Improvement of mechanical properties and thermal stability of biodegradable rice starch–based films blended with carboxymethyl chitosan
Graphical abstract
Introduction
Among the several types of biomaterials, the polysaccharide starch is an attractive material due to its good film-forming capability, biocompatibility, relatively low cost, renewability and abundance. Rice starch (RS) is a by-product of rice processing. RS is considered to be a relatively cheap source of starch. RS contains around 30% amylose, which is the linear and the more readily crystallizable component of starch compared with amylopectin. RS can be used to produce biodegradable films (Janjarasskul and Krochta, 2010; Wittaya, 2012).
Polysaccharide films are good oxygen barriers with suitable optical properties and moderate mechanical properties at low relative humidity (RH) (Bourtoom and Chinnan, 2008; Laohakunjit and Noomhorm, 2004; Whistler et al., 1984). However, starch-based films are brittle and hydrophilic, which affect and limit their processing and application (Mendes et al., 2016). Plasticizers such as glycerol, sorbitol and polyethylene glycol have been used to overcome the brittleness of starch films (Suppakul et al., 2013; Wittaya, 2012), but their use decreases the tensile strength of the films (Tantala et al., 2012a). Blending, grafting, and compounding with other materials have been used as the main techniques to overcome this shortcoming (Al-Hassan and Norziah, 2012; Bourtoom and Chinnan, 2008; Dias et al., 2010; Ghanbarzadeh et al., 2010; Mathew et al., 2006; Saberi et al., 2016).
Chitosan is a cationic polysaccharide containing β (1–4)-2-amino-2-deoxy d-glucopyranose repeating units (Wu et al., 2013). Chitosan is a deacetylated product of chitin, and is abundantly available in nature, nontoxic and biodegradable. Chitosan has high quality film-forming capability (Wu et al., 2013); however, it dissolves in acidic water, which affects the finished product by causing a bad odor (Ferreira et al., 2009). Carboxymethyl chitosan (CMCh) is a water-soluble etherified chitosan. Applications for CMCh have been reported for food (Carolan et al., 1991), drugs (Chen et al., 2004), cosmetics (Wannaruemon et al., 2013), and agriculture (Dau et al., 2016). CMCh has the capability to form films and gels, is biodegradable and soluble over a wide range of pH, has high viscosity, biocompatibility and antimicrobial activity, and low toxicity (Bukzem et al., 2016; Tantala et al., 2012b). The introduction of CMCh in blend films can increase water solubility, improve tensile strength, and provide some antimicrobial capability (Fan et al., 2006; Tantala et al., 2012b). Previous studies have shown that the incorporation of chitosan or CMCh can improve the mechanical properties of rice starch–chitosan blend films (Bourtoom and Chinnan, 2008), corn starch–chitosan blends (Mendes et al., 2016), and pullulan–CMCh blend films (Wu et al., 2013). Tantala et al. (2012b) showed that CMCh has some antimicrobial capabilities against several strains of lactic acid bacteria but not against foodborne pathogens. Thus, incorporating CMCh into rice starch–based films could provide appealing film properties, and it is the focus of this work.
To the best of our knowledge, there is no published research on the development and characterization of RS-based films that incorporate CMCh, or on understanding their end-of-life scenario and biodegradability. Thus, the purpose of this work was to evaluate the effect of incorporating CMCh on the physical and thermal properties, permeability and biodegradability of RS–CMCh blend films. The interactions and compatibility between these two polysaccharides were also studied by Fourier transform infrared spectroscopy and X-ray diffraction.
Section snippets
Materials
Native RS powder (Rose 100R) was purchased from Thai Flour Industry Co., Ltd. (Bangkok, Thailand). Shrimp chitosan flake (molecular weight range 900,000–1,300,000 Da; degree of deacetylation 98%) was purchased from Taming Enterprises (Samut Sakhon, Thailand). Sodium hydroxide, glacial acetic acid, isopropanol, ethanol and methanol were purchased from RCI Lab-scan Co., Ltd. (Bangkok, Thailand). Glycerol was purchased from Union Science Co., Ltd. (Chiang Mai, Thailand). Monochloroacetic acid was
Results and discussion
CMCh was synthesized from chitosan via carboxymethylation using sodium monochloroacetate as the etherifying agent. The degree of substitution (DS) of the CMCh was found to be 0.49. The critical DS value at which CMCh becomes soluble in water is in the range of 0.40–0.45. Above this range, CMCh solubility increases with an increase in the DS value (Chen et al., 2003). This means that the synthesized CMCh was water-soluble.
Conclusions
This work emphasized the advantages of the addition of CMCh into RS-based film. The RS-based blend films that incorporated various CMCh contents had better mechanical properties, transparency and thermal stability than the RS film, while the CMCh did not affect oxygen and water vapor permeability. The optimum CMCh content for the RS–CMCh blend film to provide strong, flexible and durable film was 50% w/w of CMCh; the RS/50CMCh film had the highest tensile strength and elongation at break, and
Conflict of interest
The authors express no conflict of interest and no competing financial interest.
Acknowledgments
This work was supported by the Thailand Research Fund through the Royal Golden Jubilee Ph.D. Program (Grant No. PHD/0063/2555) and the Graduate School and Faculty of Agro-Industry, Chiang Mai University. We wish to thank the Center of Excellence in Materials Science and Technology for financial support under the administration of the Materials Science Research Center, Faculty of Science, Chiang Mai University.
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