Binding Interaction of Porcine Pancreatic α-Amylase with waxy/amylose extender Double-mutant Rice Starch Granules Does Not Determine Their Susceptibility to Hydrolysis

The starch from waxy/amylose extender (wx/ae) double-mutant rice is almost indigestible by porcine pancreatic alpha-amylase (PPA) and behaves like resistant starch, even though PPA digests waxy mutant (wx) and wild-type (WT) rice starches easily. To clarify the relationship between binding ability and susceptibility to PPA hydrolysis, we studied the PPA adsorption capacity of wx/ae starch. Results showed that the wx/ae starch adsorbed 80% more PPA than wx and WT starches, consistent with the ratio of their specific surface areas. This suggests that PPA adsorption capacity is determined by the specific surface area of starch granules. Thus, the resistance to digestion of the wx/ae starch is not due to adsorption capacity, but to the higher-order nanostructure of the starch granules. The morphological features of digested wx/ae starch granules observed by scanning electron microscopy (SEM) suggest that the higher-order nanostructure of wx/ae starch is similar to that of resistant starches, such as high-amylose rice and potato starches.