Physical properties of chitosan films incorporated with natural antioxidants

In the past decades, as alternative to the petroleum based products, biodegradable polymers, such as chitosan, have undergone extensive investigation in order to minimize waste disposal problem. Yet, biopolymers face some constraints, namely regarding their poor mechanical and barrier properties. Thus, the purpose of this work was to test the incorporation of several natural antioxidants, both oil and water based, in the chitosan matrix and to evaluate the effect on the physical properties of the resulting biopolymers. Five essential oils (EO) (ginger, rosemary, sage, tea tree and thyme EO) and six different hydro-alcoholic extracts (HAE) (ginger, rosemary, sage, black tea, green tea and kenaf leaves HAE) were used. Thickness of films produced did not change statistically with incorporation of the natural antioxidants tested (values were found in the range 52-71 μm, including pristine chitosan with 67 μm). The color of EO films was similar to control samples, but HAE turned the material to a more saturated color and less bright. EO's and (especially) HAE's (which reduced by 15-80% the transmittance compared to pristine chitosan) improved the light barrier of films, conferring to chitosan an extra protection against oxidative processes. In general, incorporation of HAE in the chitosan increased the moisture content (from 13% in the chitosan to 25% on average of the HAE films) and solubility (from 17% in the chitosan to 22% on average of the HAE films) while swelling degree decreased (from 191% in the chitosan to 139% on average of the HAE films), due to the interaction of water, chitosan and polyphenols present on extracts used. The same trend, although with less significance, was also observed in the EO's films. HAE's and (especially) EO's (which increased the tensile strength of chitosan from 20 MPa to 26 MPa, on average) active compounds changed the mechanical properties of chitosan films. To conclude, chitosan films incorporated with natural antioxidants have shown, generally, modified physical properties. Among the tested extracts, black tea and green tea HAE's and sage, thyme and rosemary EO's can be highlighted as the most promising due to the resulting mechanical properties of the prepared films. Yet, in order to identify if these novel materials can be used by the food packaging industry, further research is demanded to evaluate the behaviour of these biopolymers when in contact with food matrices.