cross-linker that can be used and further increase in amount of cation employed does not exert significant changes in the gel properties (Chan, Lee, & Heng, 2006).
6 Page 7 of 42
Sodium alginate among the other kinds of alginates, can form films with the properties of being water-soluble, strong, glossy, tasteless, odorless, flexible, low permeable to oxygen and oils (Xiao et al., 2014). During the formation of alginate film by diffusion setting method, the Ca2+ in the cross-
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linking solution will first cross-link the film surface drawing the polymer chains closer to form a less permeable surface to the diffusion of calcium ions into the interior. So, using an optimal
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amount of cross-linker to produce matrix with the desired characteristics (instead of a matrix with a highly cross-linked surface and a less well cross-linked interior) is essential in diffusion
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setting method. In internal setting, by using CaCO3, as the source of calcium ions, the reaction between the acid and carbonate leads to the formation of CO2 which cause formation of cavities within the film. Therefore, prepared films and coating by diffusion setting which use external
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cross-linking are thinner films with smoother surface, greater matrix strength, stiffness and permeability than films and coatings prepared by internal setting which are internally cross-
M
linked (Chan et al., 2006). 3. Carrageenan
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Carrageenan and agar are 2 major groups of galactans presented in red seaweeds (de
te
Araújo et al., 2011). Carrageenans are natural hydrophilic polymers (Osorio, Molina, Matiacevich, Enrione, & Skurtys, 2011) with a linear chain of partially sulphated galactans, which presents high potential of film-forming. These sulphated polysaccharides are extracted
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from the cell walls of various red seaweeds (Skurtys et al., 2010). The most usual seaweeds for extraction of carrageenans are Kappaphycus alvarezii and Eucheuma denticulatum seaweeds (McHugh, 2003). However, some scientists extract carrageenans from Hypnea musciformis (Wulfen) Lamoroux seaweeds (Cosenza, Navarro, Fissore, Rojas, & Stortz, 2014) and Solieria filiformis (de Araújo et al., 2011). The main source of commercial carrageenan is Chondrus crispus species of seaweed known as Carrageen Moss or Irish Moss in England and Carraigin in Ireland. The name Carraigin was first used by Stanford in 1862 for the extract of Chondrus crispus. It has been used in Ireland since 400 AD as a gelation agent and as a home remedy to cure coughs and colds. The term “carrageenan” is more recent and has been used by several authors after 1950 (Necas & Bartosikova, 2013; Rosa, 1972). There are 2 different methods for carrageenan production. First, is the original method, based on extraction of carrageenan to an aqueous solution and after removing the filtrate
7 Page 8 of 42
containing seaweed residue, carrageenan should be recovered from the solution. This method was expensive and was only used until the late 1970s–early 1980s. In the second method, seaweeds are washed to remove solid impurities before treating with alkali to extract the carrageenan. After extraction, the dilute extracts (1–2 % carrageenan) are filtered, concentrated,
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and then precipitated with isopropanol to give a fibrous coagulum. The coagulum is pressed to remove solvent and washed. It is then dried and milled to an appropriate particle size.(McHugh,
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2003).
Carrageenan has many applications in food and even nonfood industries and is a high
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value functional ingredient in foods (Hambleton et al., 2009; Necas & Bartosikova, 2013). Carrageenan can be used as stabilizer (Hsu & Chung, 1999) in dairy products such as flavored milks (Necas & Bartosikova, 2013). They also can be used in water-based foods, meat products
an
(as oxygen barrier to retard lipid oxidation (Varela & Fiszman, 2011)), pet food (McHugh, 2003), infant food, and nutritional supplement beverages. The ability of suspending cocoa in
M
chocolate milk at very low concentrations (ca. 300 ppm) is unique in carrageenan (Necas & Bartosikova, 2013). They mostly have been used to delay microbial growth in gels containing antimicrobial agents (Varela & Fiszman, 2011). They are also used in pharmaceuticals,
d
cosmetics, printing, and textile industries (Cosenza et al., 2014). Preparation of edible films (Flores, 2011).
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using carrageenan is not abundant in literature because carrageenan is mostly used as a coating There are 3 main types of carrageenans differing in chemical structures and properties
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(Fig. 5): kappa carrageenan (κ-carrageenan) with a 3-linked, 4-sulfated galactose and a 4-linked 3,6-anhydrogalactose, iota carrageenan (ι-carrageenan) with a structure like the former, but with an additional sulfate ester group on C-2 of the 3,6-anhydrogalactose residue, and lambda carrageenan (λ-carrageenan) with a 2-sulfated, 3-linked galactose unit, and a 2,6-disulfated 4linked galactose unit (Cosenza et al., 2014). Carrageenans are water-soluble polymers which their solubility depend on the content of ester sulfate and the presence of potential associated cations. Higher levels of ester sulfate mean lower solubility temperature. Presence of cations such as sodium, potassium, calcium, and magnesium promote cation-dependent aggregation between carrageenan helices. The presence of anhydro-bridges in κ- and ι-carrageeenan reduces the hydrophilicity of the sugar residue and inverts the chair conformation and allows the carrageenan to undergo
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conformational transitions which are conducive to the gelation properties of carrageenans (Rhein-Knudsen, Tutor Ale, & Meyer, 2015). Iota carrageenan forms elastic and clear gels with no syneresis in the presence of calcium salts (McHugh, 2003). Its gels are thermoreversible (50–55 ˚C). The blending of iota carrageenan
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with calcium salts makes it a suitable hydrocolloid as a functional ingredient for stabilization, thickening, and gelation in dairy products such as milk gels and ice cream (Hambleton et al.,
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2009). It can form a three-dimensional network with double-helix chains. Each pair of helixes being 13.9 Å laterally spaced, therefore it has a compact, dense, and organized film structure
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(Hambleton et al., 2012). Iota carrageenan-based edible films have good mechanical characteristics. They are emulsion stabilizers and can decrease oxygen transfer, and limit surface dehydration and taste deterioration of fruits and cheeses (Hambleton et al., 2012). They can also
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be used as encapsulation agents for active substances (Hambleton et al., 2011). Lambda carrageenan dose not form gel and just forms high viscosity solutions (de Araújo et al., 2011).
