Ellagic Acid May Improve Mechanical and Barrier Properties in Films of Starch-A Review Paper

Packaging increases the shelf life of food and facilitates its handling, transportation and marketing. The main packaging materials are plastics derived from petroleum, but their accumulation has given rise to environmental problems. An alternative is the use of biodegradable materials. In this regard, starch is an excellent choice because it is an abundant and renewable source with film-forming properties. However, the films obtained from starch have some limitations with respect to their mechanical and barrier properties. Several strategies have been developed in order to improve these limitations, ranging from the addition of lipids to the modification of the polymer structure. The aim of this review was propose the use of ellagic acid as a cross-linking agent that may improves the mechanical and barrier properties in films based on exists reports that phenolic compounds interact with starch-based materials decreasing their rate of retrogradation. Furthermore, ellagic acid is a powerful natural antioxidant, which would allow the production of active packaging with antioxidant properties, in addition to the improvement of the mechanical and barrier properties of starch films. In this concern more studies such as Fourier transform infrared spectroscopy, X-ray diffraction, differential scanning calorimetry and thermogravimetric analysis are necessary to verify the structural changes and interactions between starch and ellagic acid. We expect extensive use of it in the future of packaging materials.


Introduction
Food packaging plays a key role in the conservation, distribution and marketing of food products.Packaging protects the product from mechanical, physical, chemical and microbiological damage (Falguera, Quintero, Jiménez, Muñoz, & Ibarz, 2011).Plastics are chemically synthesized polymers that are widely used in food packaging, as their production is relatively simple and inexpensive (Ghanbarzadeh, Almasi, & Entezami, 2011); however, its use has caused environmental problems (Marsh & Bugusu, 2007).An alternative is the use of edible films and coatings, which contribute to the reduction of environmental pollution.These materials can be obtained from renewable and biodegradable sources, such as polysaccharides, proteins, lipids, resins and their mixture thereof (Campos, Gerschenson, & Flores, 2011;Ribeiro, Vicente, Teixeira, & Miranda, 2007).Starch is one of the most abundant polysaccharides in nature.It has a great variety of botanical sources, and it is relatively inexpensive to isolate (Bertuzzi, Armada, & Gottifredi, 2007;Jiménez, Fabra, Talens, & Chiralt, 2012).Generally, starch films have poor mechanical properties and can be highly hygroscopic (Chiumarelli & Hubinger, 2012;Ghanbarzadeh et al., 2011).The addition of substances that generate intermolecular bonds, improving the integrity of the film, is among the strategies to improve the properties of starch films (Olivato, Grossmann, Bilck, & Yamashita, 2012).Ellagic acid, besides being a powerful antioxidant, can interact with polysaccharides as cross-linking agent, retaining its antioxidant properties (Kim et al., 2009).Generally, the cross-linking of starch involves the formation of esters between the carboxyl groups of the cross-linking agent (generally a carboxylic acid) and the OH of the glucose in starch (Kim et al., 2009;Reddy & Yang, 2010).Ellagic acid it is a polyphenolic acid without carboxyl groups ; however, it is the product of the hydrolysis of ellagitannins (glucose esters and phenolic compounds) (Ascacio-Valdés, Aguilera-Carbó, Rodríguez-Herrera, & Aguilar-González, 2013), whereby it should be possible to make this acid work as a cross-linking agent in the presence of oxidized starches, which are characterized by the presence of carboxyl groups, to obtain biodegradable films with suitable properties for use as food packaging or coating.This article describes the findings that support our hypothesis.

