Shells And Blocklets Of Three Tuber Starches As Indicated By Enzymolysis Approach

Starch is an important food composition. Its granule structure is a hot area of starch research. Shell is one of the levels of starch granule structure. Blocklet is the building block of shell. However, only limited information is available about shell and blocklet of starch granule. The shell and blocklet structures of potato, sweet potato and tapioca starch granules were explored by controlled hydrolysis with α-amylase, β-amylase and pullulanase, alone and combined. The influence of enzymolysis was investigated by comparing the properties of starches before and after enzymolysis, including particle size, amylose content, thermodynamic property, pasting viscosity, gel property, transparency and crystalline property. The morphology, shells and blocklets of starch granules were observed by using polarized microscopy and scanning electron microscopy.Potato starch granules performed far lower enzymatic hydrolyzation than sweet potato and tapioca starches, suggesting higher resistance of potato starch granules to enzymolysis, due to the lower specific surface area and higher compactness of granule surface. The value of enzymatic hydrolyzation with α-amylase was around 20 times as those with β-amylase or pullulanase, indicating that the main linkages located on granule surface were α-1, 4 glycosidic bond, while the non-reducing end and the branch points(α-1, 6 glycosidic bond) located mainly inside granules.The properties of three tuber starches, including particle size, amylose content, pasting temperature, pasting viscosity, transparency and crystalline property, changed little after enzymolysis. This indicated the similarity of structure between enzymolyzed and native granules.The granules of three tuber starches, after treatment with α-amylase alone or combined, were cracked in varying degree. There was a cavity inside the granule for both potato and tapioca starches, but not for sweet potato starch. Little granules were cracked after treatment with β-amylase or pullulanase. After treatment with α-amylase alone or combined, there were scratches on the granule surface of three tuber starches, and pores randomly distributed on the granule surface of sweet potato and tapioca starches. No pores were found on the granule surface of potato starch after enzymolysis. The results suggested that the compactness of a granule was heterogeneous, loose in interior and compact at periphery, for potato and tapioca starches. There was less difference in compactness between interior and periphery of a granule for sweet potato starch. Furthermore, the compactness of granule surface was heterogeneous for sweet potato and tapioca starches, but not for potato starch.The shell structure was observed inside the broken granules of potato and sweet potato starches and the thickness of shells were 300~400 nm and 100~280 nm, respectively. There was no obvious shell inside the broken granules of tapioca starch.The arrangement of blocklets, appeared on the granule surface of three starches after treatment with α-amylase alone or combined, was regular. The blocklets, on granule surface, were 30~65 nm with compact arrangement for potato starch, and 30~60 nm and 32~73 nm with loose arrangement for sweet potato and tapioca starches, respectively.The average particle size of potato, sweet potato and tapioca starch granules were 28.8 μm, 13.5 μm, and 12.9 μm, respectively. By comparing particle size, shell thickness and blocklet size of three starches, it was predicted that shell thickness may related to particle size, while there was no obvious relation between blocklet size and particle size.In conclusion, shells and blocklets of tuber starch granules appeared only when α-amylase was used, alone or combined. This was not the case when β-amylase or pullulanase was used alone. Thus, α-amylase was most suitable for exploring shell and blocklet structures of starch granules.