Development and characterization of bilayer films of FucoPol and chitosan
Introduction
Primary packaging, defined as “a sales unit to the final user or consumer at the point of purchase” (European Parliament and Council Directive, 1994) tends to be the most visible aspect of packaging (Barlow & Morgan, 2013). The use of synthetic non-biodegradable polymers for primary packaging was tremendous in the last century, mainly because they are low-cost and present good mechanical and thermal properties, and are good barriers to gases, aroma compounds and microorganisms. Though primary packaging is mandatory for food preservation and protection, the intensive use of synthetic plastics created serious environmental problems because they are non-biodegradable and non-renewable materials.
This severe problem can be minimized using biodegradable natural and bio-based polymers instead of the non-biodegradable synthetic ones (Muscat, Adhikari, Adhikari, & Chaudhary, 2012; Siracusa, Rocculi, Romani, & Rosa, 2008).
As such, the attention has been driven to the search of alternative materials for use in food packaging. Bio-based films are usually designed from biodegradable, non-toxic and edible polymers (e.g. polysaccharides and proteins) and lipids (Galgano et al., 2015). The use of blends and multi-layers of those materials are strategies for new composite materials development, with properties that turn them potential synthetic polymers substitutes (Debeaufort, Quezada-Gallo, & Voilley, 1998; van den Broek, Knoop, Kappen, & Boeriu, 2015).
Polysaccharides obtained from plant, algae, animal and microbial origin (e.g. starch, alginate, chitosan, gellan gum) have been widely used for edible and/or biodegradable films development (Khwaldia, Arab-Tehrany, & Desobry, 2010; Song, Murphy, Narayan, & Davies, 2009; van den Broek et al., 2015). Such films are usually poor barriers to water vapour but good barriers to gases.
FucoPol, one of the microbial polymers referred in the literature, is a fucose-rich exopolysaccharide produced by the bacterium Enterobacter A47 (Alves, Freitas et al., 2010, Torres et al., 2011). It is a high molecular weight heteropolysaccharide composed of neutral sugars (fucose, galactose, glucose), an acidic sugar (glucuronic acid) and acyl groups (acetate, succinate and pyruvate). Glucuronic acid, together with succinate and pyruvate, are responsible for the anionic character of the biopolymer (Freitas et al., 2011). FucoPol has film forming capacity and its films have been reported to be transparent, with brown tone, hydrophilic with high permeability to water vapour and good barrier properties to gases (CO2 and O2) (Ferreira et al., 2014).
Chitosan is derived from chitin, which is the most abundant natural amino polysaccharide and the second most abundant biopolymer in nature. Crustacean shells, a food industry waste, are one of the chitin main sources (Ravi Kumar, 2000). Chitosan is a copolymer of β-(1-4)-2-cetamido-d-glucose and β-(1-4)-2-amino-d-glucose units, with the latter usually exceeding 60%. It possesses a cationic character, antimicrobial properties and film forming capacity (Elsabee & Abdou, 2013). Chitosan films have a selective permeability to gases (CO2 and O2) and good mechanical properties, but are highly permeable to water vapour that limits their use in food packaging applications.
The improved properties obtained by the combination of different hydrocolloids have been reported for several systems. Blends and bilayer films of chitosan and anionic polymers have been reported to have improved mechanical and barrier transport properties comparing to single component based films. This fact was attributed to the formation of polyelectrolyte complexes through electrostatic interactions between the protonated amino groups of chitosan and the negatively charged side-chain groups in the other biopolymer at the operating pH (Elsabee and Abdou, 2013, Luo and Wang, 2014; Nowzari, Shábanpour, & Ojagh, 2013).
Improvement in mechanical properties, better performance in terms of water vapour permeability and lower water solubility have been reported for blends and bilayer films of chitosan with starch, pectin or alginate (Jindal, Kumar, Rana, & Tiwary, 2013; Luo & Wang, 2014; Xu, Kim, Hanna, & Nag, 2005), gelatin (Rivero, García, & Pinotti, 2009) or whey (Kurek, Galus, & Debeaufort, 2014), comparing to chitosan stand-alone films.
Some authors reported difficulties in total solubilization of one of the polymers in specific conditions and formation of insoluble complexes between polymers in blends preparation (Ferreira, Nunes, Delgadillo, & Lopes-da-Silva, 2009). Otherwise, bilayer systems are reported to have better water vapour barrier properties than blend films (Kurek et al., 2014, Rivero et al., 2009). In this context, the aim of the present study is to develop bilayer films in combination with chitosan in order to enhance the properties of FucoPol films. The films were characterized in terms of their optical, hygroscopic, surface, mechanical and barrier properties, envisaging their potential use in food-packaging applications.
Section snippets
Materials
FucoPol was produced and purified as described by Ferreira et al. (2014). The freeze dried FucoPol was analysed in terms of chemical composition and average molecular weight. Commercial medium molecular weight chitosan (deacetylation degree of 75–85%) was purchased from Sigma (USA). Citric acid monohydrate was purchased from VWR chemicals—BDH Prolabo (UK). Glycerol (analytical grade) was used as plasticizer and purchased from Sigma (USA).
FucoPol chemical composition
FucoPol dried samples (5 mg) were hydrolyzed (2 h at 120 °C)
FucoPol characterization
The chemical composition analysis revealed FucoPol was composed of fucose (33 mol%), glucose (25 mol%), galactose (25 mol%) and glucuronic acid (15 mol%) and a total acyl groups content of 18 wt%. The average molecular weight was 4.7 × 106 g/mol.
Films appearance, colour alteration and transparency
FucoPol, chitosan and bilayer films were uniform and homogeneous by naked eye observation. The bilayer films presented a brownish tone, as well as FucoPol stand-alone films.
The transparency of bilayer films measured at 600 nm was 5.6 ± 0.3, while that of chitosan
Conclusions
Bilayer films of FucoPol and chitosan are homogeneous, flexible and resistant. They are transparent, causing colour alterations on coloured surfaces detectable by human eye. The films presented a high swelling degree in contact with liquid water and a high water vapour permeability, which disables their used in applications that involve the direct contact with high moisture content products. However, they have shown excellent barrier properties to O2 and CO2, better than those referred for
Acknowledgements
The authors acknowledge the financial support from Fundação para a Ciência e a Tecnologia, Portugal, with projects UID/Multi/04378/2013, UID/QUI/50006/2013 and PEst-OE/AGR/UI0245/2014; and Ana R. V. Ferreira, Cristiana A. V. Torres and Filomena Freitas acknowledge the fellowships SFRH/BD/79101/2011, SFRH/BPD/87774/2012 and SFRH/BPD/72280/2010.
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