Characterisation of low-digestible starch fractions isolated from amylosucrase-modified waxy corn starch

Amylosucrase (AS) modification of starch increases the slowly digestible (SDS) and resistant starch (RS) fractions. However, the characteristics and formation mechanism of each fraction of AS-modified starch have not been determined yet. Therefore, this study isolated SDS and/or RS from AS-modified waxy corn starches and investigated their structural characteristics. The amount of SDS+RS and RS had a positive correlation with the proportion of the medium length (13-24 of degree of polymerisation) branched chains of amylopectin. The relative crystallinity increased in the order of AS-modified starch < SDS+RS < RS, while maintaining the B-type crystalline structure. The thermal transition temperature ranges of the isolated fractions were also higher than those of undigested starches. The medium branched chains of amylopectin were presumably the clincher for the SDS and/or RS formation in AS-modified starches. The principal causes of SDS and RS formation were the chain length elongation and the subsequent retrogradation-like process, respectively.