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 crosslinked 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 crosslinked 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.