Mechanical Properties and Water Vapor Permeability, a Limitation of Starch Films
The use of plastic materials, mainly derived from petroleum, shows an upward trend.About 150 million tons of plastic are produced each year throughout the world, which generates large amounts of waste that pollute the environment (Shit & Shah, 2014).Among the various applications of petroleum polymers is their use for food packaging, which is why in recent years there has been an increasing interest in developing packaging from biodegradable polymers (Mali, Grossmann, Garcia, Martino, & Zaritzky, 2002).In this sense, starch is a polymer with great potential for the manufacture of this type of material, since it is abundant, biodegradable and cheap (Mali et al., 2002;Wilhelm, Sierakowski, Souza, & Wypych, 2003).Starch is a reserve polysaccharide that is found in plant materials in the form of semi-crystalline granules which, depending on the botanical source, vary in shape, size, structure and chemical composition, which affects their functional properties and is composed of two glucose polymer molecules linked by a glycosidic bond; one is a linear molecule known as amylose, and the other a branched molecule known as amylopectin (Campos et al., 2011;Jiménez et al., 2012;Smith, 2001;Tharanathan & Saroja, 2001;Wilhelm et al., 2003).Although this polysaccharide has shown great potential as a film-forming material and has been extensively studied as such, its films are not suitable for commercial use, mainly due to their poor mechanical properties and high affinity for water (Mali et al., 2002;Schmidt, Porto, Laurindo, & Menegalli, 2013).The mechanical and barrier properties of the films prepared from polymers are determined by the structure of the polymer , its nature, the presence of polar and non-polar groups in the polymer chain, the glass transition temperature (Tg) and the degree of cross-linking (Gajdoš, Galić, Kurtanjek, & Ciković, 2000;Mrkić, Galić, Ivanković, Hamin, & Ciković, 2006).Films made only with starch are brittle; however, the addition of plasticizers such as polyols, mainly low molecular weight compounds, decreases the intermolecular attraction between adjacent chains in the amorphous region (Donhowe & Fennema, 1993), which in turn increases the flexibility and elongation of the films while decreasing their tensile strength (Jiménez et al., 2012).Moreover, it has been found that plasticizers increase the hydrophilicity of films, thereby increasing their permeability to water vapor, oxygen and other gases (Jiménez et al., 2012;Mali et al., 2002;Mali, Grossmann, & Yamashita, 2010).The high presence of OH-groups of the starch and some plasticizers makes films highly sensitive to contact with water or air at a high relative humidity, which produces an increased permeability to water vapor (Schmidt et al., 2013).According to Forssell, Mikkilä, Moates, and Parker (1997), the glass transition temperature (Tg) should be considered as the most important parameter of the mechanical properties of amorphous and semi-crystalline materials; thus, the process of recrystallization of these materials should be controlled.Materials such as films made from thermoplastic starch (gelatinized starch plus plasticizer) are able to recrystallize.This molecular rearrangement is accelerated when these materials are stored above their Tg (Mali, Grossmann, García, Martino, & Zaritzky, 2006;van Soest & Vliegenthart, 1997;Y. Zhang, Rempel, & Liu, 2014).The recrystallization of starch films, and the consequent modification of their mechanical properties and permeability, is a consequence of the retrogradation process, which is characteristic of starch-based systems (Campos et al., 2011).Retrogradation occurs after gelatinization, and it is the result of a molecular arrangement that consists in the formation of hydrogen bonds between the oxygen of the carbon atom 6 and the OH-of carbon 2 of the glucose residues from the molecules of amylose, and the OH-group of carbon 2 and the OH-of carbon 6 of the glucose residues from the short chains of the amylopectin molecule, respectively (Figure 1) (Y.Zhang et al., 2014).Under proper conditions, the molecular arrangement can be of the crystalline type (Buléon, Colonna, Planchot, & Ball, 1998).Mali et al. (2006) characterized the thermal, mechanical and barrier properties of films obtained from different starches (corn, yam and cassava) under controlled storage conditions (90 days, 64% relative humidity, 20 °C).These authors using differential scanning calorimetry noted that the type of starch

