Physical, mechanical and antibacterial properties of alginate film: Effect of the crosslinking degree and oregano essential oil concentration

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Abstract

Alginate films with different degrees of crosslinking obtained by internal gelation, and alginate films incorporated with oregano essential oil (OEO) were prepared. The impact of the degree of crosslinking caused by the use of calcium carbonate as crosslinking agent and the incorporation of OEO into the alginate films on their antibacterial, optical, mechanical, microstructural and water vapour barrier properties was evaluated.

An increase in the degree of crosslinking produced alginate films that were significantly thicker (0.031–0.038 mm) and stronger (51.9–52.9 MPa) but less elastic (2.3%) than those non-crosslinked films (0.029 mm; 39.7 MPa; 4.4%). The water vapour permeability (WVP) of the films decreased significantly only with the highest level of crosslinking.

The incorporation of OEO in alginate films affected significantly their physical properties. Thickness and percent elongation at break of the films were increased by the addition of OEO (0.036–0.042 mm and 2.7–3.7%), while the tensile strength and water vapour permeability decreased (31.1–55.5 MPa and 2.7–3.0 × 10−9 g/m s Pa).

Films incorporated with OEO were more effective against Gram-positive bacteria (Staphylococcus aureus and Listeria monocytogenes) than Gram-negative bacteria (Escherichia coli and Salmonella Enteritidis). A minimum concentration of 1.0% of OEO was necessary to ensure their antibacterial efficacy.

Highlights

► An increase in the degree of crosslinking affected the mechanical properties of films. ► The addition of essential oil improved the antimicrobial and barrier properties of films. ► Modifying the degree of crosslinking, it is possible to obtain an effective antibacterial alginate film.

Introduction

Microbial contamination of ready-to-eat products such as refrigerated meats and intermediate moisture foods is a serious concern to human health (Cárdenas et al., 2008, Cutter, 2000, Devlieghere et al., 2004). A traditional method used to control the growth of microorganism has been the application of antimicrobial dips or sprays on the surface of products. However, this has had limited success because the antimicrobial substances may interact with food components by evaporating or diffusing into bulk food (Kerry et al., 2006, Quintavalla and Vincini, 2002, Siragusa and Dickson, 1992).

One new approach to overcome these limitations could be the use of antimicrobial packaging techniques (Appendini and Hotchkiss, 2002), or the application of antimicrobial edible coatings (Zhou et al., 2010). To control undesirable microorganisms on food surfaces, volatile and non-volatile antimicrobial agents can be incorporated into packaging materials or coatings. Through this approach, it is possible to achieve greater efficiency if the antimicrobial agents are released slowly to the surface of food and remaining at effective concentrations for extended periods of time (Coma et al., 2001, Ouattara et al., 2000).

In order to meet consumer demands for more natural products and for packaging materials with low environmental impact, many researchers have focused on the incorporation of plant extracts into films, edible coatings and bio-based packaging materials (Del Nobile et al., 2008, Norajit et al., 2010, Oussalah et al., 2006, Rojas-Graü et al., 2007).

Essential plant oils and their constituents have been widely used as flavouring agents in food since early recorded history (Tiwari et al., 2009) and are categorised as Generally Recognised as Safe (GRAS) (López et al., 2007). Essential oils rich in phenolic compounds have been reported to have a wide spectrum of antimicrobial activity. Among these, clove, oregano, rosemary, thyme, sage and vanillin oils have been found to be the most effective (Holley and Patel, 2005).

Carvacrol, thymol, γ-therpinene and p-cymene are the principal constituents of oregano essential oil (Burt, 2004, Lamber et al., 2001). Its antimicrobial properties have been demonstrated in numerous studies (Avila-Sosa et al., 2010, Emiroglu et al., 2010, Ozkalp et al., 2010, Zivanovic et al., 2005, Seydim and Sarikus, 2006, Zinoviadou et al., 2009).

Alginates are natural substances extracted from brown seaweed and are composed of 1-4β-d-mannuronic acid (M) and α-l-guluronic acid (G). In polymer chains, monomers are arranged alternately in GG and MM blocks, together with MG blocks (King, 1983, Grant et al., 1973). The most interesting property of alginates is their ability to react with polyvalent metal cations, specifically calcium ions. The ions establish the cooperative association between M and G blocks, resulting in a tridimensional network where they may pack and be coordinated. This arrangement is pictured as the ‘‘eggbox’’ model (Grant et al., 1973).

Because alginate films are hydrophilic matrices, the crosslinking process with polyvalent cations has been used to improve their water barrier properties, mechanical resistance, cohesiveness and rigidity (Rhim et al., 2003, Rhim, 2004) and to delay the release of some drugs (Al-Musa et al., 1999, Chan et al., 2006, Remuñan.López and Bodmeier, 1997).

Due to the fast crosslinking process between alginate and calcium ions, localised gelling areas are produced, compromising the uniformity and quality of films. Draget et al. (1991) have proposed a technique (internal gelation) to form homogeneous matrices through the slow release of calcium ions from insoluble calcium salt in an acidified medium.

Antimicrobial films containing volatile antimicrobial compounds can be considered controlled release systems, and their effectiveness depends on the diffusion of volatile compounds through the polymer as well as their vapour partial pressure at saturation. Therefore, control of the release rates and migration of antimicrobial compounds from films, either by regulating the degree of crosslinking in the film or by using multilayer structures, could be an interesting strategy (Buonocore et al., 2003, Han, 2000). According to Cagri et al. (2001), a complete analysis of both antimicrobial and physicochemical properties is important for predicting the behaviour of antimicrobial edible films in food systems.

The objectives of this work were to evaluate the effects of the degree of internal crosslinking and the addition of oregano essential oil on the antimicrobial, optical, mechanical, microstructural and barrier properties of alginate films. The antimicrobial activity of alginate films containing different amounts of oregano essential oil was tested against Gram-negative bacteria (Escherichia coli and Salmonella Enteritidis) and Gram-positive bacteria (Staphylococcus aureus and Listeria monocytogenes).

Section snippets

Materials

The material used for film formation included food-grade sodium alginate purchased from Sigma–Aldrich Chemical Co., (St. Louis, MO, USA). Glycerol (used as a plasticising agent) and Tween 80 (used as a surfactant) were obtained from Merck Co., (Darmstadt, Germany). The analytical grade glucono-δ-lactone (GDL), calcium chloride and calcium carbonate used in the crosslinking process were purchased from Merck Co., (Darmstadt, Germany). Oregano essential oil (OEO), obtained from leaves and the

Properties of alginate film with internal gelation

The effect of the internal crosslinking of the alginate films, promoted by the slow release of calcium ions from CaCO3 in an acidified medium with GDL, on functional properties is shown in Table 1.

Conclusions

The properties of alginate films were affected by the addition of CaCO3 crosslinking. The crosslinked films showed high tensile strength, low elongation and porous microstructure compared with the control film. The addition of OEO in alginate films significantly affected their functional properties. The films obtained were less rigid, more flexible, and less transparent, with lower WVP compared with the control film. Films incorporated with OEO exhibited significant antibacterial activity

Acknowledgments

The authors thank the financial support of the Dirección de Investigación de la Universidad del Bio Bio (DIUBB) through Project 085922 3/R.

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