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So, these polysaccharides are used as cold soluble thickeners in syrups, fruit drinks, pizza sauces, and salad dressings (Milani & Maleki, 2012). Kappa carrageenan is one of the most common forms of carrageenans, which can be used in foods. It has a double-helix conformation. The
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linear helical portions can associate and form a three-dimensional gel in the presence of
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appropriate cations. This gel is freeze-thaw stable. Kappa carrageenan is also able to interact with various food proteins through electrostatic interactions and increase their aggregation stability (Lopez-Pena & McClements, 2014). Kappa carrageenan forms strong and rigid gels
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with potassium salts as well as brittle gels with calcium salts. Kappa carrageenan gels are opaque, but by sugar addition they become clear (McHugh, 2003). The mechanism of gel formation of carrageenans which is a thermo-reversible gel depends on temperature and gel-inducing agents. Structure of carrageenans at high temperature (above 80 °C) is as random coil which is due to the electrostatic repulsions between neighboring chains. Upon temperature reducing, the conformation of chains changes to helical structure. Further cooling and presence of cations lead to intermolecular interactions between the carrageenan chains which cause aggregation of the helical dimers and formation of a stable three dimensional network. For κ-carrageenan and ι-carrageenan, typically potassium and calcium respectively stabilize the junction zones between the two helixes by binding to the negatively charged sulfate groups without hindering cross-linking of the two helices (Fig. 6). However, the charged sulfate
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esters on the other side of the λ-carrageenan cause an extensive conformation via a repulsion effect of the negative SO3− groups and inhibit gelation while promoting viscosity in the solution (Rhein-Knudsen, Tutor Ale, & Meyer, 2015). Carrageenans, especially poligeenan (degraded carrageenan) have been known to induce
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colonic inflammation, (Chen, Yan, Wang, Xu, & Zhang, 2010) and to be toxic to macrophages (Thomson & Horne, 1976). However, toxicological properties of carrageenans have been shown
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at high doses that do not occur with the food additive. Average molecular weight greater than 100,000 Da with a low percentage of smaller fragments is necessary for food grade carrageenan
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(Cohen & Ito, 2002; Prajapati, Maheriya, Jani, & Solanki, 2014). 4. Edible film and coatings applications
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An ideal edible coating forms a thin layer on the surface of the coated food product and supplies an effective barrier to water, vapour, moisture, or temperature. In addition, it does not
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absorb oxygen and forms a selective barrier to gases, significantly carbon dioxide (ChlebowskaŚmigiel et al., 2008). Alginate and carrageenan are highly hydrophilic. So, they present only a limited barrier to moisture. However, they are a good barrier to fats and oils. They are also good
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oxygen barriers and can provide protection against lipid oxidation (Varela, & Fiszman, 2011).
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The diffusion of compounds within edible films and coatings depend strongly on the physicochemical properties of the compounds such as molecular weight, structure, hydrophobicity, and polarity. Permeability of compounds is affected by the interactions between
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the compounds and the film matrices. Also the mobility of the diffusing molecule has an important role on the permeability. On the other hand the diffusion of compounds within alginates and carrageenans which are water-soluble polymers can be affected by water. Water is a solvent, so it can induce a modification of the matrix structure, such as swelling of polymers, and can modify the diffusion of molecules showing an affinity for water (Hambleton et al., 2011: Hambleton et al., 2012).
The diffusion of water soluble material is expected to occur through aqueous pores or channels in the alginate film matrix. A more cross-linked film has lower or smaller pores or channels. The distribution of cross-linkages in the matrix has a significant effect on substances permeation. Inhomogeneous alginate films impede substances permeation more than homogeneous films.
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Internal setting method produces a more uniformly cross-linked matrix. However, the liberation of CO2 forms microscopic cavities in internally cross-linked films which impede diffusion of substances. The cavities are air vesicles which disrupt liquid continuity in the matrix. The internal gelation method is useful in producing cross-linked alginate films and coatings for
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retarding or controlling release of material in comparison to diffusion setting (Chan et al., 2006). The functionality and performance of edible coatings mostly depend on the production
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method of coating and the coating ability to adhere to the product surface (Dhanapal et al., 2012). Edible films can be defined as a thin protective layer for food packaging, which can be
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eaten with food (Hambleton et al., 2009). Edible films can reduce the environmental pollution; improve the sensorial properties of packaged products, decrease moisture loss, and increase the nutritional value of the foods (Zactiti & Kieckbusch, 2006).
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Alginate and carrageenan can be used to form film coating for meat and meat products. The coatings can prevent shrinkage, microbial contamination, and surface discoloration by
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delaying moisture transport (Nussinovitch, 2009). The effect of alginate gel coating as an edible susceptor in combination with salt in microwaveable chicken nuggets was investigated by Albert et al. (2012). Results showed that salty alginate coating could act as an effective susceptor during
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heating by microwave and therefore decrease cooking times. Alginate and carrageenan based
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edible coatings are effective as post-harvest treatments to maintain quality of fruits such as tomato, peach, sweet cherry, and so on. Edible coatings are able to delay ripening and can extend shelf-life of products (Hamzah et al., 2013; Valero et al., 2013). Minimally processed products
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such as fresh cut fruits are susceptible to microbial spoilage. Edible coatings are able to control the microbial spoilage by preventing microbial proliferation and delaying respiration (Mastromatteo, Conte, & Del Nobile, 2012). Alginate and carrageenan film coatings are applied to carry different functional agents in order to improve their application. For example, alginate solution diminished dye leaching out of colorimetric oxygen indicator due to its ability to ion bind with the cation dye (Vu & Won, 2013). Fei Lue (2010) also indicated that the use of alginate coating with nisin and cinnamon could maintain quality of northern snakehead fish fillets. Application of nisin containing alginate film for sliced beef could reduce the amount of Staphylococcus aureus. It could be suggested that nisin containing edible films could control the growth of pathogens at the surface of ground beef or other meat products (Millette, Tien, Smoragiewicz, & Lacroix, 2007) .
