•Normal potato starch granules showed distinguished outer shells.•The outer shells were stiffer than the inner shells.•A refined blocklet model is proposed for potato starch granules.Potato starch was heated in distilled water and acetic acid solution (pH = 1.5), respectively, then washed with anhydrous ethanol and dried. After cooked in water with certain program, high-resolution scanning electron micrographs (SEM) of broken granules showed isolated outer shells. After treatment with acetic acid, SEM images of dissilient granules exhibited outer shells with randomly assembled spherical inner blocklets. A refined blocklet model was proposed for potato starch granule, the smaller blocklets were compacted densely to form the stiffer outer shell, and the larger blocklets were packed loosely to form the inner shells.

One acetylated waxy potato starch and three acetylated normal potato starch samples were treated with aqueous calcium chloride (4 mol/L) for surface gelatinization. The remaining granules after removal of gelatinized starch were investigated on size distribution and level of acetylation. Their crystal structure, thermal property and morphological change were studied with X-ray diffractometry, differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). Greater proportions of the acetyl groups were present at the periphery than at the core of the granule. Remaining granules obtained after higher degree of gelatinization showed greater relative crystallinity and gelatinization temperature. Morphology of gelatinized starch and remaining granules differed for acetylated waxy and normal potato starch samples. The results suggest that structural divergence exists not only between acetylated waxy and normal potato starches, but also within one granule.

•A method to interrupt the gelatinization process of starch was established.•Swollen blocklets deformed to form bead-like structure, and network structure in the end.•Morphological changes of the blocklets were not simultaneously.•A schematic diagram was presented for the morphological changes of blocklets.After combined hydrolysis by α-amylase and β-amylase at room temperature, spherical blocklets in diameters of 27–60 nm were observed on the surface of tapioca starch granules by scanning electron micrography (SEM). Tapioca starch (1%, w/w, db, distilled water) was heated by using a rapid visco analyzer (RVA) in four different programs, then the samples were settled and freeze dried, respectively. The SEM images showed that the blocklets swelled at 52 °C; the swollen blocklets deformed to olive shape, and linked by molecular chains, formed bead-like structure at 62 °C; they started to merge at 72 °C (pasting temperature); then the blocklets fused together and their shapes disappeared completely, and the gel network formed at 95 °C. Furthermore, the morphological changes of the blocklets were not simultaneously.