Modification of granular potato starch by multiple deep-freezing and thawing

The influence of multiple freezing of water inside or outside the starch granule on the modification of starch properties has been investigated. Freezing in liquid nitrogen, followed by thawing was repeated in several subsequent cycles for native starch and dried starch–water suspensions (1:1.5 v/v). After such procedures, the precise structural analysis of the starch granule surfaces was carried out by high-resolution non-contact atomic force microscopy. Moreover, gelation characteristics, X-ray diffraction as well as the water and iodine sorption ability of the processed granules were estimated. Pronounced changes were found on the granule surface. After the fifth freezing, a well-ordered microstructure of the surface with distinct straight chains of fine particles of about 30 nm in diameter could be observed. The surface regularities were accompanied by some irreversible disruption of the granule inner structure indicated by changes of the starch crystallinity and in water adsorption. The freezing/thawing process also influenced the gelation characteristics, water solubility and water holding capacity, but did not change the branching characteristics of the studied starch granules.