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Hydrogels are highly hydrophilic polymer gels with the ability of swelling by absorbing and retaining large amounts of water without dissolving or losing their integrity in water. They can be used for food packaging. Rhim (2013) prepared hydrogel films composed of agar, κcarrageenan, konjac glucomannan, and nanoclay. This film showed a high water-holding and
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water vapour adsorptive capacity. These properties makes hydrogel film a suitable packaging
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film for highly moisture foods or foods contacting with high humidity condition. 5. Film/coating production methods
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Principal technologies for producing edible films are similar to those for thermoplastic structures: solvent casting and extrusion. Obviously, the conditions are different, but the principles are allied. For the coating production the main techniques are spraying and dipping
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(Skurtys et al., 2010).
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5.1. Edible film formation methods
One of the most used techniques to form edible films is solvent casting or wet process (Nussinovitch, 2009). Dispersions of edible materials are spread on a suitable base material and
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then let to be dried. During drying of the solution, solubility of the polymer decreases as a result
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of solvent evaporation, until polymer chains align themselves to form films (Skurtys et al., 2010). It is important to carefully control drying rate and environmental conditions due to their high influence on final thickness and structural characteristics of the resultant film (Flores,
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2011). Infrared drying can hasten the drying process and so is advantageous. Easy removing of film without any tearing and wrinkling is very important, which is dependent on the type of base material. For easy peeling off dried film from one edge of the base material, optimum moisture content (5–8%) is desirable (Tharanathan, 2003). The other technique is a dry process in which compression molding and extrusion are done, rely on the thermoplastic status of some polysaccharides and proteins at low moisture levels (Nussinovitch, 2009). Among three techniques of film production, only casting method exemplifies. Because films produced via this method are stand-alone. Hence, it is possible to evaluate the physical and chemical properties of film forming material alone (Lee, & Wan, 2006). Casting method has been used to form alginate films. Alginate reaction with calcium ions is fast, but casting to make film is difficult. So, 2 steps procedures are used to preparing casted
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films. The first step is to cast a partially dry alginate film solution and then immersing into a calcium solution. Alternatively, it is possible to spray calcium solution to the pre-formed film (Flores, 2011). This method was used for preparation of alginate/chitosan (Arzate-Vázquez et al., 2012), alginate/pectin (Galus, & lenart, 2013), and alginate/cellulose films (Sirvio et al., 2014).
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Cha, Cooksey, Chinnan, and Park (2003) prepared nisin-incorporated films and assayed the effect of casting and heat-press methods as film preparation methods and the type of edible
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films (methylcellulose (MC), hydroxypropyl methyl cellulose (HPMC), κ- carrageenan and chitosan) on antimicrobial activity of the films. Results showed that heat-pressed films exhibited
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less inhibitory zones in comparison with casting method. The antimicrobial activity of nisinincorporated MC films was most effective in heat-pressed films as well as chitosan films in the cast films category. Casting method was also used for preparation of polycaprolactone/alginate
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based antimicrobial films (Takala et al., 2013) and for production of pectin/ κ- carrageenan
5.2. Edible coating formation methods 5.2.1. Spraying method
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composite films containing organically modified nanoclays (Falguera et al., 2011).
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This method can be used for low viscosity coating solutions, which can be easily sprayed
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at high pressure (60–80 psi) (Dhanapal et al., 2012; Tharanathan, 2003).The drop-size distribution of sprayed coating-forming solution in classic spraying system can be up to 20 μm, whereas electrospraying can produce uniform particles of less than 100 nm from polymer and
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biopolymer solutions. Furthermore, the formation of polymeric coatings by spraying systems can be affected by other factors such as drying time, drying temperature, drying method, and so on. (Skurtys et al., 2010). 5.2.2. Dipping method
Among different coating formation methods, only dipping techniques can form high thick coating (Dhanapal et al., 2012). Dipping method is used to form coatings on fruits, vegetables, and meat products (Lu, Ding, Ye, & Liu, 2010; Tharanathan, 2003). Properties such as density, viscosity, and surface tension of coating solution are important to estimate the film thickness (Skurtys et al., 2010). In this method, a thin membranous film is formed over the product surface by directly dipping the product into the aqueous medium of coating formulations, removing, and allowing to air dry. Foam application method is another coating formation method. This method
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is usually used by applying emulsions. In here the foam will break by extensive tumbling action, and therefore uniform distribution of the coating solution will be over the product surface (Tharanathan, 2003). Dipping method has been used to coat papaya fruit by using κ-carrageenan (Hamzah et
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al., 2013) and to coat carrot by using sodium alginate (Mastromatteo et al., 2012).
The formation of edible coatings on minimally processed fruits is problematic due to the
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difficulty of obtaining a good coating adhesion to the hydrophilic surface of the cut fruit. Multilayer technique is used to overcome it by the layer-by-layer electrodeposition. In here 2 or
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more layers of material are physically or chemically bonded to each other (Skurtys et al., 2010). The layer-by-layer method was used to produce multilayer antimicrobial alginate-based edible coating for fresh-cut watermelon (Citrullus lanatus) (Sipahi et al., 2013).A multilayer
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coating was also prepared for controlled release of polyethylene terephthalate by using κ-
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carrageenan and chitosan (Pinheiro, Bourbon, Quintas, Coimbra, & Vicente, 2012). 5.2.3. Spreading method
In spreading method or brushing method, the coating solution is spread on the product.
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González-Forte et al. (2014) used this method to coat dog biscuits with 2 different coatings. On
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one hand, they spread sodium alginate solution with a brush on the surface of biscuits and then sprayed a solution of CaCl2 to form a gel. On the other hand, they spread a gelatinized suspension of corn starch on the biscuit.
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6. Composite films and coatings
Edible film and coating materials must have favorable physicochemical and sensory properties to satisfy the consumers' product acceptability (Wang, Marcone, Barbut, & Lim, 2012). As each individual component has some defects, so applying one component with another one can cover their defects. Composites are hybrid materials, which their properties are different from their individual components of which they are made. For more than one hundred years, many new composite materials had been produced annually (Simkovic, 2013). It is possible to improve the technological properties of edible films and coatings made of single component by using chemical reactions (such as cross linking) or physical treatments (such as ultrasound, heat or radiation) (Wang et al., 2010). However, film functionality can be improved by combining different proteins, polysaccharides, lipids, as well as synthetic polymers.