Oxidized Starches: Inclusion of Carboxyl Groups in the Structure
In their native state, starches may have certain limitations for specific applications, while their modification increases their range of possible applications (Simsek, Ovando-Martínez, Whitney, & Bello-Pérez, 2012;Zamudio-Flores et al., 2015).The modification of starches is done to improve or change properties such as the paste-forming profile, viscosity, gelling properties, the stability of viscosity at different pH and shear stress values, retrogradation trends, surface properties, ionic character, among others (Abbas, Khalil, & Hussin, 2010;Ayoub & Rizvi, 2009;Kaur, Ariffin, Bhat, & Karim, 2012).The chemical modification of starch involves derivatization of its molecules through etherification, esterification, cationization, cross-linking, and oxidation.The oxidation of starch is a modification that has been practiced since the early nineteenth century (Steve, Qiang, & Sherry, 2005).Its use in food has increased because oxidized starches have high stability, low viscosity and binding properties (Sánchez-Rivera, García-Suárez, Velázquez del Valle, Gutierrez-Meraz, & Bello-Pérez, 2005).The oxidized starches used in the food industry are mainly modified with sodium hypochlorite (Sánchez-Rivera et al., 2005).During the oxidation of starch with sodium hypochlorite, the hydroxyl groups of the glucose residues are oxidized to carbonyl groups (C=O) and, finally, to carboxyl groups (COOH); the presence of these groups is used as an indicator of the extent of the oxidation process (Y.J. Wang & L. Wang, 2003).In order to evaluate the effect of the concentration of the oxidizing agent (sodium hypochlorite, NaClO), Y.-J.Wang and L. Wang (2003) oxidized corn starch with NaOCl (0.25 to 3%).The authors observed that the presence of carbonyl and carboxyl groups increased with the increase in the concentration of sodium hypochlorite in the reaction.Both amylose and amylopectin were degraded during oxidation, but amylose was more susceptible to oxidation.Similar results were reported by Sánchez-Rivera et al. (2005), who oxidized banana starches with different concentrations of sodium hypochlorite.Kuakpetoon and Y.-J. Wang (2001) investigated the effect of the botanical of starch (corn, potato and rice) on the properties of starch oxidized by NaOCl.They analyzed the patterns of X-ray diffraction of the oxidized starches and found that the botanical source of starch had no effect on crystallinity, suggesting that oxidation takes place in the amorphous region of starch, which varies according to the type of starch.Kuakpetoon and Y. J. Wang (2006) evaluated the effect of amylose on the oxidation level of corn starches with different content of amylose.The authors used NaOCl (0.8, 2 and 5%) as oxidizing agent and found the highest concentrations of carboxyl and carbonyl groups in waxy starch, in which the concentration of NaOCl was higher (2 to 5%); they also reported that amylose was more sensitive to depolymerization, but that a higher content of it (70% amylose) hindered the formation of carboxyl groups.The authors explained the above by saying that it is probable that starches with up to 70% amylose do not allow the access of hypochlorite and water to their amorphous region, which would be necessary to carry out the oxidation of the OH-groups.As mentioned above, the modification of native starch changes its properties; thus, it is expected that the films obtained from modified starches also have different properties.Zamudio-Flores, Vargas-Torres, Pérez-González, Bosquez-Molina, and Bello-Pérez (2006) prepared films based on banana starch oxidized with three levels of NaOCl (0.5, 1.0 and 1.5%).Tensile strength increased with the oxidation level of starch, which probably was due to a greater oxidation.There were interactions between the carboxyl groups and the OH-of the glucose residues of starch through hydrogen bonds, which could have resulted in a greater integrity of the structure of the films.Moreover, S. D. Zhang, Zhang, Wang, and Wang (2009) evaluated the effect of the carbonyl groups on the properties of films based on oxidized corn starch plasticized with glycerol; the authors reported that the mechanical and thermal properties of the films improved with the increase of carbonyl groups, which in turn increased starch interactions (hydrogen bonds).Table 1 shows the concentration of carbonyl and carboxyl groups in starches oxidized with two concentrations of sodium hypochlorite.As can be seen, the carboxyl groups were present in greater concentration because they are the primary oxidation products.Similarly, it can also be seen that the presence of these groups depends on the type of starch.
Oxidation can also be performed with peroxides.Sangseethong, Termvejsayanon, and Sriroth (2010) oxidized commercial cassava starch (Manihot esculenta) using sodium hypochlorite and hydrogen peroxide at the same concentration (3% of oxidizing agent).They observed that the concentration of carboxyl groups concentration was higher when using sodium hypochlorite because the conversion of carbonyl to carboxyl groups was slower when peroxide was used.

Conclusions
This article proposes the use of ellagic acid as cross-linking agent; ellagic acid is a powerful antioxidant that is found in some plant tissues in the form of ellagitannins, which are esters of ellagic acid, and some polyol such as the anhydroglucose molecule.Ellagic acid is derived from the hydrolysis of a glucose ester and polyphenols presents in plants, which leads us to believe that it may participate in cross-linking reactions in starches, either reacting with the hydroxyl groups of glucose residues or with the carboxyl groups of oxidized starches, generating esters.This could improve the mechanical and barrier properties of starch-based films because cross-linking promotes strong interactions at the molecular level that strengthen the structure of the films and can reduce the interaction with water, providing bioactive films with high antioxidant capacity.
Further experiments should be carried out adding ellagic acid to the formulation of films based on native and oxidized starches in order to measure the effect of the presence of carboxyl groups in the interaction between acid and starch and its effect on the physicochemical and structural properties of the films.Ellagic acid has been used to autoclave chitosan, indicating its high stability at elevated temperatures.It would be interesting to subject native and oxidized starch to an autoclave process with ellagic acid and to elaborate films based on the obtained starches, to assess the effect of this process on the physicochemical properties of such films.FTIR studies should performed on the obtained films in order to verify the structural changes in the starch resulting from its possible interaction with ellagic acid, while X-ray diffraction studies should be done to observe any changes in the amorphous/crystalline arrangement of starch.It would also be interesting to evaluate the mechanical and barrier properties of these films to determine how these properties are modified by the inclusion of ellagic acid in the polymer matrix.Clearly, these studies could be performed simultaneously with thermal analysis by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) to evaluate the stability and thermal degradation of the polymer matrix.Finally, in order to characterize their functioning as active films, the antioxidant properties of these films should be evaluated.

Table 1 .
Content of carboxyl and carbonyl groups in oxidized starches