Effect of plasticizer content on the functional properties of extruded gelatin-based composite films
Graphical abstract
Highlights
► WVP values and the solubility of films in water were higher when the glycerol content increased. ► Glycerol content had a significant effect on the oxygen permeability. ► Glycerol content in film formulations has dramatic effects on the mechanical and barrier properties of the films. ► Films may have some commercial potential in its current form for niche food packaging applications.
Introduction
Plasticizers can be categorized as primary or secondary entities depending on the solubility of the plasticizers in highly concentrated polymers. If soluble, it is termed a primary plasticizer and able to gel the polymer rapidly under a normal processing temperature range (Vieira, da Silva, dos Santos, & Beppu, 2011). Meanwhile, secondary plasticizers have lower gelation capacities and normally blended with primary plasticizers to enhance product properties or reduce costs (Chanda & Roy, 1986).
For biopolymer-based films, plasticizers can be classified as water-soluble and water-insoluble (Siepmann, Paeratakul, & Bodmeier, 1998; Vieira et al., 2011). Apart from water, the most commonly used plasticizers in biodegradable and/or edible films are; glycerols, sorbitol, ethylene glycol (EG), vegetable oils, lecithin waxes and amino acids. In these cases, the plasticizers increase the free-volume or intermolecular spacing, molecular mobility and play an important role in increasing the flexibility and stretchability of biopolymers films produced from polysaccharide and protein sources (Sothornvit, Reid, & Krochta, 2002). Without the use of a plasticizing agent, biodegradable films tend to become brittle due to extensive intermolecular forces (forces that act between functional groups within macromolecules) involving polymer chain-to-chain interactions.
To date, the majority of studies carried out on the use of plasticizers for biopolymer film manufacture have employed casting, even for those specifically investigating gelatin (Bergo & Sobral, 2007; Cuq, Gontard, Cuq, & Guilbert, 1997; Jongjareonrak, Benjakul, Visessanguan, & Tanaka, 2006; Langmaier, Mokrejs, Kolomaznik, & Mladek, 2008; Nur Hanani, Roos, & Kerry, 2012; Thomazine, Carvalho, & Sobral, 2005; Vanin, Sobral, Menegalli, Carvalho, & Habitante, 2005). However, only a limited number of studies have investigated the effects of using plasticizers for the manufacture of gelatin-based composite films using extrusion. Krishna, Nindo, and Min (2012) studied the effect of glycerol concentration on the properties of fish gelatin films produced by extrusion followed by compression molding and found that films with higher glycerol content had the highest percent elongation at break but higher WVP values than those of solution cast films. Park, Whiteside, and Cho (2008) investigated the effects of using various plasticizers (glycerol, sorbitol and glycerol–sorbitol mix) on the mechanical and water barrier properties of extruded films derived from pork gelatin. These authors reported that stretchable films were obtained when plasticizers of glycerol or mixture of glycerol and sorbitol were used, whereas resins plasticized with sorbitol were extruded in non-stretchable sheets.
It has been reported that the type (based on compatibility) and quantity of plasticizer used in film manufacture strongly influences film formation from aqueous dispersions of polymers (Jonhson, Hathaway, Leung, & Franz, 1991). Glycerol has been identified and used as favorable plasticizer (Bertuzzi, Castro Vidaurre, Armada, & Gottifredi, 2007; Rivero, García, & Pinotti, 2010; Vanin et al., 2005) and has also been approved by the FDA (Food and Drug Administration) as a food additive (Li & Huneault, 2010). Glycerol has a high temperature tolerance, is water-soluble, polar and non-volatile (Banker, 1996; Yan, Hou, Guo, & Dong, 2012). Therefore, the objective of this study was to evaluate the influence of glycerol concentration on the mechanical and barrier properties of beef gelatin films manufactured by extrusion.
Section snippets
Chemicals
Beef skin gelatin (Bloom 220) powder with an average particle size of 1300 μm was purchased from Healan Ingredients Ltd (York, UK). Glycerol, used as the plasticizer and corn oil (CO) were obtained from Sigma Aldrich (MO, USA). For pH adjustment, sodium hydroxide (NaOH) was obtained in pellet form from Merck (Darmstadt, Germany).
Film preparation
Gelatin films were manufactured by a twin-screw co-rotating extruder (MPF model, APV Baker, Peterborough, UK). The barrel had 4 heating zones with a screw diameter of
Film thickness
The film thickness values for all gelatin films manufactured and investigated during these studies are shown in Table 1. The results indicated that the thicknesses of films were not significantly influenced by glycerol content. However, gelatin films manufactured with 1.1% glycerol had the highest thickness value of 23.5 μm.
Mechanical properties
The effect of plasticizer concentration on the mechanical properties of gelatin-based composite films is presented in Table 1. The TS of gelatin composite films decreased (P
Conclusions
It was clearly demonstrated in this study that the use of different concentrations of glycerol affected mechanical properties, WVP, OP and heat-sealability of all gelatin-based composite films extruded during this study. Increased glycerol content reduced TS but improved flexibility, thereby increasing EAB values. WVP values and the solubility of films in water were also higher when the glycerol content increased. Glycerol content had a significant effect on the oxygen permeability of films
Acknowledgments
We gratefully acknowledge the financial support from Ministry of Higher Education (MoHE) Malaysia, University Putra Malaysia (UPM) and Food Packaging Group, University College Cork (UCC).
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