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A synergistic effect of combined features of pure components may be achieved by designing composite films and coatings. Mechanical and barrier properties of composite films and coatings depend on properties of each forming polymers and their compatibility (Bourtoom, 2008; Galus
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& Lenart, 2013). 6.1. Carrageenan/ alginate – lipid composite films and coatings
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Composite films containing both lipid and polysaccharide can form packaging with good mechanical and water barrier properties (Hambleton et al., 2012). In here, lipids can be either
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used to obtaining an emulsified film by dispersing it in polysaccharide aqueous solution and then drying or can be cast as a layer on the polysaccharide film to obtain a bilayer film (Hambleton et al., 2012; Marcuzzo et al., 2010). The prior is used in most of food industry applications because
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it can improve water barrier properties and only need one step in manufacture in comparison to 3 step for latter (Hambleton et al., 2009; Hambleton et al., 2012). In addition, by using lipids to
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form emulsified films, active molecules can be encapsulated.
Carrageenan emulsion-based film was prepared to encapsulate different aroma compounds. The fat was added to the plasticized film-forming solution composed of carrageenan
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and glycerol. A mix of 10 aroma compounds was pre-solubilized in melted fat before being
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dispersed into the film-forming solution. Glycerol monostearate (GMS) was used as emulsifier. It was found that carrageenan films as encapsulating matrixes have better performances for retention of more polar aroma compounds. They can retain volatile compounds during film
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formation, and release gradually with time (Marcuzzo et al., 2010). Hambleton et al. (2012) measured permeability, sorption, and diffusion coefficient of the n-hexanal and D-limonene aroma compounds through emulsified and non-emulsified ι- carrageenan and sodium alginate based films. In another study, Fabra et al. (2012) analyzed the release of n-hexanal and Dlimonene from ι- -carrageenan films with and without lipid. They showed that D-limonene was encapsulated in the lipid phase of edible films and its release was decreased in the salt medium. 6.2. Carrageenan/alginate- polysaccharide composite films and coatings
Alves et al. (2010) studied the barrier properties of a polymeric matrix composed of κ- carrageenan and pectin with the inclusion of mica flakes. The effect of adding water as plasticizer on barrier properties of these composite films was also studied. Results demonstrated that there is a significant decrease of CO2 and O2 permeability in the films without mica flakes.
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Furthermore, above 10% of mica flakes in the composite, the barrier properties of the films were decreased. It was interested that a significant increase of both CO2 and O2 permeability was observed in hydrated films with 25% water. Biocomposite films based on 4 different wood cellulose fibers and alginate were prepared
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by Sirvio et al.(2014) and the film properties were studied. There was a good interaction between celluloses and alginate matrix. By increasing the amount of the fiber in the biocomposite films,
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thickness, tensile strength, and specific modulus of films were increased but strain and flexibility of films decreased and the mechanical properties of films were increased by increasing the
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amount of the fiber in the biocomposite films. Osorio, Molina, Matiacevich, Enrione and Skurtys (2011) produced edible film coating for fresh blueberries by incorporation of κ-carrageenan, plasticizer, and carnauba wax emulsion into the hydroxy propyl methyl cellulose and studied the
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film structure and functional properties. An increase in film structure and functionality was seen as a result. Martins et al. (2012) developed κ-carrageenan/locust bean gum blend films by casting
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method and studied films physical properties. The addition of κ- carrageenan to locust bean gum improved the barrier properties of the films. For example water vapour permeability of the films decreased. Moreover, the tensile strength of blend films enhanced. It is suggested that hydrogen
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films properties.
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bonds interactions between κ- carrageenan and locust bean gum have a significant influence in High cost of some polymers such as pullulan has limited their use. Blending them with other abundant polymers can overcome this problem. Alginate was successfully incorporated
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into the pullulan films by Xiao, Lim, & Tong (2012). It reduced not only the cost of pullulan films, but also potentially enhanced the material properties of the blend films. 6.3. Carrageenan/alginate-inorganic particles composite films and coatings
The addition of inorganic impermeable particles (for example mica flakes) into the polymer matrix enhances the barrier properties of the films. However, it makes the films less transparent and therefore not suitable for packaging applications. Reducing the size and the amount of the particles can overcome this drawback. Nanocomposites have obtained significant attention due to their improvement in the mechanical and physical properties (Alves et al., 2011). In the research done by Alves et al. (2011), composite film composed of pectin, κcarrageenan and modified nanoclays was prepared by casting method. Results indicated that the addition of 10% organically modified nanoclay particles to the polymer matrix has a positive
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impact on the barrier properties of the films. Rhim (2013) prepared a multilayer film with agar/ κ- carrageenan/clay nanocomposite and polylactide. The result indicated that the film properties of prepared film such as optical, mechanical, barrier, and thermal stability properties were
6.4. Carrageenan/alginate-fruit purees composite films and coatings
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improved.
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Production of edible films from fruit purees has some elaborations due to the presence of film-forming polysaccharides in their combination. The mechanical and barrier properties of
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those polysaccharides combine with the sensory and nutritional properties of the fruit in these films. Azeredo et al., 2012 evaluated the properties of the edible films from alginate-acerola puree reinforced with cellulose whiskers. Results showed that the whiskers could improve the
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water vapour barrier and overall tensile properties (except by elongation) of the films.
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7. Combination of different components with edible films and coatings Edible films and coatings can incorporate other components such as antibrowning, antimicrobial, antioxidant, and texture modifier agents (Bonilla et al., 2012; Osorio, Molina,
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Matiacevich, Enrione, & Skurtys, 2011; Zactiti, & Kieckbusch, 2006), colorants, flavors,
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nutrient, spices (Kang et al., 2013; Nussinovitch, 2009; Mastromatteo et al., 2012), surfactants, emulsifiers plasticizers, and so on. (Osorio, Molina, Matiacevich, Enrione, & Skurtys, 2011). In the other hand, blending with other food additives and modifier agents can extend applications of
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edible films and coatings. This is because of the weak mechanical properties (tensile strength and tensile elongation) and poor barrier properties of biodegradable films and coatings (Lee et al., 2004; Sorrentino, Gorrasi, & Vittoria, 2007) .For example, hydrocolloids are hydrophilic materials, and so are poor moisture barriers (Alves et al., 2011; Atarés et al., 2011) This property can be compensated by adding lipids, which are very good moisture barriers (Tharanathan, 2003).
