Heterogeneity of lotus rhizome starch granules as revealed by α-amylase degradation

Lotus (Nelumbo nucifera Gaertn.) rhizome starch granules have an elongated oval shape with the hilum located at one end. The morphologic characteristics were used as a direction anchor to study the heterogeneity of molecular organization of starch granules using microscopy before and after partial digestion by bacterial α-amylase (Bacillus sp.) The partially digested granule showed a single, big eroded hole at the end distant from the hilum. The enzyme-attacked end was revealed to be the loosely packed end and to be the weak point for enzyme hydrolysis. The α-amylase hydrolyzed the loosely packed central part of the granule faster than the densely packed periphery, and left an empty shell with a fish-bone-like tunnel inside. The periphery was more resistant to amylase hydrolysis and had strong birefringence. For the whole starch granule, the selectivity of α-amylase hydrolysis was low for the crystalline and amorphous regions and for amylose and amylopectin molecules. This study elucidated that the molecular organization of lotus rhizome starch granules was heterogeneous.