| Abstract: | Starches having different molecular sizes were obtained by subjecting rice starch and maize starch, independently, to oxidation with aqueous sodium hypochlorite solutions of different concentrations. These starches were then reacted with urea in a dry state to yield mainly starch carbamate. The latter was isolated from the reaction products by treatment of the latter with either water or 4% aqueous NaOH solution. It was observed that regardless of the kind and molecular size of starch, treatment with water brings about starch carbamate which swells but does not dissolve completely in water. Against this was water soluble starch carbamate isolated by ethyl alcohol precipitation from NaOH solution (after neutralization). Studies of the rheological properties of the water soluble starch carbamates revealed that starch carbamates derived from rice starches are characterized by either thixotropic or pseudoplastic character depending on the molecular structure of starch. On the other hand, starch carbamates derived from maize starch exhibit pseudoplastic behaviour by virtue of their relatively lower molecular weight and higher carboxyl content. It was also found that the apparent viscosity of starch carbamates is determined by the kind of starch, the degree of oxidation and the storing time. |
