Preparation of slowly digestible sweet potato Daeyumi starch by dual enzyme modification

• Glycogen branching enzyme and amylosucrase were employed for modification of starch.
• Branch density and chain length distribution of amylopectin were altered.
• Dual enzymatic modification enhanced the contents of SDS and RS in sweet potato starch.
• Promoted retrogradation brought slow digestion property to the modified starch.

Sweet potato Daeyumi starch was dually modified using glycogen branching enzyme (BE) from Streptococcus mutans and amylosucrase (AS) from Neisseria polysaccharea to prepare slowly digestible starch (SDS). Dually modified starches had higher SDS and resistant starch (RS) contents than control starch. The branched chain length distributions of the BE-modified starches indicated an increase in short side-chains [degree of polymerization (DP) ≤ 12] compared with native starch. AS treatment of the BE-modified starches decreased the proportion of short side-chains and increased the proportion of long side-chains (DP ≥ 25) and molecular mass. It also resulted in a B-type X-ray diffraction pattern and an increased relative crystallinity. Regarding thermal properties, the BE-modified starches showed no endothermic peak, whereas the BEAS-modified starches had a broader melting temperature range and lower melting enthalpy compared to native starch. The combined enzymatic treatment resulted in novel glucan polymers with slow digestion properties.