Visualization of pressure-shift freezing and thawing of concentrated aqueous sucrose solutions

A visualization of pressure-shift freezing of 0.7 w/w sucrose solutions was carried out at three temperatures 268, 253 and 235 K by release of pressure from 200 MPa to atmospheric value. Furthermore, pressure-shift freezing at 268 K and additionally pressure-shift thawing was carried out for a solution of 0.2 sucrose mass fraction. The solid phase observed at 268 K in the case of solution with 0.2 w/w sucrose fraction was ice I. The phase changes of 0.7 w/w sucrose solutions at 253 K and 235 K resulted in formation of spherical solids which are hypothesized to be eutectics, i.e. crystals with structure containing sucrose hydrates and water. The visual evaluation revealed differences in size and distribution of spherical crystals. The solids formed at 235 K were more numerous, more homogenously distributed and finally smaller than those observed at 253 K. It is explained by the higher supercooling of the solution in the former case, which provided higher driving force for nucleation and crystal growth.