Swelling, Solubilization and its Morphorogical Changes of Sweet Potato Starch Granules by Urea and Pullulanase

Abstract

The mild solubilizing method of sweet potato starch granules by urea and pullulanase treatment was studied with microscopical analysis. The solubilization of starch granules by urea was more effective at 20°C than at 4°C, and at 5M urea and above, their solubility increased with increasing of urea concentration. Starch solubilized at 10M urea was about 13% of original starch and about 67% of insoluble starch in 10M urea solution was further solubilized by the subsequent treatment with pullulanase. And also, the insoluble residual fraction of approx. 20% after 10M urea-pullulanase treatment were obtained. Concentrated urea affected the structure of the starch granules. At 7M urea or higher concentrations, sweet potato starch granules swelled rapidly to 2-3 times as large as their normal size. The inside parts of them were stained blue with iodine and the outside layers purple. When these granules were washed with water they lost the blue inside parts, leaving empty granular bags of the outside layers. After pullulanase treatment of the granular bags, the core like a surface layer was obtained. A model of sweet potato starch granule was proposed.