| Abstract: | The combined effect of oxidation and etherification on the molecular structure of starch was investigated. Maize and rice starches were used. Oxidation was effected using sodium hypochlorite while etherification was performed by reacting starch with acrylamide in presence of sodium hydroxide; this reaction is known as carbamoylethylation. The extent of the latter, expressed as % N, was found to increase by decreasing the molecular size of starch through oxidation. The carbamoylethyl starches so obtained could be precipitated by ethanol and they are soluble in water irrespective of the kind of starch. On the other hand, results of rheological properties indicated that they are governed mainly by the molecular size of starch. Oxidized starches with relatively high molecular size exhibit thixotropic behaviour. The degree of thixotropy decreases and changes into pseudoplasticity as the molecular size of starch decreases. The results also indicated that at constant rate of shear the apparent viscosity of carbamoylethyl starches decreases as the molecular size decreases. Furthermore, the rheological properties of carbamoylethyl starch pastes undergo considerable changes and so does the apparent viscosity when these pastes were stored for 3 days. |
