Retrogradation of Sweetpotato Starch

Hardness and the percentage of leaked water of sweetpotato starch gels after storage were investigated as indexes of starch retrogradation. Starches of some varieties of sweetpotato were retrograded rapidly, but those of others were not retrograded during storage for one week. After one month of storage, starches of all varieties were highly retrograded, but the varietal order of hardness and the percentage of leaked water were almost the same as that after storage for one week. The study of chain length distribution by gel permeation chromatography after debranching by isoamylase showed that the hardness and the percentage of leaked water from the gel were positively correlated with amylose content and the proportion of Fr 1, the fraction of the highest molecular weight, containing amylose and extremely long chains of amylopectin, and negatively correlated with the proportion of Fr 3, the fraction with the lowest molecular weight, containing shorter chains of amylopectin. In addition, it was demonstrated by high performance anion exchange chromatography that the proportion of extra-short chains (around DP 10) of amylopectin after isoamylase treatment was negatively correlated with the retrogradation index of starch. These results suggest that retrogradation of sweetpotato starch was promoted by amylose and extremely long chains of amylopectin and was inhibited by extra-short chains (around DP 10) of amylopectin.