Precipitation Behaviour of Debranched Waxy Rice Starch in the Presence of Fatty Acid

Debranched waxy rice starch (DBS) and fatty acid (FA; i.e., lauric acid and stearic acid) were dissolved and co-precipitated in acetate buffer solutions pH 3-7. The X-ray powder diffraction (XRD) patterns of precipitants obtained displayed B-type at low pH (3-5) but Vh-type at pH 7, indicating that DBS precipitated as free form at pH 3-5 but as inclusion complexes with FA at pH 7. The precipitation behaviour of DBS in the presence of FA at pH 3-7 was evaluated, using rapid visco analyzer (RVA). RVA was able to detect the increased viscosity, resulting from both the precipitations of free DBS and DBS/FA inclusion complexes. DBS/FA complexes precipitated at a higher temperature than did free DBS. RVA profiles also revealed that DBS favorably precipitated as free form at pH 3 and as DBS/FA inclusion complexes at pH 7. It was thought that more FAs dissociated at pH 7 and resulted in the increased solubility which facilitated more FA to form inclusion complex with DBS. RVA profiles clearly demonstrated that lauric acid had less potential to form inclusion complex with DBS, particularly at a pH below the pKa. In addition, it was likely that the formation and the precipitation of DBS/FA complexes occurred at a higher temperature when the pH of the mixture was increased.