Cross-linked methyl cellulose films with murta fruit extract for antioxidant and antimicrobial active food packaging

Highlights

• Active films based on methyl cellulose(MC) and murta extract were prepared by casting.

• Glutaraldehyde (GA) was used as cross-linker to increase water resistance of MC films.

• GA had a significant effect on increasing the mechanical strength of the films.

• Highest antioxidant and antimicrobial activities occurred at lowest GA concentration.

Abstract

The use of biopolymers as substitutes for non-degradable traditional plastics is an interesting alternative particularly for short-term applications, such as food packaging. Additionally, active packaging has attracted much attention as an innovative technology for food conservation. Thus, the functionality of biocomposite films based on methyl cellulose (MC) and murta fruit (MU) (Ugni molinae Turcz) extract was studied. Murta fruit is a native Chilean berry and good source of antioxidant and antimicrobial compounds. First, a MU extract with the highest antioxidant ability and polyphenolic content was selected, and active MC films were prepared by casting using glutaraldehyde (GA) to improve their water resistance. The effects of GA concentration and incorporation of MU extract on material properties and antimicrobial and antioxidant activities were examined. The addition of GA greatly decreased swelling index, improved mechanical properties and antioxidant and antimicrobial activities achieved highest potential when GA was added at lowest concentration.

Introduction

During the last years, the research on the development of active materials with antimicrobial and antioxidant properties has increased enormously with the purpose to protect food from oxidative reactions and microbial growth. Furthermore, the application of natural ingredients is a new trend in the food industry and food research, i.e., natural preservatives derived from natural sources, instead of synthetic substances and the reduction of additives in foods. Natural substances such as α-tocopherol, essential oils and plant extracts have been incorporated into polymers for the preparation of active packages (Galotto et al., 2015, Gómez Estaca et al., 2014). On the other hand, the production and the use of plastics throughout the world have grown enormously, worsening environmental impact and problems of the waste disposal. The use of biopolymers as substitutes for non-degradable synthetic polymers is becoming a sustainable alternative, particularly interesting for short-term applications, such as food packaging. Thus, the development of biodegradable active packaging containing natural extracts derived from plants is one interesting strategy largely considered by the food packaging industry (Cooper, 2013, Seydim 2006; Song, Lee, Al-Mijan, & Song, 2014). One alternative to develop bio-based materials is by film coating multilayer. A film coating is a thin polymer-based coat between 20 and 10 μm applied to a solid support. Biobased coatings offer extra advantages such as low costs due to the reduction on amount of polymer, reducing terms of CO₂ emissions, and the processing temperatures are lower, which is very useful to avoid thermal degradation of active agents. For industrial processes, the techniques can be extrusion or co-extrusion for multilayer films, lamination, and mainly roll-drying for the solvent removal of the polymer solution (Debeaufort et al., 1998, Vartianinen et al., 2014).

Among the biopolymers, anionic linear polysaccharides derived from cellulose, such as methyl cellulose (MC), are very promising because they are environmentally friendly due to their biodegradability. In the food industry, MC films promise important potential to be used for packaging applications due to their excellent film-making properties, large availability, low cost, transparency, high-strength and easy ability to be processed (Aminabhavi et al., 2008, Cooper, 2013, Rimdusit et al., 2008). Methyl cellulose are polysaccharides composed of linear chains of β(1 → 4) glucosidic units with methyl substituents that provides films that are water soluble and oil-and grease resistant and with efficient oxygen and lipid barrier properties (García et al., 2004, Nisperos-Carriedo, 1994, Park and Ruckenstein, 2001).

MC films, like most films based on biopolymers, are highly sensitive to environmental conditions and due to their hydrophilic nature, physical properties are very susceptible to moisture and biodegradability. As a result, several researchers have developed new formulations through small chemical modifications of these polymers, such as crosslinking processes which promote covalent linkages between polymer chains. Crosslinking is one of the most popular methods used to modify water-soluble polymers in order to achieve desired properties. MC can be cross-linked by means of different methods, such as radiation or by using convenient chemical cross-linkers consisting on aldehydes, polyepoxy compounds, and even tea catechins (Atala, 2002; Wach et al., 2003, Yu et al., 2015).

Glutaraldehyde, GA, is a widely used cross-linker which forms a strong covalent attachment onto a polymer surface, providing the way for a rigid structure (Hernández-Muñoz et al., 2004, Lee et al., 2005, Mansur et al., 2008, Park et al., 2000). Some polymer characteristics could be altered by crosslinking such as permeability, mechanical properties and drug releasing. The effect of addition of cross-linkers on compounds that are desired to be released has been already studied, but mainly with pharmaceutical applications (Aiedeh et al., 2006, Ranjha and Qureshi, 2014, López de Dicastillo et al., 2011a; Martinez et al., 2014, Teng et al., 2011).

In the development of active materials it is important to reach a controlled release of active components to the food to protect it from oxidation or microbial spoilage, enhancing food safety and shelf life. In this work, the effect of crosslinking on the release of antioxidant compounds from active packaging was evaluated. Release of these active agents will be completely dependent on the swelling degree of the polymer. Due to its hydrophillicity, methyl cellulose was chosen in this work to be the polymer matrix in the development of active materials for food packaging systems (Calatayud et al., 2013, Lopez de Dicastillo et al., 2011b). On the other hand, GA was selected as cross-linker agent due to its proven effectiveness in other works and to avoid degradation of phenolic compounds propitiated by irradiation (Beaulieu et al., 1999, Hirashima et al., 2013, Park and Ruckenstein, 2001, Rimdusit et al., 2008). And finally, polyethylene glycol was incorporated as plasticizer. The use of plasticizer is very important on the materials development with biodegradable polymers because they decrease the intermolecular interactions among the functional groups of the backbone chains, resulting in increased flexibility and extensibility. In food industry, polyethylene glycol (PEG) has been widely used for MC products for rolling, casting, or extruding processes because it provides several advantages due to chemical interactions between MC and PEG structures (Park and Ruckenstein, 2001, Sarkar and Walker, 1995).

Ugni Molinae, also known as “murta”, “murtilla”, “uñi” or “Chilean cranberry”, is a native plant from Chile, western Argentina, and certain regions of Bolivia. It is a wild shrub of the Myrtaceae family that bears aromatic red globular fruit that is said to combine the sweetness of a strawberry with the pungency of a guava and the texture of a dried blueberry. Previous studies have shown murta fruit is composed by high levels of flavan-3-ols, such as catechin and epicatechin, flavonols, such as quercetin and quercetin-3-glucoside, and hydroxycinnamic acid, such as caffeic acid-3-glucoside. The other compounds were anthocyanins, like cyanidin-3-glucoside and peonidin-3-glusocide, rutin, gallic acid, quercitrin, luteolin, kaempferol, p-coumaric and myricetin, that could afford interesting antimicrobial and antioxidant activities for food protection (Alfaro et al., 2013, Junqueira-Gonçalvez et al., 2015, Ruiz et al., 2010, Schreckinger et al., 2010).