7.1. Plasticizers Plasticizers are non-volatile and low-molecular weight compounds, which are added to polymers in order to reduce brittleness, impart flow and flexibility, and enhance toughness and strength for films. As a specific definition for coatings, plasticizers impact resistance of the coating and reduce flaking and cracking by improving coating flexibility and toughness. As a
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disadvantage, plasticizers generally increase film permeability to oxygen, moisture, aroma, and oils due to reducing intermolecular attractions along the polymer chains (Barreto et al., 2003; Rojas-Graü et al., 2007; Sothomvit & Krochta, 2005). Small size, high polarity, more polar groups per molecule, and more distance between
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polar groups within a molecule are plasticizer characteristics, which enhance plasticizing effects on a polymeric system. Plasticizers are generally required for polysaccharides or proteins based
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edible films (Skurtys et al., 2010).
Monosaccharides, disaccharides, or oligosaccharides (for example. fructose-glucose
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syrups, sucrose, honey, corn syrup) are commonly used as plasticizers in film systems (Azeredo et al., 2012). Polyols (for example glycerol, sorbitol, glyceryl derivatives, and polyethylene glycols) (Galus & Lenart, 2013; Hambleton et al., 2012; Hamzah et al., 2013; Myllärinen,
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Partanen, Sppälä, & Forssell, 2002; Song et al., 2011), lipids (Flores, 2011)), and derivatives (for example phospholipids, fatty acids, and surfactants) are also used as plasticizers ( Sothomvit &
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Krochta, 2005). Water can act as a plasticizer in hydrophilic biopolymers due to disruption of
7.2. Antimicrobial agents
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hydrogen bonds between polymer chains (Xiao et al., 2012).
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The incorporation of antimicrobial agents into the edible films and coatings has demonstrated to act as a stress factor to decrease pathogen growth and to protect foodstuff against spoilage flora. The use of chemical antimicrobial agents such as benzoic acid, sodium
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benzoate, propionic acid, sorbic acid, and potassium sorbate is limited in food systems due to health concerns of consumers. (Mastromatteo et al., 2012; Skurtys et al., 2010). Therefore, consumers demand for natural and healthy preservatives is caused to use of generally recognized as safe (GRAS) compounds (Flores, 2011). The most frequently used biopreservatives for antimicrobial packaging are lysozyme and nisin (Skurtys et al., 2010). Other antimicrobial compounds include organic acids (lactic, acetic, malic, and citric acids), chitosan, the lactoperoxidase system, and some plant-derived secondary metabolites such as essential oils and phytoalexins. Cassia, clove, garlic, sage, oregano, pimento, thyme, rosemary, lemongrass, scutellaria, and forsythia suspense are examples of such plants (Flores, 2011; Mastromatteo et al., 2012; Shojaee-Aliabadi et al., 2014). Edible films and coatings with antimicrobial properties can be named as active packaging (Juck et al., 2010).
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Metal nanomaterials as a noble kind of antimicrobial agents for food packaging system have received increasing attention in recent years. Among metal nanomaterials, nanosilver has been shown to be a promising antimicrobial material. Nanosilver particles can interrupt bacterial processes such as respiration and cell division and finally lead to cell death by attaching to the
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cell membranes and penetrating into bacteria. Bactericidal activity of nanosilver enhances by release of silver ions within bacterial cells. Jiang et al. (2013) investigated the effect of
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alginate/nanosilver coating on the microbial and physicochemical quality of shiitake mushroom during storage. Results indicated that the alginate/nanosilver coating has beneficial effect on the
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quality of shiitake mushroom and therefore, could be used for its preservation and shelf-life expansion. The antimicrobial property of alginate/clay nanocomposite films containing 3 essential oils (marjoram, clove, and cinnamon) against food pathogens was studied. Results
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showed that in all films marjoram had the highest antimicrobial activity (Alboofetileh et al., 2014).
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Azarakhsh et al. (2014) studied the influence of adding lemongrass essential oil to alginate based edible coating in microbial and physicochemical of fresh-cut pineapple. Results indicated that the incorporation of 0.3% (w/v) lemongrass in coating had potential to access both
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extension in the shelf-life and maintenance the quality of fresh-cut pineapple. In another study
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Takala et al. (2013) incorporated two antimicrobial formulations into alginate films, namely, A and B. The former contained organic acids mixture, rosemary extract, and Asian spice essential oil and the latter contained organic acid mixture, rosemary extract, and Italian spice essential oil.
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The inhibitory effect of each formulation on the growth of Listeria monocytogenes, Escherichia coli, and Salmonella typhimurium was investigated on fresh broccoli stored at 4°C. Results showed a good inhibitory capacity of alginate films containing formulation A against growth of mentioned bacteria.
Olaimat et al. (2014) evaluated the bactericidal property of allyl isothiocyanate incorporated into κ-carrageenan/chitosan coating against Campylobacter jejuni on fresh chicken. They found that κ-carrageenan/chitosan coatings containing allyl isothiocyanate had excellent potential to reduce C. jejuni viability on raw chicken. Chitosan and κ-carrageenan are oppositely charged polysaccharides, so a mixture of them has good barrier properties and can cause delayed release of incorporated bioactive compounds. The effect of incorporation of antimicrobial agents including nisin, Novagard CB 1, Guardian NR100, sodium lactate, sodium diacetate, and potassium sorbate into edible coatings
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including alginate, κ-carrageenan, pectin, xanthan gum, and starch on growth control of Listeria monocytogenes in poached and deli turkey products was studied by Juck, Neetoo and Chen (2010). Results showed that alginate based antimicrobial coatings can enhance the microbiological safety and quality of turkey poultry products by inhibiting the growth of Listeria
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monocytogenes. Meanwhile, the incorporation of nicin/sodium lactate and sodium lactate/sodium diacetate into the alginate coating effectively inhibited the growth of this pathogen. In another
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study, Mastromatteo et al. (2012) investigated the effectiveness of combined use of ethanol as antimicrobial compound and alginate based coating on the shelf-life of fresh carrots packed
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under passive and active modified atmosphere packaging. Results showed that the combination of dipping in ethanol and application of an alginate coating controlled both dehydration and respiration of sliced carrots. So, it caused a good preservation of sensory properties and
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prolonged the shelf-life of carrots.
