Research PaperSynthesis and characterization of cassava starch with maleic acid derivatives by etherification reaction
Introduction
Starch constitutes the carbohydrate reserve of many plants, and it is found in leaves (chloroplasts) and in the reproductive organ (amyloplasts). It is extracted commercially from grains such as corn and rice and from tubers and roots like potato and cassava. In the food industry, it is used in soups, sauces, baking products, confectionery products, and dairy products, among others. It can also be applied in pharmaceuticals, textiles, fuels, biodegradable packaging materials, and thin films of thermoplastics, among other items (Kaur, Ariffin, Bhat, & Karim, 2012). Its application is due to its action as a thickener, emulsifier, gel-former and stabilizer, modifying the texture of the emulsions, and also as a pharmaceutical excipient (Wiacek & Dul, 2015).
Starch is a biopolymer composed of glucose as repeat units. There are, however, two different structures, amylose and amylopectin (Han, 2005). An important aspect to note is that it is a renewable, low cost, widely availability material, which can replace the petrochemical polymers currently used in industry (Mensitieri et al., 2011; Muller, Pires, & Yamashita, 2012; Ye et al., 2017).
As is the case in several biopolymers, starch exhibits the characteristic of hydrophilicity, which assists in biodegradability, but also impairs other properties such as water vapor permeability and interactions with hydrophobic substances (Soares, Yamashita, Müller, & Pires, 2013; Yu, Dean, & Li, 2006).
Starch modification is a methodology used in several areas to change the micro and macroscopic characteristics. Sonication and ultrasound can be used to alter the gel-forming properties of starch (Ashokkumar, 2015). Starch modification can also be used for the encapsulation of natural substances, such as Melissa Officinalis (Mourtzinos, Papadakis, Igoumenidis, & Karathanos, 2011). Among the various starch modification objectives, enhancing the compatibility of starch with hydrophobic materials in polymeric blends intended for the production of packaging materials is a notable example (Xiong et al., 2014, Zuo et al., 2015).
There are several starch modification techniques, including: i) chemical modification; ii) physical modification; iii) enzymatic modification; and iv) genetic modification (Kaur et al., 2012, Zhu, 2015). Within each class of modification exists a great diversity, for instance, possible chemical modifications include the reactions of esterification and etherification, of which the former are more frequently used than the latter.
The chemical modification of starch is of great interest when the surface acquires hydrophobic characteristics. When this characteristic is achieved, starch can be used in several situations, as emulsions, pharmaceutical excipients, hydrophobic polymer blends and others.
In this context, the aim of this study was to prepare starch derivatives by the grafting reaction of diethyl maleate, dipropyl maleate and dibutyl maleate (derived from maleic acid) in the macromolecular chain of starch by etherification reactions (Fig. 1), based on previous studies (Bien, Wiege, Warwel, & Addition, 2001; Wokadala, Emmambux, & Ray, 2014). This etherification reaction mechanism has been selected so that the two R groups present in the reactant are available to promote a surface interaction with another hydrophobic polymer material. After the etherification reaction, the degree of substitution and the physicochemical properties of the modified starch will be evaluated. The grafted starch must present a high degree of substitution, thus providing hydrophobic characteristics for the macromolecular chain for future applications in the area of biodegradable packaging and in the encapsulation of active principles for bioactive packaging and drug delivery.
Section snippets
Materials
Cassava starch (Manihot esculento), with 22.5 ± 2.5% of amylose and 14.4 ± 0.6% of moisture, was supplied by Indemil (Diadema-SP, Brazil). Maleic acid (analytical grade), acetic acid (analytical grade) and sodium hydroxide (analytical grade) were supplied by Vetec (Duque de Caxias-RJ, Brazil). Ethanol, 1-propanol, 1-butanol (anhydrous), Amberlist 15 hydrogen and dialysis tubing (D9402) were supplied by Sigma-Aldrich (Brazil). Hydrogen peroxide (35%) and sulfuric acid (analytical grade) were
Proton nuclear magnetic resonance (1H NMR) and infrared spectroscopy (FTIR)
Figs. S1 and S2 show the 1H NMR and FTIR of the synthetized diethyl maleate, dipropyl maleate and dibutyl maleate, respectively. These compounds showed ester groups, sp2 and sp3 carbons, with similar absorption bands to unsaturated esters (COC), in the range of 1300–1220 cm−1, sp2 carbon (CC) at 1644 cm−1 and sp3 carbons CH2 and CH3 at 2965 and 2879 cm−1, respectively. All FTIR spectra of the analyzed precursors showed an absence of OH absorption band at 3500 cm−1, suggesting that the
Conclusions
The etherification reactions of starch from maleic acid derivatives showed a high degree of substitution, with a mean of two substitutions at each repeat unit. Of the three reactive hydroxyls groups present in the starch repeat unit, it can be seen from the 1H NMR analysis that the hydroxyls attached directly to the glycosidic ring were preferentially substituted because the glycosidic ring stabilized the reaction intermediately, thus allowing the maleic acid derivatives to attack.
Grafting of
Acknowledgements
The authors are grateful for the financial support provided by CAPES and CNPq.
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