Preparation and characterization of sodium caseinate films reinforced with cellulose derivatives

Abstract

Edible composite films, based on glycerol-plasticized sodium caseinate (SC) and either carboxymethyl cellulose (CMC) or cellulose acetate (CA) fibers, were respectively prepared by suspension casting. The effects of mixing SC with very low amounts (up to 3 wt%) of CMC or CA were systematically investigated through changes in morphology, surface hydrophilicity, moisture sorption, water vapor permeability, opacity, dynamic mechanical response and mechanical properties of the films. Incorporation of 3 wt% cellulose derivatives into the protein matrix led to slight but measurable decreases of equilibrium moisture contents, achieving reductions of 7.5 and 14.4% for CMC and CA reinforced films, respectively. Besides, the addition of CA contributed to the decrease of water vapor permeability, leading to a 38% falloff at 3 wt% filler. Contact angle measurements using a polar solvent ranged from 40° to 29–30° as filler concentration increased, indicating that reinforced films had higher superficial hydrophilicity than neat caseinate ones. Transparency decreased as filler concentration increased: the opacities of the most concentrated samples were 1.9 (CMC) and 2.3 times (CA) higher than that of the SC matrix. Moreover, tensile tests revealed that the addition of cellulose derivatives enhanced the tensile modulus (i.e. an 80% increase with 3 wt% CMC) and strength (i.e. a 70% increase with 3 wt% CA) although the elongation at break decreased. The differences found in the performance of both cellulose derivatives were explained in terms of water-solubility, hydrophobic character, rigidity of the fibers and quality of the filler dispersion in the protein matrix.

Introduction

Edible films based on agricultural materials have received much attention as potential packaging materials, mainly because such biodegradable films are considered to be a promising solution to environmental impact of synthetic polymer packaging (Su et al., 2010, Wang et al., 2008). Although these films are not meant to entirely replace synthetic packaging films, they do have the potential to substantially reduce the environmental burden due to food packaging, and to limit moisture, aroma and lipid migration between food components (Arvanitoyannis, 1999, Arvanitoyannis and Biliaderis, 1999, Krochta and De Muller-Johnson, 1997). Edible films can also be developed with many other functions such as carriers of substances (antimicrobial, aroma, pharmaceutical), coloring agents, or to improve mechanical handling of food (Langmaler, Mokrejs, Kolomamik, & Mladek, 2008). Among the naturally occurring edible materials, casein and casein derivatives have been extensively studied due to their low cost, availability, and complete biodegradability (Arvanitoyannis & Biliaderis, 1998). Caseinate presents thermoplastic and film-forming properties due to its random coil nature and its ability to form weak intermolecular interactions, i.e. the partially denatured peptide chains bond together primarily through hydrophilic and hydrogen bonds resulting in the formation of the protein matrix (Fabra et al., 2010a, Rhim and Ng, 2007). Moreover, casein-based edible films are attractive for food applications due to their high nutritional quality, excellent sensory properties and potential to provide food products with adequate protection from their surrounding environment (Atarés, Bonilla, & Chiralt, 2010). However, as other protein-based films, the hydrophilic nature of casein and caseinate based films limits their moisture barrier ability when compared to the commonly used synthetic plastic films (Fabra et al., 2010b, Han and Gennadios, 2005, Han and Gennadios, 2005). The second inherent problem limiting usage of pure sodium caseinate films is their inadequate mechanical properties (Ibrahim et al., 2010, Li et al., 2010, Sothornvit and Krochta, 2001). Thus, to improve the mechanical properties and moisture resistance, one of the available options can be blending/reinforcing caseinate with other polymers. In this respect, protein/cellulose derivatives composites have the potential to replace conventional food packaging leading to edible and biodegradable films and coatings. However, to keep the edible properties, the additives must be safe to eat or generally recognized as safe (GRAS). The additives should be compatible with proteins and have the ability to form films (Su et al., 2010). Water-soluble cellulose derivatives are used for packaging because of their edibility and biodegradability (García, Pinotti, Martino, & Zaritzky, 2009). At the same time they offer good barrier properties, being non-toxic, non-polluting and having low cost (Vasconez, Flores, Campos, Alvarado, & Gerschenson, 2009). Carbomethoxy cellulose (CMC), one of the most important commercially produced derivatives of cellulose (Psomiadou, Arvanitoyannis, & Yamamoto, 1996), is a typical anionic polysaccharide that has been widely used as a stabilizer in food (Togrul and Arslan, 2004a, Togrul and Arslan, 2004b) and can be a suitable additive for enhancing the properties of protein films. CMC is one of the natural water-soluble cellulose derivatives that have no harmful effects on human health (Su et al., 2010, Ma et al., 2008), therefore, it is used as a highly effective additive to improve product and processing properties in fields of application varying from foodstuffs, cosmetics, and pharmaceuticals to products for the paper and textile industries (Olaru et al., 1998, Togrul and Arslan, 2004a, Ma et al., 2008). CMC chains are linear β (1–4)-linked glucopyranose residues. Due to its non-toxicity, biocompatibility, biodegradability, hydrophilicity and good film-forming ability, CMC has been used in a high number of edible film formulations (Togrul & Arslan, 2004a). In the same way, cellulose acetate (CA), which is also made from different renewable resources of cellulose, has been widely used in oral pharmaceutical products and is regarded as a nontoxic, nonirritant and biodegradable material. Cellulose acetate had been used as a semi-permeable coating on tablets, which allows for controlled, extended release of actives (Zhou et al., 2006). It is also used in high volume applications ranging from fibers, to films, to injection molding thermoplastics due to its good physical properties, resistance to moulds and bacteria and relatively low cost (Aluigi et al., 2008, Chandra and Rustgi, 1998).

In a previous paper (Pereda, Amica, Rácz, & Marcovich, 2011), we studied the effect of adding low amounts of nanocellulose to sodium caseinate film forming solutions, with the aim of enhance the mechanical resistance and reduce the water vapor permeability (WVP) of the resulting composite films. However, the addition of non soluble cellulose particles produced only a marginal effect on WVP since there were two opposite factors affecting this behavior: both, the bubbles and pores content in the films and the path length for vapor diffusion increased as cellulose concentration increased. Consequently, the aim of this work is to enhance the dispersion of the filler into the sodium caseinate (SC) matrix by using water-soluble cellulose derivatives, in order to reduce the pore/bubble formation while maintaining the increased mechanical response. Thus, the effects of using CMC and CA to prepare reinforced films based on glycerol-plasticized sodium caseinate (SC) on the mechanical, optical and water barrier properties, microstructure and surface morphology are addressed.