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7.3. Antioxidant agents
Adding lipids to edible films and coatings in order to reduce water vapour transfer is popular. So, incorporation of antioxidants in edible films and coatings materials leads to increase
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product shelf-life by protecting foods against oxidative rancidity, degradation, and discoloration.
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It is interested that most antimicrobial agents have antioxidant properties. Natural and synthetic antioxidants are widely used in food packaging (Skurtys et al., 2010) Phenolic compounds, vitamins E and C, essential oils (Oregano and rosemary), sodium ascorbate, citric acid, and
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ferulic acid are the most common used antimicrobial compounds (Bonilla et al., 2012; Song et al., 2011).
Blanco-Pascual et al. (2014) extracted edible film-forming materials from brown seaweeds Laminaria digitata and Ascophyllum nodosum and studied the antioxidant activity of extracts. Results indicated that Ascophyllum nodosum films had higher antioxidant activity than Laminaria digitata. In another experiment Song et al. (2011) investigated the effect of incorporation of different antioxidants (vitamin C and tea polyphenols) into the alginate based edible coating on shelf-life and quality of refrigerated bream (Megalobrama amblycephala). They found that quality of treated product with vitamin C and tea polyphenols was higher than untreated ones. Vitamin C had the best effect in the case of reducing the degree of chemical spoilage, retarding water loss, and enhancing the overall sensory values of bream.
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7.4. Antibrowning agents
Great efforts have been done to increase the shelf-life of fresh cut fruits and vegetables by preventing cut surface browning. Incorporation of antibrowning agents into the films and coatings can improve colour preservation of fruits and vegetables. Ascorbic acid, citric acid, and
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some sulfur-containing amino acids (cysteine and glutathione) have been widely incorporated into edible coatings to prevent enzymatic browning (Conte, Scrocco, Brescia, & Del Nobile,
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2009; Robles-Sánchez et al., 2013)
The ability of alginate and gellan based edible coatings to carry antibrowning agents (N-
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acetylcysteine and glutathione) and the effect of antibrowning agents on water vapour resistance of coatings for fresh-cut apples were investigated by Rojas-Graü, Tapia, Rodríguez, Carmona and Martin-Belloso (2007). Results indicated that both antibrowning agents could be carried in
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the coatings and exerted their effects on fresh-cut apples. N-acetylcysteine had positive effect on water vapour resistance in the alginate coating, while in the gellan coating, its effect was
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negatine. Citric acid as an antibrowning agent was incorporated into the sodium alginate coating. The coating was used to prolong the shelf-life of minimally processed lampascioni. Results showed that the respiratory activity and the browning process of coated lampascioni were
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delayed. Result also indicated that the coated lampascioni packaged in a polyester based produce (Conte et al., 2009).
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biodegradable film showed the best performance in prolonging the shelf-life of the fresh-cut In another research, ascorbic and citric acid were used as antibrowning agents in alginate
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edible coating and their effects on color of fresh-cut mangoes was evaluated. The application of alginate coating with antibrowning agents maintained the color of fresh-cut mangoes (RoblesSánchez et al., 2013). 7.5. Other agents
Functional ingredients such as probiotics (Tapia et al., 2007), prebiotics (Rößle, Brunton, Gormley, Wouters, and Butler, 2011), minerals (Rhim, and Wang, 2014), and vitamins (RoblesSánchez et al., 2013) are other agents incorporated into the edible films and coatings in order to increase their functionalities. Edible films and coatings can also be used as a carrier to convey nutrients and nutraceuticals that are lacking or are present in only low quantity in food products (Rößle et al., 2011; Tapia et al., 2007). Flavour and pigments agents may also be incorporated
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into the edible films and coatings to improve the sensory quality of products (Skurtys et al., 2010) 8. Advantages and adverse biological effects
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As mentioned before, edible films and coatings should have no toxic effect on biological systems. Some known positive biological effects of carrageenans such as antitumor,
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immunomodulatory, anticoagulant, antithrombotic, and antiviral activities have been known. Furthermore, the antioxidant activity of all carrageenans especially lambda carrageenan is
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reported (Campo, Kawano, da Silva, & Carvalho, 2009; Prajapati et al., 2014).
Despite the positive biological effects, some researchers have focused on the adverse biological effects of carrageenans. These effects are mostly related to degraded carrageenan
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(poligeenan) which have molecular weights below 50 kDa (Chen et al., 2010; Liu, Zhan, Wan, Wang, & Wang, 2015; Prajapati et al, 2014). However, Tobacman (2001) reported the
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carcinogenic effects of undegraded carrageenans.
Degraded carrageenans are responsible for intestinal inflammation (Tobacman, 2001) and also, they are toxic to macrophages (Thomson & Horne, 1976) . Feeding 2 g/Kg body weight of
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degraded carrageenan to guinea pigs for 20-45 dayes caused ulcerative lesions (Prajapati et al,
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2014). Silva et al. (2010) evaluated the inflammatory properties of kappa, iota and lambda carrageenans. Lambda carrageenan was seen to have the most inflammatory activity and the least activity was reported for kappa carrageenan. Their results demonstrated that an increase in the
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sulfate groups led to inhanced inflammatory activity of carrageenan. However, toxicological properties of carrageenans have been shown at high doses that do not occur with the food additive. It is necessary for food grade carrageenan to have average molecular weight greater than 100,000 Da with a low percentage of smaller (Cohen & Ito, 2002; Prajapati et al, 2014).
