SOME CHARACTERISTICS OF HYDROXYPROPYLATED AND CROSS-LINKED RICE STARCH

Rice starch was cross-linked with phosphorus oxychloride (POCl3), hydroxypropylated with propylene oxide, and dual-modified with both reagents. The properties evaluated were: solubility in dimethyl sulfoxide, heat absorption during gelatinization, paste viscosity, and freeze-thaw stability. In addition, scanning electron microscopy was used to observe the morphology of the starch granules and their corresponding pastes. Cross-linking increased heat of gelatinization and shear stability, and it reduced solubility in dimethyl sulfoxide and freeze-thaw stability. Cross-linked starch exhibited a three-dimensional network structure under scanning electron microscopy. Opposite effects were found for hydroxypropylation. The hydroxypropylated starch paste had a planar structure. The procedure used to prepare dual-modified starches also affected the product properties. Cross-linking reduced the degree of subsequent hydroxypropylation, but hydroxypropylation increased the degree of subsequent cross-linking. The results indicate that hydroxypropylation of rice starch took place inside starch granules.