Impact of amylose content on the starch branch chain elongation catalyzed by amylosucrase from Neisseria polysaccharea

In this study, three rice starches with different amylose content were isolated from three rice varieties, followed by chain elongation using amylosucrase from Neisseria polysaccharea (NpAS). Herein, the chain elongation could induce the starch precipitation during the enzymatic reaction. With a higher content of amylose, an earlier precipitation occurred, leading to the decrease of transglycosylation efficiency of the enzyme. Results from X-ray diffraction and thermal property analysis indicated that the crystalline structure of the NpAS-modified starches largely formed at the precipitation point. The elongation at the surface of the starch precipitates, however, would not enable the formation of crystallites. In addition, the amylose might participate in the recrystallization of starch molecules. Digestion kinetics revealed that all of the NpAS-modified starches contained rapidly and slowly digested fractions, the latter of which had a digestion rate being dependent on the structural stability of crystallites (length of double helices). These findings may provide an efficient way to produce chain-elongated starches with desirable functionality and digestibility.