Unlike carrageenans, the majority of articles have been just mentioned the advantages of alginate. However, it should be noted that only highly purified alginates have been reported to have no immunogenic response in mice. This means some impurities including heavy metals, endotoxins, proteins, and polyphenolic compounds may cause immunogenic responses (Lee, & Mooney, 2012). Alginates are known as dietary fiber (Houghton et al., 2015). The positive effects of alginates on intestinal absorption and colonic health have been reported by Dettmar, Stugala, &
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Richardson (2011). Some research has been reported the pharmaceutical activity of alginate molecule. This ability refers to alginates with high mannuronate residues which could induce cytokine production 10 times more than alginates containing high guluronate blocks (Lee, &
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Mooney, 2012). Conclusion
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Consumer demands for more natural foods, and also for environmental protection have excited the development of new packaging materials. Edible films and coatings have been
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received considerable attention over the last years due to their possibility to use as edible packaging materials over synthetic ones and to reduce the environmental pollution. Besides, development of edible films and coatings can reduce the post-harvest losses and also can provide
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less expensive packaging materials for industry and lower the prices of the food products. Edible films and coatings are defined as thin layers of materials used on food products that have
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important effects on their conservation, distribution, and marketing. Edible films and coatings can protect the product from mechanical damage, physical, chemical, and microbiological activities. Such films can be a carrier of antioxidants, antimicrobial, nutraceuticals, and
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flavorings agents or other additives to improve the mechanical integrity, handling, and quality of
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food products. Different biopolymers such as polysaccharides, proteins, and their blends are applied to form edible films and coatings. Among these biopolymers, alginates and carrageenans have been frequently used in recent years due to their good barrier properties to oxygen, carbon
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dioxide, and lipids as well as their superb mechanical properties (tensile strength and elongation at break). Commercialization of biopolymer films is limited due to their high sensitivity to moisture and their compatibility with other emergent stress factors such as high pressures, electric fields, ultrasound, microwave radiation, and gamma radiation. Technical information on edible films and coatings is far from adequate, so the food scientist has the formidable task of developing a film for each food application. Acknowledgments The authors would like to acknowledge the Isfahan University of Technology for providing an excellent library facilities and support for the preparation of review article.
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Rico-Pena, D. C., & Torres, J. A. (1991). Sorbic acid and potassium sorbate permeability of an edible methylcellulose-palmitic acid film: water activity and pH effects. Journal of Food Science, 56(2), 497-499. Rößle, C., Brunton, N., Gormley, R. T., Wouters, R., & Butler, F. (2011). Alginate coating as carrier of oligofructose and inulin and to maintain the quality of fresh-cut apples. Journal of Food Science, 76(1), 19-29. Robles-Sánchez, R. M., Rojas-Graü, M. A., Odriozola-Serrano, I., González-Aguilar, G., & Martin-Belloso, O. (2013). Influence of alginate-based edible coating as carrier of antibrowning agents on bioactive compounds and antioxidant activity in fresh-cut Kent mangoes. LWT - Food Science and Technology, 50(1), 240-246. Rojas-Graü, M. A., Avena-Bustillos, R. J., Olsen, C., Friedman, M., Henika, P. R., & MartínBelloso, O., et al. (2007). Effects of plant essential oils and oil compounds on mechanical, barrier and antimicrobial properties of alginate–apple puree edible films. Journal of Food Engineering, 81(3), 634-641. Rojas-Graü, M. A., Tapia, M. S., Rodríguez, F. J., Carmona, A. J., & Martin-Belloso, O. (2007). Alginate and gellan-based edible coatings as carriers of antibrowning agents applied on fresh-cut Fuji apples. Food Hydrocolloids, 21(1), 118-127. Rosa, M. D. (1972). Biological properties of carrageenan. Journal of Pharmacy and Pharmacology, 24, 89-102. Schnepp, Z., Wimbush, S. C., Mann, S., & Hall, S. R. (2010). Alginate-madiated routes to the selective synthesis of complex metal oxide nanostructures. CrystEngComm, 12, 14101415. Shojaee-Aliabadi, S., Hosseini, H., Mohammadifar, M. A., Mohammadi, A., Ghasemlou, M., & Hosseini, S. M., et al. (2014). Characterization of kappa-carrageenan films incorporated plant essential oils with improved antimicrobial activity. Carbohydrate Polymers, 101, 582-591. Silva, F. R. F., Dore, C. M. P. G., Marques, C. T., Nascimento, M. S., Benevides, N. M. B., Rocha, H. A. O., Chavante, S. F., & Leite, E. L. (2010). Anticoagulant activity, paw edema and pleurisy induced carrageenan: action of major types of commercial carrageenans. Carbohydrate Polymers, 79, 26-33. Simkovic, I. (2013). Unexplored possibilities of all-polysaccharide composites. Carbohydrate Polymers, 95(2), 697-715. Sipahi, R. E., Castell-Perez, M. E., Moreira, R. G., Gomes, C., & Castillo, A. (2013). Improved multilayered antimicrobial alginate-based edible coating extends the shelf life of fresh-cut watermelon (Citrullus lanatus). LWT - Food Science and Technology, 51(1), 9-15. Sirvio, J. A., Kolehmainen, A., Liimatainen, H., Niinimaki, J., & Hormi, O. E. (2014). Biocomposite cellulose-alginate films: Promising packaging materials. Food Chemistry, 151, 343-351. Skurtys, O., Acevedo, C., Pedreschi, F., Enronoe, J., Osorio, F., & Aguiler, J. M. (2010). Food hydrocolloid edible films and coatings. Nova Science Publishers, Inc (US). Song, Y., Liu, L., Shen, H., You, J., & Luo, Y. (2011). Effect of sodium alginate-based edible coating containing different anti-oxidants on quality and shelf life of refrigerated bream (Megalobrama amblycephala). Food Control, 22(3-4), 608-615. Sorrentino, A., Gorrasi, G., & Vittoria, V. (2007). Potential perspectives of bio-nanocomposites for food packaging applications. Trends in Food Science and Technology, 18(2), 84-95.
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Figure caption: Figure 1. Structural characteristics of alginates: (a) alginate monomers, (b) chain conformation, (c) block distribution (Draget, & Taylor, 2011). Figure 2. Formation of “eggbox” during gelation of alginate (Schnepp, Wimbush, Mann, & Hall, 2010).
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Figure 3. Intermolecular and intramolecular interactions of alginate and metal cations (Hassan, Gobouri, & Zaafarany, 2013).
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Figure 4. Two gel preparing methods: diffusion setting (a) (Juarez, Spasojevic, Faas, & de Vos, 2014), internal setting (b) (Tumarkin, & Kumacheva, 2009).
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Figure 5. Chemical structures of carrageenans.
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Figure 6. The gelation mechanism of κ-carrageenan in the presence of potassium ions (Rhein-Knudsen et al.,2015).
