Chemical modification of cassava starch by carboxymethylation reactions using sodium monochloro acetate as modifying agent

Carboxymethylation of cassava starch was carried out in three different reaction media: iso-propanol (IP), dimethylsulfoxide (DMSO) and water. Monochloroacetic acid (MCAA) and sodium monochloroacetate (SMCA) were employed as modifying agents. With the objective of obtaining the best compromise between a high degree of substitution (DS) and a low degradation of the starting polymer (indicated by a high zero-shear viscosity of the carboxymethyl starch (CMS) in aqueous solution), the following reaction conditions were systematically varied: temperature, reaction time and reactants ratios. The carboxymethylation reactions carried out using DMSO or water yielded water soluble derivatives while those performed in IP were insoluble. When SMCA was used as a modifying agent, CMS with higher DS and viscosities were produced as compared to those obtained with MCAA. On the other hand, factors such as temperature and time increased the DS but reduced viscosity values. From all the studied conditions, those which produced the best results were: water as a solvent at 60oC for 6 hours and a starch/SMCA molar ratio of 1:3. Under these conditions the derivative produced exhibited a DS of 0.32 and a zero shear viscosity (o) of 0.073 Pa·s (for 2% CMS in water). DS values of up to 0.47 could be obtained in DMSO but with a o value as low as 0.006 Pa·s (for 2% CMS in water).