Active chitosan–polyvinyl alcohol films with natural extracts
Graphical abstract
Highlights
► Active chitosan–PVA films were developed. ► Mint extract/pomegranate peel extract used as active ingredient. ► Films had improved tensile strength and permeability was not altered. ► The natural extracts conferred antioxidant properties to the film. ► Films exhibited antibacterial activity against S. aureus and B. cereus.
Introduction
Packaging is an important factor in food industry and is dominated by petroleum-derived polymers. However, the amount of research involving the production and characterization of biodegradable films has increased substantially, mainly due to interest in minimizing the ecological impact caused by the use of synthetic packaging materials. Several biopolymers have been exploited to develop eco-friendly food packaging materials (Siracusa, Rocculi, Romani, & Rosa, 2008). A significant proportion of research on these films has been made using biopolymers from renewable sources, i.e. products or by-products derived from agriculture or from agro-industries. Usually, films based on biopolymers are highly sensitive to environmental conditions and generally present low mechanical resistance. As a result, several researchers have developed films based on mixtures of biopolymers and synthetic polymers (Ghorpade, Gennadios, Hanna, & Weller, 1995).
Chitosan (Ch) is a very promising biopolymer because it is environmentally friendly due to its biodegradability and has good film forming properties. In the food industry, Ch films promise immense potential to be used as active packaging material due to its antimicrobial activity, non-toxicity and low permeability to oxygen (No, Meyers, Prinyawiwatkul, & Xu, 2007). Some synthetic polymers from non-renewable sources are also biodegradable, such as polyvinyl alcohol (PVA). PVA is a synthetic, water soluble polymer with excellent film forming, emulsifying, and adhesive properties. It also imparts good tensile strength (TS) and biodegradability and hence has been used in many biomaterial applications. PVA has also been approved for use in packaging meat and poultry products by the USDA (DeMerlis & Schonek, 2003). Ch contains free hydroxyl and amine groups, and is therefore miscible with PVA due to the formation of hydrogen bonds.
Polymer blending is one of the most effective methods to have new material with desired properties. Films formed by blending of polymers usually results in modified physical and mechanical properties compared to films made of individual components. Since synthetic polymers are easily obtained and have low production cost, blending of natural and synthetic polymers improves the cost–performance ratio of the resulting films. Blending of synthetic polymers, such as PVA (Bahrami, Kordestani, Mirzadeh, & Mansoori, 2003), polycaprolactone (Sarasam, Krishnaswamy, & Madihally, 2006) with Ch have been reported to improve mechanical properties of Ch films. In order to enhance utility of these films, addition of active agents is being investigated to improve food safety and quality. In terms of active agents that can be incorporated into films, plant extracts have received much attention as they contain high concentrations of phenolic compounds that possess strong antioxidant properties. Earlier studies in our laboratory have shown that mint extract (ME) and pomegranate peel extract (PE) have good antioxidant potential (Kanatt, Chander, & Sharma, 2007, 2010). These extracts effectively scavenged DPPH, hydroxyl and superoxide radical and their scavenging ability was comparable to the synthetic antioxidant, butylated hydroxytoluene (BHT). ME/PE also had good reducing power and iron chelation capacity. Hence, the objective of this study was to develop composite active films from Ch and PVA incorporated with ME/PE. Optical, mechanical, barrier, antioxidant and antimicrobial properties of these films were also investigated.
Section snippets
Materials
Commercial Ch from shrimp shells with a molecular weight of 1.86 × 105 Da and minimum deacetylation degree of 90% was purchased from Mahatani Chitosan Pvt. Ltd. (Veraval, India). 2, 2, diphenyl 1-picryl hydrazyl (DPPH) and catechin were purchased from Sigma Chemical Co. (St. Louis, MO). PVA (molecular weight 1.4 × 105 Da) and microbiological media were procured from HiMedia (Mumbai, India). All other reagents used were of analytical grade and procured from S. D. Fine Chemicals (Mumbai, India).
Preparation of mint extract and pomegranate peel extract
Appearance and film thickness
The composite films formed from Ch and PVA were visually homogeneous with no brittle areas or bubbles and could be easily peeled from the casting plates. Ch and PVA were miscible in all blends used in this study and this was attributed to formation of strong inter-molecular hydrogen bonds between Ch and PVA molecules (Kumar et al., 2010). Film thickness is a very important parameter in determining film's physical properties. The films had an average thickness of 70 ± 2 μm. Ratio of Ch/PVA and
Conclusions
This study demonstrated that active Ch–PVA films can be made by incorporation of ME/PE. The extracts introduced excellent antioxidant activities to the films. The release of phenolics from the films was temperature dependent and maximum amount of phenolics were released from the films at 37 °C. The incorporation of these extracts into the Ch–PVA films also improved their tensile strength without significantly affecting their barrier properties. The films showed antibacterial activity against
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