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Table 1
Table 1. Main hydrocolloids used for the formation of edible films and coatings (Milani & Maleki, 2012; Skurtys et al.,
Principal function
Agar (E406)
Gelling agent
Alginate (E400-404)
Gelling agent
Carrageenan (E 407)
Gelling agent
Carboxymethyl cellulose (E466)
Thickener
Hydroxypropyl cellulose (E463)
Thickener and emulsifier
Hydroxypropyl cellulose (E464)
Thickener
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Type of hydrocolloid
Methyl cellulose (E461)
Thickener,
emulsifier
and
gelling agent
an
Polysaccharide
2010).
chitosan
Gelling
and
antimicrobial
agent
M
Arabic gum (E414) Guar gum (E412)
starches
Fish gelatin
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protein
Bovine gelatin
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Pectin (E440)
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Xanthan gum (E415)
Emulsifier Thickener Thickener Gelling agent Thickener and gelling agent Gelling agent Gelling agent
Pig gelatin
Gelling agent
Whey protein
-
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Table 2
Table 2. Various edible films and coatings from alginates and carrageenans Type of hydrocolloid
Application
Product
Main results
Alginate
Film
Microwaveable
Increasing
food
efficiency
Coating
Fuji apple
Less browning
Packaging(film)
Broccoli
Inhibitory effect on growth
Takala,
of pathogens
Khan, and Lacroix (2013)
warming
Alginate/clay
nanocomposite/
Film
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-
Coating
Alginate/Lemongrass essential oil
Hosseini, and Abdollahi
turkey products
monocytogenes
Coating
(2014)
of
listeria growth,
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Κ-carrageenan/pectin/mica flakes
Κ-carrageenan/chitosan/
model
Film
Coating
Juck, Neetoo, and Chen (2010)
good adherent and stable film Less chemical and bacterial
Song, Liu, Shen, You, and
bream
spoilage
Luo (2011)
Extention shelf life
Azarakhsh,
fresh-cut pineapple
Carrageenan
Rezaei,
refrigerated
d
coating
growth
Inhibition
te
Alginate/antioxidant
Salmieri,
Alboofetileh,
Poached and deli
M
Alginate
Vu,
Inhibitory effect on bacterial
an
essential oil
and
Rojas-Graü et al. (2007)
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antimicrobial compounds
Salvador,
Fiszman (2012)
agent polycaprolactone/alginate/
Albert,
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Alginate/gellan/antibrowning
Source
Osman,
Ghazali, Tan, and Mohd Adzahan (2014)
Papaya
-
Reduction
moisture
loss,
Hamzah, Osman, Tan,
delayed ripening, retention
and Mohamad Ghazali
of firmness
(2013)
Improvement
of
barrier
Alves et al. (2010)
properties -
Dependent
release
of
bioactive compound (methylene
methylene
blue
on
blue)
concentration gradient and polymer
relaxation
Pinheiro et al. (2012)
of
nanolayers Cellulose/alginate
Film
-
Good mechanical and barrier
Sirvio,
Kolehmainen,
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properties
Liimatainen,
Niinimaki,
and Hormi (2014) Alginate/pectin
Film
-
Homogenous
and
Galus and Lenart (2013)
transparent film Coating
Dog biscuits
-
González-Forte,
Bruno,
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Alginate
and Martino (2014)
alginate/
i-carrageenan
Film
-
Dependence of permeability,
Hambleton et al. (2011)
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Sodium
/aroma compound
diffusion, and structure of
us
film on aroma compound characteristics Film
-
-
isolate (WPI) /gelatin(G) Κ-carrageenan/agar/clay
Film
-
Coating
(2010).
Improving film properties
Rhim (2013)
UV-activated
Reduction of dye leaching
Vu and Won (2013)
oxygen indicator
into water
M
/polylactide Alginate
Wang, Auty, & Kerry,
an
Sodium alginate (SA) /whey protein
Film
alginate-acerola puree/cellulose
Alginate-apple essential oils
Film
Ac ce p
Κ-carrageenan/locust bean gum
puree/
plant
-
te
Alginate/pullulan
d
films
Film
Film
-
Moisture sensitive
Xiao,
Lim,
and
Tong
(2012) Improving barrier properties
Martins et al. (2012)
and tensile strength -
Improvement of tensile and
Azeredo, Miranda, Rosa,
water
Nascimento,
vapor
barrier
properties of films -
Antimicrobial
and
de
Moura (2012)
activity,
no
Rojas-Graü et al. (2007)
adverse effect on mechanical properties of film
alginate/nano-Ag
Coating
Shiitake
Spoilage
mushroom
improvement
Reduction, of
sensory
Jiang, Feng, and Wang (2013)
atribiutes, lower weight loss i-carrageenans
film
Encapsulating
Suitable
aroma compound
encapsulation
for
flavor
Hambleton,
Fabra,
Debeaufort,
Dury-Brun,
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and Voilley (2009) i-carrageenans/ aroma compound
film
-
Controlled release of aroma
Marcuzzo,
componds,
Debeaufort, and Voilley
encapsulation
Coating
cinnamaldehyde
fresh-cut
Shelf-life
watermelon
extention,
Sipahi,
Castell-Perez,
Inhibitory effect on growth
Moreira,
Gomes,
of
psychrotrophics,
Castillo (2013)
Robles-Sánchez,
ip t
Alginate/beta-cyclodextrin/ trans-
(2010)
cr
capacity
Sensidoni,
and
coliforms, yeast and molds Coating
Fresh-cut
Kent
Coating
Alginate/chitosan
Film
te
glucomannan
compound in water
breasts
Ac ce p
agar/k-carrageenan/konjac
aroma compound
d
isothiocyanate/ mustard extract
Fast
chicken
Film
Fresh spinach
release
Rojas-
Graü, Odriozola-Serrano, González-Aguilar,
and
Martin-Belloso (2013)
Encapsulating
Fresh
high
antioxidant activity
M
film
κ-carrageenan/chitosan/allyl
retention,
an
mangoes
i-carrageenan
Color
us
Alginate /ascorbic acid/ citric acid
of
aroma
Fabra, Chambin, Voilley, Gay,
and
Debeaufort
(2012) Inhibition of C. jejuni, lactic
Olaimat,
Fang,
acid bacteria, and aerobic
Holley (2014)
and
bacteria growth Antimicrobial
and
Rhim and Wang (2013)
antifogging film with high water holding capacity
-
Rougher structure
Arzate-Vázquez
et
al.
(2012)
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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