Rice stubble as a new biopolymer source to produce carboxymethyl cellulose-blended films
Introduction
Biopolymer films have been explored recently due to their biodegradability and potential uses as an environmentally friendly packaging material. Polysaccharides are the main source of biopolymers, especially carboxymethyl cellulose (CMC) (Bifani et al., 2007; Mali, Grossmann, García, Martino, & Zaritzky, 2006). CMC is a water-soluble cellulose derivative which can be used alone or combined with other materials into an edible film; for example, CMC films blended with konjac glucomannan (Cheng, Karim, & Seow, 2008), modified starch (Ghanbarzadeh, Almasi, & Entezami, 2010) and corn starch (Kibar & Us, 2013).
Basically, CMC-based film provides a poor moisture barrier and poor mechanical properties, (brittleness) due to its hydrophilic character (Bifani et al., 2007, Petersson and Stading, 2005). Thus, plasticizers are required to overcome the film brittleness caused by intermolecular forces (Bifani et al., 2007). Glycerol is widely used to improve film flexibility and extensibility as it adds increased mobility of polymer chains (Bifani et al., 2007; Cao, Yang, & Fu, 2009; Mali et al., 2006). For example, glycerol increased the elastic modulus and elongation of CMC films (Ghanbarzadeh and Almasi, 2011). However, glycerol also increased the hydrophilicity of the film resulting in higher permeability (Ghanbarzadeh et al., 2010). Fatty acids, lipids, and vegetable oils have been incorporated as an emulsion film to increase water barrier effectiveness (Ma, Tang, Yin, Yang, Qi et al., 2012; Ma, Tang, Yin, Yang, Wang et al., 2012). Among vegetable oils, olive oil is an interesting source of monounsaturated fatty acids which has positive health benefits, especially natural antioxidants (López-Miranda, Pérez-Martínez, & Pérez-Jiménez, 2006) to improve the film water barrier as shown in whey protein film (Javanmard, 2008), gelatin-based films (Ma, Tang, Yin, Yang, Qi et al., 2012; Ma, Tang, Yin, Yang, Wang et al., 2012) and chitosan emulsion films (Pereda, Amica, & Marcovich, 2012). Therefore, it is useful to investigate the effect of olive oil in CMC-based films as to date, there is no information available on this topic. This will benefit not only improving the film properties but will also provide nutrition when the edible film is applied directly to food products.
Recently, renewable sources to obtain CMC have been continuously explored as biodegradable and edible films and coatings (Haq, Hasnain, & Azam, 2014; Ma, Tang, Yin, Yang, Qi et al., 2012). CMC from sugar beet pulp (Toğrul and Arslan, 2004) and CMC film from durian rind cellulose (Rachtanapun, Luangkamin, Tanprasert, & Suriyatem, 2012) are example. In Thailand, rice stubble constitutes a large amount of the agricultural waste residues left in the field after harvest and when they are burnt, there are undesirable effects on environmental pollution and human health. Rice stubble consists of mainly cellulose and to date, no study has synthesized CMC from rice stubble. Therefore, the successful synthesis of CMC from rice stubble may be a replacement for the commercial CMC used in the food industry which must be imported from other countries. Moreover, conversion into CMC will add value to local agricultural waste from rice production.
Thus, the aims of this work were: (i) to select the desirable concentration of each plasticizer (glycerol, olive oil, and mixture of glycerol and olive oil) for commercial CMC (CMCc)-based films; and (ii) to study the feasibility of CMC from rice stubble (CMCr) as a replacement of CMCc and as a renewable source for biodegradable film and coating materials.
Section snippets
Raw materials
Rice stubble (Oryza sativa cv. Khao Dawk Mali 105), residues in a local rice field (Ratchaburi, Thailand), was washed with tap water, cut into small pieces (0.1–0.5 cm long), and then dried in an hot air oven (RF 115, RedLINE, Germany) at 60 °C for 10 h.
Commercial carboxymethyl cellulose (CMCc, CEKOL® 700 Cellulose gum, purity 99.5%, DS 0.75–0.85, molecular weight 270,000 Da) was donated from Winner Group Enterprise Plc. (Bangkok, Thailand). Extra light olive oil (BERTOLLI, Italy) was purchased
Glycerol
The MC of films significantly increased with the glycerol concentration (Table 2). Compared to the control film, glycerol increased the film solubility. However, the amount of glycerol did not affect any further reduction in the film solubility. Due to the hydrophilic nature, glycerol interacted with water molecules resulting in a higher MC and solubility (Ghanbarzadeh and Almasi, 2011, Haq et al., 2014; Jouki, Khazaei, Ghasemlou, & HadiNezhad, 2013). Similar results have been represented in
Conclusion
The first successful renewable biopolymer edible film was prepared from CMCr with glycerol and olive oil as the plasticizer. Incorporation of 20% glycerol or olive oil, or a combination of 10% glycerol and 10% olive oil into CMCc-based films showed equivalent film properties (moisture content, solubility, water vapor permeability, and tensile properties). At most, 50% CMCr can be replaced for CMCc to form blended film incorporated with 10% glycerol and 10% olive oil. This film was smooth and
Acknowledgement
The authors are thankful to the Center of Advanced Studies in Industrial Technology, Kasetsart University, Thailand for a financial support.
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