Effects of phosphorylation and cross-linking on the pasting properties and molecular structure of sago starch

Native sago starch was phosphorylated with 5% sodium tripolyphosphate (STPP) or cross-linked with 4% phosphorous oxychloride (POCl3) at pH 8 to 11. The modified starches were characterized for degree of substitution (DS), pasting properties, crystallinity and molecular structure. The results indicated that DS of phosphorylated sago starch prepared at pH 9 was 0.008, which was the highest value among those prepared at any other pH. For cross-linked sago starch, increase in the pH of the reaction mixture resulted in increase in DS. The swelling properties indicated unrestricted swelling of phosphorylated sago starch, while that of cross-linked sago starch showed restricted swelling. The restricted swelling might be related to the formation of inter-molecular bridges by phosphate group. The phosphorylated and cross-linked sago starches had degrees of crystallinity ranging from 22.96-24.76% and a C type of X ray diffraction pattern. The 31P NMR spectrum of phosphorylated sago starch (DS 0.008) indicated that the phosphorylation resulted in monostarch phosphate showing the phosphate groups attached at C-6 and C-3, while that of cross-linked sago starch (DS 0.018) showed a signal for distarch phosphate with a small proportion of cyclic monostarch phosphate.